PRINCIPLES OF SELECTION OF ABRASIVE TOOL CHARACTERISTICS

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

It affects the selection of:
• abrasive material
         • for machining steel and malleable cast iron the synthetic corundum is preferred in case of conventional abrasive tools, and CBN in case of superhard abrasive tools
         • for machining sintered carbides, ceramics, concrete, hard, brittle cast iron the silicon carbides are preferred in case of conventional abrasive tools, and synthetic diamond in case of superhard abrasive tools This is important for selection of:
• grain size
          • fine grains are used on hard and brittle materials
          • coarse grains are used on soft and ductile materials
• abrasive tool hardness
         • low hardness grades are preferred on hard material
         • high hardness grades are preferred on soft materials

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

It affects the selection of: 
• grain size
       • coarse grain for a quick removal of heavy stock, high depth of grinding and high surface roughness requirements
       • fine grain for finishing and low surface roughness
• bond
      • vitrified bonds for precision grinding but also resinoid bonds that enable surface finishing to be performed
      • rough grinding and cutting-off - only resin bond and reinforced resin bond. At low operating speeds the vitrified bonds for rough grinding may be used as well. In case of superhard wheels for cutting-off and profile grinding also electroplated wheels can be used. 

It affects the selection of: 
• bond
         • standard flat grinding wheels with vitrified bond are designed for conventional operating speeds of 35 [m/s].
         • wheels can be also designed for increased speeds, which are: 35 < V =< 50 [m/s] - depending on the bond.
         • and high speeds: 50 < V =< 100 [m/s]
         • for resin bonded wheels, the conventional speed is 35, 40 and 50 [m/s],
         • the increased speeds are: 50 < V =< 63 [m/s]
         • and high speeds: 63 < V =< 80, 100 [m/s]
Note:
Increased operating speed results in a higher "dynamic hardness" and conversely, lower operating speed results in more soft grinding.
In case the increased and high operating speeds are necessary, consult this with the manufacturer.
It is not allowed to exceed the permissible operating speed specified on the grinding wheel.
In case of superhard wheels, working speeds recommended due to the nature of the grinding operation are used.

It affects the selection of:
• grain size
         •fine grains for small, narrow contact areas
        • rough grains for large contact areas
• abrasive tool hardness
         • higher hardness grade for small, narrow contact areas and vice versa.

It affects the selection of:
• the hardness of the abrasive tool
         • when grinding wet, it is acceptable to use abrasive tools one grade harder than in dry grinding

It affects the selection of:
• abrasive material
          • rough grinding (snagging) operations requires: normal aluminium oxide 95A and zirconia aluminium oxide or black silicon carbide 98C
         • finishing grinding of high quality hard tempered steels requires: brittle and fine abrasives: white aluminium oxide 99A, pink aluminium oxide CRA, monocorundum M or their mixtures. CBN is used in case of superhard wheels
         • in grinding operations of average difficulty semi-friable aluminium oxide 97A and mixtures can be used
         • grinding carbides and ceramic materials require green silicon carbide 99C or synthetic diamond. 

It affects the selection of:
• hardness grade of grinding wheel
            • on more powerful grinders, higher hardness grinding wheels can be used
Note:
If the power of the grinding machine is not sufficient, then the rotational speed of the grinding wheel will be reduced, and this will cause lower cutting ability of the wheel, which may result in higher pressures, high temperature, and may lead to damage of the grinding wheel.
Example: a cutting-off machine should have a power of 3 [kW] per 100 [mm] of grinding wheel diameter, which means that in case of a grinding wheel 41 – 300, a total power required is 9 [kW].

Surface roughness - is a measure of the texture of a solid body surface. It is quantified by the vertical deviations of a real surface from its ideal form. Roughness value of the workpiece surface depends on material grade and on the way of its machining.

ADDITIONAL FACTORS AFFECTING THE GRINDING RESULT

Important factors:
• grinding parameters: feed rate, workpiece speed, depth of cut
• characteristics of grinders: rigidity, technical condition, grinding pressures
• dressing parameters and conditions
• qualifications of the grinder’s.operator

Practical tips

Write down the characteristics of the abrasive tool and the name of its manufacturer that you are about to install on the grinder. If it is properly selected for the technological task, it is enough to repeat the order of the abrasive tool. If you are not satisfied with the results, it will be the starting material for analysis with the supplier service.

Keep in mind that replacing the tool used so far with the same characteristics but from another manufacturer, may not guarantee the same machining results. This is closely related to the differences in technologies applied by manufacturers, the raw materials they use and the process parameters. Especially before ordering a larger batch, a new tool should be tested under specific grinding conditions.

Note:

Examples of the application for different materials and grinding operations are provided in the data sheets.

 

The rules presented are not without exceptions, but for the most part they are confirmed in practice.