forstbroaching (1)

8/10/2019 ForstBroaching (1)

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Oswald Forst GmbH & Co. KG, 42659 Solingen

Schtzenstrae 160, Telefon 0212 /409-130

Fax 0212/409-180

We reserve the right to make changes without notice.

We retain all rights and copyright, especially to the translated version. Reproduction and

mechanical or photographic reproduction of any kind is prohibited, whether in full or in part.

Copyright 2000 by Oswald Forst GmbH & Co. KG, Solingen.

Notes on broaching

MANUAL

Internet: http://www.forst-online.de

E-mail: [email protected]


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2

Foreword

Oswald Forst first published a manual on broaching in 1932 under the title Broaching,

a Guide for the Works Manager and the Designer.

The first Forst Handbook appeared thirty years later, in 1962, and provided

comprehensive information about the state of broaching technology. This book was

completely revised and reprinted in 1970.

The Forst Handbook is regarded as a standard work and text book in technical circles.

The comprehensive developments that have taken place since 1970 have caused us to

undertake a revision of our book and to issue it in a new form as the Forst Manual.

The new form simplifies our desire to update this book at specific intervals to the state

of the art.

Solingen, January 2000

Revised by:

H. Holstein in collaboration with R. Melcher, F. Stamm and D. Voigt


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4

Foreword to the 1st edition of the Forst Handbook

The present Forst Handbook was written as a result of a wish that was often expressed

among customers due to the importance of broaching. This covers both the technical

and the economic side of broaching, and gives a summarised view of the range of

products produced by our company. The Forst Handbook is intended to provide help in

solving the manifold problems encountered by the planning engineer and by the

process engineer in production. In addition, it should also provide information to all

interested parties concerning the manifold applications of the broaching process.

The Theory of the Broaching Process section is largely based on the results of

research from the Laboratory for Machine Tools and Industrial Administration at the

Technical University of Aachen. We wish to express our thanks to Professor

Dr. H. Opitz and Dr. H. Rohde, who wrote his dissertation about broaching, for providing

documents to us. We would also like to thank Dr. K. Schnert, Bielstein and Mr.

W. Wei, Cologne, for their help in the section on the feasibility of broaching various

types of materials.

Solingen, January 1962


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5

Foreword to the 2nd edition of the Forst Handbook

Since the first edition of our Forst Handbook appeared in 1962, there have been

dramatic developments in the area of broaching technology as a whole, which has been

primarily marked by so-called High Speed Broaching, i.e., the broaching of steel with

cutting speeds of more than 20 m/min., and the rapid automation of the process. The

development of a new generation of broaching machines in connection with ever better

devices, especially for the automatic handling of work pieces, has not only meant

greatly increased economy for the broaching process, but also a considerable

improvement in precision when using broaching in connection with more refined

production methods in the production of tools and equipment.

All this impelled us to completely revise and expand the first edition of our handbook so

as to bring our customers up to date within this second edition on the state of the art of

broaching technology and the standardisation that has been carried out in this area.

In addition to almost ten years of experience at our company in the area of high speed

broaching, gained as a result of comprehensive in-house research, this knowledge of

metal processing technology is primarily based on the results of research carried out by

the Laboratory for Machine Tools and Industrial Administration at the Technical

University of Aachen. We therefore wish to express our thanks to Professor Dr. H.

Opitz and Professor Dr. W. Knig, and also to Dr. M. Schtte, who wrote a dissertation

on Broaching at Higher Cutting Speedsand who also helped us greatly on the Theory

of the Broaching Processsection.

Solingen, September 1970


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6

Oswald Forst in Solingen, known for decades as a leading manufacturer of broaching

machines, broaching tools and broach sharpening machines, has always given new

incentives to broaching technology.

A quick look at the history of the company:

The company was established in 1909 by Oswald Forst, who registered his company in

1914. After initial tests with a wide range of products, Forst concentrated on broaching

machines from 1918 on the advice of machine trading company Alfred H. Schtte that

operated worldwide, whereby Schtte undertook the sales and marketing.

Initially, horizontal mechanically-driven broaching machines were produced. In addition,

and as a result of technical developments, Forst began the use of hydraulically-driven

units using oil from 1928 on. Initially, these were horizontal machines, and later vertical

machines in single- and twin-cylinder form. A separate plant was set up in 1940 for the

production of drive units, the Energators, known for short as ENOR drive units.

The main plant in Solingen was destroyed in a bombing raid in 1944. It was onlypossible to rebuild it after the currency reform in 1948, which took into account future

developments by making use of opportunities to expand. Additional production facilities

were established through the associate company Forst Broachbeing set up in 1957 in

the UK, and also through Dagger Forst in India, our joint venture partners since 1965.

How have matters developed subsequently?

The introduction of the high-speed broaching process in the 1960 s led to a

breakthrough in the broaching process for mass production. Automatic broachsharpening machines, which have been built since 1970, brought about a considerable

improvement in tool life.

The first helical broaching machine to broach helically-toothed inner gearwheels for

automatic gearboxes in cars was supplied by Forst in 1973. This complex technology

was systematically developed in order to successfully keep pace with the constantly

increasing requirements of users. The market leadership in Europe that was attained

could even be expanded up to the present day.


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8

Table of contents

Prefaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

The History of the company . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6

Table of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8

1. The basics of broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

1.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

1.2. Results that can be attained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

1 Surface quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

2 Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

3 Tool life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

4 Economic factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

2 . Theory of broaching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

2.1. Basic features and parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

1 Surface quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

2 Tool wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

3 Forces when broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

4 Chip formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

2.2. Factors affecting broaching results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

1 Tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .282 Work pieces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

3 Machines and devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

3. Broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

3.2. Design of broaches and systematic classification of commonly used cutting schematics .45

1 Single stepping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

2 Group stepping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

3 Back taper on broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

4 Internal broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

5 Forst full form monoblock broach for the broaching of gears . . . . . . . . . . . . . . . . . .53

6 External broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

3.3. Cutting materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

4. Notes on tool design and machine planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59

4.1. Calculation of broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

1 Basic design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60

2 Cutting schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61

3 Tooth geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62


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9

4 Rise per tooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63

5 Chip space size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

6 Selection of the chip space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

7 Cutting forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67

8 Calculation of tensile stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68

5. Instructions for broaching operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71

5.1. Broaching machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71

5.2. Cooling and lubrication when broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71

1 Basic principles of cooling and lubrication when broaching . . . . . . . . . . . . . . . . . . .71

2 Water-miscible metalworking fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72

3 Broaching oils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73

4 General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73

5.3. Care and maintenance of broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

1 Storing broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

2 Determining the end of the tool life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

3 Maintenance of broaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76

4 Machines that can be used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84

5.4. Defects when broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84

1 Defects due to the work piece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84

2 Defects due to the tools . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86

3 Defects due to the machine and devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88

4 Defects due to the metalworking fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89

5 Problems in broaching; searching for the causes . . . . . . . . . . . . . . . . . . . . . . . . . . .90

6. Hard broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95

6.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95

6.2. Main areas of application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96

1 Bearing area fraction of surface and quality of the jointed connection, e.g.,

gears with shafts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .96

2 Automatic joining in assembly lines for transmissions . . . . . . . . . . . . . . . . . . . . . . .97

3 Internal hard broaching as a basis for the hard machining of gear teeth . . . . . . . . .97

4 Precision in sliding gear such as synchro sleeves for transmissions . . . . . . . . . . . .98

6.3. Tools for hard broaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99

6.4. Economic aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99

6.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .100

7. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101

7.1. Symbols and units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101


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1.1. Definition

Broaching is a shaping process that makes

use of cutting. Broaches have a number of

teeth placed one after another, which have

a specific rise with respect to the previousteeth. In the case of the generally linear

relative motion of the tool with respect to

the work piece, the material of the work

piece is removed by the teeth coming into

contact one after another. The thickness of

the chip depends on the rise per tooth. In

the case of helical broaching, the linear

movement is overlaid by a rotary

movement around the longitudinal axis of

the broach.

