Truly centreless grinding

Specialist Birmingham-based engineering firm GB Precision has added centreless grinding to its extensive range of machining capabilities, with the purchase of a Unison Dedtru 388 which can grind to micron tolerances.

The ISO 9001 certified company has an established reputation as a subcontractor with particular expertise in high accuracy grinding, with three CNC state-of-the-art Studer machines already installed on its shopfloor.

However, as director, Paul Turner, explains, the new Dedtru offers specific additional benefits.

He comments: “When it comes to achieving the best roundness, dimensional tolerance and surface finish for small, delicate or very slender parts, we believe centreless grinding can provide a really good solution.”

Among the many so called ‘black arts’ of engineering processes, grinding is perhaps one of the least understood, and within that group, centreless grinding is more mysterious than most. However, the principle of the process is long-established, the first centreless grinding machines having been developed over 50 years ago.

In contrast to other grinding systems, in centreless grinding the workpiece is not mechanically held during the operation and therefore does not need centre holes or fixtures at the ends – in other words the term describes exactly what is involved.

The principle of the process is that the workpiece rotates between a high-speed grinding wheel and a slower speed, smaller diameter regulating wheel and is supported on its own outer diameter by a blade. The regulating wheel, which is made of soft material, such as rubber and often incorporates granular material to provide good traction, serves as both a driving and braking element, with the workpiece rotating at a constant and uniform speed, in the opposite direction to the grinding wheel.

The main benefit of centreless grinding is its ability to produce excellent roundness with extremely close diametrical tolerances, and high quality surface finishes in many different materials including ceramics, fibreglass, tungsten and Teflon as well as hardened tool steels. However additional advantages include the fact that, because workpieces are supported directly beneath the grinding cut, and along the length of the item, there is little work deflection, enabling long, slender components to be accurately ground.

A wide range of items from many industry sectors are suitable for centreless grinding, including bushings, carbide rods, rollers, cylinders, pins, pivots, spools, tubes and valve components.

Mr Turner explains: “Although centreless grinding is often associated with mass production applications, the process offers real benefits for GB Precision’s customers in the production of short run, specialist components, which may be small, or delicate and liable to deform.

“To take one example we have just used the Dedtru to machine a set of very small rings, which were found to have become slightly distorted when they returned from the heat treatment process. If we had tried to regrind these using our standard equipment the pressure of the jig itself would have added further distortion. By using the centreless grinder, we could grind the external diameter with no distortion.”

GB Precision






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