The first sliding-head turn-milling centre to be installed at the Ferndown, Dorset factory of Air Bearings Ltd (ABL) is a Citizen Cincom L20-VIIILFV. Notably, the machine has brought in-house the production of nearly all shaft-type components, saving around £8000 per month previously spent on subcontract Swiss-type turning. As a consequence, the machine paid for itself within 18 months.
ABL only purchases top-end machine tools to meet the levels of precision required and ensure the rotational motion of its air bearing spindles to within a couple of microns. The shaft assembly, with its six key parts, is especially critical.
ABL’s senior production controller Dave Stacey says: “Take the collet, for example, which is produced from 13 mm diameter tool steel bar. The concentricity of the front bore to the taper is tied up to 30 µm TIR. Dimensional tolerances on diameter and length need to be within 50 µm, or sometimes 25 µm to allow post machining to sub-micron accuracy, while there is a 6 µm limit in the bore.
“Originally, before our decision to use subcontract services, these collets were machined in-house in two operations – turning and boring on a fixed-head lathe and then drilling of eight radial holes on a machining centre,” he continues. “This time-consuming process led to our pre-finishing department only producing the quantity needed, which could be as low as 15-off, yet external heat treatment and stress relieving before final finishing still cost £250 a time, irrespective of component quantity.
“Now, with single-hit turn-milling of the collets on the Cincom L20, we run off typically 500, representing three months’ supply, at a fraction of the cost of subcontracting them out, added to which we can take full advantage of the fixed-cost heat-treatment service.”
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www.citizenmachinery.co.uk