Progress in the work during broaching and

the shape and dimensions of the broaching

depend on the design of the broaches and

the equipment used. A feed motion, such

as is found in turning, milling, or shaping,

etc., is not necessary. Either the work piece

or the tool can be moved. The machining

direction is generally horizontal or vertical.

Depending on whether broaching is to be

done from a hole or from an external

shape, a distinction is made between

internal and external broaching. In the case

of internal broaching, a broach is pulled or

pushed through a hole to produce the

predetermined profi le. In the case of

external broaching, the broach is passed

along work pieces fixed to suitable devices.

A special instance for external broaching is

pot broaching. In this case, work pieces arebroached around their circumference while

they are being pushed through a tubular

tool holder with broaches arranged around

the inner side. Internal broaching can be

used instead of internal turning, internal

grinding, reaming, drilling, shaping, etc.,

and external broaching instead of milling,

shaping, grinding and similar machining

methods.

1.2. Results that can be attained

1.2.1 Surface quality

As is described in more detail in section 2.

Theory of broaching, the surface qualityof broached parts depends on a number

of different factors. The material to be

broached and its micro structure, the

cutting speed, the metalworking fluid used,

and the state and design of the broach

greatly affect the surface quality.

When broaching steel, normally roughness

values (Rz) of 6 to 25 m can be maintained

within long term operation if, as is normal in

back taper broaching, the surfaces areproduced by the flank of the tools. Reduced

surface roughness is possible on a case by

case basis by taking special care, such as

by finishing with the main cutting edges of

the teeth. In any case, it is possible to

achieve substantially better surface quality

when broaching light metals and various

bronze alloys than is the case when

broaching steel.

The roughness increases as the tools get

blunter. The roughness thus also

determines the end of the tool life and thus

the time to change the tool.

1.2.2 Tolerances

As is also the case with other chip-

removing machining processes, the tightest

production tolerances can only be attained

in broaching by making a correspondingeffort in long-term operation. The primary

factor that affects the precision of

broaching, in addition to the production

tolerances of the tools and tool holders and

also the overall machine unit, is the shape

and the material of the work pieces.

Dimensional deviations are increased by

wear. Due to increasing wear of the cutting

edges and the higher passive forces

resulting from this the elasticity of shape

will lead to dimensional deviations.

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1. The basics of broaching

1.


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Higher strength of materials and un-

favourable micro structures promote wear

of the back off faces (clearance faces),

which shifts the cutting edge and produces

increased passive forces. In the case of

internal broaching, this has less effect withregard to profile dimensions in the area of

the normal splines and serrations. In the

case of external broaching, any positional

changes of the tool to the work piece affect

the results of the broaching. Thus

adjustable wedge strips in the tool slide

and also play-free guide systems are

advantageous (see section 5.4. Defects

when broaching).

Internal broaching

Given the prerequisite that the work pieces

are sufficiently stiff and that the material is

suitable for broaching, i t is normally

possible to obtain ISO dimensional quality 8,

and with an increased amount of finishing it

is also possible to obtain quality 7.

When broaching splines and serrations, it

is generally possible to obtain DIN 5480

quality 8, whereby the quality can be

regarded as the limiting profile between the

greatest individual dimension and the

smallest plug gauge profile. Quality level 7

is only possible here with increased effort.

Overall qualities of class 8 as per DIN 3960-

3962 can be obtained when broaching ring

gears, depending on the profile itself and

the shape of the workpiece, whereby the

qualities of individual items can besubstantially better (e.g. pitch errors).

Since it is hardly possible to prevent

internal broaches from drifting during the

broaching process, it is possible to have

small profile deviations. Parts that require

very precise runout must be clamped in the

broached profi le after the broaching

operation and then finish-machining must

be done. It is possible to take work pieces

into which a toothed profile has to be

broached for finish-machining in terms of

the internal diameter as well if the broaches

have an alternate finishing cutting section,

i.e., the profile teeth and the teeth for

broaching of the internal diameter alternate

with each other and thus a concentric

internal diameter for the profile can beproduced.

External broaching

The production precision for external

broaching depends on a larger number of

influences that can affect it when compared

to internal broaching. The machine and

clamping devices also affect the results, in

addition to the tool tolerances and the

insertion accuracy of the tools in the toolholders. Apart from constant errors caused

by production tolerances, there is also

a tolerance variation range. This variation

range depends above all on the play in the

moving parts that affect the results of the

broaching and also the stiffness of the

overall setup and the work pieces.

It is necessary to distinguish in external

broaching between the shape tolerances of

the broached profiles, the pitch tolerances

if multiple profiles are broached into a work

piece, and the positional tolerances of the

profiles with respect to the reference faces

of the work pieces. The shape tolerances

are dependent on the machine, tools,

fixtures and also on the work pieces, the

positional tolerances generally depend on

the work pieces. The accuracy of the

unmachined part and the clamping options

also affect these positional tolerances.Under normal conditions, it is possible to

comply with ISO qualities 7 to 9 in long

term operation; tighter pitch tolerances than

0.03 to 0.05 mm should not be called for;

the positional tolerance is between 0.03 mm

and 0.2 mm, depending on the properties

of the work piece.

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1.2.3 Tool life

Under normal conditions, i.e., flawless tools,

materials that are easy to cut and suitable

metalworking fluid, it is possible to achieve

a total length of cuts of 80 to 250 m beforethe tool has to be sharpened.

The life of the tool depends on a wide

variety of factors. The individual parameters

affecting this are discussed in detail in

sections 5.4. Defects when broaching, 2.

Theory of broachingand 5.2. Cooling and

lubrication when broaching. In addition to

determining when the tool life has come to

an end, it is also advisable to read section

5.3. Care and maintenance of broaches.

1.2.4 Economic factors

The basic factors for economical production

are:

- increasing the quality of the products,

reducing scrap and thus increasing

process reliability,

- reducing production times, thus increasing

capacity, and consequently

- reducing costs related to the product,

- and relieving human beings of mental and

physical stress.

The quality of the products depends in the

first instance on complying with thedimensional requirements and the surface

quality.

The use of high-quali ty broaching

machines, broaches and fixtures for

internal and external machining ensure

compliance with dimensions within tight

tolerances and with high surface quality.

Due to the relatively low wear of the tool,

the process is good for high volume

production, which is generally the

prerequisite for economical operation for

the process of broaching.

Compliance with dimensions for broached

work pieces cannot be affected much by

the operating personnel for the machines

that have been set up, and likewise there is

relatively little effect on the amount of

scrap. It is only necessary to reckon with a

number of wasted work pieces during test

broaching on machines that have just been

set up. This presupposes that the work

pieces meet the requirements of the

broaching process in terms of the designand technical properties.

The cutting capacity for broaching is

significantly higher even with low cutting

speeds than with comparable processes,

since the chip volume per tooth coming into

contact is large and at the same time many

teeth are engaged. Roughing and finishing

operations are generally done in one

operation, if multiple broaching operations

are not required due to the very high

volume of chips produced.

Normally, subsequent finishing operations

are not needed.

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2.

