ABRASIVE WHEELS

A grinding wheel is an expendable wheel that is composed of an abrasive compound used for

various grinding (abrasive cutting) and abrasive machining

operations. They are used in grinding machines

CHARACTERISTICS

• Abrasive Grains• Grain Size• Wheel Grade• Grain Spacing• Wheel bond

Abrasive Grains

• The actual abrasive, is selected according to the hardness of the material being cut.

Aluminum Oxide (A)Silicon Carbide (S)Ceramic (C)Diamond (D, MD, SD)Cubic Boron Nitride (CBN)

NOTE

• Grinding wheels with diamond or Cubic Boron Nitride (CBN) grains are called super abrasives.

• Grinding wheels with Aluminum Oxide (corundum), Silicon Carbide or Ceramic grains are called conventional abrasives.

Grain Size

• Grain size, from 8 (coarsest) to 1200 (finest), determines the physical size of the abrasive grains in the wheel. A larger grain will cut freely, allowing fast cutting but poor surface finish. Ultra-fine grain sizes are for precision finish work.

Table Coarse Medium Fine Very fine

8 30 80 220

10 36 100 240

12 46 120 380

14 54 150 320

16 60 180 400

20 500

24 600

Wheel Grade• Wheel grade, from A (soft) to Z (hard),

determines how tightly the bond holds the abrasive.

• Grade affects almost all considerations of grinding, such as wheel speed, coolant flow, maximum and minimum feed rates, and grinding depth.

D to H > SoftI to P > MediumR to V > Hard

Grain Spacing

• Grain spacing, or structure, from 1 (densest) to 16 (least dense).

• Density is the ratio of bond and abrasive to air space.

• A less-dense wheel will cut freely, and has a large effect on surface finish.

• It is also able to take a deeper or wider cut with less coolant, as the chip clearance on the wheel is greater.

Wheel Bond• Wheel bond, how the wheel holds the abrasives,

affects finish, coolant, and minimum/maximum wheel speed.

• Vitrified (V)• Resinoid (B)• Silicate (S)• Shellac (E)• Rubber (R)• Metal (M)• Oxychloride (O)

TYPES OF GRINDING WHEEL• Straight Wheel• Cylinder or Wheel Ring• Tapered Wheel • Straight Cup• Dish Cup• Saucer Wheel• Diamond Wheel• Mounted Points• Cut Off Wheels

BRITISH STANDARD MAKING SYSTE M FOR GRINDING WHEEL

KEY POINTS………….1• Soft wheel for hard material and hard wheel for soft

material• Rough Grinding: Medium to Hard Grade• Precision Grinding needs soft grades.• Higher the wheel speed, harder the wheel required.• Harder grades used when coolant applied• Abrasive wheels has an inherent risk of Breakage• Wheel should be handled carefully to prevent

dropping and bumping• Wheel must be stored in racks or bins constructed to

suit the type and size.

KEY POINTS……………2

• Store room should be kept warm or dry.• Manufacturer attach a tag or label to each

wheel with the specifications and safe working speed.

• Abrasive wheels of less than 55m.m (2¼”) must not be use in factory without permission.

• Small dia, high speed.• Large dia, low speed

TYPES OF MACHINE ACCORDING TO SPINDLE SPEED

• The constant speed spindle. The maximum working speed

• A spindle operating at more than one specified working speed. Each working speed.

• A spindle operating at an infinite number of speed. The maximum and minimum working speed

ANGULAR EXPOSURE OF GUARD

• The maximum angular exposure of bench and floor stand grinding= 90°

• Wheel below the horizontal plane of the spindle= 125 °

• Cylindrical grinders= 180 °• Surface Grinders= 150 °• Portable Grinders= 180 °

FLANGES• Exposed arc of a peripheral abrasive wheel

exceeds 180° a wheel guard is practically ineffective

• Protective FLANGES are used.• The same taper as the wheel and should be

made of Mild Steel. • Inspected regularly• Maintained in good condition.• It is most important that both the flanges are the

same diameter

WORK RESTS

• Stand and bench type grinder• Kept adjusted as close as possible to the

wheel face• The maximum opening of 3m.m.• Securely Clamped

STARTING AND STOPING

• Power driven wheels should only be used in machines provided with an efficient Starting and Stopping device placed in safe position with in reach of operator.

• Operator, enable the machine to be easily stopped in an emergency

BEARINGS

• Should be maintained in good condition.• Sufficiently Lubricated.• Loose bearings can cause vibration.• Damaged, distorted or sprung, can cause the

wheel to be out of balance.

MAINTANANCE• Regular maintenance of following things should be done regularly: Wheel Bearings Safety guard Work rest Flanges Washers Spindle nuts Belts Stopping and starting devices Governors Any safety shields or screens

DRESSING• When the cutting action is reduced DRESSING of the

wheel is necessary.• DRESSING of the wheel should be done by proper

dressing tool.• Work rest must be correctly adjusted to give proper

support to the tool.• Hold the tool rigidly.• Dressing a wheel restores: Cutting Balance Trueness

MOUNTING………1• Must be properly mounted.• Clean the wheel first by brush before mounting.• Vitrified wheels should be checked by tapping it with non-

metallic implement.• Unsound abrasive wheel could be dangerous, sound

abrasive wheel can be mounted.• Wheel should be easily fit, do not mount so tightly or

loosely.• Sound wheel should be mounted on power driven machine

for which its was intended• Before mounting, check the maximum permissible spindle

speed

MOUNTING………2• When wheel is to be secured by a single central spindle nut, the

rotation for tightening the nut should be Opposite to the direction of wheel rotation.

• Wheel should be mounted between suitable flanges which should not be less than one third diameter of the wheel.

• Recessed on the side next to the wheel, to ensure that the clamping pressure is not exerted near the hole in the wheel.

• Wheels with large holes should have the corners of the flange wheel seating are undercut

• All straight sided wheels should have paper washer between flanges and the wheel.

• The inner mild steel Flanges should be Secured to the spindle by keying and shrinking.

MOUNTING………3• Mounting a straight sided abrasive wheel with a small

hole, to use recessed flanges with paper washer and have the recessed side of the flanges towards the wheel.

• Do not start a machine fitted with a new wheel until the guards are properly fixed and secure in the correct position.

• Screws used to mount abrasive discs, must be long enough to engage the anchored nut threads sufficiently.

• While mounting the wheel manufacturer’s instructions should be strictly observed.

REMOUNTING THE USED WHEEL

• Before remounting the used wheel it should be given the ring test and any fragments of the old paper washers removed.

• Remove any loose grains of abrasive from the surface of an abrasive wheel before remounting.

• If left on the wheel will cause high spots, leading to sever local stresses when the wheel is mounted

• Remounting can cause local stresses in the wheel which increasing its tendency to BURST.

Safety

Hand outs