What Precision Machine Shops Do: Grinding
Grinding is a machining process using abrasive surfaces to remove material from metal workpieces. On the surface (pun intended) grinding may seem different than other machining processes, but it still works through chip formation and removal–just like sawing, milling, broaching and most other techniques. Grinding can produce surfaces conforming to rough or extremely close tolerances. Because of its versatility, grinding is used for simple gate removal in castings as well as advanced finishing processes like polishing and sharpening.
How Grinding Works
From a physical standpoint most machining processes remove metal through compression and sheer: a cutting point compresses the workpiece material until it breaks off (sheers) in the form of a chip. When a saw sheers a chip, the blade's teeth carry it away from the workpiece. Similarly, sharp points on grinding wheels or belts sheer chips, which are then carried away from the workpiece in the cavities of the grinding wheel. Grinding's principal difference is that instead of using metal cutting tools, it utilizes abrasive materials that supply the sharp points needed to sheer metal chips.
Tools Used for Grinding
Grinding can either be accomplished using a grinding wheel or a grinding belt. The composition of a grinding wheel depends on its application, but most are formed by combining abrasive materials with bonding agents and molding the mixture into the shape of a wheel.
Bonding agents can be ceramic, in which case wheels are fired in a kiln to harden after forming. Plastic-based or rubber-based bonding agents are also used if a more flexible grinding wheel is desired. High-polish wheels can be formed using fine abrasive grains and shellac at high temperatures. To form the toughest grinding wheels, called superabrasives, manufacturers coat metal wheels with extremely hard grains like diamond or cubic boron nitride (CBN) through a process called electroplating.
Structure is a term referring to the spacing of the abrasive particles on a surface. Grinding surfaces with low-density structure, or open structure, have larger chip cavities and so are able to remove material more quickly. However, open structure abrasives do not provide the same smooth surface finish as denser ones. Grade refers to the strength of the bonds holding together abrasive particles in a grinding wheel.
In belt grinding, abrasive materials are affixed to a flexible backing and cut into belts. Belt grinding can be used for different applications, but works under the same principles as wheel grinding.
Beyond belt grinders and wheel grinders, grinding machines can take on a number of different configurations. All grinding machines must be able to spin the grinding wheel at high speeds, or spin the wheels that carry the grinding belts, and must offer support to hold the workpiece. Rough grinding is often accomplished by holding the workpiece by hand, but precise operations require a more sophisticated apparatus.
Types of Grinding
While grinding is generally used as a finishing process to remove small amounts of material with a high degree of precision, certain grinding techniques have been developed that rival milling in their ability to remove material quickly. Some types of grinding in use today include:
- Snagging: a rough process using open-structure wheels, often used for removal of gates, risers and rough spots from castings. Snagging is used when accuracy is not a priority.
- Creep-feed grinding: a type of grinding that removes material quickly, at a rate comparable to milling. Modern versions require constant dressing of the grinding wheel (explained below).
- Cylindrical grinding: a process used to grind cylindrical surfaces and contours of a workpiece. Cylindrical grinding is capable of finishing parts within extremely close tolerances.
Maintaining Grinding Wheels
Grinding wheels are considered consumables because they wear during regular use and must eventually be replaced. Based on the composition of the grinding wheel and the application, operators can predict how quickly a grinding wheel will wear. In automated grinding operations, wear rates must be factored into machine movements so that each piece is ground consistently even as the grinding surface changes shape.
Grinding wheel wear is actually a necessary part of the process, because it allows old, dull grains to strip away from the wheel and reveal fresh, sharp grains. Often, however, dull grains fail to leave the wheel and chips lodge in the cavities. When this occurs, the wheel must be dressed to fracture the dull grains and allow the chips to fall from the wheel. Dressing involves placing a specialized tool against the grinding wheel that knocks old grains out of the composite.
Truing is another issue that can propagate during normal grinding operations. Grinding wheels that are out of true are unbalanced, either by uneven grinding or by chips clogging the cavities unevenly. In many cases, dressing can solve both problems.
Since grinding accidents are often caused by unbalanced grinding wheels that can break machines and sometimes injure operators, regular truing and dressing are extremely important. In order to facilitate and automate the process, modern CNC grinding machines include automatic dressers built in to the system.
When Grinding is Used
The grinding process is extremely versatile, and almost any size and shape of workpiece can be finished on modern grinding equipment. However, grinding is usually not as efficient at removing material as other machining processes, so grinding is often reserved for the final steps of manufacturing. Two exceptions are continuous-dress creep- feed grinding (CDCF) and cut-off grinding. Cut-off grinding actually mimics sawing, but uses an abrasive blade instead of a toothed one.
Sawing and grinding are often used together for gate removal on cast parts. Once a casting solidifies, the operator cuts away the gating with a saw. He or she then grinds the residual material on the casting to create a smooth surface. Gate grinding is usually accomplished with open-structure grinding wheels to allow for rapid removal of material, and is followed by additional grinding with finer grain for improved surface finish.
Grinding is also commonly used in machine tool production. Tool steel is very hard and often cannot be cut by other non-abrasive machine tools. Superabrasives can be used to form, sharpen and refurbish tools made from nearly any material, including carbide.
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Written by Nik Seyferth
Nik is Sales Manager at Eagle CNC Technologies in Muskegon, MI.