3 Grinding Process

IPE 331: Production Processes Lecture sheet-3

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GRINDING & RELATED OPERATIONS

Grinding

The grinding process consists of removing material from the work-piece by the use of a rotating wheel that has a surface composed of abrasive grains. Grinding is considered to be the most accurate of the existing machining processes. Grinding processes are used when high accuracies, close dimensional tolerances, and a fine surface finishes are required. Hard materials can also be machined.

Grinding Machines

Fig: Horizontal spindle reciprocating table surface grinder

Wheel guard

Work table

Workpiece

Saddle

Wheel head

Column

Bed


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Fig: Vertical spindle reciprocating table surface grinder

Cutting conditions in grinding

The cutting velocity V in grinding is very high. It is related to the rotational speed of the wheel by, V = πDN, where D is the wheel diameter, and N is the rotational speed of the grinding wheel.

Depth of cut d is called infeed and is defined as the distance between the machined and work surfaces.

As the operation proceeds, the grinding wheel is fed laterally across the work surface on each pass by the workpart. The distance at which the wheel is fed is called a crossfeed. The crossfeed is actually the width of cut w. The crossfeed multiplied by infeed determines the cross-sectional area of cut, CSA = crossfeed×infeed = wd

The cross-sectional area in grinding is relatively small compared to other machining operations The workpart moves past the wheel at a certain linear or rotational velocity called a feed Vw.

The material removal rate, MRR, is defined by MRR = Vwwd


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Common Grinding Wheels

A grinding wheel is made of abrasive grains held together by a bond. These grains cut like teeth when the wheel is revolved at high speed and is brought to bear against a work piece. The properties of a wheel that determine how it acts are the kind and size of abrasive, how closely the grains are packed together and amount of the bonding material.

Grinding Wheels-Designation and selection

A grinding wheel is made of abrasive grains held together by a bond. These grains cut like teeth when the wheel is revolved at high speed and is brought to bear against a work piece. The properties of a wheel that determine how it acts are the kind and size of abrasive, how closely the grains are packed together and amount of the bonding material.

The way the abrasive grains, bonding material, and the air gaps are structured, determines the parameters of the grinding wheel, which are

abrasive material, grain size, bonding material, wheel grade, and wheel structure.

To achieve the desired performance in a given application, each parameter must be carefully selected.


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1) Abrasive material:

These are hard materials with adequate toughness which act as cutting edges for a sufficiently long time. They also have the ability to fracture into smaller pieces when the for force increases, which is termed as friability. This property gives the abrasives the necessary self sharpening capability. The abrasives that are generally used:

Abrasives Work material Aluminum-Oxide…….. hardened steels, HSS, steels, cast iron Silicon-Carbide……….. HSS, cemented carbides, aluminum, brass, brittle materials Cubic-Boron-Nitride.. tool steels, aerospace alloys Diamond………………… ceramics, cemented carbides

2) Grain size:

Compared to a normal cutting tool, the abrasives used in a grinding wheel are relatively small. The grain size of the abrasive particle is an important parameter in determining surface finish and material removal rate. Small grit sizes take a very small depth of cut and hence provide a better surface finish while larger grain sizes permit larger material removal rates. The abrasive grains are classified in a screen mesh procedure. In this procedure smaller grit sizes have larger numbers and vice versa. Grain sizes used in grinding wheels typically range between 6 and 600. Grit size 6 is very coarse and size 600 is very fine. Finer grit sizes up to 1000 are used in some finishing operations.

