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Vacuum pump casings machined in half the time


20-06-2013

Author: Admin

Edwards / NCMT synergy realises a step improvement in milling performance
 
Production and assembly of vacuum equipment designed by Edwards Group plc (formerly BOC Edwards) has been transferred from the UK to Korea and the Czech Republic. The company's former factory in Burgess Hill has been transformed into its Group Technical Centre, becoming the world's leading vacuum science R&D facility, which includes a machining process development centre.
 
A notable success of this centre in early 2013 was the development of an alternative process for machining the chambers of a new, multi-stage vacuum pump that enables semiconductor, pharmaceutical, petrochemical and other manufacturers to use considerably less power in their production operations.
 
According to Senior Manufacturing Engineer at Edwards, Reuben Day, the time needed to rough- and finish-mill the chambers in the two halves of the pump's SG cast iron stator has been reduced by a massive 94 per cent. The process has been validated on a Makino horizontal machining centre, three of which exist in the Machining Process Development cell for carrying out R&D cutting trials at Burgess Hill.
 
Edwards has had a long term relationship with Japanese manufacturer's UK agent, NCMT, which was called in to advise on how to reduce the unacceptably long cycles required for milling the chambers in the new pump.
 
Coincidentally, NCMT was appointed last year as UK agent for Kaiser boring bars, made in Switzerland, which Edwards has been using for many years. Engineers from both NCMT and Kaiser visited Burgess Hill under strict non-disclosure agreements. Discussions centred on the single-point cutting tools that were being used to machine the radius and faces of the swept volumes that form the chambers in the inlet and exhaust castings.
 
Fortuitously, Kaiser works closely with German milling tool and indexable insert manufacturer, Avantec, which specialises in producing bespoke cutters for difficult machining applications, often involving high positive rake tooling to reduce cutting forces and produce top quality surface finishes. It was clear that such a tool would be ideal for machining the swept volumes in Edwards' vacuum pump.
 
Three side-milling cutter bodies of different diameters were supplied to accept 10 mm wide, ground, carbide-coated indexable inserts for either roughing or finishing, the latter inserts being developed especially by Avantec for the application. The smaller cutter accommodates five inserts on each side while the other two bodies have six inserts per face, all secured in deep pockets that expose only the edge of the inserts.
 
The top cutter face contains right handed inserts for machining one side of a chamber while the reverse of the tool has left handed inserts for machining the opposite side of the chamber. To avoid an insert being placed on the wrong face of the mill, Avantec has coloured the right handed, top inserts gold and the others black. All finishing inserts have special wiper geometry to achieve the 1.6 Ra surface finish in the chamber.
 
Avantec applications engineers also visited Burgess Hill to conduct trials on the Makinos, which lasted three days, to ascertain how best to mill the two faces and diameter of the chambers, each of which is a different size. Not only was surface finish important, but dimensional tolerance even more so in order to achieve sub 100 microns clearance between the chamber surfaces and the rotor that is subsequently fitted.
 
The final, optimised results, after Mr Day redesigned the tool holder’s back end to be a one-piece unit, were a cycle time reduction of 96 per cent to rough the chambers from cast and a 93 per cent reduction to finish the same features.
 
Previously, roughing was 30 per cent of the total cycle time using a conventional side-and-face milling cutter capable of holding three 6 mm wide inserts per side. The problem was that inserts had to be removed to leave one on each face, as only in this way could chatter be avoided and surface finish preserved. So essentially, the tool was fly-cutting each side of the chamber.
 
The finishing cycle was 34 per cent of the total cycle time and again a fly-cutting method was employed, using a tool with a single brazed carbide insert made at Burgess Hill.
 
The new process using the Avantec cutters consumes just 6 per cent of the original cycle times. As chamber milling (roughing plus finishing) used to account for around two-thirds of total program run time on a casting, the 94 per cent reduction translates into a 60 per cent reduction in the overall cycle.
 
Mr Day commented, "We are working to design intent and using production fixtures.
 
"After the process has been transferred to our Korean factory, machining will be carried out using our tooling package and programs on state of the art Makinos, so capability is assured.
 
"Edwards has the same Zeiss co-ordinate measuring machines in Korea as we use here in the UK, so 100 per cent inspection of the production castings will be carried out to the same standard as in our R&D facility."
 
In developing the new, highly successful machining process, what really focussed the minds of the team at Burgess Hill were the tight tolerances that have to be held in a shop floor environment. In addition to sub 100 microns stator / rotor clearance, they include: ± 30 microns on the overall length of the casting; ± 16 microns on dimensions from the datum face; 25 microns squareness; ± 30 microns on the bores; and ± 10 microns on dowel hole positions. The latter two features are machined with Kaiser tools.
 
In traditional manufacture of multi-stage, dry vacuum pumps, separate stages each containing one chamber are dowelled and bolted together. Edwards' one-piece design comprising two mating castings avoids the risk of cumulative errors as well as the cost of assembling the stages. However, it creates the problem of how to access the internal chambers with long-reach tooling while avoiding deflection, vibration and consequent loss of accuracy.
 
On the shop floor in Korea, the tolerances will be maintained by the robust tooling solution developed at Group Technical Centre in Burgess Hill combined with strict maintenance regimes and the inherent accuracy of the Makino horizontal machining centres, which have temperature controlled ballscrews, slideways, spindle and coolant.

In conclusion, Mr Day said, "This application is an example of innovation and engineering efficiency excellence. We have developed a high added value machining process for cost effective production.
 
"It is a benefit to have a dedicated, fully equipped R&D facility to carry out this type of work, avoiding the need to interrupt production to borrow valuable time on production machines.
 
"Under those circumstances, we would almost certainly have entered production of our new pump with a significantly longer cycle time and less capable process for machining the casing.
 
"That would have meant further capital investment further down the line as production volumes ramped up, which would have ended up costing us a lot more money in the long run."
 
The next project Mr Day and his team will undertake in the machining process development centre is to optimise production of two key components for the pump rotors, which will be machined on multi-axis CNC mill/ turning centres in Korea.
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