CHAPTER 5 OF MACHINE SHOP BASICS

CHAPTER 5
GRINDING
NOTES TAKEN WHILE STUDYING CHP5 OF MACHINE SHOP BASICS
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Grinding
By definition, grinding is the process of disintegrating a material and reducing it into small particles of dust by crushing or attrition.
Manufacture of Grinding Wheels
Grinding wheels are made from sandstone, occurs in nature or from man-made abrasives. Natural grindstones are mostly used in glass, cutler, and woodworking edge-tool industries. However, manufactured abrasives wheels have largely replaced natural grindstones in these industries. Because manufactured grinding wheels can be operated at higher speeds, and the grain size, hardness and structure can be controlled.
Manufactured Abrasives Grinding Wheels
Most grinding wheels are made from aluminum oxide and silicon oxide, these are prepared in a similar way. The process starts out with ore from an electric furnace rough crushed into lumps about six inches in diameter. From there the next step is to the abrasive mill, the lumps are further reduced in size to about three fourths in diameter. The final step they are ran through a series steels crushing rolls, the pieces are reduced to grain size for use in grinding wheels and coated abrasive products.
Iron impurities are removed by using magnetic separators, fine abrasive dust and foreign particles are removed by washing with stream and hot water. Washing is important because clean grains mix uniformly with the bond, after washing the abrasive in then dried in continues rotary driers.
The abrasive grains are passed over a series of vibrating screens in the range of 10 to 600 this is the mesh size, which a particle grain will pass though. A screen having 10 mesh openings per linear inch is called a 10-grit size and will have 100 openings per square inch.
Abrasive grains are inspected as they come from the mill for capillary, uniformity of size, strength, and weight per unit volume. The grains are uniformly distributed throughout the bond in the wheel. The structure refers to relative spacing as dense, medium or open, depending on the percentage of abrasive or pores. Wheels of medium structure are best for hard, dense materials open structures are best for heavy cuts and soft, ductile materials that are easily penetrated and require good chip clearance.
Shapes of Grinding Wheels
Grinding wheels are manufactured in nine standard shapes and 12 faces

Straight
Recessed one side
Recessed two sides
Cylinder
Straight cup
Flaring cup
Tapered Two sides
Dish
Saucer

Twelve Standard Grinding wheel faces
Method of Mounting Grinding Wheels
Before mounting grinding wheels should be checked for balance and out of balance wheels cause excessive vibration, causing chatter marks on the ground surface and excessive wear on the bearings and spindle.
Truing and Dressing the Grinding Wheel
To be running true on its own spindle, a wheel should have enough of the cutting face removed in preparation for grinding. With use a wheel will become glazed or loaded then it needs to be dressed, this restores its original shape and clean cutting face. There are three types of wheel dresses used on precision grinding machines. The diamond tool, abrasive wheel and mechanical dresser, the common is the diamond wheel on all types of precision grinders.
Bond types in grinding wheels
The bond in the grinding wheel holds the grains together and supports them while they cut. The greater the amount of bond with respect to the abrasive, the heavier the coating of bond around the grains the harder the wheel. Hard and soft are terms that actually refer to the strength of the bonding wheel. The three types of grinding wheel bond are:

Vitrified
Organic (resinoid, rubber, shellac)
Silicate
Vitrified-Bond Grinding Wheels

Approximately two-thirds of the manufactured grinding wheels are made with a vitrified bond composed of clays and feldspars, and are selected for their fusibility. During the burning process manufacture, a temperature of 1270’f is reached, which is high enough to fuse the clays into molten glass condition. When cooling a span or post of this glass connects each abrasive grain to its neighboring grains to make a strong rigid grinding wheel.
Organic-Bonded Grinding Wheels
Organic-bonded grinding wheels are made from resinoid , rubber of shellac. Resinoid wheels are used in high-speed rough-grinding operations and are the largest percentage of organic-bonding wheels. Straight wheels are used on bench and pedestal grinding, cup wheels and cones are used for cleaning castings and for weld grinding with portable grinders. Reinforced cutoff wheels are used on both cutting-off machines, cutting bar stock and cutting concrete blocks and other masonry materials.
Grinding Wheel Markings
In 1944 a standard system for marking grinding wheels was adopted by grinding wheel manufacturers throughout the country. The markings consists of six parts arranged in this order

o Abrasive type
o Grain size
o Grade
o Structure
o Bond Type
o Bond Modification Symbols

Abrasive types: Abrasive types fall into two groups, letters are used to identify them

o A-Aluminum Oxide
o C-Silicon Carbide

Grain Size: Grain size in wheels range in size from 10 (course) to 600 (fine)
Grade: Grade is indicated by letters of the alphabet ranging from A to Z (soft to Hard)

Grade A to H Soft
I to P Medium
Q to Z Hard

Structure : Structure spacing or grain spacing in grinding wheels is indicated by a number from 1 to 12. The higher numbers indicates more open grain spacing
examples

3 in dense or close grain structure
8 is a wide grain spacing

Bond or Process: the following letters designate the type of bond.

V-Vitrified
B-Retinoid
R-Rubber
E-Shellac
S-Silicate

Bond Modification Symbols: These indicate a particular type of bond that distinguishes it as a variation from a basic bond.

VG (Norton G type of Vitrified bond)
B11 (Norton 11 type of resinoid bond)

Factors Affecting Grinding Wheel Selection
Six items need to be taken into consideration when selecting grinding wheels

1. Abrasive type
2. Grain size
3. Grade
4. Structure
5. Bond type
6. Manufacture’s Record

Hardness of the Metal to be Ground
Abrasive: Aluminum oxide is best suited for grinding steel and steel alloys.
Silicon Carbide grinding wheels are most efficient for grinding cast iron, non-ferrous metals, and non-metallic materials.
Grit Size: Fine grit is best for hard, brittle, and difficult to penetrate materials. Course grit is best for soft ductile, easily penetrated materials.
Grade: For very hard dense materials, use a relatively soft grade-grinding wheel. Hard materials resist the penetration of the abrasive grains and cause them to dull quickly. A soft grade wheel enables worn, dull grains to break away and expose newer, shaper cutting grains. Harder grade grinding wheels should be used for soft, easily cut materials.
Amount of Stock to be removed and Finish Required
Grit size and bond are important in the selection of a wheel depending on the amount of stock to be removed and the finish required.
Grit size: For rough grinding or rapid removal of stock, use a coarse grit. If a high finish and close tolerances are required, use a fine grit size.
Bond: A vitrified bond is best suited for a fast cutting and a commercial finish. Resinoid, rubber of shellac is best for obtaining a high finish.
Operation (Wet or Dry): Generally wet grinding permits the use of grinding wheels at least one grade higher than for dry grinding without danger of burning the work from heat of friction. Water speeds up the work and reduces dust.
Wheel speed: Grinding wheel speed is very important in selecting a wheel and bond as follows.
For speeds, less than 6500 surface feet per minute (sfpm) use vitrified-bonded wheels.
For speeds above 6500, sfpm use an organic-bonded wheel (resinoid, rubber or shellac).
Areas of Grinding Contact
The area of contact between the wheel and the work influences grit size and grade. In general the smaller the area of contact, the harder the grinding wheel should be. For a large area of contact, a coarse grit wheel should be used. If the area of contact is small then a fine-grit wheel should be used.
Severity of the Grinding Operation
Factors that influence the choice of abrasive:
Use a tough abrasive, “regular aluminum oxide for grinding steel and steel alloys under severe conditions.
Use a relatively mild abrasive for light grinding on hard steels.
Use a intermediate abrasive for grinding job of average severity