ch8 grinding

7/29/2019 Ch8 Grinding

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Abrasive ProcessesAbrasive machining involves material removal by the action of hard abrasive particles that are usually in the form of a bondedwheel.

These operations are generally used as finishing operations ,although in some cases they are used for high metal removingrates rivaling of conventional machining operations.

These processes include grinding, honing, lapping, super finishing, polishing and buffing.


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Process Part Geometry Surface roughness(R a) in m

Grinding Flat, Externalcylinder

0.406 1.60.203 0.406

Honing Round hole 0.102 0.813

Lapping Flat or slightlyspherical

0.0254 0.406

Super finishing Flat, Externalcylinder

0.0127 0.203

Polishing Different shapes. 0.0254 0.813

buffing Different shapes. 0.0127 0.406


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The reasons for the commercial and technological importance

of the abrasive processes are;1. They can be used on all types of materials , ranging from

soft to hard materials.

2. Some of theses processes can produce extremely finesurface finish . (see the previous table) .

3. The dimension of the product can be held to extremelyclose tolerances .


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Grindingit is a material removal processes in which the abrasive particles

are contained in a bonded grinding wheel that operates at veryhigh surface speeds.

The grinding operation is much like the milling operations.Cutting occurs either the periphery or the face of the grindingwheel, similar to the peripheral milling and face milling. The

peripheral grinding is much common than the face one.

Grindingwheel Grinding

wheel


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Milling Grinding

The cutting edges

Defined number of cutting edges (teeth)

The abrasive is smaller andmore numerous (undefinednumber of grains).

Cutting speed Much higher

Cutting angles Defined cuttingangles

Randomly oriented and can possess very high negativerake angles

Reshaping of the

cutting edges

The dull abrasive particles

either fracture to create a freshcutting edges or pulled out toexpose a new grains.

The difference between the milling operations and the grindingoperations are;


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Grinding wheelThe grinding wheel is the tool with which the grinding action is performed.

It consists of abrasive particles and bonding materials, (as shown in the Fig.).

The bonding material holds the particles in place and build the shape and structure of thewheel.

These two components and the way they arefabricated determine the parameters of thegrinding wheel;

1. Abrasive materials. 2. Grain size

3. Bonding materials. 4. Wheel grade5. Wheel structure


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1- Abrasive materials

General properties of abrasive materials are;

1. High hardness2. Wear resistance

3. Toughness

4. Friability; it is the ability of the abrasive material to fracture,when the cutting of the grain becomes dull, thereby exposing anew cutting edge (sharp cutting edge).


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Abrasivematerials

Marking Using

Aluminum Oxide(Al 2O3)

A Steel, other ferrous materials andhigh strength alloys.

Silicon carbide(SiC)

C Ductile metals (AL, brass, andstainless steel), and brittle materials(CI, and certain ceramics).

Cubic born

nitride (CBN)

B Hard materials (hardened tool

steels and aero space alloys).Diamond D Hard abrasive materials (ceramics,

cemented carbides and glass).


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2- Grain size

Its an important parameter in determining the surface finish and metalremoving rate.

1. Grain size surface finish

2. Grain size material removal rate .

3. Harder work materials required smaller grain size while the softer materials required larger grit size. Grit size is measured using ascreen mesh. Smaller grit size has larger numbers.

The selection of the grit size must take into consideration the above 3

points.


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3- Bonding materials

The bonding material holds the particles in place and build the shape and

structure of the wheel.The desirable properties of bonding materials are;

1. Strength (withstand centrifugal forces).

2. Toughness (resist shattering in shock loads).

3. Hardness.

4. Temperature resistance (withstand high temperature).

5. Hold abrasive material in place rigidly to accomplish the cutting forceswhile allowing the grain that are worn to be dislodged so that newgrains can be exposed.


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Bondingmaterials

Marking Using

Vitrified bond V Strong, rigid, resist temp., relativelyunaffected by water and oil that used ingrinding fluids.

Silicate bond S Limited to situations in which heat generatedmust be minimized.

Rubber bond R most fixable, used in cut-off wheels.Resinoid bond B Very high strength, used for rough grinding

and cut-off operations.

Shellac bond E Strong but not rigid, for good finish.

Metallic bond M Common for diamond and cubic born nitride,usually bronze.


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4- Wheel Structure

Refers to the relative spacing of the abrasive grains in the wheel.

It is measured on a scale ranges from open to dense (the ratio between thegrains proportion to bond materials proportion).

Open is used for chip clearance.

Dense one is used for surface finish and dimensional control.

Vg + V b + V p = 1.0

Vg grains proportion

V b bond materials proportion

V p voids (pores) proportion


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Wheel Grade

Indicates thewheels

bond strength in retaining the abrasive grits duringcutting. Dependent on the present of bonding materials (V b).

It is measured on a scale ranges from soft to hard .

Soft one, lose the grains readily, is used for low materials removal rates

and when machining hard work material. Hard one, retains the grains, is used for high materials removal rates andwhen machining soft work material.