2.1. Basic features and parameters

Broaching tools have a number of teeth

arranged after each other (cutting edges),

by which the rise per tooth determines the

chip load h (depth of cut). The face angle and the clearance angle depend on thetype of material to be broached, the

clearance angle also depends on whether

the broach tooth is in the roughing and the

finishing section or in the reserve section of

the tool. The width of the land bf differs for

the reserve section or the other cutting

sections and also for the various kinds of

materials to be broached. The lands are

parallel to the axes in the reserve section,

and in the roughing and finishing sectionthey are arranged in ascending order by

the amount of the land angle on the back

off face (negative angle) to the end of the

tool (Fig. 3).

b width of cut

c depth of chip space

e thickness of tooth

bf width of land on the flank

bf width of land on the face

h rise per tooth

t pitch

r radius of cutting face

clearance angle

f land angle on clearance (back off) face face anglef land angle on the face inclination angle

15

2. Theory of broaching

Fig. 3


16/102

2.

2.1.1 Surface quality When broaching tough materials (steel,

heavy metal alloys), strain hardened and

brittle layers of the material are deposited

on the cutting face before the cutting edge.

These wedge-shaped build up edges

prevent the contact of the cutting edge withthe work piece and in practical terms take

over this function themselves (Fig. 4).

These built-up edges are subject to

fluctuations. They increase in a periodic

rhythm and parts migrate about the

underside of the chip and also between the

clearance face and the cut surface,

whereby the latter increase the wear of the

clearance faces. The particles that are

pushed into the surface of the work piece

thus lend it a scaly appearance (Fig. 5) andmake the surface quality worse. The run-in

surfaces of all broached parts are free of

built-up edge particles. The upper layers

only break off once the built-up edge has

reached a specific height. The distribution

of the scales is somewhat regular across

the other parts of the broached surface.

The roughness of the machined surface

can be reduced to a certain extent by

increasing the cutting speed, depending on

the material or the previous treatment of

the material. To what extent this increased

speed can affect the surface quality is

shown in Fig. 6 by the example of C 45

steel. This steel was subjected to various

forms of heat treatment. The illustration

shows the roughness when broaching soft

annealed samples according to the cutting

speed. In addition to the specific cutting

speeds, polished cross-sections weretaken through the point where the chip is

produced and polished cross-sections were

taken across the broached surface. The

fact that the roughness values decrease

rapidly with the broaching speed after

reaching a maximum at approx. 10 m/min

is of a special value in practice, since it was

only possible previously to use very low

cutting speeds and constantly a great deal

of t ime is required in broaching soft

annealed work pieces with regard to the

surface quality.

16


Fig. 4 23:1

Micro-section photograph of a chip root under thefollowing broaching conditions:C 45, S 6-5-2, = 2,= 15, = 0, v = 6 m/min, h = 0,1 mm, dry cutting,w = 260 mm

Fig. 5

Broached surface with scale formation, material:

C 45 soft annealed, v = 5 m/min, h = 0,04 mm,

dry cutting

Point of entry

directionofbroaching


17/102

2.

As can be seen from the polished cross-

sections through the point where the chip is

created, there are build up edges of

relatively pointed and unstable shape to be

observed in the area of cutting speeds

between 5 and 15 m/min, and these

protrude well above the cutting edge.

Within this area there likewise appear

larger scales on the cutting surface which

can be allocated to the maximumroughness values. At higher speeds, the

build up edges take on a flat and extended

appearance which only protrudes a little

above the cutting edge. The built-up edge

particles which have migrated with the cut

surface are still only relatively small so that

the roughness values are correspondingly

reduced.

Not all materials show an improved surface

quality with increasing cutting speed. The

roughness values were not significantly

less, for example, in the case of materials

with a ferritic-pearlitic grain structure when

using speeds over 10 m/min. Figs. 7a and

7b show a number of diagrams on the

attainable surface quality according to the

cutting speed for a series of different

materials and grain structures.

17


Fig. 6

Effect of cutting speed on surface quality when broaching C45 steel, soft annealed

Material: C45 G

Cutting material: high-speed steel M 34

= 2, = 15, = 0, h = 0.04 mm,Coolant: cutting oil t = 12.5 mm

Cutting speed

Roughnessvalue


18/102

2.

The good surfaces which could be

observed when broaching free cutting steel

are notable. This is due to a large extent to

the higher sulphur content, since it is

normally necessary to reckon with poorer

surfaces for steel with comparably low

carbon contents.

In addition, Fig. 7a shows that the

roughness is reduced by about half in the

case of grey cast iron of a higher strength.

The reason for this is the significantly finer

distribution of graphite plates and the fine

strip-like formation of pearlite plates.

High alloy steels such as X20CrMo13

behave somewhat like a hardened and

tempered steel in terms of surface quality,

i.e., it is possible to achieve similar or lower

roughness values in the area of higher

broaching speeds.

18


Fig. 7a

Attainable surface quality when broaching various types of material

Material: 9 S Mn 28

Material: 100 Cr6 soft annealed

Material: grey cast iron

Cutting speed v (m/min)

Cutting material: high-speed steel S 2-9-2-8

= 2, = 15, = 0, h = 0.04 mm, t = 12.5 mmCoolant: cutting oil

Roughnessva

lue


19/102

2.

What is interesting is the way that chip

compression depends on the cuttingspeed. The chip is subjected to the

minimum amount of compression at the

maximum roughness value and vice versa.

A significant factor that influences the

surface quality of a broached surface is the

rise per tooth of the tool. The rise per tooth

to be selected depends greatly on the

material to be machined and the profile of

the surface to be broached. It is possible to

achieve good surface quality over the entire

cutting speed range, as is shown in Fig. 8

by making a suitable selection of the rise

per tooth in the finishing section of the

broach. The chip load (depth of cut)

however must not be reduced too much, i.e.

to significantly below 0.01 mm, since the

teeth no longer cut but only press and thus

wear more quickly. The broaching results

concerning dimensional accuracy and

surface quality can be negatively affected.

19


Fig. 7b

Effect of various microstruktures on surface quality when broaching

Material: C45 N

Material: C45 soft annealed

Material: C45 V


Cutting material: high-speed steel S 2-9-2-8

= 2, = 15, = 0, h = 0.04 mm, t = 12.5 mmCoolant: cutting oil

Roughne

ssvalue


20/102

2.

Broaching tools are used in many cases up

to a width of wearmark of approx.

B = 0.4 mm. It is therefore the influence of

the total length of cuts on the surface

quality that is of great interest, since the

wear of the tool that increases with the total

length of cuts affects the surface quality of

the work pieces. As can be seen from the

diagram of Fig. 9, the roughness values

increase more strongly at the start and then

change to a flatter curve. Approximately

similar roughness values can be found for

the material being investigated at all cutting

speeds after a total length of cuts of 150 m.

After a specifically longer total length of

cuts (not shown here), the wear starts to

increase progressively and the roughnesslikewise. The significant factor of the

surface quality is therefore the raggedness

of the cutting edge produced by wear and

the formation of a built-up edge.

Similar results were found when broaching

grey cast iron. The surface quality was

slightly better at a cutting speed of

30 m/min, viewed over the total length of

cuts, compared to 10 m/min.

20


Fig. 8

Surface quality when broaching with various tooth

rise values (chip load)

Fig. 9

Relationship between total length of cuts and surface

quality for various cutting speeds

Material: C45 V

Cutting material: high-speed steel S 2- 9-2- 8

= 2, = 15, = 0, t = 12.5 mmCoolant: cutting oil

Cutting speed v (m/min) Total length of cuts w

Roughnessvalue

Averageroughnessvalue


21/102

2.

