Grinding: A Machining Process
Grinding is a machining process used to improve surface finish, cut hard materials, and tighten the tolerance on flat and cylindrical part surfaces by removing a very small amount of material. While the grinding process has been vastly improved upon with the addition of CNC, it is still a very slow way to remove stock from a part. It is a near must when you need to achieve one of the following:
- A very high surface finish.
- Very tight tolerances on both flat and round areas. (0.0005 to 0.000050 inches)
- Flatness over large surface areas requiring compliance within 0.001 inches.
- Removal of finish materials is difficult utilizing other machining processes due to the material’s hardness.
There are a variety of grinding methods and machines that perform them. A few of these machining methods are Surface Grinding, Cylindrical Grinding, Internal Grinding, Centerless Grinding and Vertical Surface Grinding among others.
Grinding operations of all kinds like outside diameter (OD) grinding and internal diameter (ID) grinding are also being done on CNC grinders. CNC has even opened up a new technology, c ontour grinding, previously infeasible due to technology constraints.
CNC grinders are equipped with a computer consisting of one or more microprocessors and storage units, a user interface for programming and other interfaces that enable the CNC grinding machines to work in conjunction with CAD/CAM software systems. All of this enables a CNC machinist to instruct the CNC grinding machine to execute the exact movements necessary to create the precision ground part.
Centerless grinding is utilized when you need to grind material from the outside diameter of a cylindrical part or bar without the use of centers. Because no centers are drilled into the ends of the part or bar to assist with work holding the part is held in place during the grinding process by the grinding wheel, a work rest blade and the regulating wheel. Pressure is applied during the process to remove a desired amount of material. Utilizing this method, parts can be ground speedily and without much human intervention. The maintenance required may be as simple as feeding the parts into the grinder, occasionally dressing the wheels and adjusting the size settings so that parts maintain tolerance.
The CNC cylindrical grinding process is also used for grinding the outside diameters of a part. Configured much like a lathe, the cylindrical grinder has a head and tail stock. The grinding wheel is typically positioned at the back of the machine and traverses the length of the part during the grinding process. The cylindrical part or bar is often held during this process on centers which have drilled into the part in preparation for grinding. CNC variables that need to be managed to achieve the desired finish and tolerance include grinding wheel speed, work speed and the traverse speed.
Surface grinders are used to grind flat parts into the proper level of finish and tolerance. The surface grinder has a simple configuration that consists of a movable table with a horizontally mounted spindle above it. The table (often magnetic) typically acts as the work holding device in the surface grinding operation. The surface table moves left to right and in and out ensuring that the grinding wheel covers the entire work area and does not linger too long in one portion of the surface. The horizontal spindle that has the grinding wheel attached to it may be adjusted in height to determine the amount of material to be removed in one pass. Successive adjustments are typically necessary to finish the part.
If your part requires extreme accuracy from inside diameters then chances are that it requires internal grinding. Internal grinding can achieve close tolerances even if your part has been pre-hardened. Internal grinding can be accomplished on a wide range of hole sizes.
While the size of the internal grinder is determined by the work holding diameter and the length of the grinding stroke careful attention must be paid to the length of cut to ensure accuracy. The basic configuration of an internal grinder consists of a work drive (which holds the part), the workpiece or the part being ground and the grinding wheel which is mounted on a mandrel. Typically internal grinding requires higher RPMs than other grinding methods in order to cut efficiently.
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