The Principles Of Centerless Grinding
Centerless grinding was developed over half a century ago. This shaping technology is an outstanding method of maintaining extremely close diametrical tolerances under high-production conditions. The process can handle diameters from 0.003" to over 3".
The basic components of a centerless grinding machine are the grinding wheel, the feed or regulating wheel and the work support blade. To the layman there seems no apparent way of controlling the roundness of the work. The cutting pressure that is developed by the grinding action forces the work down against the work support blade as well as the regulating wheel. The regulating wheel serves as both a frictional driving and braking element, with the work rotating at a constant and uniform speed. Centerless grinders use negative work speed, which means the rotation of the work and that of the grinding wheel are in different directions. The speed of the regulating wheel is varied to suit the operation being performed. The diameter of the work being ground is determined by the distance between the surfaces of the grinding wheel and the regulating wheel, but a constant diameter does not always mean a perfect cylinder.
What are the factors that make for true roundness? The position of the work is governed by the position of the support blade. Suppose the blade top is flat and the centerline of the part is in line with center of the grinding and regulating wheels. The periphery of the wheels plus the top of the support blade form three sides of a square. Grinding with this simple setup will generate a constant diameter, but not necessarily in a cylindrical form. In its extreme, the shape formed would be a three-arc triangle, having a constant diameter but not round. The reason for this is that while a high spot is in contact with the regulating wheel, the spot diametrically opposite is being ground, thus grinding a low spot on the part. This phenomenon can be corrected by making two changes: first, the height of the support blade should be raised, and second, the blade top should be angled. Now when a high spot on the work contacts the regulating wheel, the corresponding low spot ground will not be directly opposite. The error will progressively diminish and roundness will quickly be achieved.
Corrective action depends on many variables, such as the angle of the support blade, the size of the work and its hardness, the speed of the regulating wheel and other factors. All things being equal, work rotating at faster speeds reaches true cylindrical shape faster than work rotating slowly. The limiting factor is when the work starts to chatter. Chatter is caused when the blade is set too high or the speed too fast. Work speed can be controlled by the speed of the regulating wheel.
Depending on the shape and condition of the workpiece to be ground, the machine setup of the grinding wheel, the regulating wheel, work support blade and feeding mechanism will vary. The four basic methods of centerless grinding in use are throughfeed, infeed, endfeed and infeed throughfeed combination.
Throughfeed grinding is used to grind straight, cylindrical workpieces which have no interfering projections. This method is a continuous production operation. To provide continuous, automatic throughfeed, the axis of the regulating wheel is tilted relative to the axis of the grinding wheel. The feed rate is controlled by the speed, angle and diameter of the regulating wheel. The parts are guided into and out of the machine in a straight line. If this line is not perfectly straight, taper and/or concavity conditions can result on the workpiece.
Infeed (Plunge) Grinding is used to grind workpieces which have projections or shoulders, multiple diameters or other irregular shapes which preclude the use of throughfeed grinding. The wheels are opened to allow for the part to be loaded (from above if multi-diameters). Most often the part is fed along with the blade and regulating wheel to a set finish position against a stationary grinding wheel. The grinding wheel must have sufficient width to cover the entire surface being ground. In many cases, the desired shape is cut into the face of the wheel either by single point diamond trueing or by diamond roll crush trueing. Diamond roll trueing is used on intricate shapes which are difficult to form with a single point diamond.
Endfeed Grinding is used principally on tapered work. The grinding wheel, regulating wheel and work support blade are set at a fixed relationship to each other. All three are shaped to meet the desired taper of the part to be ground. The work is fed from the front of the machine and is ground until it reaches an end stop.
Infeed-throughfeed combination is for parts which are more conveniently ground in one pass, but have too large an amount of stock for the conventional throughfeed method. It is also for grinding the smaller diameter of two diameter parts, where the portion to be ground exceeds the width of the grinding wheel.
Due to shorter loading times, actual grinding time is decreased compared with other methods of grinding. Once the centerless grinder has been properly set-up for a particular run, no further adjustment or additional set-ups are needed as opposed to centertype (cylindrical) grinding. This alone offers substantial savings. The workpiece is under constant, rigid support while in the grinding mode, making it feasible to take heavier cuts and grind fine pins, rods and wires. When grinding easily distorted workpieces, the centerless grinder is most advantageous because of its lack of axial thrust. Automatic feeding offers continuous production of large quantities of smaller size workpieces. Centerless machines are designed for simplicity which in itself cuts down on machine maintenance.
The blade is an essential and important component in centerless grinding. The workpiece is held in contact with the grinding and regulating wheels by an adjustable blade made of various materials. The selection of the proper blade will affect the roundness and quality of the surface finish obtainable. Four types of support blades in general use are: aluminum bronze, chilled iron, high speed steel and tungsten carbide. For the majority of centerless grinding operations, the 30° angular top support blade is used. It should be noted however, that as the diameter of the workpiece to be ground increases, the angle of the blade decreases. The length of the blade is determined by the width of the wheels and thickness should be slightly less than the diameter of the workpiece. Work ground below the centerline may come out of round as well as work ground too high above the centerline. Long lengths generally have to run lower than short pieces due to the possibility of warpage or whip in the rods. Shims are used to adjust the height of the blade.
Many types of products and materials may be ground on the centerless grinder. The use of diamond wheels makes grinding carbide plug gages, ceramic wafers, tungsten filament, teflon rods, glass stoppers, fiberglass rods, ferrite cores, etc. economically feasible. Numerous automatic components such as piston pins, steering knuckle pins, and shock absorber shafts lend themselves ideally to centerless grinding. Along with the proper grinding media, there seems to be no limitation to the kind of material that can be ground.
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