2.1.2 Tool wear

Wear is produced during broaching by the

friction between the clearance faces of the

tool teeth and the work piece and also

between the cutting face and the chip thatis curl ing, favoured by the cutting

temperature and the high specific face

pressures. At the same time the cutting

edges are rounded off. The wear that

increases with the number of broached

work pieces reduces the surface quality

and the ability of the work piece to maintain

dimensions and the pulling force increases.

The displacement of the cutting edge as a

result of wear causes a deviation in

dimensions. The rounding-off radii of thecutting edges are around 3 to 8 m in the

case of sharp tools and from 20 to 50 m in

the case of blunt tools. What is primarily

interesting is the tool wear when broaching

at higher cutting speeds.

The upper limit of the area of application

when using tool steel and high-speed steel

is determined by the rapid reduction of high

temperature strength by a loss of hardness

of the martensitic grain structure. Since this

temperature l imit can be reached byincreasing cutting speed, it is important to

know how high the temperatures are that

occur for various working conditions in the

area of the tool cutting edge.

Fig. 10 shows the effect of the cutting

speed, the rise per tooth and metalworking

fluid on the cutting temperature. The

temperatures were determined with the aid

of the one chissel process, which supplies

a medium temperature value for the entirearea of the contact zone.

21


Fig. 10

Cutting temperatures when broaching steel

Material: C45 VCutting material: high-speed steel S 2-9-2-8 = 2, = 15, = 0, t = 12.5 mm


Dry cutting Rise per tooth h = 0.04 mm


Dry

Oil

Emulsion 1:5

Cuttingtemperaturet

Cuttingtemperaturet


22/102

2.

As can be seen from the left-hand diagram

(Fig. 10), temperatures of 500C to 600C

were reached over a range of 20-30m/min

for a rise per tooth of 0.08 mm. These

conditions must therefore be regarded as

the upper limit of the area of application ofhigh-speed steel tools; i t is however

possible to achieve a reduction in

temperatures by using metalworking fluid,

as can be seen from the right-hand diagram.

When making broaching tests on C 45V

material with broaches made of high-speed

steel S 2-9-2-8 it was possible to determine

that there were different values of wear of

the cutting edge on the clearance faces

of the teeth after a total length of cuts of200 m, depending on the cutting speed.

When illustrating the wear of the cutting

edge B for a cutting speed v, this showed a

distinct minimum wear between

v=20 m/min and v=30 m/min. This means

an increase in l i fe of the tool when

broaching at approx. 25 m/min (the usual

cutting speed for high-speed broachingis

approx. 24 m/min) compared to low-speed

broaching. This increase in tool life is only

possible however if the rise per tooth does

not exceed a specific value due to the

associated increase in temperature at the

tool cutting edge. The maximum admissible

rise of the teeth for high-speed broaching is

further dependent on the length of cut,

since the cutting temperatures increase

with an increasing length of contact of the

tool teeth.

With the development of broaching using

high cutting speeds and the resulting

higher contact zone temperatures, it is

necessary to test with the individual work

operations to see to what extent cemented

carbide tools can be used for broaching.

Broaches with cemented carbides have

been used with success for the external

broaching of cast iron.

On the basis of several series of tests, the

best results were attained with cemented

carbide K 20. As can be seen from the

juxtaposition of the wear curves of hard

metal and high-speed steel tools (Fig. 11),

the clearance face wear is significantlylower with cemented carbide. The

scalloped knock-outs that occur on the

cutting edge of the tool can be attributed to

the lower toughness of the cemented

carbide compared to high-speed steel and

the interrupted cutting. The development of

extremely fine grain (< 1 m) and ultra-fine

grain (< 0.5 m) grades with their improved

mechanical properties also make cemented

carbides interesting for the cutting of steel.

Chipping of the cutting edge was not

observed when broaching grey cast iron

with cemented carbide, so here i t is

possible to recommend the use of

cemented carbide tools for the external

broaching of flat surfaces without any

reservations.

22



23/102

2.

23


Fig. 11

Comparison of clearance face wear when broaching with tools made of cemented carbide and high-speed steel

Broaching path w

Material: C45 NCutting speed v = 50 m/minRise per tooth h = 0.06 mmCoolant: cutting oil

High-speed steel S 2-9-2-8 = 2, = 15, = 0

Cemented carbide K20(single-tooth tool)

= 5, = 10, = 15

wearofthecuttingedge

B


24/102

2.

2.1.3 Forces when broaching

During the machining process, the

machine, the tool and the work piece are

subjected to the forces required to remove

the chips. In the case of broaching, thecutting force must be taken on the one

hand by the broaching tool and the drive

unit, and on the other hand by the machine

frame and the clamping table. The back

force represents a significant parameter in

the design of the work piece clamping

device in the case of external broaching.

The resulting force (the total force exerted

by a cutting tooth) which acts on a broach

tooth without an inclination angle during

cutting, can be divided into two

components. In the movement direction

there is the cutting force Fc, which isapplied by the pulling force of the machine,

and in addition the back force Fp which is

applied vertically (Fig. 12). This is taken up

by the work piece during internal

broaching, the tool is supported all around

by the walls of the work piece. In the case

of external broaching the work piece and

the tools are supported by the fixture and

the machine.

24


Fig. 12

Geometrical components of the total force (resultant force) exerted by the tooth during orthogonal cutting

F Resultant force

- total force exerted by

the tool

Fp Back force

Fn Perpendicular force on

the faceFc Cutting force

Ft Tangential force on the

face

bK Width of contact zone

h Rise per tooth

hch Thickness of chip

Shear angle Angle of friction on the

face

Face angle Clearance angleWork piece

Chip

Cutting wedge


25/102

2.

The actual pulling force of the broaching

machine varies periodically during the

broaching stroke, since a specific number

of cutting edges come in and out of contact

one after another. The variations in the

required force depend on the cutting cross

section and the specific cutting force and

also on the pitch, i.e., the distance of the

teeth from each other, and the broaching

length. The pulling force can be made moreeven by arranging the teeth at an

inclination angle (Fig. 13). At the same

time, additional side forces also occur

which act on the tool and the work piece.

An increase in pulling force during

machining can provide conclusions on the

bluntness of the tools.

As is known from other cutting processes,

an increase of the cutting speed has an

effect on the amount of the cutting force

components (Fig. 15).

There is a sequence concerning the

absolute amount of the forces for the

various heat treatment states, which can be

explained less by the strength of the

materials than by the process of theformation of the chip. A soft annealed steel

may have the lowest strength, but it is

subjected to the maximum chip compression

and forms the largest contact zone bk (see

Fig. 15) between the tool and the chip and

for this reason it shows the highest cutting

force components.

25


Fig. 13

Theoretical course of the cutting force over the

length of the cut for various inclination angles

(as per Schatz)

Fig. 14

Pulling force diagram when broaching work pieces of

various lengths

length of the cut l

Broaches

Bro

achingtensileforceFM

Broaching stroke

Ck 45; S 6- 5- 2; = 2, = 10, t = 12 mm,

v = 6 m/min, square: 18.88 mm; emulsion

Cutting

forceFc


26/102

2.

2.1.4 Chip formation

With improvements in the grades of high-

speed steel, the use of cemented carbidetools and increases in broaching speed, it

has also become more difficult to control

and monitor the curling of the chips at

higher speeds.

The chips produced in broaching can

cause damage to the tool or the work piece

and thus affect the work process under

certain circumstances, for example, as they

jam in the chip space and cannot be

flushed out by the metalworking fluid. Thechip shape is significantly determined,

among other things, by the strength of the

material being broached and its micro

structure. In general, it is necessary to

reckon with more strongly curled chips as

the strength increases.

