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Centreless Grinding is a method of grinding exterior cylindrical, tapered, and formed surfaces on workpieces that are not held and rotated on centres. The principal elements of an external centreless grinder are the grinding wheel, regulating or back up wheel and the work rest. Both wheels are rotated in the same direction The work rest is located between the wheels. The work is placed upon the work rest, and the latter, together with the regulating wheel is fed forward, forcing the works against the grinding wheel. The axial movement of the work past the grinding wheel is obtained by tilting the regulating wheel at a slight angle from horizontal. An angular adjustment of 0 to 8 or 10 degrees is provided in the machine for this purpose. Centreless Grinding is classified into three types:
- Throughfeed grinding,
- Infeed grinding,
- End feed grinding.
Throughfeed grinding with a centreless grinder is common in the Bearing (mechanical), manufacturing industry. Outer rings of bearing casings can be ground in this way to a very high degree of accuracy especially outer rings which are ground in stages along a Centreless Grinding Line These Stages are named Rough, Semi-finish and Finish, each grinding stage is designed to remove enough stock material from the casing so that the next stage does not encounter any problems such as burning or surface chatter, the finish stage achieves the final dimension for the outer or inner ring (depending on the type of bearing casings being produced). Each grinding wheel at all of the aforementioned stages has a varying degree of abrasive quality (finish being the finest grade) to achieve the appropriate stock removal for the next stage and final surface finish required. Bearing casings are introduced to the grinding action via means of a transfer from the delivery system to a pair of infeed rollers, these infeed rollers are tapered to a certain angle so that the casings are driven forward until the regulating wheel and grinding wheel catch them and slow them to their grinding speed which can be altered by speed control of the regulating wheel. The casings are constantly rotating and are fed into the grinding area to prevent separation which can cause finish/size problems or even a "bump" that can potentially crack or destroy casings and will damage the grinding and regulating wheels. Whilst grinding, the bearing cases run through the grinding stages in one long tube of casings that is showered with a cutting fluid on a hardened steel blade with an angled, highly ground surface held on a horizontal plane between the grinding wheel and regulating wheel, often named a Work Rest Blade, the height of which perfectly aligns the bearing casing with the horizontal centreline of the grinding wheel creating a flawless ground finish, the work rest blade height can be altered using packing bars place underneath the blade, height adjustments must be made depending on the diameter of the casings being ground. Each casing exits the grinding zone at high speed onto a conveyor that delivers them to whatever storage and/or inspection arrangement a manufacturer may have, inspection is also carried out by the operator of the centreless line, by checking finish appearance, diameter, squareness and roundness by use of a Dial Test Indicator in varying configurations, size allowances are permitted but are extremely tight depending on the customers requirements and can vary plus or minus within microns of finish diameter, Sizes can be adjusted on all grinding stages via a compensation button which can be pushed to remove extra microns in 5, 2, and 1 micrometre or micron blocks, the grinding wheel can move away at the same compensation to make the casings bigger if so required if the casing size moves from the operators target, and as the grinding wheel wears. Because a centreless grinding line has typically three grinding machines the operator must be in complete control and therefore must be checking for blockages in transfers grinding exits and packing areas, also sizes and quality must constantly be checked, so the operator is constantly moving up and down the line and checking for problems and quality. Safety features include a emergency stop button which immediately moves the grinding wheel and its revolutionary axis away from the ground rings, because of the wheels momentum, it is not stopped but the power is cut, the wheel cannot be reactivated until the emrgency stop is reset. Any casings between the wheels at this time must be removed and inspected, and must be reintroduced to the machine to be reground or discarded as scrap depending on their condition, the work rest blade must also be inspected and changed if necessary depending on the reason the emergency stop was hit, afterwards the machine is reactived, size is achieved (depending on the grinding stage) and the operatins is restarted.
- Please note that the above description is only one of several centreless grinding configurations and methods employed by manufacturers throughout the world and is not necessarily the standard.
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