pipe thread cutiing machine

PIPE THREAD CUTIING MACHINE

in partial fulfillment of the requirement for the award of degree of

BACHELOR OF ENGINEERING

IN

MECHANICAL ENGINEERING

BY

Under the guidance of----------------------------

2004-2005

DEPARTMENT OF MECHANICAL ENGINEERING

CERTIFICATE

Register number: _________________________

This is to certify that the project report titled “POWER

GENERATION USING SPEED BRAKE” submitted by the

following students for the award of the degree of bachelor of

engineering is record of bonafide work carried out by them.

Done by

Mr. /Ms._______________________________

In partial fulfillment of the requirement for the award of degree in

Bachelor of Mechanical Engineering

During the Year – (2004-2005)

_________________ _______________

Head of Department Guide

Coimbatore –641651.

Date:

Submitted for the university examination held on ___________

_________________ ________________

Internal Examiner External Examiner

---------------------------------------------------------------------------------

ACKNOWLEDGEMENT---------------------------------------------------------------------------------

ACKNOWLEDGEMENT

At this pleasing moment of having successfully

completed our project, we wish to convey our sincere

thanks and gratitude to the management of our college

and our beloved chairman

…………………………………………………, who provided all

the facilities to us.

We would like to express our sincere thanks to our

principal ………………………………………, for forwarding

us to do our project and offering adequate duration in

completing our project.

We are also grateful to the Head of Department

Prof. …………………………………….., for her constructive

suggestions & encouragement during our project.

With deep sense of gratitude, we extend our earnest

& sincere thanks to our guide

…………………………………………………….., Department of

Mechanical for her kind guidance & encouragement

during this project.

We also express our indebt thanks to our TEACHING

and NON TEACHING staffs of MECHANICAL ENGINEERING

DEPARTMENT,……………………….(COLLEGE NAME).

SYNOPSIS

In ordinary hand threading the tap may be shaken so that thread may not be

formed correctly. The thread may be formed in angular position. There is also a

change for the breakage of tap. So a skilled person is need for tapping. This

process is also taking more time. The same conditions apply for threading also.

With the use of this machine, all this disadvantages can be

avoided. Even an unskilled person can be making a thread by using this

machine. By just setting the work piece in the chuck, the thread can be

formed easily by rotating the hand wheel. The time consumed is very

less. Both internal and external thread can be formed accurately by

using this machine. Suitable sized tape and die sets are used to form

the required internal and external thread respectively.

INTRODUCTION

The most important element of any machine is screw threads

without screw threads. There can not be any development in the field

of engineering. Even machine tools operated by computers. Lead

screw threads for moving different slides. The size of screw threads for

different purposes varies a fraction of mm to as high as 300mm. These

screw threads are made in quantities from one to several millions.

Some screws are to be very accurate and some screws can be very

rough. A number of processes are available for manufacturing screw

threads. The correct process is selected so that the required quantity

and quality of screw threads are produced at the lower cost. Those

basic processes are used in the manufacture of threads. They are

casting process can be used for producing both external and internal

threads. Die casting or investment casting methods are used for

producing screw threads in plastics.

Metal cutting process like thread milling, thread grinding and

thread chasing are used for producing large variety of screw threads in

different sizes. Thread rolling process is used in the mass production of

screw threads in ductile materials. Only external threads can be

produced in this process. The process is generally restricted to produce

simple threads in standard forms.

A C MOTORS

An electric motor is an electromechanical device that converts

electrical energy into mechanical energy. Most electric motors operate

through the interaction of magnetic fields and current-carrying

conductors to generate force. The reverse process, producing electrical

energy from mechanical energy, is done generators such as an

alternator or a dynamo some electric motors can also be used as

generators, for example, a traction motor on a vehicle may perform

both tasks. Electric motors and generators are commonly referred to as

electric machines.

Electric motors are found in applications as diverse as industrial

fans, blowers and pumps, machine tools, household appliances, power

tools, and disk drives. They may be powered by direct current, e.g., a

battery powered portable device or motor vehicle, or by alternating

current from a central electrical distribution grid or inverter. The

smallest motors may be found in electric wristwatches. Medium-size

motors of highly standardized dimensions and characteristics provide

convenient mechanical power for industrial uses. The very largest

electric motors are used for propulsion of ships, pipeline compressors,

and water pumps with ratings in the millions of watts. Electric motors

may be classified by the source of electric power, by their internal

construction, by their application, or by he type of motion they give.

The physical principle behind production of mechanical force by

the interactions of electric current and a magnetic field, Faraday’s law

of induction, was discovered by licheal Faraday in 1831. Electric motors

of increasing efficiency were constructed from 21 through the end of

the 19th century, but commercial exploitation of electric motors large

scale required efficient electrical generators and electrical distribution

networks. First commercially successful motors were made around

1873.

Some devices convert electricity into motion but do not generate

usable mechanical were as a primary objective, and so are not generally

referred to as electric motors. For example, magnetic solenoids and

loudspeakers are usually described as actuators and transducers,

respectively, instead of motors. Some electric motors are used to

produce torque or force.

4.3.1 PRINCIPLE OF OPERATION

A large percentage of AC motors are induction motors. This

implies that there is no current supplied to the rotating coils (rotor

windings). These coils are closed loops which have large currents

induced in them. Three-phase currents flowing in the stator windings

leads establish a rotating magnetic field in the air gap. This magnetic

field continuously pulsates across the air gap and into the rotor. This is

a single phase representation of windings and current flow.

Fig 4.4 A C Motor

The rotor consists of copper or aluminum bars connected together at

the ends with rings.

Fig 4.5 Rotor

As magnetic flux cuts across the rotor bars, a voltage is induced in

them, much as a voltage is induced in the secondary winding of a

transformer. Because the rotor bars are part of a closed circuit

(including the end rings), a current circulates in them. The rotor current

in turn produces a magnetic field that interacts with the magnetic field

of the stator. Since this filed is rotating and magnetically interlocked

with the rotor, the rotor is ragged around with the stator field.

4.3.2 SPECIFICATIONS

Voltage : 230v

Frequency : 50Hz

Hase : single

Feed : 1440 rpm

: 180w

WORM GEAR BOX

Worm Gear

Worm Gears

Backed by 50 years of experience with precision mechanical parts, PIC Design carries an impressive line of worm gears, designed and sold as worm and gear pairs. We offer worm and gear design expertise, many standard sizes to choose from, and custom manufacture of worm gears to print. In addition, we can modify standard worm gears when a full custom build isn’t an option.

What varieties of worm gears do you offer

We feature an extensive selection of worm gear options, including:

Right Hand Worm Gears Left Hand Worm Gears Anti-Backlash Worm Gears Worm and Wheel Sets Miniature Worm Gears 303 Stainless Steel Worms Bronze Worm Gears

What sizes and specifications do you offer for worm gears

With both standard and custom worm gears available in the below ranges, we can meet just about any need for gear reduction, offering ratios up to 360:1. Details include:

Pitch - 48 and 64Ratios - 7.5:1 to 360:1Worm Starts - 1, 2, 4Gear OD - 5/8’’ to 5 5/8’’Bores - 1/8’’ to ¼’’

Typical Worm Gear Applications

Working primarily in the manufacturing, electronics, optics, and control systems industries, our worm gears are used in motion applications where precision is paramount, including lens positioning and adjustment, and sensor drives. Worm gears are also used in the following products:

Encoders Potentiometers Cameras Microscopes Turntables

We are always available to answer any additional questions you may have about our worm gear offerings. Please contact PIC Design today for more information or a quotation.

Precision Bearings

A leader in the mechanical product manufacturing industry, PIC Design features a wide selection of precision bearings, with custom and stock products available to suit any customer need. As with all of our other mechanical components, our bearings are compatible with many other PIC products – if you have any questions on compatibility or corresponding parts, don’t hesitate to get in touch with us. Our 50 years of experience allow us to expertly assist with any part selection issues you may have. In addition, we have full access to the RBC Bearings product and knowledge base, allowing us to pass on this additional benefit to you.

What Are the Specification Ranges for Your Bearings

With many, many part numbers in stock, we are one of your best resources to get the bearing that you need, when you need it. Our stock ball bearings are available within the following ranges; certain custom options can be manufactured as well.

Bore - .040’’ to 1.5’’Outside Diameter - .125’’ to 2.625’’Grade - ABEC 1, 3, 7Type - Shielded, Sealed, OpenMounting - Plain, FlangedRadial Load - Up to 3000 lbs

What Types of Bearings Do You Carry

Our extensive inventory and custom capabilities allow us to carry all varieties of bearings, in materials such as 440 C stainless steel, 52100 steel, sintered bronze, and PTFE. Bearing options include, but aren’t limited to:

Sleeve Bearings Ball Bearings

Cam Followers Plain Bearings Bearing Spacers Precision Balls Thrust Bearings Thin Section Bearings Rod Ends Extended Inner Race Bearings Linear Bearings

We can also offer full bearing and component assembly services.

