What is a grinding machine?
Grinding machines generally use an abrasive, rotating wheel to slowly but very accurately remove material from the part or parts in question. Where milling can be used to remove large amounts of material quite quickly, grinding is more of a refining process which results in a superior finish, at extremely high tolerances (within a ten thousandth of an inch).
Grinding is not only used for finishing flat materials, but can also be utilised to refine cylindrical parts, or even create intricate grooves. There are a number of grinding machines available on the market, depending on the types of materials being ground, and the results required.
When drilling cylindrical objects, in order to maintain concentricity, a cylindrical grinder is necessary. There are several options for cylindrical grinding machines.
- Centre-type cylindrical grinders
These types of grinders use a set of hardened ‘centres’ or pins which hold the material at either end, and allow the grinding wheel to grind along the entire length of the part. The part and the abrasive wheel rotate in the same direction, and a concentric, cylindrical shape is achieved.
- Chuck-type cylindrical grinders
Rather than centres which hold the part in place, chuck-type grinding uses a ‘chuck’ to hold the part tight, allowing a small grinding wheel to grind its interior.
- Centreless grinders
Centreless grinders, as the name suggests, do not utilise a set of centres to hold parts in place. Available in both internal and external variations, centreless grinders are popular as they are very quick to set up and do not require the mounting of the parts on centres. Instead, in the instance of exterior centreless grinding, the part is mounted atop a simple resting blade, while a regulating wheel assists in keeping the part moving in the correct direction. On the other hand, an internal centreless grinder utilises two ‘support rolls’ to hold the part in place, while a grinding wheel works within its interior.
Generally used for grinding flat surfaces, the surface grinder uses a horizontal or vertical abrasive wheel and a table upon which the material rests. The table may either move back and forward (reciprocate) or spin in circles, when sliding the object beneath the grinding wheel. While surface grinders are used primarily to finish flat surfaces, it is also possible to produce grooves in the material.
Surface grinders are available in four formats:
- Vertical wheel and reciprocal table
- Vertical wheel and rotating table
- Horizontal wheel and reciprocal table
- Horizontal wheel and rotating table
In order to maintain the life of metal tools in the shop, regular and accurate tool grinding is a must. Specialised ‘tool and cutter’ grinders are similar to centre-type grinders; however, their major components can be moved in order to achieve precise angles and shapes. Tool grinders can be used with CNC (computer numerical control) machines, in order to maintain the precision and accuracy necessary for maintaining tools in the workshop.
What is creep feed grinding?
In contrast to regular grinding operations, which involve a quickly rotating wheel removing minimal material, creep feeding involves reducing the spin of the wheel and removing greater amounts of material. Creep feeding can be used in both surface and cylindrical grinding operations, and can negate the need for other metal-removal processes such as milling, which also remove large amounts of material. Using creep grinding rather than milling means being able to maintain the superior surface finish, for which grinding is preferred.
Offhand and snagging grinding
When the need arises to remove larger amounts of material, without the necessity for a precise finish, offhand and snagging grinding may be performed. Using a regular grinding machine, parts can either be placed atop the worktable, or else held by hand (offhand) and ground by the abrasive wheel until the appropriate amount of material is removed. Another option for offhand grinding, is to use an abrasive belt instead of a grinding wheel—this means that the process is much gentler and is less likely to damage the piece being worked on.