Multi-spindle turn-milling at ATP

Sub-contractor is planning to have nine ‘multis’ in an area that is currently a cam-auto shop

Posted on 15 Oct 2015 and read 3493 times
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In 2012, turned-parts sub-contractor Auto Turned Products (ATP) — a long-time user of cam-type multi-spindle autos and CNC single-spindle lathes — decided to evaluate CNC multi-spindle turn-milling. A German-built Index MS 32 six-spindle CNC auto with a 32mm bar capacity — and an opposed spindle for end-working — was bought.

Since that time, the machine — supplied by Index’s UK agent Geo Kingsbury Ltd, Gosport (www.geokingsbury.com) — has been largely dedicated to producing a specific family of automotive components in batch quantities ranging from 250,000 down to 10,000; this work would otherwise have been done by six single-spindle lathes.

ATP’s experience with the first CNC multi was so positive that it decided to standardise on this type of equipment for all turning-machine purchases in the foreseeable future. It installed a 40mm-capacity Index MS 40 in July 2014 and a second similar model in August this year. Eventually, the company plans to have nine Index CNC multis installed in an area that is currently a cam auto shop within its factory at Round Spinney, near Northampton. All of the company’s 40 single-spindle cam-type lathes were sold or scrapped in March 2015, and the stock of 22 multi-spindle cam autos will be gradually reduced, although a few will be retained to carry out simple long-running jobs.

Production engineer Vic Pais, who is in charge of all machine tools at the factory, described the transition to CNC multi-spindle machining as extremely rewarding. He added that ATP would have struggled to cope with the significantly raised level of throughput at the factory without the high output from the CNC multis.

Financial manager Eddie Craddock says the first Index multi paid for itself within two years, adding that “buying a CNC multi involves a considerable investment, so embarking on a business plan to own nine of them is not for the financially faint-hearted.

“Nevertheless, we have calculated that investing in these machines is justified, not only because the high output reduces unit production costs but also because of the speed of programming and set-up, which reduces the economical batch size to the low thousands. Around 80% of our work is for the automotive sector, to which the Index MS machines are devoted. They produce batches of components down to typically 10,000-off for prestigious marques that include Jaguar, Land Rover, Rolls-Royce, Bentley and Aston Martin. That said, we are now targeting petrochemical work, because the difficult materials used in that industry play to the strengths of the Index multis— and we would consider runs as low as 3,000-off because the machines are so quick to change over.”

Confirming this, Mr Pais said: “If the same bar is used, a setter can re-tool an Index multi for a new job in a couple of hours, which increases to about half a day if the bar size needs to be changed over as well. The equivalent time for a cam multi could be two weeks.”

In the 50-plus years since it was founded, ATP has used a wide range of turning machines, including cam-type and CNC multi-spindle autos, as well as twin-opposed-spindle and single-spindle lathes — both sliding-head and fixed-head types. The company says that, generally speaking, the output from one CNC multi is equivalent to that of three or four single-spindle CNC lathes, but within a smaller footprint and with lower operator overhead.

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Comparing a CNC multi with a cam-type multi, Mr Pais says the former can produce more-complex components three-times faster than the latter. This calculation takes into account the 95% up-time of the CNC spindles compared with 60% for a cam multi — and the latter’s long resetting time. For a given output, the factory space taken up by Index multis is three-times less than that required by cam multis — plus there is a consequent reduction in power consumption. Furthermore, for the same output, there is a three-fold saving in labour when comparing the Index CNC multi with cam-type multis. This will lower ATP’s head-count as time progresses, and a policy is already in place of not replacing staff who retire.

Mr Pais adds: “These are not the only benefits. The quality of finish on machined components is far superior on an Index multi compared with all other lathes we use. Unlike with CNC multis, cam-type equivalents suffer from having to use the same feeds and speeds at each spindle position, compromising cutting conditions at some stations — and they can’t perform second operations. They also have to use costly form tools to machine certain features that are easily produced on a CNC equivalent using single-point cutting with low-cost indexable inserts. “For example, we transferred the production of fairly complex ‘body mount’ spacers in S355 steel to the MS 40. Previously, form tools wore out quickly on the cam multi and the surface finish deteriorated, so we sometimes found ourselves producing scrap, whereas the Index multi machines them without a problem.”

Vastly longer tool life on an Index multi — sometimes by a factor of 30 or more compared with all other types of lathe — is another significant advantage for ATP in terms of both reduced tooling costs and less production down-time
for replacing cutters. The harder and tougher the material being machined, the greater the benefit. Tool longevity is partly a result of the Index multis’ 80-bar through-tool coolant (with particle filtration to 20µm), and also down to the rigidity and vibration resistance of the machine construction.

Mr Pais highlights a particular job where an M14 tap performs 100,000 operations on an Index multi before it is routinely replaced, even though it would operate satisfactorily for longer. The same job on a cam multi requires the tap to be replaced every 3,000-4,000 parts. In another case, an end mill lasts for 2,000 components when machining steel components on a sliding-head lathe, yet it cuts 21,000 parts on an Index multi. A further instance is an insert that can part-off 300 components on a slider but 4,000 on a CNC multi.

Not only is productivity increased through fewer tool changes, there is also less bar wastage, as cutter reliability reduces the chance of the tool breaking and producing scrap. Stock savings can be substantial, especially when tool breakages are avoided during unattended running.

In conclusion, Mr Pais said: “Index multis are the future for our company. They inject big numbers of high-accuracy components into our system in a short space of time and are economical for shorter batches. The only problem we have is loading 3.2m bar or tube into the six-channel bar magazines quickly enough, as the machine gets through stock at an incredible rate, especially when a relatively simple part is coming out every few seconds.”





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