What Are Typical Bearing Applications?

Our bearings are used in an enormous range of mechanical applications. Past customers have used our bearings in the following ways, just to name a few:

Control Systems (Sensor Drives) - Encoders, Potentiometers, ResolversResearch/Quality - Gauges, DialsElectronics - Gyroscopes, MotorsManufacturing - Rollers, Slides

Gears

Whether the need is to transfer motion or transmit power, PIC Design has the complete range of gearing to fulfill any application requirement. Standard gears and assemblies are available for operation on parallel and right-angle shafts, with gear racks for linear motion applications.

Gear’s one stop solutions right from concept designing to final production & performance testing. We cater to industrial needs for power transmissions! Top Gear has hard won reputation for quality products & production excellence through highly skilled

& trained workforce of more than 200 people supported by more than 70 Sub Contractors.

The technical skills of the employees enhanced through regular training programs, continued investment in the latest equipments for advance manufacturing capability, fulfilling customer commitments of prompt delivery their changing demands through our resourceful planning system & our continuous efforts has resulted in long term relations with retained customers for more than decade !

Top Gear offers comprehensive drive solutions to various industrial sectors which include petrochemicals, material handling, power plants & thermal stations, textile, plastic, machine tools, steel plant, chemical & pharmaceuticals, construction equipments, earth moving machines & have special expertise in one stop solutions for sugar plants. Top Gear has capability to offer wide range of drive solutions and range starts from 0.17 Kw & 100 Nm to 750 Kw 4500 KNm capacity gear boxes!

Specifications

Size Description Drawing Number

LS Worm Gear Actuator VM-WG-S

top

Dimension Drawings/Standard Materials of Construction

(contact factory for optional materials)


Above Ground - Butterfly Valve

4" - 16" Above Ground Worm Gear for BFVMaterials of Construction List

VM-3A08VM-3A08-M

18" - 20" Above Ground Worm Gear for BFVMaterials of Construction List

VM-3G24VM-3G24-M

18" - 36" Above Ground Worm Gear for BFV Materials of Construction List

VM-3H24VM-3H24-M


VM-3L24VM-3L24-M


VM-3P24VM-3P24-M

Buried Service - Butterfly Valve

4" - 20" Buried Service Worm Gear for BFV Materials of Construction List

VM-4A02VM-4A02-M


VM-4F02VM-4F02-M


VM-4H02VM-4H02-M


VM-4M02VM-4M02-M

Above Ground - Plug Valve

4" - 12" Above Ground Worm Gear for Plug Valve VM-7A08

Materials of Construction List VM-7A08-M

14" - 18" Above Ground Worm Gear for Plug Valve Materials of Construction List

VM-7E18VM-7E18-M


VM-7G12VM-7G12-M


VM-7M18VM-7M18-M

24" Above Ground Worm Gear for Plug Valve Materials of Construction List

VM-7N30VM-7N30-M


VM-7R24VM-7R24-M

Buried Service - Plug Valve

4" - 12" Buried Service Worm Gear for Plug Valve Materials of Construction List

VM-8A02VM-8A02-M


VM-8E02VM-8E02-M


VM-8J02.5VM-8J02.5-M


VM-8R02VM-8R02-M

Spur Gear - Plug Valve

20" - 24" Spur Gear for Plug Valve Materials of Construction List

VM-SG3VM-SG3-M

top

Instruction Manuals


3" - 24" Butterfly Valve BFV-OM1

30" - 108" Butterfly Valve BFV-OM2

3" - 48" Plug Valve CCPV-OM2

top

Technical Data

Description Drawing Number

Worm Gear Actuator Mounting Positions VM-2000-WGA

Manual Actuators White Paper

top

By William P. Crosher

The concept of the "worm gear" dates back to ancient times. Over the centuries, the design and use of this gear has evolved and improved. It describes a gear that contains a spiral or "worm" like groove in it. Its early applications mainly involved the drawing of water, but today it has many varied applications-from power transmission to manufacturing.

This comprehensive professional reference on the subject covers not only the design and manufacture of worm gears, but also issues regarding performance, maintenance, failure analysis, as well as applications.

The author has extensive experience in the field and has written this book for gear designers and manufacturers, gear users, as well as for mechanical engineering students.

Publisher: ASME Publish Date: 2002 Pages: 300 Language: English ISBN: 0791801780

Atlas Gear Company has been servicing the Special Machine Builders Industry since 1946. Located in the Detroit suburb of Madison Heights, Michigan, our 15,000 square foot facility has provided the solution to many machine builders' problems over the years. We are proud of the many specialties that we can offer to the industry. Whether it be small parts production or one of our many other services, our well trained and experienced technicians provide you with the maximum quality, service and in-house production control. Our versatility, flexibility and our vast range of machining capabilities have made us an industry leader. We continue to expand and are pleased to offer many

http://www.atlasgear.com/01.htm



additional Sales Offices for your convenience.

We welcome you to browse through our site and familiarize yourself with our many services. We look forward to adding your companies name to our distinguished list of satisfied customers.

Please don't hesitate to call one of our Sales

Representatives or fax us with your unique problem and I'm sure you will be pleased with our solution.

Introduction

A worm gear is used when a large speed reduction ratio is required between crossed axis shafts which do not intersect. A basic helical gear can be used but the power which can be transmitted is low. A worm drive consists of a large diameter worm wheel with a worm screw meshing with teeth on the periphery of the worm wheel. The worm is similar to a screw and the worm wheel is similar to a section of a nut. As the worm is rotated the wormwheel is caused to rotate due to the screw like action of the worm. The size of the worm gearset is generally based on the centre distance between the worm and the wormwheel.

If the worm gears are machined basically as crossed helical gears the result is a highly stress point contact gear. However normally the wormwheel is cut with a concave as opposed to a

http://www.atlasgear.com/?gclid=CISM7L2Wv7QCFUx66wodA04AHw

straight width. This is called a single envelope worm gearset. If the worm is machined with a concave profile to effectively wrap around the wormwheel the gearset is called a double enveloping worm gearset and has the highest power capacity for the size. Single enveloping gearsets require accurate alignment of the worm-wheel to ensure full line tooth contact. Double enveloping gearsets require accurate alignment of both the worm and the wormwheel to obtain maximum face contact.

Diagram showing the different worm gear options available.

The double enveloping (double throat/double globoid ) option is the most difficult to manufacture and set up. However this option has the highest load capacity, near zero backlash capability, highest accuracy and extended life capability.

A more detailed view showing a cylinderical worm and an enveloping gear. The worm is shown with the worm above the wormwheel. The gearset can also be arranged with the worm below the wormwheel. Other alignments are used less frequently.

Nomenclature

As can be seen in the above view a section through the axis of the worm and the centre of the gear shows that , at this plane, the meshing teeth and thread section is similar to a spur gear and has the same features

αn = Normal pressure angle = 20o as standard γ = Worm lead angle = (180 /π ) tan-1 (z 1 / q)(deg) ..Note: for α n= 20o γ should be less than 25o

b a = Effective face width of worm wheel. About 2.m √ (q +1) (mm)b l = Length of worm wheel. About 14.m. (mm)c = clearance c min = 0,2.m cos γ , c max = 0,25.m cos γ (mm) d 1 = Ref dia of worm (Pitch dia of worm (m)) = q.m (mm)d a.1 = Tip diameter of worm = d 1 + 2.h a.1 (mm)d 2 = Ref dia of worm wheel (Pitch dia of wormwheel) =( p x.z/π ) = 2.a - d 1 (mm)d a.2 = Tip dia worm wheel (mm)h a.1 = Worm Thread addendum = m (mm)h f.1 = Worm Thread dedendum , min = m.(2,2 cos γ - 1 ) , max = m.(2,25 cos γ - 1 )(mm)m = Axial module = p x /π (mm)m n = Normal module = m cos γ(mm)M 1 = Worm torque (Nm)M 2 = Worm wheel torque (Nm)n 1 = Rotational speed of worm (revs /min)n 2 = Rotational speed of wormwheel (revs /min)p x = Axial pitch of of worm threads and circular pitch of wheel teeth ..the pitch between adjacent threads = π. m. (mm)p n = Normal pitch of of worm threads and gear teeth (m)q = Worm diameter factor = d 1 / m - (Allows module to be applied to worm ) selected from (6 6,5 7 7,5 8 8,5 9 10 11 12 13 14 17 20 )p z = Lead of worm = p x. z 1 (mm).. Distance the thread advances in one rev'n of the worm. For a 2-start worm the lead = 2 . p x

R g = Reduction Ratioμ = coefficient of frictionη= Efficiency

Vs = Worm-gear sliding velocity ( m/s)z 1 = Number of threads (starts) on wormz 2 = Number of teeth on wormwheel

Worm gear design parameters

Worm gears provide a normal single reduction range of 5:1 to 75-1. The pitch line velocity is ideally up to 30 m/s. The efficiency of a worm gear ranges from 98% for the lowest ratios to 20% for the highest ratios. As the frictional heat generation is generally high the worm box must be designed disperse heat to the surroundings and lubrication is an essential requirement. Worm gears are quiet in operation. Worm

gears at the higher ratios are inherently self locking - the worm can

drive the gear but the gear cannot drive the worm. A worm gear can provide a 50:1 speed reduction but not a 1:50 speed increase....(In practice a worm should not be used a braking device for safety linked systems e.g hoists. . Some material and operating conditions can result in a wormgear backsliding )

The worm gear action is a sliding action which results in significant frictional losses. The ideal combination of gear materials is for a case hardened alloy steel worm (ground finished) with a phosphor bronze gear. Other combinations are used for gears with comparatively light loads.