Fig. 16 shows a number of photographs

which illustrate the formation of the chip

during the broaching process. The spirals

produced in the broaching of normalizedand hardened and tempered steels may

indeed increase with increasing cutting

speed, but the chip shape itself can be

regarded as good up to 50 m/min. On the

other hand, the soft annealed material once

again shows an extended chip which tends

to jamming, especially at higher speeds.

It is necessary to pay special attention to

the design of the tooth space when

broaching soft annealed steels, whichfrequently tend to produce chips of this

shape. In the case of external broaches,

the shape of the chip can be especially

favourably influenced by grinding a chip

breaker, for example. In other cases in

which this particular measure is not

feasible, it is necessary to provide a means

of help by grinding larger face angles and

increasing the chip space and thus the

tooth pitch.

26


Fig. 15

Resultant force components when broaching steel

Resultantforcecom

ponents

Resultantforcecomponents

Cutting material: high-speed steel S 2- 9- 2- 8

= 2, = 15, = 0, t = 12.5 mmWidth of cut b = 10 mm

Coolant: cutting oil

Cutting force Fc Cutting force Fc

Material:C45 V

Back force Fp

Material: C45 soft annealedC45 VC45 N

Rise per tooth h = 0.04 mm

Cutting speed v Rise per tooth h

Back force Fp


27/102

2.

27


Fig. 16

Chip formation when broaching steel

Material: C45 N

Cutting material:high-speed steel S 2-9-2-8; Dry cutting

Material: C45soft annealed

Material: C45 V

= 2, = 15, = 0;h = 0.1 mm; t = 12.5 mm( )

v = 50 m/min

v = 5 m/min

v = 30 m/min

5 mm

5 mm


28/102

2.

2.2. Factors affecting broaching

results

Criterias to evaluate the results of

broaching are:

- the surface quality of the work pieces,

- the shape and dimensional accuracy of

the work pieces,

- the edge durability,

- the amount of power required,

- the formation of chips.

2.2.1 Tools

The cutting geometry influences the results

of the broaching to a significant extent. In

general, the roughness of the broached

surfaces is less with an increase in the face

angle (Fig. 17). Nonetheless, the amount of

the face angle has limits depending on the

material to be processed, on the one hand

due to the stability of the tooth and on the

other hand due to the life of the tool. In

high-speed broaching an increase on the

face angle by 3 to 5 with respect to the

normal values for low cutting speeds has

proved to be advantageous.

28



29/102

2.

The clearance angle and the inclinationangle have hardly any influence at all onthe surface quality, but on the other hand

the rounding radius of the cutting edge is of

considerable significance, since the

roughness value of the broached surfaces

becomes greater as bluntness increases

(Fig. 18).

Fig. 19 shows how the roughness is a

function of the total length of cuts and thus

is associated with increasing wear when

broaching 16MnCr5 steel.

29


Fig. 17

Comparison of roughness value ranges in relation to the cutting geometry in plunge-cutting, single-tooth broaching

and external broaching

Fig. 18

Roughness value Rt in relation to the cutting edge rounding radius for C45 steel

Fig. 19

Roughness in relation to the total length of cuts for the external broaching of 16MnCr5 steel (regression line)

Material: C45 V

Tool: high-speed steel

S 12-1-4-5, S 6-5-2

Cutting conditions: v = 6 m/min

Dry cutting: h = 0.05 mm

Plunge-cutting

Single-tooth broaching

External broaching

RoughnessvalueRt

Face angle

Dry cutting

RoughnessvalueRt

RoughnessvalueRt

Single-tooth broaching

Rounding radius

Total length of cuts w


30/102

2.

If one takes into consideration that there

can be, for example, a rise per tooth of 10

to 20 m in the finishing section of

broaches, then it can be seen that the

cutting edge rounding radius and rise per

tooth are of the same order of magnitude.There is a significant reduction of the face

angle due to wear of the cutting face, which

leads to a reduction of work piece surface

quality. The effect of the rounding-off of the

cutting edge is also greater in the finishing

section of broaches due to the smaller rise

per tooth compared to the roughing section.

Fig. 20 shows the way that the roughness

depends on the rise per tooth and therounding-off of the cutting edge.

30


Fig. 20

Roughness value Rt in relation to the rise per tooth and rounding radius of the cutting edge

Fig. 21a

Cutting and back forces in relation to the face angle in single-tooth broaching

Fig. 21b

Tool: S 6-5-2, = 2, = 15Material: C45 V, Cutting speed v = 6 m/min

Rise per toothRoughnessvalueRt

Face angle Face angle

Dry cutting

Cuttingforce

Backforce

150

300

450

N/mmh = 0,1 mm

150

300

450

N/mm

0,02

0,05

0,08

h = 0,1 mm

0,02

0,05

0,08


31/102

2.

The clearance angle and the angle of

inclination have practically no effect at all

on the formation of built-up edges. The

number of scales increases with an

increase in the face angle, and decreases

approximately linear with an increase in thecutting edge radius. The roughness

becomes less as the number of scales per

length unit increases.

While the clearance angle has no effect on

the cutting force, this is reduced by about 1

to 1.5% per degree of increase in face

angle. The back force is reduced by an

average of around 2% per degree of

increase in face angle. It thus shifts the

ratio of the cutting force to the back force(Fig. 21). An increase in cutting force is

likewise linked with the increase in the

bluntness of the cutting edge. If the tool life

is not to be considered as having come to

an end due to the work piece surface

quality having become too bad or because

tolerances have been exceeded, and thus

subsequent sharpening is required, the

criteria for subsequent sharpening are an

increase in pulling force by around 25 to

40%. The higher back force means it is

necessary to reckon with an elastic

expansion of the work pieces, with

reductions in dimensions when carrying out

internal broaching. The work pieces are

pressed more firmly in the case of external

broaching, which can lead to the

corresponding dimensional deviations.

The cutting material and the kind of heat

treatment it had received influence theresults of broaching, primarily with respect

to the tool l i fe. On one hand this is

necessary to require a high degree of

resistance to wear, and on the other hand

the cutting edges must not crack off or

break. Hardness, resistance to wear and

the toughness of the cutting material must

also be retained with increasing cutting

temperatures, especially in the area of high

broaching speeds. For that reason,

broaching tools are generally made from

high-speed steels today.

High-speed steels alloyed with cobalt have

been used with good results, primarily

when broaching at increased cutting

speeds. The increased tool life that has

often been observed is due to the higher

resistance to tempering and the higherresistance to wear of these tools at higher

cutting temperatures when compared to

cobalt-free steels.

Alloyed tool steels for cold working with a

chromium content of approx. 12% and a

carbon content of 2.1% are ranked below

high-speed steels in terms of the tool life

that can be reached and cannot be

recommended apart from a few exceptional

cases. Assuming that hardening has beencarried out perfectly with the corresponding

tempering treatment, a hardness of 63 to

66 RC can be aimed for with high-speed

steels.

When nitriding (bath nitriding, ion-nitriding)

ready-ground broaches made of high-

speed steel, it is necessary to use nitriding

depths of 0.02 to max. 0.03 mm. Greater

nitriding depths are unsuitable, since they

lead to flaking-off of the extremely brittle

iron nitride, which results in cracking-off at

the cutting edge. The hardness of a perfectly

nitrided layer is 1075 - 1150 HV0.05. It is

possible to some extent to achieve higher

tool lives here. The nitride layer has a

beneficial effect as a result of the reduction

of the frictional resistance when cutting

such material that have a tendency towards

cold welding. Adhesions of the material of

the work piece to the flanks, lands andclearance faces of the broach teeth are

reduced. The lower coefficient of friction of

the nitr ide layer is also used when

broaching highly hardened and tempered

work pieces if deep broaching profiles are

to be produced.

