The Hertfordshire sub-contractor Unicut Precision has installed a pair of Index C200 lathes that will help it to fulfil a contract — potentially worth £5 million — from Eaton Hydraulics that quadruples the number of valve cartridge bodies to be manufactured by the company over the next four years.

Key to this contract is the high performance of the Index lathes, which significantly lower the production cost per part.

When a manufacturer installs a new machine tool, it is usual for time to be trimmed off production cycles by a combination of increasing feeds and speeds, adding one or more axes to lower the number of set-ups, and introducing more-efficient tooling.

Reducing idle times through faster axis movements is cited as another contributor to productivity improvement, but it is seldom the sole factor.

The use by Unicut of the two Index C200 lathes — supplied by Geo Kingsbury Ltd (www.geokingsbury.com), the sole agent in the UK and Ireland for Index CNC mill-turn centres — is one of those rare examples where the whole benefit derives from minimising non-cutting times.

Cycles at Unicut’s Welwyn Garden City factory are being slashed by an average of 35% across a family of hydraulic-valve cartridge bodies due to the sheer speed of the movements on the Index bar autos.

Down-time between batches is also lower compared with the machines previously used for producing valve cartridge bodies, due to the use of the manufacturer’s proprietary quick-change VDI tool-holders and pre-settable front ends, which also keep productivity high when tools need to be exchanged during longer batch runs.

Proven parameters

All previously used feeds and speeds, as well as the tooling, have been kept the same on the Index turning centres, as the parameters have been optimised by Unicut over many years when producing similar types of carbon-steel component for Eaton.

The C200s have three tool turrets which can be in cut simultaneously, so their cutter deployment is equivalent to those of a different make of fixed-head lathe and a sliding-head lathe that are also being used to produce the valve bodies.

The 35% shorter cycles are down to the ability of the Index machines to minimise the amount of time consumed travelling from one cut to the next. This is due to a combination of fast turret indexing, 1g axis acceleration to 50m/min in all axes plus similarly fast deceleration, and short travels within the machining area.

Unicut’s managing director, Jason Nicholson, says the last point is crucial: “The C200 is an out-and-out bar machine with a compact configuration and the nimbleness of a slider. I suppose you could fit a chuck, but you wouldn’t want to.

“Chip-to-chip times between successive operations in a program are around half those of other fixed-head lathes we use; they are also shorter than those of our sliders. Everything is fast on these Index machines, even the C-axis engagement and the synchronous transfer between main spindle and counter-spindle.

“It is probable that we would have ended up with the Index C200s because of their performance. However, bearing in mind we were shopping for a high-end lathe, I thought it strange that Geo Kingsbury was the only supplier willing to carry out time trials on our components, which were completed satisfactorily at the Index factory in Esslingen.

“The fact that the C200s came with a Fanuc control option was an added bonus, not only lowering the machine price but also providing better compatibility with other Fanuc and Mitsubishi controls on our shopfloor and allowing the possibility of swapping programs.”

Tool-holder variety

Another facet of Index that caught Mr Nicholson’s eye was the variety of VDI tool-holders it manufactures in-house — both static and driven — which he described as vast and not on offer from other potential machine suppliers.

Furthermore, W-shaped grooves ground by Index into the face of the turrets at each tool position and mating W-grooves on the tool holders provide a more rigid and accurate clamping system than the block, dowel pin and screw arrangement of conventional back-ends.

Pre-set Capto front ends for the holders are helping to minimise change-over times between and during batches; a quarter turn releases the tool for exchange, which is completed in moments. Repeatability is also excellent (the fact that offsets are never entered into the control confirms the precision of this location), and Index guarantees 8µm repeatability for any tool in any station around a turret.

Access to the top two turrets is convenient, as the machine bed is vertical, not slanted, which means there is no need to stretch any further into the working area than when re-tooling the bottom turret. Mr Nicholson says: “Our operators really like this, because set-ups are much quicker, and coolant doesn’t drip all over them.”

It is interesting to note that the turrets on the C200, two of which incorporate a Y axis, are not driven by conventional compound slides but by kinematically actuated steel plates that slide directly over the machine bed.

This design means that the distance between the turret centre-line and the point at which it is driven is shorter, minimising vibration while ensuring precise metal cutting, good surface finish and long tool life.

New factory unit

Unicut’s two Index C200s are fed with bar from an Iemca magazine; they operate alongside another fixed-head lathe and a sliding-head lathe to form a lean production cell in a new 4,400ft2 factory unit devoted to the Eaton contract.

A dedicated inspection room has also been added. This houses a Tesa Scan for measuring external dimensions and a Mitutoyo CMM for internal features.

Over 120 measurements have to be made on some of the valve bodies. Dimensional tolerances are as tight as 20µm, and a 0.05µm surface finish is required in the reamed and burnished bores.

Currently, 66 different bodies are undergoing PPAP (production part approval process), and full production is due to start at the beginning of September. Around 400,000 bodies, produced from bar ranging in diameter from 28 to 64mm, will be required annually in batches of 1,000- to 10,000-off. They will be delivered in a two-week lead time to meet Eaton’s just-in-time supply-line requirement.

The components will be produced 24/5 and at weekends ‘in one hit’ on all of the turning centres. The Index C200s, with their ability to take so much cost out of producing the components, are to be used as much as possible — especially for those valve bodies required in larger quantities.

High-pressure coolant ensures that the copious quantities of chips are removed efficiently from the working area.

To avoid damaging the finished components, a conveyor transports them (after ejection) from the counter-spindle to a Hydrafeed Rota-Rack parts accumulator.

The sliding-head lathe in the cell will be used for shorter runs of sub-32mm-diameter variants, while the other fixed-head lathe will be devoted to producing bodies towards the top end of the size range, which are also needed in smaller quantities.