Specifications

BS721 Pt2 1983 Specification for worm gearing — Metric units. This standard is current (2004) and provides information on tooth form, dimensions of gearing, tolerances for four classes of gears according to function and accuracy, calculation of load capacity and information to be given on drawings.

Worm Gear Designation

Very simply a pair of worm gears can be defined by designation of the number of threads in the worm ,the number of teeth on the wormwheel, the diameter factor and the axial module i.e z1,z2, q, m .

This information together with the centre distance ( a ) is enough to enable calculation of and any dimension of a worm gear using the formulea available.

Worm teeth Profile

The sketch below shows the normal (not axial) worm tooth profile as indicated in BS 721-2 for unit axial module (m = 1mm) other module teeth are in proportion e.g. 2mm module teeth are 2 times larger

Typical axial modules values (m) used for worm gears are

0,5 0,6 0,8 1,0 1,25 1,6 2,0 2,5 3,15 4,0 5,0 6,3 8,0 10,0 12,5 16,0 20,0 25,0 32,0 40,0 50,0

Materials used for gears

Material Notes applications

Worm

Acetal / Nylon Low Cost, low duty Toys, domestic appliances, instruments

Cast Iron Excellent machinability, medium friction.

Used infrequently in modern machinery

Carbon Steel Low cost, reasonable strengthPower gears with medium rating.

Hardened Steel High strength, good durabilityPower gears with high rating for extended life

Wormwheel

Acetal /Nylon Low Cost, low duty Toys, domestic appliances, instruments

Phos Bronze Reasonable strength, low friction and good compatibility with steel

Normal material for worm gears with reasonable

efficiency

Cast Iron Excellent machinability, medium friction.

Used infrequently in modern machinery

Backlash / quality Grades

A worm gear set normally includes some backlash during normal manufacture to allow for expansion of the gear wheel when operating at elevated temperaturs. The backlash is controlled by adusting the gear wheel tooth thickness.

BS 721 includes a table of backlash limits related to the accuracy grade. The standard lists 5 accuracy grades.

AGMA and DIN provide a similar grading system

Grade 1 relates to critical applications where minimum backlash is required i.e instruments /metering

Grade 2 relates to precision drives such as machine tools

Grade 3,4,5 relates to industrial drives with working temperatures of about 120o C

Design of a Worm Gear

The following notes relate to the principles in BS 721-2 Method associated with AGMA are shown below..

Initial sizing of worm gear.. (Mechanical)

1) Initial information generally Torque required (Nm), Input speed(rpm), Output speed (rpm).2) Select Materials for worm and wormwheel.3) Calculate Ratio (R g)4) Estimate a = Center distance (mm)5) Set z 1 = Nearest number to (7 + 2,4 SQRT (a) ) /R g

6) Set z 2 = Next number < R g . z 1

7) Using the value of estimated centre distance (a) and No of gear teeth ( z 2 ) obtain a value for q from the table below. (q -value selection)8) d 1 = q.m (select) ..9) d 2 = 2.a - d 1

10) Select a wormwheel face width b a (minimum =2*m*SQRT(q+1))11) Calculate the permissible output torques for strength (M b_1 and wear M c_1 )12) Apply the relevent duty factors to the allowable torque and the actual torque13) Compare the actual values to the permissible values and repeat process if necessary14) Determine the friction coefficient and calculate the efficiency.15) Calculate the Power out and the power in and the input torque

16) Complete design of gearbox including design of shafts, lubrication, and casing ensuring sufficient heat transfer area to remove waste heat.

Initial sizing of worm gear.. (Thermal)

Worm gears are often limited not by the strength of the teeth but by the heat generated by the low efficiency. It is necessary therefore to determine the heat generated by the gears = (Input power - Output power). The worm gearbox must have lubricant to remove the heat from the teeth in contact and sufficient area on the external surfaces to distibute the generated heat to the local environment. This requires completing an approximate heat transfer calculation. If the heat lost to the environment is insufficient then the gears should be adjusted (more starts, larger gears) or the box geometry should be adjusted, or the worm shaft could include a fan to induced forced air flow heat loss.

Formulae

The reduction ratio of a worm gear ( R g )

R g = z 2 / z 1

eg a 30 tooth wheel meshing with a 2 start worm has a reduction of 15

Tangential force on worm ( F wt )= axial force on wormwheel

F wt = F ga = 2.M 1 / d 1

Axial force on worm ( F wa ) = Tangential force on gear

Output torque ( M 2 ) = Tangential force on wormwheel * Wormwheel reference diameter /2

M 2 = F gt* d 2 / 2

Relationship between the Worm Tangential Force F wt and the Gear Tangential force F gt

Relationship between the output torque M 2and the input torque M 1

M 2 = ( M 1. d 2 / d 1 ).[ (cos α n - μ tan γ ) / (cos α n . tan (γ + μ) ) ]

Separating Force on worm-gearwheel ( F s )

Sliding velocity ( V s )...(m/s)

V s (m/s ) = 0,00005236. d 1. n 1 sec γ = 0,00005235.m.n (z 1

2 + q 2 ) 1/2

Peripheral velocity of wormwheel ( V p) (m/s)

V p = 0,00005236,d 2. n 2

Friction Coefficient

Note: The values of the coeffient of friction as provided in the table below are based on the use of phosphor bronze wormwheels and case hardended , ground and polished steel worms , lubricated by a mineral oil having a viscosity of between 60cSt, and 130cSt at 60 deg.C .

Cast Iron and Phosphor Bronze .. Table x 1,15 Cast Iron and Cast Iron.. Table x 1,33 Quenched Steel and Aluminum Alloy..Table x 1,33 Steel and Steel..Table x 2

Friction coefficients - For Case Hardened Steel Worm / Phos Bros Wheel

Sliding Speed


Sliding Speed


m/s μ m/s μ

0 0,145 1,5 0,038

0,001 0,12 2 0,033

0,01 0,11 5 0,023

0,05 0,09 8 0,02

0,1 0,08 10 0,018

0,2 0,07 15 0,017

0,5 0,055 20 0,016

1 0,044 30 0,016

Efficiency of Worm Gear

The efficiency of the worm gear is determined by dividing the output Torque M2 with friction = μ by the output torque with zero losses i.e μ = 0

First cancelling [( M 1. d 2 / d 1 ) / M 1. d 2 / d 1 ) ] = 1 Denominator = [(cos α n / (cos α n . tan γ ] = cot γ

η = [(cos α n - μ tan γ ) / (cos α n . tan γ + μ ) ] / cot γ

= [(cos α n - μ .tan γ ) / (cos α n + μ .cot γ )]

Graph showing worm gear efficiency related to gear lead angle ( γ )

Self Locking

Referring to the above graph , When the gear wheel is driving the curve points intersecting the zero efficiency line identify when the worm drive

is self locking i.e the gear wheel cannot drive to worm. It is the moment when gearing cannot be moved using even the highest

possible torque acting on the worm gear. The self-locking limit occurs when the worm lead angle ( γ ) equals atan (μ). (2o to 8o )

It is often considered that the static coefficient of friction is most relevant as the gear cannot be started. However in practice it is safer to use the, lower, dynamic coefficient of friction as this comes into play if the gear set is subject to vibration.

Worm Design /Gear Wear / Strength Equations to BS721

Note: For designing worm gears to AGMA codes AGMA method of Designing Worm Gears

The information below relates to BS721 Pt2 1983 Specification for worm gearing — Metric units. BS721 provides average design values reflecting the experience of specialist gear manufacturers. The methods have been refined by addition of various application and duty factors as used. Generally wear is the critical factor..

Permissible Load for Strength

The permissible torque (M in Nm) on the gear teeth is obtained by use of the equation

M b = 0,0018 X b.2σ bm.2. m. l f.2. d 2.

