Rules for selection of abrasive tool characteristics to grinding operations

Specified in Table 2, page 11 combinations of abrasive - grain size - hardness grade - structure - bond type enable designing and manufacture of abrasive tools with a lot of different characteristics.

Moreover, a very wide range of industrial grinding operations and requirements require various abrasive tools, with properly adjusted parameters, to be available. An appropriate selection of abrasive tool characteristics so that they meet the requirements is the prerequisite for success.

Rules for selection of abrasive wheel or segment characteristics are as follows.

1. Worked material. Type and condition

  • chemical composition
  • hardness
  • type of treatment prior to grinding operation

This is important for selection of:

  • Abrasive material type
    • as a general rule, the synthetic corundum abrasive is preferred on steel and cast iron
    • silicon carbides are preferred on sintered carbides, ceramics, concrete, hard, brittle cast iron, etc.
  • Grain size
    • Fine grains are used on hard and brittle materials
    • Coarse grains are used on soft and ductile materials
  • Hardness grade
    • low hardness grades are preferred on hard material
    • high hardness grades are preferred on soft materials

2. Type and nature of grinding operation

  • rough grinding (snagging)
  • cutting-off
  • precision grinding (rough or finish) and consequently: rate of stock removal and required surface roughness.

This is important for selection of:

  • Grain size
    • coarse grain for a quick removal of heavy stock, high depth of grinding and low surface roughness requirements
    • fine grain for finishing and high surface roughness requirements
  • Bonds
    • vitrified bonds for precision grinding but also resinoid bonds that enable surface finishing to be performed
    • rough grinding and cutting-off - only resinoid and reinforced resinoid bonds. At low operating speeds the vitrified bonds for rough grinding may be used as well.

3. Operating speed of grinding wheel

This is important for selection of:

  • Bond - see Table 2, page 11
    The standard vitrified bond straight grinding wheels are intended for operation at operating speed of 35 [m/s] (40 and 45 [m/s] depending on the grain size and hardness grade).
    For resinoid bond, the conventional operating speeds are correspondingly 50 and 63 [m/s].
    Increased speeds are: 45, 50 and 63 [m/s] - depending on the bond.
    High speeds are: 80, 100 [m/s]

Notes:

  • The increased operating speeds result in a higher “dynamic hardness” and, in contrary, the lower operating speeds result in the more soft grinding.
  • In case the increased and high operating speeds are necessary, consult this with the manufacturer.
  • The maximum safety operating speed, specified on the grinding wheel must not be exceeded.


4. Contact area between the grinding wheel and ground material

This is important for selection of:

  • Grain size
    • fine grains for small, narrow contact areas
    • coarse grains for large contact areas
  • Hardness grade
    • higher hardness grade for small, narrow contact areas and vice versa

5. “Dry” grinding or “wet” grinding (with coolant)

This is important for selection of:

  • Hardness grade
  • As a general rule, the wet grinding enables use of one grade higher hardness in compare with dry grinding.

6. Difficulty rate of grinding operation

This is important for selection of:

  • Abrasive material
    • heavy conditions of rough operation (snagging) requires normal aluminium oxide 95A and zirconia aluminium oxide ZrA or black silicone carbide 98C.
    • brittle, refined abrasive materials - noble aluminium oxide, white 99A, chromium aluminium oxide CrA, monoco¬rundum MA and their mixtures are used for finish grinding of hard, hardened high-quality steel
    • intermediate abrasives: semi-noble aluminium oxide 97A and mixtures are used for grinding operations of medium working conditions and for special applications
    • green silicon carbide 99C is used for grinding of sintered carbides and ceramics.

7. Grinding machine power

This is important for selection of:

  • Hardness grade of grinding machine
    • the higher power of grinding machine, the higher hardness grade of grinding wheel

Note:
When the power of grinding machine main motor is not sufficient, then the rotational speed of grinding wheel will be lowered and the grinding properties of the grinding wheel will be reduced which, in turn, may results in higher pressures, high temperatures and, as a consequence, damage of grinding wheel.
Example: The cutting-off grinder must be of 3 [kW] in power per each 100 [mm] of grinding wheel diameter which for the grinding wheel 41-300 makes the total power of main motor of 9 [kW].

8. Approximate relationship between surface roughness and grain size

 

Roughness number

Ra [µm]

Grain size

Rz [µm]

46

60

80

100

120

150

180

220

7

1,25

×

 

 

 

 

 

 

 

6,3

8

0,63

 

×

×

 

 

 

 

 

3,2

9

0,32

 

 

×

×

×

 

 

 

1,6

10

0,16

 

 

 

 

×

×

×

 

0,8

11

0,08

 

 

 

 

 

 

×

×

0,4

 

9. The following additional factors also affect on grinding results:

  • grinding parameters: feed, speed of workpiece, depth of grinding
  • grinding machine features: rigidity, technical condition, pressures during grinding
  • dressing parameters and conditions
  • skill of operator-grinder

The above mentioned rules have certain exceptions but, they are commonly proven by practice.

10. Additional, practical guides

  • Record characteristics and manufacturer’s name of grinding wheel being mounted. If it occurs suitable then it will be sufficient to repeat the purchase order. If not, it will be the material for analysis for the supplier service.
  • Substitution of symbols from grinding wheel designations from different suppliers by a designation of “Andre” may be a certain approximation but does not ensure the same grinding results. This results from different materials and parameters of manufacturing processes used by the manufacturers. Therefore, tools from a new supplier require to be tested in specified grinding conditions, especially before ordering a larger batch of the tools.

Note:
Examples for application of particular grinding wheels and segments for various materials and grin¬ding operations are presented in the following catalogue sheets.