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Posted on Aug 17, 20206
Centerless grinding is a machining process that uses abrasive cutting to remove material from the workpiece. Centerless grinding differs from centering grinding in that no spindle or chuck is used to locate and secure the workpiece; the workpiece is clamped between two rotating grinding wheels, and the speed of their rotation relative to each other determines the speed at which material is removed from the workpiece.
Centerless grinding is typically used in place of other grinding processes in operations where many parts must be processed in a short time.
In centerless grinding, the workpiece is held between two discs rotating in the same direction at different speeds and a clamping platform. One disc, known as a grinding wheel, sits on a fixed axis and rotates so that the force applied to the workpiece is directed downward onto the clamping platform. This wheel typically performs the grinding operation at a higher tangential speed than the workpiece at the point of contact. The second wheel, known as the adjuster wheel, is movable. The wheel is positioned to exert lateral pressure on the workpiece and typically has either very rough or rubber bonded abrasive that will trap the workpiece.
The speed of the two wheels relative to each other provides the grinding action and determines the speed at which material is removed from the workpiece. During operation, the workpiece rotates with the adjusting wheel at the same linear speed at the point of contact and (preferably) without slippage. The grinding wheel turns faster, slipping past the surface of the workpiece at the point of contact and removing chips of material as it passes.
There are three forms of centerless grinding, differing primarily in the method used to feed the workpiece through the machine.
With centreless through grinding, the workpiece passes through the grinding wheels completely, going in on one side and going out on the other. The adjusting disc in through grinding is inclined away from the plane of the grinding wheel so as to provide an axial force component to guide the workpiece between the two wheels. Pass-through grinding can be very efficient because it does not require a separate feed mechanism; however, it can only be used on parts with a simple cylindrical shape.
In centerless grinding with front feed, the workpiece is axially fed into the machine from one side and rests on the end stop; a grinding operation is performed and then the workpiece is fed in the opposite direction to exit the machine. Face feed grinding is best for tapered parts.
Feed-less centerless grinding is used to grind workpieces with relatively complex shapes, such as an hourglass shape. Before starting the process, the workpiece is manually loaded into the grinder and the adjusting wheel is brought into place. The complexity of the part shapes and wheel shapes required to accurately grind them prevent the workpiece from being axially displaced through the machine.
Centerless grinding uses specially designed centerless grinders. Such a machine will always include a grinding wheel, an adjusting wheel, and some workpiece support means. Modern machines may require computer numerical control to enable automation and improve precision. The grinding wheels are interchangeable to allow for different grains and shapes. Machines designed for pass-through grinding operations will allow the angle of the adjuster wheel to be adjusted to accommodate different sized parts.
Centerless grinding is critical in the production of many high-volume auto parts. These include valve sliders, control rods, camshafts, crankshafts, pistons, bushings and rollers. In addition, centerless grinding is used to manufacture parts for the hydraulic and fluid control industries, the medical and aerospace industries - in fact, any industry where roundness and extreme accuracy of cylindrical surfaces are required.
For those who "chip" each day, milling without center may seem a mystery, but it is a fairly simple process. In this article, we'll go over how it works, where and when it should be used, and give you advice on how to apply this well-known technology.
:: Read more : Centerless Grinding and Precision Machining
Before the development of centreless grinding, round parts were ground between the centers or gripped with a chuck or gripper. Centerless grinding does not require such fixing methods. The parts are fed between the grinding wheel and a smaller regulating wheel while resting on an angled workpiece support—a blade-like device that sits between the opposing wheels.
During grinding, the force of the grinding wheel pushes the workpiece against the adjusting wheel and against the support. The control wheel determines the rotational speed of the workpiece. Tilt it a few degrees and the workpiece will be pulled through the wheels and out of the machine, a technique known as through grinding. Input grinding is the second available centerless grinding technology. The adjusting wheel pulls the part to the dead resistance located on the prop blade. The grinding wheel, which often contains a profile, is then fed into the part until the final part size is reached.
Cylindrical grinding will always be needed, but centerless grinding has several advantages. Since there is no need to place the part between centers or clamp it to a chuck, parts can be quickly loaded into the grinder for increased throughput. The workpiece is held securely between the wheels and the support rail, allowing you to grind long, thin pieces.(Entire lengths of bars are often centerless ground for use on Swiss CNC lathes.) And because the wheel adjustment is diametric, rather than radial - as is the case with cylindrical grinders - any infeed errors are halved, which increases precision.
For centreless parts, less finishing abrasive is generally required as the workpiece tends to find its own center when it first contacts the wheels. Unfortunately, this means that concentricity with previously machined holes and other features can be a problem, one of the major drawbacks of centreless grinding. Another problem is the extended set-up time, due to the handling and screwing in of large wheels and special work supports may be required.
Centerless grinders are often classified according to the inclination of the machine bed. The most popular are machines with a horizontal bed, which provide easy access for wheel dressing. Angular and even vertically inclined beds are used for grinding heavy workpieces because of the potential for a greater grinding force and less abrading of the workpiece by the work support.
Some builders classify their machines by type of grinding zone. In machines with a moving zone or three-stroke zone, the grinding wheel is stationary during operation, and the X, Z and B axis movements (grinding wheel tilt) are controlled from the adjustment side. This arrangement provides a compact structure and excellent rigidity. Unfortunately, more frequent adjustment of the work support is required as the grinding wheel wears down, although this is easily compensated for because it is located on a separate CNC carriage.
Centerless grinders with a stationary zone include a stationary or minimally adjustable support for the base, and the grinding wheels and adjustment discs run on their own X and Z axis guides.
The cross-slide machines feature a similar axle arrangement and a fixed dressing arrangement, which maximizes precision and allows quick set-up thanks to improved ergonomics by changing the wheels towards the operator. Dual grinding machines deliver a two-step process that combines centerless grinding with grinding between centers, alleviating concentricity concerns.
Regardless of the machine design, the workpiece is guided along the outer surface during centerless grinding. For this reason, roundness can be a problem, especially for lobbying and triangulation. These conditions are sometimes not easy to spot. The use of a micrometer or other two-point measuring device can indicate that the part is within dimensional tolerance, even though it is actually a cloverleaf shape.
Such form errors can be reduced and almost completely eliminated by a solid knowledge of grinding. Adjusting wheel centerlines and support heights, applying appropriate infeed speeds, limiting wheel forces, and dressing methods have a huge impact on part quality, as do factors such as machine design and spindle stiffness.
Only by developing a stable and predictable grinding process can successful results be achieved. For example, increasing the rotational speed of the wheel lowers the grinding forces while improving the surface roughness of the workpiece and the wear of the wheel, but also increases heat generation. This may require better lubrication to avoid damaging the workpiece.
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