( example 87,1 Nm = 0,0018 x 0,48 x 63 x 20 x 80 )

X b.2 = speed factor for bending (Worm wheel ).. See Belowσ bm.2 = Bending stress factor for Worm wheel.. See Table belowl f.2 = length of root of Worm Wheel toothd 2 = Reference diameter of worm wheelm = axial moduleγ = Lead angle

Permissible Torque for Wear

The permissible torque (M in Nm) on the gear teeth is obtained by use of the equation

M c = 0,00191 X c.2σ cm.2.Z. d 21,8. m

( example 33,42 Nm = 0,00191 x 0,3234 x 6,7 x 1,5157 x 801,8 x 2 )

X c.2 = Speed factor for wear ( Worm wheel )σ cm.2 = Surface stress factor for Worm wheelZ = Zone factor.

Length of root of worm wheel tooth

Radius of the root = R r= d 1 /2 + h ha,1 (= m) + c(= 0,25.m.cos γ )R r= d 1 /2 + m(1 +0,25 cosγ)

l f.2 = 2.R r.sin-1 (2.R r / b a)Note: angle from sin-1(function) is in radians...

Speed Factor for Bending

This is a metric conversion from an imperial formula..X b.2 = speed factor for bending = 0,521(V) -0,2

V= Pitch circle velocity =0,00005236*d 2.n 2 (m/s)

The table below is derived from a graph in BS 721. I cannot see how this works as a small worm has a smaller diameter compared to a large worm and a lower speed which is not reflected in using the RPM.

Table of speed factors for bending

RPM (n2) X b.2 RPM (n2) X b.2

1 0,62 600 0,3

10 0,56 1000 0,27

20 0,52 2000 0,23

60 0,44 4000 0,18

100 0,42 6000 0,16

200 0,37 8000 0,14

400 0,33 10000 0,13

Additional factors

The formula for the acceptable torque for wear should be modified to allow additional factors which affect the Allowable torque M c

M c2 = M c. Z L. Z M.Z R / K C

The torque on the wormwheel as calculated using the duty requirements (M e) must be less than the acceptable torque M c2 for a duty of 27000 hours with uniform loading. For loading other than this then M e should be modified as follows

M e2 = M e. K S* K H

Thus uniform load < 27000 hours (10 years) M e ≤ M c2 Other conditions M e2 ≤ M c2

Factors used in equations

Lubrication (Z L)..Z L = 1 if correct oil with anti-scoring additive else a lower value should be selected

Lubricant (Z M).. Z L = 1 for Oil bath lubrication at V s < 10 m /s Z L = 0,815 Oil bath lubrication at 10 m/s < V s < 14 m /s Z L = 1 Forced circulation lubrication

Surface roughness (Z R ) ..Z R = 1 if Worm Surface Texture < 3μ m and Wormwheel < 12 μ m else use less than 1

Tooth contact factor (K C

This relates to the quality and rigidity of gears . Use 1 for first estimateK C = 1 For grade A gears with > 40% height and > 50% width contact = 1,3 - 1,4 For grade A gears with > 30% height and > 35% width contact = 1,5-1,7 For grade A gears with > 20% height and > 20% width contact

Starting factor (K S) ..K S =1 for < 2 Starts per hour=1,07 for 2- 5 Starts per hour=1,13 for 5-10 Starts per hour=1,18 more than 10 Starts per hour

Time / Duty factor (K H) .. K H for 27000 hours life (10 years) with uniform driver and driven loadsFor other conditions see table below

Tables for use with BS 721 equations

Speed Factors

X c.2 = K V .K R Note: This table is not based on the graph in BS 721-2 (figure 7) it is based on another more easy to follow graph. At low values of sliding velocity and RPM it agrees closely with BS 721. At higher speed velocities it gives a lower value (e.g at 20m/s -600 RPM the value from this table for X c.2 is about 80% of the value in BS 721-2

Table of Worm Gear Speed Factors

Note -sliding speed = Vs and Rotating speed = n2 (Wormwheel)

Sliding speed K V Rotating Speed K R

m/s rpm

0 1 0,5 0,98

0,1 0,75 1 0,96

0,2 0,68 2 0,92

0,5 0,6 10 0,8

1 0,55 20 0,73

2 0,5 50 0,63

5 0,42 100 0,55

10 0,34 200 0,46

20 0,24 500 0,35

30 0,16 600 0,33

Stress Factors

Table of Worm Gear Stress Factors

Other metal P.B. C.I. 0,4% 0,55% C.Steel

(Worm) C.Steel C.Steel Case. H'd

Metal(Wormwheel)

Bending(σbm ) Wear ( σ cm )

MPa MPa

Phosphor BronzeCentrifugal cast

69 8,3 8,3 9,0 15,2

Phosphor BronzeSand Cast Chilled

63 6,2 6,2 6,9 12,4

Phosphor BronzeSand Cast

49 4,6 4,6 5,3 10,3

Grey Cast Iron 40 6,2 4,1 4,1 4,1 5,2

0,4% Carbon steel 138 10,7 6,9

0,55% Carbon steel 173 15,2 8,3

Carbon Steel(Case hardened)

276 48,3 30,3 15,2

Zone Factor (Z)

If b a < 2,3 (q +1)1/2 Then Z = (Basic Zone factor ) . b a /2 (q +1)1/2 If b a > 2,3 (q +1)1/2 Then Z = (Basic Zone factor ) .1,15

Table of Basic Zone Factors

q

z1

6 6,5 7 7,5 8 8,5 9 9,5 10 11 12 13 14 17 20

1 1,04 1,04 1,05 1,06 1,08 1,10 1,12 1,13 1,14 1,16 1,202 1,26 1,31 1,40 1,50

5 8 2 5 4 7 8 7 3 8 2 8

20,991

1,028

1,055

1,099

1,144

1,183

1,214

1,223

1,231

1,25 1,28 1,32 1,361,447

1,575

30,822

0,890,989

1,109

1,209

1,261,305

1,333

1,351,365

1,3931,422

1,442

1,532

1,674

40,826

0,830,981

1,098

1,204

1,701

1,381,428

1,46 1,49 1,5151,545

1,571,666

1,798

50,947

0,991

1,051,122

1,216

1,315

1,417

1,49 1,55 1,611,632*

1,652

1,675

1,765

1,886

61,131

1,145

1,172

1,221,287

1,351,438

1,521

1,588

1,625

1,6941,714

1,733

1,818

1,928

7 1,316

1,34 1,371,405

1,452

1,541,614

1,704

1,725 1,74 1,761,846

1,98

8 1,437

1,462

1,51,557

1,623

1,715

1,7381,753

1,778

1,868

1,96

9 1573

1,604

1,648

1,72 1,7431,767

1,79 1,88 1,97

10

1,681,728

1,7481,773

1,798

1,888

1,98

11

1,732

1,7531,777

1,802

1,892

1,987

12

1,76 1,781,806

1,895

1,992

13

1,784

1,806

1,898

1,998

14

1,811

1,9 2

Duty Factor

Duty - time Factor K H

Impact from Prime moverExpected lifehours

K H

Impact From Load

Uniform Load

Medium Impact

Strong impact

Uniform Load Motor Turbine Hydraulic motor

1500 0,8 0,9 1

5000 0,9 1 1,25

27000 1 1,25 1,5

60000 1,25 1,5 1,75

Light impactmulti-cylinder engine

1500 0,9 1 1,25

5000 1 1,25 1,5

27000 1,25 1,5 1,75

60000 1,5 1,75 2

Medium Impact Single cylinder engine

1500 1 1,25 1,5

5000 1,25 1,5 1,75

27000 1,5 1,75 2

60000 1,75 2 2,25

Worm q value selection

The table below allows selection of q value which provides a reasonably efficient worm design. The recommended centre distance value "a" (mm)is listed for each q value against a range of z 2 (teeth number values). The table has been produced by reference to the relevant plot in BS 721 ExampleIf the number of teeth on the gear is selected as 45 and the centre distance is 300 mm then a q value for the worm would be about 7.5

Important note: This table provides reasonable values for all worm speeds. However at worm speeds below 300 rpm a separate plot is provided in BS721 which produces more accurate q values. At these lower speeds the resulting q values are approximately 1.5 higher than the values from this table. The above example at less than 300rpm should be increased to about 9

Table of Center distances "a" relating to q values and Number of teeth on Worm gear z 2

Number of Teeth On Worm Gear (z 2)

q 20 25 30 35 40 45 50 55 60 65 70 75 80

6 150 250 380 520 700

6.5 100 150 250 350 480 660

7 70 110 170 250 350 470 620 700

7.5 50 80 120 180 240 330 420 550 670

8 25 50 80 120 180 230 300 380 470 570 700

8.5 28 90 130 130 180 220 280 350 420 500 600 700

9 40 70 100 130 170 220 280 330 400 450 520

9.5 25 50 70 100 120 150 200 230 300 350 400

10 26 55 80 100 130 160 200 230 270 320

11 25 28 55 75 100 130 150 180 220 250

12 28 45 52 80 100 130 150 100

13 27 45 52 75 90 105

AGMA method of Designing Worm Gears

The AGMA method is provided here because it is relatively easy to use and convenient- AGMA is all imperial and so I have used conversion values so all calculations can be completed in metric units..