3) Bond

The bonding material holds the abrasive grains and establishes the shape and structural integrity of the grinding wheel. Desirable properties of the bond material include strength, toughness, hardness, and temperature resistance. Bonding materials commonly used in grinding wheels include the following:

Vitrified bond: Vitrified bonding material consists chiefly of ceramic materials. Most grinding wheels in common use are vitrified bonded wheels. They are strong and rigid, resistant to elevated temperatures, and relatively unaffected by cutting fluids;


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Rubber bond: Rubber is the most flexible of the bonding materials. It is used as a bonding material in cutoff wheels;

Resinoid bond: This bond is made of various thermosetting resin materials. They have very high strength and are used for rough grinding and cutoff operations;

Shellac bond: Shellac-bonded grinding wheels are relatively strong but not rigid. They are often used in applications requiring a good finish;

Metallic bond: Metal bonds, usually bronze, are the common bond material for diamond and CBN grinding wheels. Diamond and CBN abrasive grains are bond material to only the outside periphery of the wheel, thus conserving the costly abrasive materials.

4) Grade:

The grade is also called the hardness of the wheel. This designates the force holding the grains. The grade depends on the kind of bond, structure of wheel and amount of abrasives. Harder wheels hold the abrasive grains till the grinding for increases to a great extent. Denoted by A to Z.

Soft wheels are generally used for hard materials and hard wheel for soft materials.

5) Structure

The wheel structure indicates spacing of the abrasive grains in the wheel. It is measured on a scale that ranges from open to dense, generally denoted by numbers. Open structure means more pores and fewer grains per unit wheel volume, and vice versa.

The spacing between the grains allows the chips to collect. This helps avoiding the loading of grinding wheel. Open structure are used for high stock removal and consequently produce rough surface. While denser structure is used for better surface finish and dimensional precision.

Shape and size of grinding wheel

Grinding wheels are marked with a standardized system of letters and numbers, which specifies the parameters of the grinding wheel.

Grinding wheels are available in a variety of shapes and sizes the most popular shown in the figure:


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Identifying Grinding Wheels

Standard Marking System for Aluminum-Oxide and Silicon-Carbide Bonded Abrasives


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Standard Marking System for Cubic-Boron-Nitride and Diamond Bonded Abrasives

Grinding Operations

Three types of grinding based on the type of surface produced,

1) Surface grinding 2) Cylindrical grinding 3) Centreless grinding

Surface grinding

Surface grinding is most common of the grinding operations. A rotating wheel is used in the grinding of flat surfaces. Types of surface grinding are vertical/horizontal spindle and rotary/reciprocating tables.


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Cylindrical grinding

Cylindrical grinding (external) is also called center-type grinding and is used in the removing the cylindrical surfaces and shoulders of the workpiece. Both the tool and the workpiece are rotated by separate motors and at different speeds. The axes of rotation tool can be adjusted to produce a variety of shapes.

Cylindrical grinding (Internal) is used to grind the inside diameter of the workpiece. Tapered holes can be ground with the use of internal grinders that can swivel on the horizontal.

Three types of feed motion are possible according to the direction of feed motion, Traverse feed grinding (also through feed grinding, cross-feeding) in which the

relative feed motion is parallel to the spindle axis of rotation.


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Plunge grinding in which the grinding wheel is fed radially into the workpiece. A combination of traverse and plunge grinding in which the grinding wheel is

fed at 45o to grind simultaneously the cylindrical part of the workpiece and the adjacent face. This methods provides a precise perpendicular mutual position of both surfaces.

Centerless grinding

Centerless grinding is a process for continuously grinding cylindrical surfaces in which the workpiece is supported not by centers or chucks but by a rest blade. The workpiece is ground between two wheels. The larger grinding wheel does grinding, while the smaller regulating wheel, which is tilted at an angle i, regulates the velocity Vf of the axial movement of the workpiece. Centerless grinding can also be external or internal, traverse feed or plunge grinding. The most common type of centerless grinding is the external traverse feed grinding, illustrated in the following figure.

Typical parts made by centerless grinding are roller bearings, piston pins, shafts, and similar components. Parts with variable diameters, such as bolts, valve tappets, and distributor shafts, can be ground by plunge centerless grinding. Sleeve-shaped parts and


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rings can be ground by the internal centerless grinding, in which the workpiece is supported between three rolls and is internally ground.