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5- Grinding wheel specification (ANSI 1977)


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Grinding wheel shapes


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- In the cutting, parameters and the grinding wheel parameters have adirect effect into;

1- Surface finish

2- Forces and energy

3- Temperature of the work surface

4- Wheel wear

5- Machining time.

There are two types of feed;

Cross feed; motion of work table (w)

In feed; the motion of the wheel into the work (d)


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Surface finish

It is affected by the size of the individual chips (note that

small grit size get better surface finish). Number of chips formed;

C is grits/mm 2 (from 0.1 to 10)

That mean that the increase of V and C for a given w

increase the number of chips/time and improve the finish.

C .w.V nc


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Forces and energy

wd V

V F

U wc

U specific energy, F c cutting force.

Specific energy in the grinding operations is much greater that in other conventional machining because;

1. The size effect in the machining.

2. High negative rake angle of the individual grains.

3. Not all the grits are engaged in actual cutting (cutting, plowing, andrubbing, Fig. 8.5).

Example 8.1 page 193


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Temperatures at the work surface

The grinding process generates high temperature because the size

effect and the high negative rake angle, this with high friction.

This high temperature can have several damaging effects;

1. Bums and cracks;

Bum is a discolorration on the surfaceCracks are perpendicular to the wheel speed direction2. Softening of the work surface.

3. Residual stresses.

The proper application of the cutting fluids reduce friction andremove heat and washing away chips.


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Wheel wear

In the grinding wheel there are three types of wear (Fig. 8.6);

1. Grain fracture, 1st

region.2. Attritions wear, 2 nd region.

3. Bond fracture, 3 rd region.

When the wheel in the 3 rd region, it should be re-sharping (dressing

operation) to;1. Breaking off dulled grits.

2. Removing the chips.

Grinding ratio (GR) is between 95 to 125, depending on wheel speedwhich reduce the wear, while at too high speed both wear and temp.are increased and the grinding ratio is reduced with the surface finish.


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Machining time

bw

bw

d

hi

l L

wV

LiBT

wm

3/1

3/2

1000

2 B work width

b grinding wheel width

l work length

allowance (10mm)

h total depth

d infeed

V w the work speed

w cross feedExample 8.2 page 199


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Grinding operations

Used to finish products created with other operations.

Shapes to be grinded are plain surfaces, internal and externalcylinders, and contour surfaces.

The contour surfaces, such as thread, are created by a special formedwheels, which are created in the tool rooms.

1- Surface Grinding

Performed using either the periphery or the flat surface of the grindingwheel (face grinding).

The peripheral grinding is performed with a horizontal axis machines,while the face grinding is performed with a vertical axis machines, asshown in the figure.


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Horizontal spindle with reciprocating worktable. Horizontal spindle with rotary worktable.

vertical spindle with reciprocating worktable. Vertical spindle with rotary worktable.

Note the direction of the cross feed and in-feed


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2- cylindrical Grindinga- External cylindrical grinding

the feed motions are either traverse feed (the wheel or

workpiece or both) or plunge cut.

b- Internal cylindrical grinding

l d


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3- Centerless Grinding

The workpiece is not held

between center, then it used for high production rate machining,no holding time.

The regulating wheel rotates at

lower speed and is inclined at aslight angle I, the feed rate of thiswheel depending on its diameter,rotating speed, and its angle.

I N D f r r r sin

External Centerless Grinding


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The internal centerless grinding has two supportingrolls instead of the rest blade in the external one.

Internal Centerless Grinding

4 C F dG i di


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d

Feed

4- Creep Feed Grinding

In-Feed are 1000 to 10000 timesgreater then conventional one

Feed rates are reduced about thesame proportion.

High MRR and productivity

because of the continuouscutting.

Advantages;

1. High MRR

2. Improve accuracy

3. Reduced temperatures.


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Related abrasive processes

1- Honing

To finish the bores of internalcombustion engines, bearing, hydrauliccylinders, and gun barrels.

Performed with a set of bondedabrasive sticks.

Its motion is combination of rotationand linear motion.

Speed between 15 to 150 m/min.

Grit size between 30 and 600 Tool Operation


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2- Lapping

Used to produce surface finish of extremeaccuracy and smoothness.Used in production of optical lenses,metallic bearing surface, and gages.It used a fluid suspension of very smallabrasive particles (in oils or kerosene)

between the work and the lapping tool.Common abrasives are aluminum oxide andsilicon carbide with grit size between 300and 600.The tool takes the inverse shape of the work

part.Materials such as steel, CI, copper, lead andwood.


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3- Superfinishing

Like honing but;

1. Shorter strokes

2. Higher frequancy, lower pressure

3. Lower workpiece speed.

4. Smaller grit size.

5. Only one stick is used


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4- Polishing and Buffing

Polishing is used to remove scratches and burrs and smooth roughsurfaces by means of abrasive grains attached to the polishing wheelrotating at high speed around 2250 m/min.

The wheel, flexible wheel, is made of leather, canvas, felt and papers.

And grit size of 20 to 80 for rough, 90 to 120 for finish.

Buffing is similar to polishing with softer wheels, finer abrasivematerials.

Speed ranging from 2400 to 5100 m/min.

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