Good proportions indicate that for a centre to centre distance = C the mean worm dia d 1 is within the range Imperial (inches)

( C 0,875 / 3 ) ≤ d 1 ≤ ( C 0,875 / 1,6 )

Metric ( mm)

( C 0,875 / 2 ) ≤ d 1 ≤ ( C 0,875 / 1,07 )

The acceptable tangential load (W t) all

(W t) all = C s. d 20,8 .b a .C m .C v . (0,0132) (N)

The formula will result in a life of over 25000 hours with a case hardened alloy steel worm and a phosphor bronze wheel

C s = Materials factorb a = Effective face width of gearwheel = actual face width. but not to exceed 0,67 . d 1

C m = Ratio factor C v = Velocity factor

Modified Lewis equation for stress induced in worm gear teeth .

σ a = W t / ( p n. b a. y )(N)

W t = Worm gear tangential Force (N)y = 0,125 for a normal pressure angle α n = 20o

The friction force = W f

W f = f.W t / (. cos φ n ) (N)

γ = worm lead angle at mean diameterα n = normal pressure angle

The sliding velocity = V s

V s = π .n 1. d 1 / (60,000 )

d 1 = mean dia of worm (mm)n 1 = rotational speed of worm (revs/min)

The torque generated γ at the worm gear = M b (Nm)

T G = W t .d 1 / 2000

The required friction heat loss from the worm gearbox

H loss = P in ( 1 - η )

η = gear efficiency as above.

C s values

C s = 270 + 0,0063(C )3... for C ≤ 76mm ....Else

C s (Sand cast gears ) = 1000 for d 1 ≤ 64 mm ...else... 1860 - 477 log (d 1 )

C s (Chilled cast gears ) = 1000 for d 1 ≤ 200 mm ...else ... 2052 -456 log (d 1 )

C s (Centrifugally cast gears ) = 1000 for d 1 ≤ 635 mm ...else ... 1503 - 180 log (d 1 )

C m values

NG = Number of teeth on worm gear.NW = Number of starts on worm gear.mG = gear ration = NG /NW

C v values

C v (V s > 3,56 m/s ) = 0,659 exp (-0,2167 V s )

C v (3,56 m/s ≤ V s < 15,24 m/s ) = 0,652 (V s) -0,571 )

C v (V s > 15,24 m/s ) = 1,098.( V s ) -0,774 )

f values

f (V s = 0) = 0,15

f (0 < V s ≤ 0,06 m/s ) = 0,124 exp (-2,234 ( V s ) 0,645

f (V s > 0,06 m/s ) = 0,103 exp (-1,1855

WORKING

PRINCIPLE

In this machine the work is threaded by holding the work

piece in three jaw self-centering chuck. The tool is held in the

adapter. The adapter is fitted on the hallow shaft. By giving

force and turning the handle the spindle moves towards the

work piece. By moving this external thread is formed on the

work piece. After the formation of thread, the handle is

rotated in the reverse direction; the die is removed from the

work piece.

ADVANTAGES

1. Accurate threading can be done

2. Setting and operating time is less

3. Number of skilled labour is less

4. Centering of work is easy

5. Both internal and external treading can be done

6. It is used for mass production.

DISADVANTAGES

1. Only single start thread can be done.

2. Threading can be done by

Screw Thread Cutting

SCREW THREAD CUTTING

Screw threads are cut with the lathe for accuracy and for versatility. Both inch and metric screw threads can be cut using the lathe. A thread is a uniform helical groove cut inside of a cylindrical workpiece, or on the outside of a tube or shaft. Cutting threads by using the lathe requires a thorough knowledge of the different principles of threads and procedures of cutting. Hand coordination, lathe mechanisms, and cutting tool angles are all interrelated during the thread cutting process. Before attempting to cut threads on the lathe a machine operator must have a thorough knowledge of the principles, terminology and uses of threads.

Figure 3-73. Screw thread terminology.

Screw Thread Terminology

The common terms and definitions below are used in screw thread work and will be used in discussing threads and thread cutting.

External or male thread is a thread on the outside of a cylinder or cone.

Internal or female thread is a thread on the inside of a hollow cylinder or bore.

Pitch is the distance from a given point on one thread to a similar point on a thread next to it, measured parallel to the

Figure 3-74. Screw thread types.

Angle of the thread is the angle formed by the intersection of the two sides of the threaded groove.

Depth is the distance between the crest and root of a thread, measured perpendicular to the axis.

Major diameter is the largest diameter of a screw thread.

Minor diameter is the smallest diameter of a screw thread.

Pitch diameter is the diameter of an imaginary cylinder formed where the width of the groove is equal to one-half of the pitch. This is the critical dimension of threading as the fit of the thread is determined by the pitch diameter (Not used for metric threads).

Threads per inch is the number of threads per inch may be counted by placing a rule against the threaded parts and counting the number of pitches in 1 inch. A second method is to use the screw pitch gage. This method is especially suitable for checking the finer pitches of screw threads.

A single thread is a thread made by cutting one single groove around a rod or inside a hole. Most hardware made, such as nuts and bolts, has single threads. Double threads have

axis of the cylinder. The pitch in inches is equal to one divided by the number of threads per inch.

Lead is the distance a screw thread advances axially in one complete revolution. On a single-thread screw, the lead is equal to the pitch. On a double-thread screw, the lead is equal to twice the pitch, and on a triple-thread screw, the lead is equal to three times the pitch (Figure 3-74).

Crest (also called "flat") is the top or outer surface of the thread joining the two sides.

Root is the bottom or inner surface joining the sides of two adjacent threads.

Side is the surface which connects the crest and the root (also called the flank).

two grooves cut around the cylinder. There can be two, three, or four threads cut around the outside or inside of a cylinder. These types of special threads are sometimes called multiple threads.

A right-hand thread is a thread in which the bolt or nut must be turned to the right (clockwise) to tighten.

A left hand thread is a thread in which the bolt or nut must turn to the left (counterclockwise) to tighten.

Thread fit is the way a bolt and nut fit together as to being too loose or too tight.

Metric threads are threads that are measured in metric measurement instead of inch measurement.

Screw Thread Forms

The most commonly used screw thread forms are detailed in the following paragraphs. One of the major problems in industry is the lack of a standard form for fastening devices. The screw thread forms that follow attempt to solve this problem; however, there is still more than one standard form being used in each industrial nation. The International Organization for Standardization (IS0) met in 1975 and drew up a standard metric measurement for screw threads, the new IS0 Metric thread Standard (previously known as the Optimum Metric Fastener System). Other thread forms are still in general use today, including the American (National) screw thread form, the square thread, the Acme thread, the Brown and Sharpe 29° worm screw thread, the British Standard Whitworth thread, the Unified thread, and different pipe threads. All of these threads can be cut by using the lathe.

The IS0 Metric thread standard is a simple thread system that has threaded sizes ranging in diameter from 1.6 mm to 100 mm (see Table 7-8 in Appendix A). These metric threads are identified by the capital M, the

The American (National) screw thread form is divided into four series, the National Coarse (NC), National Fine (NF), National Special (NS), and National Pipe threads (NPT), 11 series of this thread form have the same shape and proportions. This thread has a 60° included angle. The root and crest are 0.125 times the pitch. This thread form is widely used in industrial applications for fabrication and easy assembly and construction of machine parts. Table 7-9 in Appendix A gives the different values for this thread form.

The British Standard Whitworth thread form thread has a 55° thread form in the V-shape. It has rounded crests and roots.

The Unified thread form is now used instead of the American (National) thread form. It was designed for interchangeability between manufacturing units in the United States, Canada, and Great Britain. This thread is a combination of the American (National) screw thread form and the British Whitworth screw thread forms. The thread has a 60° angle with a rounded root, while the crest can be rounded or flat. (In the United States, a flat crest is preferred.) The internal

nominal diameter, and the pitch. For example, a metric thread with an outside diameter of 5 mm and a pitch of 0.8 mm would be given as M 5 x 0.8. The IS0 metric thread standard simplifies thread design, provides for good strong threads, and requires a smaller inventory of screw fasteners than used by other thread forms. This IS0 Metric thread has a 60° included angle and a crest that is 1.25 times the pitch (which is similar to the National thread form). The depth of thread is 0.6134 times the pitch, and the flat on the root of the thread is wider than the crest. The root of the ISO Metric thread is 0.250 times the pitch (Table 7-9).

thread of the unified form is like the American (National) thread form but is not cut as deep, leaving a crest of one-fourth the pitch instead of one-eighth the pitch. The coarse thread series of the unified system is designated UNC, while the fine thread series is designated UNF. (See Table 7-9 in Appendix A for thread form and values.

