Cam-operated automatic lathes all over the UK are being replaced by multi-axis CNC sliding-headstock lathes, as OEMs and subcontractors realise 'one-hit', 'lights-out' working brings back work.

Cam auto's all over the UK are being replaced by multi-axis CNC sliding-headstock lathes, as OEMs and subcontractors realise that they can harness the machines' one-hit, lights-out production capabilities to compete with low-wage economies. A recent convert is connector manufacturer, Harwin, Portsmouth, which invested in seven Star sliders during an 18-month period to mid 2005. At the same time it sold off more than 20 cam-operated lathes, leaving 50 or so on site to be replaced over the next four to five years.

The production output from six of the Star lathes, all entry-level SB-16 bar machines, equals that of the previous 20+ cam auto's.

The seventh CNC slider, a Star SR-20RII, was bought specifically to produce a family of 20 high-voltage posts which form part of a new, 90-degree, co-axial connector developed by Harwin.

The latter machine has resulted in a 60% saving in the cost of machining the connector posts compared with the price previously paid to a subcontractor, which was manufacturing the components in two or three operations.

As a result, it was having difficulty holding tolerance, and delivery was often slow.

On the Star multi-axis lathe, the post is reliably mill-turned to the required accuracy in a single visit to the machine.

According to its size, the component is produced from 16mm or 20mm diameter brass bar.

In the main spindle, operations involve turning the front face, profile turning the OD, drilling a hole down the centre, slotting, and milling a box-shaped body.

After synchronous pick-up in the counter spindle and part-off, material is milled away from the body to leave two parallel legs at 90 degrees to the spindle centerline, a cross hole is drilled and a slot is milled to break into both this hole and the central bore.

Depending on variant, the total cycle time is from 40 to 60s, the majority being prismatic machining rather than turning, such is the versatility of these lathes.

Said Ricky Phillips, Turning Shop manager, 'Our cam auto's formed Harwin's total manufacturing capability up to the 1980s, when the company invested in high-speed stamping for producing higher volume connectors.

The trouble was that production runs were starting to fall, which made setting up cam machines uneconomic, so in 1987 we bought an early Star RNC CNC slider which was easier to set and could compete on speed.

Its reliability has been second to none and the machine is still in constant use today, producing accurate parts round-the-clock.

Batch sizes continued to get smaller as specials became more common and customers were less willing to hold stock.' Phillips and his team were having to spend four to eight hours resetting a cam auto which would sometimes run for only one day to produce, say, 5,000-off connector parts.

He says that a run of at least 50,000-off is needed to justify resetting a cam machine.

Now, Harwin is able to machine small batches of connector parts economically on the Stars, even when cycle times are as short as 10s, as set-up takes typically one to two hours for a complete changeover'.

Phillips said: 'However, with space for so many static and driven tools in the working area and by standardising on a few bar sizes, set-up can be as quick as 15 minutes if the bar and tools are unchanged and a program simply needs editing at the machine.

The ability of the Stars to generate a large throughput of parts, 24 hours a day including unattended operation overnight, leaves spare capacity to run non-connector subcontract work, such as components for the automotive, aerospace and medical industries.

In these cases, 5,000-off is a large batch and one-offs are frequently undertaken.

Drawing tolerances are down to +/- 0.03mm, although +/- 0.01 to 0.02mm is routinely held even over an entire lights-out shift, according to Phillips, who commented that this level of accuracy gives a stable platform for meeting customers' Cpk requirements.

He continued, 'The transition from cam auto to CNC slider has been very smooth and our operators readily took to the Stars, which they find easy to program at the Fanuc control or off-line.

Service from Star's Melbourne head office has been impressive, with any small problems usually sorted out over the telephone.

Stockholding of tooling and spares is also good, and we have used their applications engineers to help us out with the occasional program for new prototypes, such as the 90-degree connector post.' As a postscript, Phillips commented generally on the relative merits of sliding- and fixed-head turning for connector manufacture.

When the early Star RNC turning centre was installed, Harwin also invested in a CNC fixed-head lathe and carried out comparative trials.