Creep-feed grinding

Grinding has traditionally been associated with small rates of material removal and finishing operations. However, grinding can also be used for large-scale metal removal operations similar to milling, shaping, and planing. In creep-feed grinding, the depth of cut d is as much as 6mm, and the workpiece speed is low (1 to 0.025 m/min). The wheels are mostly softer grade resin bonded with open structure to keep temperatures low and an improved surface finish.. Creep-feed grinding can be economical for specific applications, such as grinding cavities, grooves, etc.

In the creep feed process, the idle time (stopping the wheel and table reversal) gets reduced since the grinding operation is completed in one pass.

The cutting forces and consequently the power requirement increases in the case of creep feed grinding. It is necessary to continuously dress the grinding wheel (to reduce the wheel dullness) for efficient operation. This however causes wheel wear and makes it necessary to adjust the wheel head.

The grinding fluids used are oil based I view of the low grinding speeds employed. However, the volume of grinding fluid is much more compared to conventional grinding in view of the high heats generated in the process.

Figure: (a) Schematic illustration of the creep-feed grinding process. Note the large wheel depth of cut. (b) A shaped groove produced on a flat surface in one pass by creep-feed grinding. Groove depth can be on the order of a few mm. (c) An example of creep-feed grinding with a shaped wheel.

Other Finishing Operations

Achieve surface finish up to mirror like Very close dimensional precision Assigned as the last operation after conventional and abrasive machining process


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Honing

Honing is a finishing operation used to improve the form tolerance of an internal

cylindrical surface – in particular, it is used to improve the cylindricity.

The honing tool is a metal bar holding a set of grinding stones arranged in a circular

pattern. The tool brushes along the cylindrical part surface by rotating and moving up-and-

down along its axis.

You can identify a honed surface by looking for the helical cross-hatched scratch marks on

the part surface.


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Superfinishing

Superfinishing is a finishing operation similar to honing, but it involves the use of a single abrasive stick. The reciprocating motion of the stick is performed at higher frequency and smaller amplitudes. Also, the grit size and pressures applied on the abrasive stick are smaller. A cutting fluid is used to cool the work surface and wash away chips.

In superfinishing, the cutting action terminates by itself when a lubricant film is built up between the tool and work surface. Thus, superfinishing is capable only of improving the surface finish but not dimensional accuracy.The result of these operating conditions is mirror like finishes with surface roughness values around 0.01 μm. Superfinishing can be used to finish flat and external cylindrical surfaces.

Fig: Schematic illustration of the superfinishing process for a cylindrical part:

(a) Cylindrical (b) Centerless.

Lapping

Lapping is a finishing operation. The lapping tool is made of metal, leather, or cloth,

impregnated with very fine abrasive particles. For preparing the surface of silicon wafers,

lapping operations use a flat metal disc that rotates a small distance above the part.


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The gap is filled with slurry containing fine abrasive grains. The rotation of the disc causes

the slurry to flow relative to the part surface, resulting in very fine surface finish. This

process gives dimensional tolerances of ≥ 0.5μm, and surface finish of up to 0.1 μm.

The process is employed to get: Extreme accuracy of dimension Correction of minor imperfection of shape Refinement of the surface finish Close fit between mating surfaces.

(a) Schematic illustration of the lapping process.

(b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

Polishing and buffing

Polishing is a finishing operation to improve the surface finish by means of a polishing wheel made of fabrics or leather and rotating at high speed. The abrasive grains are glued to the outside periphery of the polishing wheel. Polishing operations are often accomplished manually. Buffing is a finishing operation similar to polishing, in which abrasive grains are not glued to the wheel but are contained in a buffing compound that is pressed into the outside surface of the buffing wheel while it rotates. As in polishing, the abrasive particles must be periodically replenished. Buffing is usually done manually, although machines have been designed to perform the process automatically.


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Polishing is used to remove scratches and burrs and to smooth rough surfaces while buffing is used to provide attractive surfaces with high glossy finish.

