CNC sliding-headstock lathes reduce cycle times

Two CNC sliding-headstock lathes operated by Rodmatic are able to mill-turn components of medium to high complexity as quickly as the cam-type, six spindle automatics on site, Star has announced.

Brian Steatham, owner and managing director of Rodmatic, made the claim and added that the turned finish achieved using the Stars, mainly on stainless steel and steel components, is so good that subsequent grinding is frequently eliminated, lowering unit production cost.

A multi-spindle auto takes around two days to reset, so batch sizes above 30,000-off are needed to justify the time investment.

By contrast, sliding-head lathes can be set in a couple of hours, and so much smaller runs are economic.

Rodmatic has produced batches as low as seven-off on the Stars, although runs can be up to 5,000-off.

In this connection, Steatham has the following tip for precision-turned parts manufacturers: when producing larger batches on CNC sliding-head lathes, think of using form tools alongside single-point cutting tools to reduce cycle times.

Rodmatic often does just that, using its experience of wire-eroding and grinding form tools for the multi's and transferring the technology to the sliding-head machines.

Reductions in cycle times are achieved, typically of around 30 per cent.

One component regularly produced, a profile-turned and bored steel shaft for a European customer in the fluid power sector, is machined in part by form tools in an overall cycle of 37s.

This is more than 38 per cent faster than the 60s it used to take by exclusively single-point cutting.

Tolerances of +/- 2.5 micron are routinely held on the sliding-head machines, which would be difficult to achieve on the multi's.

Equipped with a 4m bar magazine and high-pressure coolant to allow long periods of unattended running, the first Star was a nine-axis SV-32 of 32mm bar capacity, installed mid-2007.

It was followed in 2008 by a similarly equipped seven-axis SR-20RIII for mill-turning parts up to 20mm diameter.

They joined three multi-axis CNC fixed-head lathes for producing larger components.

When asked why he did not continue down the route of fixed-head lathes to satisfy the market's increasing demand for smaller size, high-accuracy mill-turned parts in lower volumes, Steatham said that, in his opinion, a sliding-head lathe is twice as fast as an 'equivalent' fixed-head lathe - in other words, one with a counter spindle and two turrets containing driven tooling.

He added: 'Major factors slowing fixed-head lathes are their inability to cut with more than two tools simultaneously, and long turret-indexing times compared with the rapid infeed of gang and cross-working tools on sliding-head machines.' He also likes the advanced mill-turning and simultaneous end-working capabilities of modern sliding-headstock lathes, as well as their traditional strength - that of producing shaft components accurately due to the support given by the guide bush at the point of cutting.

During machining trials against competitive sliding-head lathes, the Star machines were inherently faster.

In addition, Steatham felt that their extra weight and rigidity would allow the production of more accurate components.