The nitriding of broaches is, however, only

carried out in a small number of cases and

has been substituted in the meantime by

coating technology.

31



32/102

2.

If broaching problems cannot be

fundamentally resolved by a coating of the

broach, as was already done with nitriding,

then it is possible to achieve notable

increases in the tool life under specific

conditions by making use of coatings, sincecoated broaches show significantly lower

coefficient of friction compared to nitrided

ones. The problems that are encountered

with coated broaches can often be

attributed to the fact that there is insufficient

stability (wall thickness) of the work pieces,

especially during internal broaching that

produces a springing-off (breathing) that

exceeds the thickness of the chip.

Previously, broaches were coated with TiN,Ti(C, N) or Ti(Al, N) with coating thicknesses

of 1 to 4 m and had a hardness of

between 2300 and 3500 HV according to

the type of coating. The type of coating that

gives the best broaching results cannot

generally be determined in advance and

must be tested case by case if it is not

possible to fall back onto a sufficient

amount of experience. It is possible to

subsequently remove the coat (uncoating)

and to apply other ones.

Broaches that are to be subsequently

coated require additional working steps in

manufacture to ensure that the coating can

be applied properly.

It can be assumed that further coatings will

be developed in the near future in which

multiple layers of coats of hard material will

be combined with coats of soft material thatto some extent retain lubricant. This will

certainly be of interest for broaching

processes.

The use of broaches with cemented

carbide cutting edges has been restricted

up until today to a number of special cases.

Thus, cemented carbide finds application in

the broaching of bearing shells for internal

combustion engines and also in the

broaching of parts made of grey cast iron in

the automotive industry. Cemented carbide

has shown itself to be superior to high-

speed steels in respect to the higher tool

lives attained.

Satisfactory results have not been obtained

to date when broaching steel with the useof cemented carbide broaches. While it

was possible to obtain significantly lower

wear of the clearance faces by selection of

the right grade of hard metal together with

a suitable cutting geometry with respect to

that used for high-speed steel to some

extent, on the other hand, thermo cracks

(comb cracks) were produced, and this

problem was also associated with cracking-

off at the cutting edge which made the tools

unusable. These manifestations are knownto be due to the thermal and mechanical

alternating stresses on the tool cutting

edges. The lower toughness of cemented

carbide does not permit large face angles,

as will be usual for broaches made of high-

speed steel, and this means that it is not

possible to meet the surface quali ty

requirements in the cutting of steel with the

usable cutting speeds employed. The use

of cemented carbide broaches is therefore

not to be recommended for the cutting of

steel (see 2.1.2).

It is also necessary to take into

consideration that it is not possible to

manufacture and produce broaches with

cemented carbide cutting edges for all

possible cases that will be encountered for

reasons of tool design and also due to the

manufacturing possibilities. There is a

further restriction of the possibilities forapplication due to the question of cost-

effectiveness, since the manufacture and

sharpening of such broaches is very costly.

32



33/102

2.

2.2.2 Work pieces 1)

The material of the work pieces affects the

results of the broaching to a significant

extent. Since it is primarily steel that is

machined, we have most experience of thismaterial and for this reason we will only

discuss the broaching performance of

steels and steel-related materials.

In the production of steel, there is a

domestic and international intertwining in

order to remain competit ive on an

international basis and to improve on

competitiveness. The purchasing of steel

scrap of various origins and the exchange

of ingots and blooms and remelted blocksfor the production of semi-finished products

for parts for high-volume mass production

has been a widespread practice. The

differing technical equipment in the

steelworks, especially in the areas of steel

scrap preparation, melting, hot-shaping,

heat treatment, etc., means that it is not

possible to achieve the consistency of the

semi-finished product that is so important

for high-volume mass production.

Steel materials are to a very large extent

produced by melting in metallurgical terms.

Even if the relevant type of steel shows the

same stipulated tolerances in analysis and

thus meets the requirements set, specific

tramp elements that can be brought in due

to the make-up of the different batches can

exert a significant effect on the broaching

results.

Semi-finished products are produced by

ingot casting or continuous casting.

Depending on the method of manufacture,

pieces separated off from the hot-formed

unnotched specimens or tube, and also

parts which have been forged or hot-

extruded or ring-rolled, are available for

broaching in the form of blanks, and always

invariably after some form of pre-treatment.

In addition, there is an increasing number

of parts which have been produced by

sintering or extrusion or cold-drawing.

Extruded or cold-drawn work pieces

generally do not have any further final heat

treatment after the last pressing or drawing

operation so as not to reduce the increase

in apparent yield point and tensile strength

produced by the cold forming. Cold-drawnwork pieces have a high residual stress

and thus produce poor results when

broached. Sintered materials have differing

levels of porosity and often include a high

portion of non-metallic inclusions which

also have a strongly adverse effect on

broaching.

When taking a basis for an assessment of

the broaching properties, on the one hand

there is the surface quality that can beobtained for the broached work piece, and

on the other hand there is the question of

the life of the broaches. Since broaching is

a finish machining process, the surface

quality that can be attained is of paramount

importance. Both factors are affected and

depending on the strength of the material,

its chemical composition, its degree of

purity and any previous treatment, i.e., heat

treatment and any hot or cold drawing; in

other words, its actual grain structure.

Very high strengths in the material as a

result of its chemical composition and the

type of heat treatment or cold hardening as

a result of cold forming increase the

broaching force and thus the amount of

wear of the broach. Signs of wear are

shown first at the corners of the profile

teeth. Increasing wear then adversely

affects the quality of the work piecesurfaces (measured as roughness) and

increases in turn the broaching force. It is

thus necessary to constantly pay attention

to the pulling force, since an increase in

this with respect to the normal value means

that inferences can be drawn both to the

state of the material and of the tool.

1) Besides work piece the expressions component or

part are in use.

33



34/102

2.

At this point it is necessary to refer to

section 2.1.3 Forces when broachingand

Fig. 15, which illustrate the influence of the

micro-structure in the hardened and

tempered, normalized and soft annealed

states. It is not possible to make directinferences in terms of the broaching results

to be expected merely on the basis of the

strength of the material, it is far more

necessary to take into consideration the

chemical composition and the grain

structure.

The work pieces to be broached are

available in varying states of annealing or

tempering and hardening, depending on

the type of alloy. Which of these statesapplies for the broaching process is not

determined in practice only from the point

of view of the best broaching properties,

but primarily according to the allocation of

broaching within the overall machining

process for the work pieces, and above all

through design-related criteria.

To make a rough sub-division, the following

can be taken as the prerequisites for

proper heat treatment:

The normalized state is favourable for

unalloyed case hardening and heat-

treatable steels and also for constructional

steels as per DIN EN 10 025 as long as the

strength of around 700 N/mm2 (approx.

200 HB) is not exceeded and the carbon

content is not less than around 0.15%.

It is also possible to obtain generally good

results when broaching at increased cutting

speeds when taking into account all the

factors. If there is primarily pearlite, the

roughness increases with an increase in

cutting speed and reaches a value thatremains practically constant above 10 to

15 m/min (the researched area was up to

v = 50 m/min). If there is primarily ferrite,

then this shows a maximum in the

roughness value over a cutting speed

range of between approx. 5 and 15 m/min.

