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Accurate collet chuck helps machine thin discs

Edited by the Manufacturingtalk editorial team Dec 10, 2002

The accuracy and reliability of a collet-style chuck enabled the highly accurate machining of 'discs' and secured an overseas order for the machine tool supplier concerned.

The accuracy and reliability of a Hainbuch collet-style chuck supplied by workholding specialist Leader Chuck Systems has combined with the quality build specification of a Cincinnati Hawk driven tool CNC lathe to solve a particularly demanding turning problem faced by Cincinnati Machine UK.

Machining 25mm diameter by 7.5mm thick 'discs' from Super Invar alloy bar, the Leader-Cincinnati combination has consistently met the stringent concentricity and runout tolerances of +/- 0.01mm in generating wall thicknesses of just 0.4mm, as well as achieving face parallelism requirements of 5 microns.

As a result, the Birmingham-based machine tool builder has secured an order for two Hawk 150M driven tool lathes for export to China.

With Z and X axes travels on the Hawk 150M of 425mm and 205mm, and a C axis programmable accuracy of 0.001 deg, the Hawk CNC lathe is inherently accurate due to its high rigidity, courtesy of the single-piece 45 deg cast bed coupled with the use of X-axis precision linear scale.

But, according to Cincinnati sales project engineer, Pete Robbins, "without the Hainbuch system, we would never have been able to consistently maintain the demanding tolerances on such small, thin walled parts".

He continues: "A conventional chuck used in a conventional way simply wouldn't have been able to securely grip the component without creating deformation during machining." With a clamping accuracy of up to 0.004mm TIR, the Hainbuch system differs from standard chucks in being able to securely hold short component lengths due to its positive clamping action - where the clamping head is pulled back into a perfectly mating taper to securely hold the part both axially and radially.

It was this feature, in particular, that played a key role in enabling Cincinnati to maintain the necessary machining tolerances, after the Cincinnati applications team adjusted the applied chuck pressure to maintain clamping force while eliminating the possibility of deformation.

Pete Robbins again: "After first operation parting off, the stage two set-up is carried out on the Hawk using carbide tooling for a sequence of trepanning and internal grooving operations.

"This is where about 90 per cent of material is removed, leaving a central 4mm spigot but machining to depths of 4.3mm and generating undercuts to create 0.4 mm walls and, for example, leaving a back face of just 0.5mm thick.

"Crucially, the inner faces of the disc also have to be parallel to each other to within 5 microns." Commenting that it was probably one of the most challenging turning tasks he has seen, Pete Robbins concludes: "There is no doubt that our success in winning the order depended not only on the accuracy and rigidity of the Cincinnati Hawk CNC lathe but also on Leader Chuck Systems and the capabilities and features of its Hainbuch collet system.".