Adding capacity with multi-spindle automatic

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So the purchase in December 2015 of a German-built Index six-spindle, CNC turning centre from UK agent, Geo Kingsbury, might seem an unusual choice. Such machines are only economical when the quantity of turn-milled parts is much higher.

Formula 1 motorsport contracts account for around three quarters of the throughput at Salcey Precision’s facility in Hartwell, near Northampton. The remainder involves the manufacture of similar parts for road-going supercars in larger volumes, currently 12,000 parts per month. Winning this work provided the rationale for investment in the CNC multi. According to SPE’s managing director, Richard Alcock, the machine is quick to set up and program, reducing economical batch size to the low 1,000s.

He says: “It was a fourfold increase in parts demand that very much dictated the new manufacturing solution. It all started in 2011 with an order for 3,000 supercar valve shims per month and we bought a 20mm bar capacity, sliding head CNC lathe to fulfil the contract. By last year, the quantity had risen to 12,000 per month.

“In 2012, we won further business from the same customer that added a stainless steel valve train component. It was needed in similar monthly quantities to the shim, which at the time was 3,000, so we acquired another slider, this time of 32mm capacity without guide bush, to meet the demand.

“It is this job, which has also quadrupled to 12,000 per month, that has been transferred to the multi-spindle auto. The increase in productivity has been dramatic. Cycle time is more than 80% faster, having fallen from 91 seconds on the slider to just 16.4 seconds on the multi.”

This brings the economics of investing in the Index MS40C six spindle auto into focus. Without it, a second 32mm sliding head lathe would have been needed and both lathes would have had to run for two shifts rather than one, raising the prospect of employing another operator.

Now that the Index multi has taken over, the very high output allows it to complete one months’ work in a week, without the need for further staff. It leaves a lot of spare capacity during the day which, together with the potential for unattended running overnight, will allow SPE to take on a considerable amount of extra turn-milling up to the machine’s 40mm bar capacity. Moreover, with feasible batch quantities now relatively low, the chance of winning such work is enhanced.

The valve train component is produced to PPAP (production part approval process) requirements from 25mm diameter, 3m long EN24T bar. The cycle involves front-end working followed by machining on the reverse after part-off using a scara synchro spindle and a dedicated end-working tool station. Geo Kingsbury wrote the program as part of a turnkey supply package that also included training SPE’s staff.

A tapered bore through the component needs to be machined to a particularly tight tolerance, as size, concentricity and taper as measured by a dial bore gauge must be within ± 0.05mm. It means that a positional limit 10 times better, i.e. ±5µm relative to other features, has to be maintained along the length of the bore.

So critical is the accuracy that SPC (statistical process control) of the bore is carried out on the shopfloor by measuring one sample component in every 42, while full inspection on a coordinate measuring machine is performed on one part in 84. A process capability (Cpk) of 1.33 or better is achieved.

A characteristic of a multi-spindle automatic is that six spindles (sometimes more) are operating within the footprint of one lathe. This can be another driver for adopting the technology, especially if space on a shopfloor is limited as it is at SPE’s Hartwell facility. As only one operator is needed to run a multi, lower staffing costs per spindle also helps justify machine purchase.

Mr Alcock concludes: “We chose a multi from Index, as the German company is widely regarded as a premium manufacturer. We visited the production facility in Deisizau, near Stuttgart, with staff from the UK agent, Geo Kingsbury and were very impressed with the manufacturing operation and the level of technology that goes into the machines.

“In particular, we like the ability to adjust the spindle speed and feed rates independently at each of the six stations. It gives us considerable flexibility in our choice of tooling as well as optimising productivity and component quality.”

Salcey Precision Engineering www.salceyprecision.co.uk

Geo Kingsbury www.geokingsbury.com