GRINDING
Grinding operations employ abrasive grains in various forms for the removal of material and metal to produce of different types of surface finishes. Abrasive grains held together with a hard, durable bonding material make grinding wheels, abrasive stones, and sticks. Abrasives can also be applied on fibrous sheets, paper, or cloth with the principle materials being aluminum oxide and silicon carbide. Crushed diamonds called Bort are also used as abrasive grains.
Common Applications
Sanding, tumbling, belt grinders, core grinders, horizontal double spindle disc grinders, vertical spindle disc grinders, jack grinders, portable grinders, small radial grinders, surface grinders, swing grinders, grinding benches, and grinding tables.
Feed Materials
- Particulate from the object being finished including a variety of ferrous and non-ferrous metals such as aluminum, cast iron, steel, titanium as well as non-metals such as brake shoe, composites, and rubber.
- Grinding Wheel Structure and Abrasives. The grinding wheel structure is the relative spacing of abrasive grains in a given volume of a grinding wheel and depends upon the abrasive grains, the bonding material, and the voids. A good structure will provide proper chip clearance so that as the abrasive grains cut chips out of the work the chips are thrown away from the face of the wheel. Wheels with a wide grain spacing are used generally when the contact area between work and wheel is large, material being ground is soft, or when rapid removal is required. Narrower grain structures are generally suitable when fine finishes are required or when contact area is small and the material being ground is hard.
- Grinding Wheel Bonds are the materials that hold the abrasive grains together. It is the amount of bond used that determines the “hardness” or “softness” of the wheel. The heavier the coating of bond around the abrasive the harder the wheel is rated. Grinding wheel bonds are of six general types:
- Vitrified: Seventy-five percent of the grinding wheels manufactured has this type of bond. These wheels are porous, free cutting, and unaffected by water, acids, oils, heat, or cold.
- Silicate: These wheels release their abrasive grain readily making them relatively mild acting. Best used where heat must be kept to a minimum such as in grinding edge tools.
- Shellac: These wheels are capable of producing high finishes on work such as camshaft and steel rolls. They are also used for grinding cutlery and for thin-cut wheels. They are not suited for heavy-duty grinding.
- Rubber: Rubber wheels are used as feed wheels on centrless grinding machines. They are used for producing high finishes on ball bearing races for cut-off wheels where minimum burr is important as well as snagging wheels where a high quality finish is required, such as on stainless steel bullets and welds. Rubber bond lends itself to making strong wheels 0.8mm thick and less.
- Resinoid: Resinoid wheels are used in foundries, welding shops and billot shops for high speed rough grinding. The stock removal rate is generally in direct proportion to the speed of the wheel. These wheels can be operated at speeds as high as 9500 rpm.
- Magnesite (Magnesium Oxychloride): This bond is used in wheels and segments for certain types of disc grinding.
Dust Characteristics
A combination of coating materials, metallic particulate, and grinding wheel abrasives and wheel bond agents are all generated during the grinding process. The dust will likely be abrasive, possibly agglomerative, and may often pose a potential fire and explosion risk.
Notes
Due to the variety of work and types of grinding machines employed, it is necessary to develop hoods adaptable to the particular machine in question, and such hoods should be located as close as possible to the operation. Do not mix ferrous and non-ferrous metals in the same exhaust system. Consult applicable NFPA codes.