Auto Turned Products (ATP) specialises in turned parts, presswork and assemblies for major manufacturers in the motor industry. Indeed, it is a preferred supplier to Jaguar Land Rover and has Rolls-Royce, Bentley and Aston Martin among its customers.
Operating from 55,000ft
2 premises in Round Spinney, near Northampton, the company celebrated its 50th anniversary last year with the purchase of a 32mm-capacity Index multi-spindle CNC automatic from Gosport-based Geo Kingsbury Machine Tools Ltd
(www.gkholdings.com). CNC production engineer Vic Pais says: “Before we bought the CNC multi, I read an article about a UK sub-contractor that runs four similar machines with just one operator. I thought it was a publicity stunt, but it is true; one person can look after that number of Index multi-spindles once they are set up and running — despite their apparent complexity.
“Each machine offers the production capacity of three to four CNC twin-opposed-spindle turning centres in a compact footprint, so one operator can preside over a production output that is equivalent to that of 12-16 lathes. It would require four operators to run that number of sliders or fixed-head bar autos, added to which the machines would occupy much more space on the shopfloor — and consume more power.”
While reduced labour cost is one compelling reason for investing in a CNC multi, another is dramatically prolonged tool life — in some instances by a factor of 10. Moreover, the harder and tougher the material being machined, the greater the benefit. For example, an £11 end mill lasts for 2,000 components when machining steel components on a sliding-head lathe at ATP; the same tool can cut 21,000 parts on the Index MS32. Another instance is a £12 insert that can part-off 300 components on a slider but 4,000 on the CNC multi. Mr Pais says tool longevity is partly a result of the MS32’s 80-bar through-tool coolant (with particle filtration to 20µm) and partly down to the rigidity and vibration resistance of the multi compared with the lighter CNC sliders.
Increased profits
Mr Pais also says that, apart from the cost savings resulting from buying fewer cutters, two related benefits increase the bottom line further for any given production run. First, longer tip life on the Index multi means fewer tool changes, reducing down-time. “Two hours can be saved by not having to swap an end mill nine extra times; and the same applies to other tools, resulting in significantly more parts in the bin at the end of a shift — the more so when cycle times are short.
“Second, bar wastage is lower, as a 10-fold increase in tool life cuts the chance of producing scrap by an order of magnitude. Savings can be substantial, especially when tool breakages are avoided during unattended running.”
To reinforce the advantages of reduced machine stoppage and bar wastage on the CNC multi, ATP uses a piece counter for each tip, adjusted according to the type of material being processed and the cutting conditions. Tool replacement before failure has proved highly effective at maximising production output.
Based on savings in the three key areas of machine down-time, tooling and labour, when comparing a twin-opposed-spindle CNC lathe with the Index MS32, ATP calculates that the multi will pay for itself in two years, based on running 80hr a week. The company’s shift pattern allows for operation 100hr a week, so the amortisation period could be even shorter.
Driving investment
One ATP component in particular, which is fairly complex and required in large quantities, has been instrumental in driving the company’s machine tool investment. It was originally produced on cam-type multi-spindle autos, of which ATP still has 22 operating. The company was prompted to invest in twin-opposed-spindle fixed-head CNC lathes (of which there are 20 on site) due to the long set-up time on the cam multis, which was anything from three days to two weeks. Compromised component finish and an inability to perform second operations on the cam multis when machining complex parts presented further problems.
The fixed-head machines worked well, but the increased speed and large tool capacity offered by similarly configured sliding-head lathes sent ATP along that investment path in pursuit of greater efficiencies. There are currently 25 twin-opposed-spindle CNC sliders producing parts at ATP. The Index CNC multi is thus the fourth method to date for producing these complex parts; it is also the most cost-effective.
Mr Pais quotes comparative one-hit machining figures to support this statement, based on a couple of components for which PPAP (production part approval process) has already been completed on the Index MS32. Compared with machining an EN1A leaded-steel part on a CNC slider, the six-spindle Index multi delivers it in less than a third of the time. In another example, an EN16T high-tensile-steel component requires a run time on the MS32 that is 77% shorter than the cycle time on another make of sliding-head turning centre.
Increased up-time
This is not the whole story, however, as up-time on cam multis is only around two-thirds, whereas the spindles on the CNC multi run for well over 90% of available time. Factoring in machine efficiency, the MS32 is well over four-times more productive than its cam-type counterpart. “The Index multi can produce large numbers of components in a short space of time. The only problem we have is feeding the 3.2m bar or tube into the Iemca magazine quickly enough,” says Mr Pais. “The machine simply eats stock — particularly when a relatively simple part is coming out every few seconds.”
That said, installing a CNC multi involves a change of mind-set due to the overall investment required. Mr Pais says that changing over the MS32 for new work can take up to half a day, if a new size of bar is involved.
However, the machine is back in production in minutes if it is a repeat job from bar of a similar size. Under such circumstances, the economical batch size is appreciably lower. Indeed, Richard Kingsbury — managing director of Geo Kingsbury Machine Tools — says that some customers in the UK are re-setting these multi-spindle machines daily for batch runs of fewer than 8,000 parts.
Compared with a cam multi, a further advantage with CNC multi-spindle turning is the ability to set feeds and speeds independently at all spindle positions, so that optimum cutting conditions can be maintained at each. Moreover, every C-axis spindle is served by two tool slides that can undertake boring, internal turning and external turning, so the user is not restricted to designating one type of tool in any position; this helps to optimise flexibility and reduce machining times.