Compositional specifications

Specification of a grinding wheel ordinarily meanscompositional specification. Conventional abrasive grindingwheels are specified encompassing the followingparameters. 1) the type of grit material 2) the grit size 3) the bond strength of the wheel, commonly known as wheel hardness 4) the structure of the wheel denoting the porosity i.e. the amount of inter

grit spacing 5) the type of bond material 6) other than these parameters, the wheel manufacturer may add their own

identification code prefixing or suffixing (or both) the standard code.

• The size of an abrasive grain is identified by a grit number, which is a function of sieve size: the smaller the grain size, the larger the grit number. For example, grit number 10 is typically regarded as very coarse, 100 as fine, and 500 as very fine. Sandpaper and emery cloth also are identified in this manner, as you can readily observe by noting the grit number printed on the back of the abrasive paper or cloth.

• Aluminum oxide: Carbon steels, ferrous alloys, and alloy steels.• Silicon carbide: Nonferrous metals, cast irons, carbides, ceramics, glass,

and marble.• Cubic boron nitride: Steels and cast irons above 50 HRC hardness and high

temperature alloys.• Diamond: Ceramics, cemented carbides, and some hardened steels.

GRIT

It’s to specify the size of abrasive grains

1. Coarse

2. Medium

3. Fine

4. Very fine

Grit Size -Coarse grit -Soft or ductile materials when fast grinding is required

-Fine grains -Hard and brittle materials -Good surface finish

• The size of abrasive grain is indicated in terms of the mesh,coarser grains by low numbers and finer grains by high numbers.

• Coarse grits are in use for soft, ductile, stringy materials for fast stock removal rough grinding, large contact area, high grinding pressure.

• Finer grits in use for obtaining smooth finish, hard & brittle materials, small contact area and form holding of small & narrow corners

• The grain size affects material removal rate and the surface quality of work piece in grinding.

• Large grit- big grinding capacity, rough workpiece surface • Fine grit- small grinding capacity, smooth workpiece surface

GRADE It’s to specify the hardness of abrasive grains

1. Soft 2.Medium 3.Hard

•The worn out grit must pull out from the bond and make room for fresh sharp grit in order to avoid excessive rise of grinding force and temperature. •Therefore, a soft grade should be chosen for grinding hard material. •During grinding of low strength soft material grit does not wear out so quickly. Therefore, the grit can be held with strong bond so that premature grit dislodgement can be avoided.•Rough grinding: Medium to Hard Grade•Precision grinding needs soft wheels •Higher the wheel speed, harder the grade required •Harder grades used when coolant appliedThe worn out grit must pull out from the bond and make room for fresh sharp grit in order to avoid excessive rise of grinding force and temperature. a soft grade should be chosen for grinding hard material. On the other hand, during grinding of low strength soft material grit does not wear out so quickly. Therefore, the grit can be held with strong bond so that premature grit dislodgement can be avoided.

STRUCTURE

It’s to specify the spacing between the abrasive grains

1. Open 2. Dense

•The structure should be open for grinding wheels engaged in high material removal

to provide chip accommodation space.

•The space between the grits also serves as pocket for holding grinding fluid.

•On the other hand dense structured wheels are used for longer wheel life, for holding

precision forms and profiles.

•Open structure for soft and ductile materials

•Surface grinding requires more open structure than cylindrical grinding Rough

grinding requires open structure

•Fine finish needs close structure

Marking as per ANSI (American National Standards Institute)

Designation of Grinding Wheels

Indian standard marking system for grinding wheels

Contains 6 parameters

Abrasive Type

Grit

Grade

Structure

Bond Type

Manufacturer’s identity number

C 54 F 4 S 40

As per BIS (Bureau of Indian Standards)

Structure / concentration

• The structure should be open for grinding wheels engaged in high material removal to provide chip accommodation space

• The space between the grits also serves as pocket for holding grinding fluid.

• On the other hand dense structured wheels are used for longer wheel life, for holding precision forms and profiles.

Selection of Grinding Wheels

Selection of grinding wheel mainly depends upon

Constant Factors

1. Material to be ground

2. Amount of material to be removed

3. Area of Contact

4. Type of Grinding Machine

Variable Factors

1. Wheel Speed

2. Work Speed

3. Condition of the Machine

4. Personal Factors

&

Reconditioning of Grinding Wheels

After prolonged use, the grinding wheel gets

LOADED & GLAZED

Reconditioning of Grinding Wheels

After prolonged use, the grinding wheel gets

LOADED & GLAZED

Such grinding wheels can be reconditioned by

DRESSING & TRUING

Reconditioning of Grinding Wheels

1. LOADED & GLAZED

2. DRESSING & TRUING

Dressing• Dressing is the conditioning of the wheel surface which ensures that grit

cutting edges are exposed from the bond and thus able to penetrate into the workpiece material.

• in dressing attempts are made to splinter the abrasive grains to make them sharp and free cutting and also to remove any residue left by material being ground

• Dressing therefore produces micro-geometry. • The structure of micro-geometry of grinding wheel determine its cutting

ability with a wheel of given composition• Dressing can substantially influence the condition of the grinding tool. • Truing and dressing are commonly combined into one operation for

conventional abrasive grinding wheels, • but are usually two distinctly separate operation for superabrasive wheel.

• Grinding wheels wear unevenly under most general grinding operations due to uneven pressure applied to the face of the wheel when it cuts.

• Also, when the proper wheel has not been used for certain operations, the wheel may become charged with metal particles, or the abrasive grain may become dull

• before it is broken loose from the wheel bond. in these cases, it is necessary that the wheel be dressed or trued to restore its efficiency and accuracy.

• Dressing is cutting the face of a grinding wheel to restore its original cutting qualities.

• Truing is restoring the wheel’s concentricity or reforming its cutting face to a desired shape.

• Both operations are performed with a tool called an abrasive wheel dresser

• Dressing is the process of Conditioning worn grains on the surface of a grinding wheel by producing sharp new edges on grains so that they cut more effectively.

• Truing, which is producing a true circle on a wheel that has become out of round.

• Dressing is necessary when excessive wear dulls the wheel (calledglazing because of the shiny appearance of the wheel surface) or when the wheel becomes loaded.

For softer wheels, truing and dressing are done separately, but for harder wheels (such as CBN), both are done in one operation.Loading of a grinding wheel occurs when the porosities on the wheel surfaces

(Fig. 26.9) become filled or clogged with chips from the work piece. Loading can occur in grinding soft materials or from improper selection of

wheels or process parameters. A loaded wheel cuts inefficiently and generates much frictional heat, which

results in surface damage and loss of dimensional accuracy of the work piece.

Truing

• Truing is the act of regenerating the required geometry on the grinding wheel, whether the geometry is a special form or flat profile.

• Therefore, truing produces the macro-geometry of the grinding wheel.

• 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.