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Centerless grinding is an OD grinding (outer-diameter grinding) process where the workpiece is supported by a working blade. The workpiece is located between a high-speed grinding wheel and a smaller-diameter slow-speed adjusting wheel. The interaction between the grinding wheel, the adjusting wheel, and the working blade determines whether or not the process is successful.
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Although centerless grinders have existed for nearly a century, many people are still struggling with the basic principles of the process and its advantages. In this article, we will cover all the basics of centerless grinding for you!
Centerless grinders do not have many moving parts but owe most of their functions to some basic physical principles. This makes centerless grinding a relatively easy operation, suitable for finishing the OD of a workpiece holder between two rotating cylinders: an adjustable wheel that controls the speed and feed rate of the part of a linear stroke.
Because the workpiece is firmly supported, there will be no deflection during the grinding process. Centerless grinding is continuous because the loading time is short compared to grinding between centers. Long stretches can be ground continuously, and even a great deal of small sections can be ground automatically by different feeder attachments. Furthermore, the centerless grinder can work reliably at high speeds. This makes the technology suitable for a wide range of aerospace, automobile, military, medical, and other uses.
Below is a short clip of long bar stock centerless grinding performed with the ECG-1808S by E-tech Machinery.
Long Bar Centerless Grinding, E-tech Machinery
The disadvantage of centerless grinding is that you cannot use multiple axes on the workpiece. However, in many parts, this process addresses the processing limitations in terms of size, material, and surface finish.
For example, if a part is not round on a lathe, and the diameter of the part is too small or the center cannot be installed, you can obtain roundness by centerless grinding technology. Also, during centerless machining, there is no axial thrust on the workpiece, which means it can be used to grind fragile long pieces and parts that may otherwise be deformed.
The main difference between the two most commonly used centerless grinding methods is how the workpiece is fed by the centerless grinding machine. Through-hole grinding is generally used for parts that have a uniform roundness over the entire length of the part. In this method, the workpiece moves along the rack between two wheels and is driven at a slight angle relative to the grinding wheel.
For grinding cylindrical sections with holes or complicated structures, such as gear shafts, feed grinding, or plunge grinding is used. Here the holder of the workpiece must be shaped to accommodate the part’s form. The grinding wheel and adjusting wheel must be trimmed to match the contour cut required by the part. The adjusting wheel rotates the par while pushing it towards the grinding wheel, and it rotates at a faster speed. The greater the speed difference, the faster the removal speed.
Another key factor in centerless grinding is the choice of grinding wheel. In addition to providing different diameters and widths/thicknesses, centerless grinding wheels also have different grain types and sizes, usually using super abrasives such as polycrystalline diamond and cubic boron nitride. These super abrasives and silicon carbide grinding wheel materials have advantages in the centerless grinding of very hard metals for the following reasons: the grinding wheel itself is durable and can maintain sharpness for a longer time.
They have high thermal conductivity and can maintain their shape at high contact temperatures and high rotational speeds. The purpose of dressing the grinding wheel is to round and “flatten” the grinding wheel. At the same time, the dressing will expose new sharp particles to help the abrasive cutting action. Stationary dressing tools are still widely used. If your machine is equipped with a fixing tool, make sure to use a high-quality trimming nib. The rotary dressing system is the best choice for the production of centerless grinding. They provide a more consistent process and allow more abrasive options such as high ceramic concentration and super abrasive wheels.
Coolant is used in centerless grinding not only to keep the grinding wheel cool but to remove heat from the contact area between the workpiece and the grinding wheels. Oil is usually the best coolant. It is recommended to use high-quality ester-based high-performance grinding oil. However, the oil may not always be practical, so water-soluble and synthetic coolants may be required in some applications. The coolant speed must match the wheel speed.
Grinding with Coolant, E-tech Machinery
The nozzle should provide a continuous flow of coolant. This means that the coolant coming out of the nozzle looks like a solid rod without any air entrainment. This can be achieved by a supply pipe of the correct size with a limited number of bends in the coolant line. Finally, a properly designed nozzle will help reduce turbulence and air entrapment. Keep the coolant as low as possible at ambient temperature. The size of the storage tank should be able to meet the coolant flow rate, and the coolant will not be interrupted during the continuous grinding cycle. Filtering is necessary to eliminate reworked wear debris.
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