The American National 29° Acme was designed to replace the standard square thread, which is difficult to machine using normal taps and machine dies. This thread is a power transmitting type of thread for use in jacks, vises, and feed screws. Table 7-9 lists the values for Acme threads.

The Brown and Sharpe 29° worm screw thread uses a 29° angle, similar to the Acme thread. The depth is greater and the widths of the crest and root are different (Table 7-9 in Appendix A). This is a special thread used to mesh with worm gears and to transmit motion between two shafts at right angles to each other that are on separate planes. This thread has a self-locking feature making it useful for winches and steering mechanisms.

The square screw thread is a power transmitting thread that is being replaced by the Acme thread. Some vises and lead screws may still be equipped with square threads. Contact areas between the threads are small, causing screws to resist wedging, and friction between the parts is minimal (Table 7-9 in Appendix A).

The spark plug thread (international metric thread type) is a special thread used extensively in Europe, but seen only on some spark plugs in the United States. It has an included angle of 60 with a crest and root that are 0.125 times the depth.

Different types of pipe thread forms are in use that have generally the same characteristics but different fits. Consult the Machinery's Handbook or a similar

The four fits are described as follows:

Class 1 fit is recommended only for screw thread work where clearance between mating parts is essential for rapid assembly and where shake or play is not objectionable.

Class 2 fit represents a high quality of thread product and is recommended for the great bulk of interchangeable screw thread work.

Class 3 fit represents an exceptionally high quality of commercially threaded product and is recommended only in cases where the high cost of precision tools and continual checking are warranted.

Class 4 fit is intended to meet very unusual requirements more exacting than those for which Class 3 is intended. It is a selectivefit if initial assembly by hand is required. It is not. as yet. adaptable to quantity production.

Thread Designations

In general. screw thread designations give the screw number (or diameter) first. then the thread per inch. Next is the thread series containing the initial letter of the series. NC (National Coarse). UNF (Unified Fine). NS (National Special). and so forth. followed by the class of fit. If a thread is left-hand.

reference for this type of thread.

THREAD FIT AND CLASSIFICATIONS

The Unified and American (National) thread forms designate classifications for fit to ensure that mated threaded parts fit to the tolerances specified. The unified screw thread form specifies several classes of threads which are Classes 1A, 2A, and 3A for screws or external threaded parts, and 1B, 2B, and 3B for nuts or internal threaded parts. Classes 1 A and 1 B are for a loose fit where quick assembly and rapid production are important and shake or play is not objectionable. Classes 2A and 2B provide a small amount of play to prevent galling and seizure in assembly and use. and sufficient clearance for some plating. Classes 2A and 2B are recommended for standard practice in making commercial screws. bolts. and nuts. Classes 3A and 3B have no allowance and 75 percent of the tolerance of Classes 2A and 2B A screw and nut in this class may vary from a fit having no play to one with a small amount of play. Only high grade products are held to Class 3 specifications.

Four distinct classes of screw thread fits between mating threads (as between bolt and nut) have been designated for the American (National) screw thread form. Fit is defined as "the relation between two mating parts with reference to ease of assembly. " These four fits are produced by the application of tolerances which are listed in the standards.

the letters LH follow the fit. An example of designations is as follows:

Two samples and explanations of thread designations are as follows:

No 12 (0.216) -24 NC-3. This is a number 12 (0.216-inch diameter) thread. 24 National Coarse threads per inch. and Class 3 ways of designating the fit between parts. including tolerance grades. tolerance positions. and tolerance classes. A simpler fit.

1/4-28 UNF-2A LH. This is a l/4-inch diameter thread. 28 Unified Fine threads per inch, Class 2A fit, and left-hand thread.

Metric Thread Fit and Tolerance

The older metric screw thread system has over one hundred different thread sizes and several ways of designating the fit between parts. including tolerance grades. tolerance positions. and tolerance classes. A simple system was devised with the latest ISO Metric thread standard that uses one internal fit and two external fit designations to designate the tolerance

tool bit must be ground for the exact shape of the thread form. to include the root of the thread (Figure 3-75).

For metric and American (National) thread forms. a flat should be ground at the point of the tool bit (Figure 3-76). perpendicular to the center line of the 600 thread angle. See the thread form table for the appropriate thread to determine the width of the Sat. For unified thread forms. The tip of the tool bit should be ground with a radius formed to fit the size of the root of the thread. Internal unified threads have a flat on the tip of the tool bit. In all threads listed above. The tool bit

(class) of fit. The symbol 6H is used to designate the fit for an internal thread (only the one symbol is used). The two symbols 6g and 5g6g are used to designate the fit for an external thread. 6g being used for general purpose threads and Sg6g used to designate a close fit. A fit between a pair of threaded parts is indicated by the internal thread (nut) tolerance fit designation followed by the external thread (bolt) tolerance fit designation with the two separated by a stroke. An example is M 5 x 0.8-Sg6g/6H. where the nominal or major diameter is 5 mm. the pitch is 0.8 mm. and a close tit is intended for the bolt and nut. Additional information on ISO metric threads and specific fits can be found in any updated engineer's handbook or machinist's handbook.

THREAD CUTTING TOOL BITS

Cutting V-threads with a 60 degrees thread angle is the most common thread cutting operation done on a lathe. V-threads with the 60 degree angle are used for metric thread cutting and for American (National) threads and Unified threads. To properly cut V-shaped threads. the single point

should be ground with enough side relief angle and enough front clearance angle (Figure 3-76). Figure 3-77 illustrates the correct steps involved in grinding a thread-cutting tool bit.

Figure 3-75. V-shaped thread cutter.

Figure 3-76. Relief angles on a thread cutting tool bit.

Figure 3-77. Grinding a thread cutting tool bit.

Figure 3-78. Common gages for checking threading tool bits.

For Acme and 29° worm screw threads, the cutter bit must be ground to form a point angle of 29°. Side clearances must be sufficient to prevent rubbing on threads of steep pitch. The end of the bit is then ground to a flat which agrees with the width of the root for the specific pitch being cut. Thread-cutting tool gages (Figure 7-78) are available to simplify the procedure and make computations unnecessary.

To cut square threads, a special thread-cutter bit is required. Before the square thread-cutter bit can be ground, it is necessary to compute the helix angle of the thread to be cut (Figure 7-79). Compute the helix angle by drawing a line equal in length to the thread circumference at its minor diameter (this is accomplished by multiplying the minor diameter by 3.1416 [pi]). Next, draw a line perpendicular to and at one end of the first line, equal in length to the lead of the thread. If the screw is to have a single thread, the lead will be equal to the pitch. Connect the ends of the angle so formed to obtain the helix angle.

The tool bit should be ground to the helix angle. The clearance angles for the sides should be within the helix angle. Note that the sides are also ground in toward the shank to provide additional clearance.

The end of the tool should be ground flat, the flat being equal to one-half the pitch of the thread to produce equal flats and spaces on the threaded part.

When positioning the thread-cutter bit for use, place it exactly on line horizontally with the axis of the workpiece. This is especially important for thread-cutter bits since a slight variation in the vertical position of the bit will change the thread angle being cut.

Figure 3-79. Thread tool bit for square threads.

Figure 3-80. Positioning thread cutter bit.

The thread-cutter bit must be positioned so that the centerline of the thread angle ground on the bit is exactly perpendicular to the axis of the workpiece. The easiest way to make this alignment is by use of a center gage. The center gage will permit checking the point angle at the same time as the alignment is being effected. The center gage is placed against the workpiece and the cutter bit is adjusted on the tool post so that its point fits snugly in the 60° angle notch of the center gage (Figure 3-80).

In cutting threads on a lathe, the pitch of the thread or number of threads per inch obtained is determined by the speed ratio of the headstock spindle and the lead screw which drives the carriage. Lathes equipped for thread cutting have gear arrangements for varying the speed of the lead screw. Modern lathes have a quick-change gearbox for varying the lead screw to spindle ratio so that the operator need only follow the instructions on the direction plates of the lathe to set the proper feed to produce the desired number of threads per inch. Once set to a specific number of threads per inch, the spindle speed can be varied depending upon the material being cut and the size of the workpiece without affecting the threads per inch.

with the lead screw. A control is available to reverse the direction of the lead screw for left or right-hand threading as desired. Be sure the lead screw turns in the proper direction. Feed the cutter bit from right to left to produce a right-hand thread. Feed the cutter bit from left to right to produce a left-hand thread.

