Excelling at machining plastics

These materials present challenges when machining them — particularly the resulting long and stringy swarf

Posted on 08 Oct 2015 and read 2243 times
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Many sub-contractors offer to machine components from plastics as well as metal, but they are different disciplines and it can be difficult to excel at both. Plastic materials present special challenges, not least the long and stringy swarf that is normally produced during turning.

Chip-breaking is almost impossible, whatever tooling is used, except when machining acetals with relatively high feeds and speeds.

However, the Italian CNC lathe builder Biglia — represented in the UK by Kenilworth-based Whitehouse Machine Tools Ltd (www.wmtcnc.com) — offers a range of lathes that allows the sub-spindle to be offset downwards and to the front of the machine by up to 115mm, instead of being co-linear with the main spindle.

The main reason for Biglia providing this feature is to eliminate the possibility of interference between the two (or three) turrets when they are working simultaneously in certain configurations. However, Nylaplas Engineering (www.nylaplas.com) — based in Nailsea near Bristol — has identified an additional advantage that results from offsetting the spindles: while a plastic component is being turned in the main spindle, the coils of swarf, which tend to move horizontally to the right before falling away, do not interfere with the simultaneous machining of a parted-off component in the sub-spindle, as it is lower and — more importantly — offset horizontally.

If the spindles are on the same level, the right-hand machining area invariably becomes covered in swarf from the left-hand side, which is detrimental to second-operation machining, compromising both accuracy and surface finish.

On the other hand, the ability to move the sub-spindle back in line with the main spindle is also useful, as it allows the former’s B axis to come into play. A long component can be gripped and rotated at both ends to avoid it deflecting during turning and cross-milling or drilling.

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Nylaplas is a global supplier of thermoplastic machined components and stock shapes to the nuclear, defence, aerospace, pharmaceutical and semiconductor industries.

Technical director Andrew Bassett, who runs the family-owned firm with brother James and mother Leonora, said: “We discovered the particular benefits of this feature of Biglia lathes by chance when we installed our first model 15 years ago.

“It had two C-axis spindles served by two turrets with live tooling, and it proved very productive in the manufacture of a long-running job involving turning molybdenum disulphide-filled nylon bar into carriage rollers for a rail motion system.

“We needed to produce 120,000 of the 48mm-diameter x 25mm-long components per year. We only had a single-spindle single-turret CNC lathe at the time; we needed a more productive machine with in-cycle second-operation capability to lower the unit manufacturing cost and keep the work from going to China.”

The Biglia lathe managed to secure the carriage roller contract for Nylaplas, taking 15sec out of what would have been a 63sec cycle on the single-spindle lathe. Driven tooling was not needed for this particular job, but it has proved invaluable for a multiplicity of other work that has been put on the machine; most of this work has been in the 30-70mm-diameter range. Smaller components are generally produced on sliding-head lathes.

Tolerances can be as tight as 0.05mm, but a variation of a couple of degrees Celsius can cause some engineering plastics — and advanced thermoplastics — to expand or contract by more than that, so temperature management is crucial. One-hit machining is a major advantage in this respect, and most plastics are annealed before machining to stress-relieve them and help stabilise them.

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When the time came (in March 2015) to replace the Biglia lathe, Mr Bassett reviewed the market again but decided in favour of another of the Italian-built turning centres. A 10-axis B465 T2 Y2 equipped with an LNS Quick Load Servo short-bar magazine was bought. As the machine’s designation implies, it benefits from a Y-axis CNC movement on both turrets, each of which has 12 live stations.

An additional improvement on the latest Biglia lathe is a twin conveyor arrangement on the output side, rather than a parts catcher; a component emerging from the machine on one conveyor is directed onto a second conveyor at right angles that carries it into a waiting container at the front (by the time a bar remnant arrives on the first conveyor, the control has already told the second conveyor to reverse direction so that the bar end is routed to another container at the rear).

Control is provided by a Fanuc 31i-B CNC system, which allows shop-floor programming of even quite intricate parts — as well as 3-D simulation. More-complex components are programmed off-line, using a OneCNC CAD/CAM package.

“The Biglia machine’s design has proved ideal for machining our plastic material — including laminates, PTFEs and PEEKs — for a decade and a half, and continues to do so,” says Mr Bassett. “The technical input from Whitehouse has also proved useful.

“For instance, they showed us how to adapt a peck-drilling macro in the control to generate a chip-breaking action. It is good for roughing plastics, especially when carrying out balanced turning using a tool in both turrets. As an added bonus, they managed to find a buyer in Italy for our second-hand Biglia lathe, which was in good condition despite being 15-years old.

“It had only worked on a single-shift basis cutting plastics.”

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