Wheel defects

Major and inevitable defects in grinding are glazing of grinding wheels. After the continuous use grinding wheel becomes dull or glazed. Glazing of the wheel is a condition in which the face or cutting edge acquires a glass like appearance. That is, the cutting points of the abrasives have become dull and worn down to bond. Glazing makes the grinding face of the wheel smoother and that stops the process of grinding. Sometimes grinding wheel is left ‘loaded’. In this situation its cutting face is found being adhering with chips of metal. The opening and pores of the wheel face are found filled with workpiece material particles, preventing the grinding action. Loading takes place while grinding workpiece of softer material.

Wheel dressing and truing

Dressing: The remedy of glazing and loading is dressing of grinding wheels. Dressing removes the loading and breaks away the glazed surface so that sharp abrasive particles can be formed again ready for grinding. Different type of dressing operations are done on a grinding wheel. Dressing therefore produces micro-geometry. The structure of micro-geometry of grinding wheel determines its cutting ability with a wheel of given composition. Dressing can substantially influence the condition of the grinding tool.

Truing: Truing is the act of regenerating the required geometry on the grinding wheel, when, it becomes worn and break away at different points. Therefore, truing produces the macro-geometry of the grinding wheel. Truing makes the wheel true and concentric with the bore. Truing is also required on a new conventional wheel to ensure concentricity with specific mounting system. In practice the effective macro-geometry of a grinding wheel is of vital importance and accuracy of the finished workpiece is directly related to effective wheel geometry.


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Truing and dressing are commonly combined into one operation for conventional abrasive grinding wheels, but are usually two distinctly separate operation for superabrasive wheel.

Wheel Balancing

Due to continuous used a grinding wheel may become out of balance. It con not be balanced either by truing or dressing. Here it is important to explain the meaning of a balanced wheel. It is the coincidence of centre of mass of wheel with it axis of rotation.

Wheels which are out of balance produce poor quality of surface and put undue strains on the grinding machine. Balancing of wheel is normally done at the time of its mounting on the grinding machine with the help of moving weights around a recessed flange.

Grinding time estimation (horizontal axis)

A surface grinding operation with a horizontal axis grinding machine is shown in the following figure. The grinding wheel will have to traverse beyond the actual workpiece by a distance termed as a the approach allowance, A which is given by the following equation.

dDd

2

d

2

D2

2

DA


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The value of the approach distance is very small since the depth of cut d is very small in grinding. However, to allow for the table reversal at each end of the table stroke, the radius of the grinding wheel is assumed as the approach allowance.

Thus the time for one pass

Number of passes required

Grinding time estimation (Vertical axis)

A surface grinding operation with a vertical axis grinding machine is shown in the following figure. The grinding wheel will have to traverse beyond the actual workpiece by a distance termed as a approach allowance, A which is given by the following equation.

Workpart

G. Wheel

rate feed Table

DiameterLength

rate Crossfeed

Width


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The value of the approach distance is very small since the depth of cut d is very small in grinding. However, to allow for the table reversal at each end of the table stroke, the radius of the grinding wheel is assumed as the approach allowance.

Thus the time for one pass

Mathematical problems

Example 1: Using a horizontal axis surface grinder a flat surface of C 65 steel of size 100 * 250 mm is to be ground. The grinding wheel used is 250 mm in diameter with a thickness of 20mm. Calculate the grinding time required. Assume a table speed of 10 m/min and wheel speed of 20 m/s.

Solution:

The approach distance =125 mm

Time for one passs=

Assuming an infeed rate of 5 mm/pass number of passes required= 100/5=20.

Total grinding time= 20 * 0.05=1 minutes.

Example 2: For the above example, if a vertical axis surface grinder is to be used, calculate the grinding time. The wheel to be used is 200 mm in diameter with a wheel thickness of 20 mm.

Solution:

Given, W= 100 mm and D=200 mm

Here, W=D/2

So, The approach distance, A =D/2=100 mm

Total grinding time =

rate feed Table

ALength

minutes 0.05100010

250250

minutes 0.045100010

200250

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3 Grinding Process