The location and amount of the roughness

maximum are to a certain extent dependent

on the proportion of alloying elements and

also the cutting geometry and the

metalworking fluid. The roughness thendecreases with increasing cutting speed.

The formation of chips is good with

normalized grain structures and curled

chips are formed. The normalized condition

of hypoeutectoid steels is characterised by

the fact that the ferrite and pearlite are

distributed equally in the microstructure.

The crystallite should be medium-fine and

unaligned (similar to Figs. 22 and 23 for

C10 and Figs. 24 and 25 for 15CrNi6). The

illustration of the pearlite in Figs. 24 and 25

is to be regarded as normal for a nickel-

alloy case hardening steel, since this has a

more sorbitic character, as has been shown

in practice.

34


Fig. 22 100:1C10, ferrite with pearlite islands

The proportion of pearlite corresponds to

the carbon content

Fig. 23 500:1C10, ferrite with pearlite islands


35/102

2.

Figs. 26 and 27 show the normalized

structure of heat-treatable steel 34Cr4 at

100 x and 500 x magnification. Steels in

this condition are generally good for

broaching and also offer no difficulties if the

crystallite is somewhat coarser.

It should however be noted that with an

increasing proportion of ferrite (low-carbon

steels) the formation of a built-up edge is

promoted and that there can be cold

welding of particles of the material onto the

flanks and lands of the tool teeth. Scaly

surfaces and larger amounts of tearing-out

are the result. At the same time the friction

increases between the tool and the work

piece. This is shown by an increase in

pulling force under certain circumstancesand ultimately by damage to the tool. In this

case, however, coarse-grain annealing

promises an improvement in the results.

However, it is very often not considered for

technical reasons and also for economic

reasons. Further improvements can also be

achieved by hardening and tempering, but

this is often likewise regarded as

uneconomic. Determining which steels and

which pre-treatment gives the bestmachining properties and which ones are

economically feasible can often only be

determined by tests, if at all possible with

the original work pieces, since it is not

possible to find a formula that is valid for all

cases.

The hardened and tempered condition

provides good results in the case of steels

for hardening and tempering, since they

have achieved the most homogeneousmicrostructure if the heat treatment was

carried out properly.

35


Fig. 24 100:1

15CrNi6, ferrite, pearlite and sorbite

Fig. 25 500:1

15CrNi6, ferrite, pearlite and sorbite

Fig. 26 100:1

34Cr4, pearlite, ferrite

Fig. 27 500:1

34Cr4, pearlite, ferrite


36/102

2.

There is also a clear dependency of the

surface quality on the cutting speed when

broaching hardened and tempered steels,

but in any case it is not as strongly marked

as in the case of soft annealed grain

structures (see Fig. 7).

The hardened and tempered condition

does not produce any difficulties when

broaching if the teeth encounter a

homogeneous structure (tempered

martensite). This is always the case if the

work pieces have formed fine needle-

shaped martensite in the hardening

process when seen over the entire polished

cross-section. This then precipitates

extremely finely spread carbide (temperingstructure) during the subsequent tempering

with an increasing tempering temperature

and represents an almost homogeneous

state of the steels. The hardened and

tempered structure of C45 can be seen in

Figs. 28 and 29. This shows tempered

martensite; the structure is free of ferrite.

Work pieces which have been produced

from precipitation hardening ferritic pearlitic

steels (so-called PHFP steels) are being

encountered to an increasing extent for

broaching. The alloying elements included

in these materials, and vanadium in

particular, are added in proportions such

that it is possible to aim for strength by

means of a controlled accelerated cooling

(BY-annealing; BY = beyond yield strength)

with partial utilisation of the hot-forming

temperature, which match steels produced

by the classic but significantly more

expensive hardening and tempering

treatment process (hardening + high

tempering). However, the micro-structuresproduced by BY-annealing differ

considerably from the structure produced

by classic hardening and tempering

processes.

As has already been mentioned, the

classic hardening and tempering

structures are formed homogeneously

under the corresponding conditions. The

grain structures produced by BY annealing

consist of more than 80 % fine lamellarislands of pearlite (sorbite), depending on

the carbon content. A further component of

the grain is ferrite, which surrounds the

pearlite like a net. Such high proportions of

pearlite (sorbite) are more difficult to broach

than structures produced by hardening and

tempering. It is scarcely possible to carry

out any cutting at all by broaching in the

case of a bainitic structure, which can be

produced under specific cooling conditions

with BY-annealing. Unfortunately, the

savings that can be achieved by BY-

annealing mean that it can be expected

that a higher proportion of components

treated in this way will be encountered for

broaching in the future.

36


Fig. 28 100:1

C45, tempered martensite

Fig. 29 500:1

C45, tempered martensite


37/102

2.

The grain structure produced in PHFP

steels via BY-annealing to achieve higher

strengths in a more cost-effective way

resemble in terms of their amount of

pearlite normalized unalloyed steels above

approx. 0.6% carbon content. As describedbelow, excessively high proportions of

pearlite are less meaningful in terms of the

broaching properties, and soft annealing is

recommended instead.

The soft annealed state seems

advantageous if the carbon content

exceeds an proportion of approx. 0.6% in

the case of unalloyed steels. If the steels

have been alloyed, then there is a pressure

towards the use of soft annealing as thealloying content increases, since both the

increasing strength of the normalized

states and also the increasing amount of

carbide and its distribution make the

broaching process technically and

economically more difficult. In terms of all

the grain structures, within soft annealing

the roughness depends most strongly on

the cutting speed (see Fig. 6). It is

therefore recommended to broach at

cutting speeds either of less than 3 m/min

or more than 20 m/min.

Fig. 30 shows the soft annealed grain

structure of C60W3 unalloyed tool steel. As

the carbon content increases, in a

normalized state strip-like (lamellar streaks)

pearl i te with i ts very hard and britt le

cementite lamellar streaks exist inincreasing quantities, which both increases

the tensile strength and also increases the

wear of the broach. The amount of pearlite

is increased not only by the carbon content

but also by the content of metallic alloying

elements, primari ly carbide-forming

elements such as chromium, molybdenum,

tungsten and vanadium. Thus soft

annealing must be recommended for

alloyed steels for carbon contents lower

than 0.6%.

Assuming that flawless heat treatment has

been applied, then it is also necessary to

reckon with the fact that the ideal grain

structure is still not present in many ways

and therefore the consequence will be

differing results. On the basis of our

observations, we refer to a number of

typical instances which provide an

explanation of the difficulties and poor

results encountered when broaching.

In many cases the defect does not lie

within the broach but in deficiencies in the

grain structure and the hardness of the

material to be broached.

Poor broaching can be expected if the

grain structure is banded. In such cases it

is necessary to reckon with increased

production of built-up edges and theirconsequent effects. Above al l , when

broaching in the direction of the bands, this

then produces adhesions of the material on

the flanks and the lands to produce

unclean surfaces of the work piece, which

in extreme cases makes the work piece

unusable. Since the banding structure also

reduces the technical properties in the

cross-direction, work pieces with such a

grain structure should be rejected if at all

possible.

37


Fig. 30 500:1

C60W3, granular pearlite (cementite)


38/102

2.

According to the present state of

knowledge, the crystal segregations that

form during the solidifiation of the steel melt

lead to this banding of the grain structure.

This banding structure is produced by the

extension of the globulitic or dendritic

segregated crystals of the cast grain

structure during the hot forming. Fig. 31

shows the dendritic structure of a case

hardening steel (15CrNi6) in a polished

cross-section at 10 x magnification. The

same sample shows a very clear banding

structure in a longitudinal polished section

(Fig. 32).