Direction of feed. For cutting standard 60° right-hand threads of the sharp V-type, such as the metric form, the American (National) form, and the Unified form, the tool bit should be moved in at an angle of 29° to the right (Figure 3-81), (Set the angle at 29° to the left for left-hand threads). Cutting threads with the compound rest at this angle allows for the left side of the tool bit to do most of the cutting, thus relieving some strain and producing a free curling chip. The direction is controlled by setting the compound rest at the 29° angle before adjusting the cutter bit perpendicular to the workpiece axis. The depth of cut is then controlled by the compound rest feed handle.

Figure 3-81. External threading setup.

For Acme and 29° worm threads, the compound rest is set at one-half of the included angle (14 1/2°) and is fed in with the compound rest. For square

The carriage is connected to the lead screw of the lathe for threading operations by engaging the half nut on the carriage apron

threads, the cutter bit is fed into the workpiece at an angle perpendicular to the workpiece axis.

THREAD CUTTING OPERATIONS

Before cutting threads, turn down the workpiece to the major diameter of the thread to be cut and chamfer the end. Engineering and machinist's handbooks have special tables listing the recommended major and minor diameters for all thread forms. These tables list a minimum and a maximum major diameter for the external threads, and a minimum and maximum minor diameter for internal threads. Table 7-10 in Appendix A lists the most common screw thread sizes. The difference between the maximum and minimum major diameters varies with different sizes of threads. Coarse threads have a larger difference between the two than fine threads. It is common practice, when machining threads on the lathe, to turn the outside diameter down to the maximum major diameter instead of the minimum major diameter, thus allowing for any error.

The workpiece may be set up in a chuck, in a collet, or between centers. If a long thread is to be cut, a steady rest or other support must be used to help decrease the chance of bending the workpiece. Lathe speed is set for the recommended threading speed (Table 7-2 in Appendix A).

After making the first pass check for proper pitch of threads by using one of the three methods in Figure 3-84. After each pass of the threading tool bit, the operator must move the threading tool bit out of the threaded groove by backing out the compound rest handle, taking note of the setting. Traverse the carriage back to the start of the thread and move the compound rest dial back to the original setting plus the new depth of cut. At the end of each cut, the half nut lever is usually disengaged and the carriage returned by hand. (The cross slide dial can also be used to move the tool bit in and out, depending on the preference of the operator.)

After cutting the first depth of thread, check for the proper pitch of threads by using one of the three methods in Figure 3-84. If the thread pitch is correct as set in the quick-change gearbox, continue to cut the thread to the required depth. This is determined by measuring the pitch diameter and checking the reference table for-the proper pitch diameter limits for the desired tit.

Some lathes are equipped with a thread chasing stop bolted to the carriage which can be set to regulate the depth of cut for each traverse of the cutter bit or can be set to regulate the total depth of cut of the thread.

When the thread is cut the end must be finished in some way. The most common means of finishing the end is with a specially ground or 45 degree angle chamfer cutting bit. To produce a rounded end, a cutter bit with the desired shape should be specially ground for that purpose.

Metric Thread Cutting Operations

Metric threads, are cut one of two ways by using

Figure 3-82.Thread chasing dial.

To cut threads, move the threading tool bit into contact with the work and zero the compound rest dial. The threading tool bit must be set at the right end of the work; then, move the tool bit in the first depth of cut by using the graduated collar of the compound rest. Position the carriage half nut lever to engage the half nut to the lead screw in order to start the threading operation. The first cut should be a scratch cut of no more than 0.003 inch so the pitch can be checked. Engaging the half nut with the lead screw causes the carriage to move as the lead screw revolves. Cut the thread by making a series of cuts in which the threading tool follows the original groove for each cut. Use the thread chasing dial, Figure 3-82, to determine when to engage the half nut so that the threading tool will track properly. The dial is attached to the carriage and is driven by means of the lead screw. Follow the directions of the thread chasing dial, Figure 3-83, to determine when to engage the half nut lever.

the lathe, designed and equipped for metric measurement or by using a standard inch lathe and converting its operation to cut metric threads. A metric measurement lathe has a quick-change gear box used to set the proper screw pitch in millimeters. An inch- designed lathe must be converted to cut metric threads by switching gears in the lathe headstock according to the directions supplied with each lathe.

Most lathes come equipped with a set of changeable gears for cutting different, or nonstandard screw threads. Follow the directions in the lathe operator manual for setting the proper metric pitch. (A metric data plate may be attached to the lathe headstock.) Most lathes have the capability of quickly attaching these change gears over the existing gears then realigning the gearing. One change gear in needed for the lead screw gear and one for the spindle, or drive gear.

THREADS PER INCH

TO CUT

Even Number

of

Threads

When to Engage

Split Nut

Engage at any

Graduation on

The Dial

diameter to the desired diameter measurement. Convert the linear pitch in millimeters, to threads per inch by dividing the linear pitch of 2.5 by 25.4 to get the threads per inch ( 10.16 TPI).

Now a 8-13 TPI thread micrometer can be used to measure the pitch diameter for this metric thread. To sum up how to convert metric threads to inch measurement:

Odd Numbers Of

Thread

Engage at Any

Main Division

Fractional Number

Of

Threads

½ Threads, e.g. 11 ½

Engage at any other

Main Division 1 & 3 or 2 & 4

Other Fractional Threads

Engage at Same Division

Every Time

Thread that are A

Multiple of the

Number of the

Threads per Inc

in the Lead Screw

Engage at Any Time That

Split Nut Meshes

Figure 3-83. Thread chasing dial instructions.

The metric thread diameter and pitch can be easily measured with a metric measuring tool. If there are no metric measuring tools available, the pitch and diameter must be converted from millimeters to inch measurement, and then a inch micrometer and measuring tools can be used to determine the proper pitch and diameter. Millimeters may be converted to inch measurement either by dividing millimeters by 25.4 inches or multiplying by 0.03937 inches.

For example, a thread with a designation M20 x 2.5 6g/6h is read as follows: the M designates the thread is metric. The 20 designates the major diameter in millimeters. The 2.5 designates the linear pitch in millimeters. The 6g/6h designates that a general purpose fit between nut and bolt is intended. Therefore, to machine this metric thread on a inch designed lathe, convert the outside diameter in millimeters to a decimal fraction of an inch and machine the major

Convert major diameter from millimeters to inch measure.

Convert pitch and pitch diameter to inch measure.

Set quick change gears according to instructions.

Set up the lathe for thread cutting as in the preceding paragraphs on screw thread cutting, Take a light trial cut and check that the threads are of the correct pitch using a metric screw pitch gage. At the end of this trial cut, and any cut when metric threading, turn off the lathe and back out the tool bit from the workpiece without disengaging the half-nut- lever. Never disengage the lever until the metric thread is cut to the proper pitch diameter, or the tool bit will have to be realigned and set for chasing into the thread.

After backing the tool bit out from the workpiece, traverse the tool bit back to the starting point by reversing the lathe spindle direction while leaving the half-nut lever engaged. If the correct pitch is being cut, continue to machine the thread to the desired depth.

NOTE: If the tool bit needs to be realigned and chased into the thread due to disengagement, of the half-nut lever or having to remove the piece and start again, then the lathe must be reset for threading. Start the lathe, with the tool bit clear of the workpiece engage the lever. Allow the carriage to travel until the tool bit is opposite any portion of the unfinished thread; and then turn off the lathe, leaving the engaged. Now the tool bit can be set back into a thread groove by advancing the cross slide and reference. Restart the lathe, and the tool bit should follow the groove that was previously cut, as long as the half-nut lever stays engaged.

Figure 3-84. Checking threads per inch.

TAPERED SCREW THREADS

Tapered screw threads or pipe threads can be cut on the lathe by setting the tailstock over or by using a taper attachment. Refer to the references for taper per inch and nominal measurements of tapered thread forms. When cutting a tapered thread, the tool bit should be set at right angles to the axis of the work. Do not set the tool bit at a right angle to the taper of the thread. Check the thread tool bit carefully for clearances before cutting since the bit will not be entering the work at right angles to the tapered workpiece surface.

MEASURING EXTERNAL V-SHAPED

SCREW THREADS

The fit of the thread is determined by its pitch diameter. The pitch diameter is the diameter of the thread at an imaginary point on the thread where the width of the space and the width of the thread are equal. The fact that the mating parts bear on this point or angle of the thread, and not on the top of it, makes the pitch diameter an important dimension to use in measuring screw threads.

The thread micrometer (Figure 3-85) is an instrument used to gage the thread on the pitch diameter. The anvil is V-Shaped to fit over the V-thread. The spindle, or movable point, is cone-shaped (pointed to a V) to fit between the threads. Since the anvil and spindle both contact the sides of the threads, the pitch diameter is gaged and the reading is given on the sleeve and spindle where it

Thread micrometers are marked on the frame to specify the pitch diameters which the micrometer is used to measure. One will be marked, for instance, to measure from 8 to 13 threads per inch, while others are marked 14 to 20, 22 to 30, or 32 to 40; metric thread micrometers are also available in different sizes.