The same grain structure can be seen in

Figs. 33 and 34 at 100 x magnification in a

polished cross-section and longitudinal

section.

When cooling down from the austenite

domain, the beginning of the transformation

and the course of the transformation in the

banding structures in areas of differing

composition vary. Non-metallic inclusions

act as nucleations for the formation of ferrite.

In the example of C45 steel, the differing

formations of the ferrite bands in the pearlitic

matrix can be seen in Figs. 35 and 36.

Apart from a low crystal segregation

solidifying of the steel ingots, band-free or

low banding grain structures can be

produced by various methods of heat

treatment. The best method is diffusion

annealing, since the crystal segregation

and thus the cause of the formation of

banding structure is remedied. The type of

diffusion annealing depends on the degree

of deformation and thus on the distance of

the bands. Since this is very expensive,

this generally excludes it in practice.

By applying an accelerated continuous

cooling of the steels out of the austenite

domain, in which it is necessary not to go

38


Fig. 31 10:1

15 CrNi6, dendritic structure in cross-

section

Fig. 32 10:1

15 CrNi6, banding structure in longitudinal

section

Fig. 33 100:1

15 CrNi6, cross-section

Ferrite, pearlite, dendritic structure

Fig. 34 100:1

15 CrNi6, longitudinal section

Ferrite, pearlite, banding structure


39/102

2.

below a specific minimum cooling speed,

the banding structure can be prevented or

reduced. This accelerated cooling in the

temperature range of the pearl i te

transformation suppresses the diffusion of

carbon and thus an orientated separation.

However, heat treatment of this type does

not remove the causes of this banding

structure, and any subsequent heat

treatment allows these bands to reappear

at once.

The transformation behaviour of the steels

determines when an accelerated

continuous cooling is to be done, since the

intermediate stage must not be crossed

under any circumstances. It is necessary to

prevent an undesired intermediate stage

structure with its disadvantageous effectson the wear of the tool after cooling-down.

It is necessary to carry out a stepped

cooling-down with isothermic holds in the

temperature range of the maximum

transformation speed at the pearlite stage

in the case of alloyed steels that are

reluctant to be transformed. It is cooled

down to this temperature at an accelerated

rate, whereby the final cooling after the end

of transformation can be of any desired

type (e.g., in the air).

A factor that affects the critical cooling-

down speed is the size of the austenite

grain, which again depends on the

austenitisation temperature. A higher

temperature makes the austenite coarser

and leads to a better homogenisation of the

austenite. The minimum speed in cooling

can be reduced so as to prevent the

formation of banding structures.

It should be shown in the case of

34CrAlMo5 nitriding steel (Figs. 37 and 38)

how the presence of a banding structure

39


Fig. 35 100:1

C45, longitudinal section

Pearlite with ferrite bands

Fig. 36 100:1

C45, longitudinal section

Pearlite, ferrite, banding structure,

consistently narrow

Fig. 37 100:1

34CrAlMo5, hardened and tempered

Tempered martensite with residual ferritebanding, longitudinal section

Fig. 38 500:1

34CrAlMo5, hardened and tempered

Tempered martensite with residual ferritebanding, longitudinal section


40/102

2.

can affect the hardened and tempered

grain structure. An excessively low

austenitizing temperature meant that the

remaining ferrite could not be transformed

fully so that the hardened and tempered

structure, which is arranged in bands,shows large amounts of ferrite.

The soft annealed state (Fig. 39) is

completely unsuited for the broaching

process in the case of steels with a low

carbon content. It is necessary to reckon in

such cases with strong adhesion of the

material to the broaches, which leads to the

usual difficulties.

In the same way, poor broaching conditions

prevail when the pearlite begins to change

over to the granular form (Figs. 40 and 41)

as a result of an excessively low cooling-

down speed during normalizing. It is also

necessary to reckon with smearingin suchcases.

The formation of a flawless hardened and

tempered grain structure depends on the

austenitisation temperature and the holding

time at this temperature, the cooling-down

speed, and the right tempering treatment.

The most suitable state however does not

exist if full hardening is not possible due to

the wall thickness of the work pieces. Whilethe conditions may be good in the outer

zone of the work pieces, there is, however,

a mixed grain structure in the direction

of the core that exhibits alternating

constituents of varying hardnesses on a

case by case basis, and this can be seen

clearly in the results of the broaching.

40


Fig. 39 500:1

C15, ferrite with granular pearlite

Structure soft annealed

Fig. 40 500:1

15CrNi6, ferrite,

Pearlite transforming to granular form

Fig. 41 500:1

C45, beginning of degeneration of lamellar

pearlite, ferrite


41/102


42/102


43/102


44/102


45/102

3.

3.1. Introduction

A definition of the broaching process and a

number of basic statements about broaches

was given in sections 1.1 and 2. In addition

DIN 1415, Sheet 1, makes a classificationof tools by the type of surface to be

produced and also gives a listing of the

descriptions of the individual broach types

and details about the tool nomenclature.

There are also the following additional DIN

standard sheets:

DIN 1409 High-speed steel

broaching tools

Technical deliveryconditions

DIN 1416 Broaching tools;

design of tooth and tooth

space 1)

DIN 1417 Pull ends and tail ends of

internal broaches.

DIN 1417 replaces the

old standard DIN 1415,

Pages 3 to 6, which is no

longer applicable for new

designs

DIN 1418 Pullers for broaches with

pull ends and tail ends as

per DIN 1417

DIN 1419 Internal broaches with

interchangeable round

broaching shells

DIN 8589, Part 5 Manufacturing processes,

cutting; broaching

1) Besides tooth space the expressions chip

space or gullet are in use.

3.2. Design of broaches and

systematic classification of

commonly used cutting

schematics

Broaches are multi-toothed tools whoseteeth have a designed rise with respect to

the previous teeth. It is possible to

distinguish between the roughing, finishing

and reserve section of the teeth of the

broach. There can be several different

roughing, finishing and reserve sections

within a broach. In the case of profile

broaches there is no finishing section,

since generally the profile is produced by

the minor cutting edges (flanks) of the

teeth. All broaches have a reserve section.

The arrangement of the teeth on a broach

and thus the cutting scheme can be

described as stepping. In principle there

are only two basic types, namely depth

stepping and lateral stepping; all other

types of stepping are combinations or

variations of the two basic types. It is

meaningful to further subdivide the

stepping options, into single or group

steppings according to the type of surface

to be produced through the major or minor

cutting edges. In the definition of the

individual cutting schematics, i t is

necessary to proceed on the basis of the

direction of the rise of the teeth as related

to the work piece surfaces.

45

3. Broaches


46/102

3.

3.2.1 Single stepping

In most cases depth stepping is made use

of for broach teeth (Fig. 49). The broaching

process is thus similar to plunge-cutting.

The material is cut off in layers by the

broach cutting edges penetrating vertically

into the surface of the work piece. Since a

large length of cutting edge can be

produced, depending on the profile to be

produced, the total cross-sectional area of

the cut is large despite a relatively small

rise of the teeth. Broaches with depth

stepping can therefore be shorter than

those of other types.

We refer to lateral stepping (Fig. 50) if the

rise of the broach teeth runs parallel to the

surface of the work piece. The main cutting

edges are vertical or at a small angle to the

surface of the work piece. Since the total

cross-sectional area of the cut is small,

depending on the thickness of the layer to

be removed by roughing and the maximum

permissible rise per tooth, a greater

number of teeth is required on the broach

then for depth stepping, which naturally

entails l

forstbroaching (1)

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