The procedure in checking the thread is first to select the proper micrometer, then calculate or select from a table of threads the correct pitch diameter of the screw. Lastly, fit the thread into the micrometer and take the reading.

The 3-wire method is another method of measuring the pitch diameter for American National (60 degree) and Unified threads. It is considered the "best' method for extremely accurate measurement. Page A-28 in Appendix A shows three wires of correct diameter placed in threads with the micrometer measuring over them. The pitch diameter can be found by subtracting the wire constant from the measured distance over the wires. It can be readily seen that this method is dependent on the use of the "'best'" wire for the pitch of the thread. The "best" wire is the size of wire which touches the thread at the middle of the sloping sides. in other words, at the pitch diameter. A formula by which the proper size wire may be found is as follows: Divide the constant 0.57735 by the number of threads per inch to cut. If. for example, 8 threads per inch have been cut, we would calculate 0.577358 = 0.072. The diameter of wire to use for measuring an 8-pitch thread is 0.072.

can be read by the operator.

Figure 3-85. Thread micrometer.

The wires used in the three-wire method should be hardened and lapped steel wires. they, should be three times as accurate as the accuracy desired in measurement of the threads. The Bureau of Standards has specified an accuracy of 0.0002 inch. The suggested procedure for measuring threads is as follows:

After the three wires of equal diameter have been selected by using the above formula, they are positioned in the thread grooves as shown on page A-28 in Appendix A. The anvil and spindle of an ordinary micrometer are then placed against the three wires and the reading is taken. To determine what the reading of the micrometer should be if a thread is the correct finish size. use the following formula (for measuring Unified National Coarse threads): add three times the diameter of the wire to the diameter of the screw; from the sum, subtract the quotient obtained by dividing the constant 1.5155 by the number of threads per inch. Written concisely, the formula is:

m = (D +3 W)-1.5155

n

Where m = micrometer measurement over wires,

D = diameter of the thread,

n = number of threads per inch,

W = diameter of wire used

Example: Determine m (measurement over wires) for 1/2 inch, 12-pitch UNC thread. We would proceed to solve as follows:

where W = 0.04811 inch

D = 0.500 inch

n=12

M= PD+CPD=M-C

M = measurement over the wires

PD = pitch diameter

C = N constant (This is found in Table 7-11 in Appendix A)

The "best" wire size can be found by converting from inch to metric, or by using Table 7-11 in Appendix A.

An optical comparator must be used to check the threads if the tolerance desired is less than 0.001 inch (0.02 mm). This type of thread measurement is normally used in industrial shops doing production work.

CUTTING INTERNAL THREADS

Internal threads are cut into nuts and

Then m = (0.500+ 0.14433) - 155155

12

m = (0.500 + 0.14433) -0.1263

m = 0.51803 inch (micrometer measurement)

When measuring a Unified National Fine thread, the same method and formula are used. Too much pressure should not be applied when measuring over wires.

Metric threads can also be checked by using the three-wire method by using different numerical values in the formula. Three-wire threads of metric dimensions must have a 60° angle for this method.

castings in the same general manner as external threads. If a hand tap is not available to cut the internal threads, they must be machined on the lathe.

An internal threading operation will usually follow a boring and drilling operation, thus the machine operator must know drilling and boring procedures before attempting to cut internal threads. The same holder used for boring can be used to hold the tool bit for cutting internal threads. Lathe speed is the same as the speed for external thread cutting.

Figure 3-86. Internal thread cutting.

To prevent rubbing, the clearance of the cutter bit shank and boring tool bar must be greater for threading than for straight boring because of the necessity of moving the bit clear of the threads

CUTTING EXTERNAL ACME THREADS

The first step is to grind a threading tool to conform to the 29° included angle of the thread. The

when returning the bit to the right after each cut.

The compound rest should be set at a 29° angle to the saddle so that the cutter bit will feed after each cut toward the operator and to his left.

Although the setup shown in Figure 3-86 would be impractical on extremely large lathes, it allows a degree of safety on common sized machines by having the compound ball crank positioned away from any work holding device that would be in use on the lathe, eliminating the possibility of the operator's hands or the compound rest contacting the revolving spindle and work holding devices.

Figure 3-87. Left-hand threading.

Cutting 60° left-hand threads. A left-hand thread is used for certain applications where a right-hand thread would not be practicable, such as on the left side of a grinder where the nut may loosen due to the rotation of the spindle. Left-hand threads are cut in the same manner as right hand threads, with a few changes. Set the feed direction lever so that the carriage feeds to the right, which will mean that the lead screw revolves opposite the direction used for right-hand threading. Set the compound rest 29° to the left of perpendicular. Cut a groove at the left end of the threaded section, thus providing clearance for starting the cutting tool (see Figure 3-87). Cut from left to right until the proper pitch dimension is achieved.

tool is first ground to a point, with the sides of the tool forming the 290 included angle (Figure 3-88). This angle can be checked by placing-the tool in the slot at the right end of the Acme thread gage.

Figure 3-88. Acme thread cutting tool bit.

If a gage is not available, the width of the tool bit point may be calculated by the formula:

Width of point= 0.3707P -0.0052 inch

Where P = Number of threads per inch

Be sure to grind this tool with sufficient side clearance so that it will cut. Depending upon the number of threads per inch to be cut, the point of the tool is ground flat to fit into the slot on the Acme thread gage that is marked with the number of threads per inch the tool is to cut. The size of the flat on the tool point will vary depending upon the thread per inch to be machined.

After grinding the tool, set the compound rest to one-half the included angle of the thread (14 1/2°) to the right of the vertical centerline of the machine (Figure 3-89). Mount the tool in the holder or tool post so that the top of the tool is on the axis or center line of the workpiece. The tool is set square to the work, using the Acme thread gage. This thread is cut using the compound feed. The depth to which you feed the compound rest to obtain total thread depth is determined by the formula given and illustrated in Table 7-9 in Appendix A. The remainder of the Acme thread-cutting operation is the same as the V-

threading operation previously described. The compound rest should be fed into the work only 0.002 inch to 0.003 inch per cut until the desired depth of thread is obtained.

Figure 3-89. Acme and 29 degree worm thread setup.

Figure 3-90. Using one wire to measure an acme

The formulas used to calculate Acme thread depth are in Table 7-9 in Appendix A. The single wire method can be used to measure the accuracy of the thread (Figure 3-90). A single wire or pin of the correct diameter is placed in the threaded groove and measured with a micrometer. The thread is the correct size when the micrometer reading over the wire is the same as the major diameter of the thread and the wire is placed tightly into the thread groove. The diameter of the wire to be used can be calculated by using this formula:

Wire diameter = 0.4872 x pitch

0.4872 x 1/6= 0.081 inch

Cutting the 29° worm screw thread (Brown and Sharpe). The tool bit used to cut 29° worm screw threads will be similar to the Acme threading tool, but slightly longer with a different tip. Use Table 7-9 in Appendix A to calculate the length of the tool bit and tip width. The cutting is done just like cutting an Acme thread.

CUTTING SQUARE THREADS

Because of their design and strength, square threads are used for vise screws, jackscrews, and other devices where maximum transmission of power is needed. All surfaces of the square thread form are square with each other, and the sides are perpendicular to the center axis of the threaded part. The depth, the width of the crest, and root are of equal dimensions. Because the contact areas are relatively small and do not wedge together, friction between matching threads is reduced to a minimum. This fact explains why square threads are used for power transmission.

Before the square thread cutting tool can be ground, it is necessary first to determine the helix angle of the thread. The sides of the tool for cutting the square thread should conform with the helix angle of the thread (Figure 3-79).

For cutting the thread, the cutting edge of the tool should be ground to a width exactly one-half that of the pitch. For cutting the nut, it should be from 0.001 to 0.003 of an inch larger to permit a free fit of the nut on the screw.

The cutting of the square thread form presents some difficulty. Although it is square, this thread, like any other, progresses in the form of a helix, and thus

Thus, if 6 threads per inch are being cut, the wire size would be:

assumes a slight twist. Some operators prefer to produce this thread in two cuts, the first with a narrow tool to the full depth and the second with a tool ground to size. This procedure relieves cutting pressure on the tool nose and may prevent springing the work. The cutting operation for square threads differs from cutting threads previously explained in that the compound rest is set parallel to the axis of the workpiece and feeding is done only with the cross feed. The cross feed is fed only 0.002 inch or 0.003 inch per cut. The finish depth of the thread is determined by the formula.

pipe thread cutiing machine

Documents