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Copyright © 2004, Emerald Group Publishing Limited
Reductions in turbine component grinding costs
Reductions in turbine component grinding costs
Keywords: Turbines, Grinding, Cost reduction
Using a technique known as Viper grinding for the production of turbine blades and nozzle guide vanes, subcontractor Teleflex Aerospace Manufacturing UK reports that it has reduced multiple operations on up to ten different machines to one or two set-ups on a Makino A55 horizontal machining centre, supplied by NCMT. Savings are so great that Teleflex Aerospace has been able to offer price reductions to its customers and still recoup the capital investment in 18 months (Plate 1).
Plate 1 Working area of the Makino A55 at Teleflex Aerospace, showing a nozzle guide vane that is now machined in two clampings using the Tyrolit Viper grinding wheel. Diamond rolls for dressing the wheel are mounted on the rotary table
One of the very few quantum-step advances in production technology in recent years, vitreous improved performance extreme removal (Viper) grinding was developed by Rolls-Royce and is capable of stock removal rates up to eight times those achievable when conventionally grinding nickel alloys using a plated CBN wheel. Very high pressure coolant at 100 bar has to be directed at the point of grinding, so the machining centre manufacturer must take care to strengthen the guarding to protect the operator. Slideway covers must also be modified to prevent ingress of swarf and coolant, which would otherwise quickly damage the machine.
Said Phil Chadwick, Customer Service/Quality Manager at Teleflex Aerospace in Dunnockshaw, near Burnley, which is believed to be the first subcontractor in Europe up to use Viper grinding, “We felt that the Makino, with its double skin construction and specially guarded slideways, was the best machine to use and so it has proved. We also liked the professionalism of NCMT's support team, especially the sales staff and applications engineers”.
The first “engine” set of 16 nozzle guide vanes for a new customer was completed in just 6 weeks of the machine going live, allowing Teleflex Aerospace to meet a tight delivery schedule. Using traditional grinding techniques, a cell containing ten machines would have been used in employing five operators to clamp every part once on each machine. It would have taken 2 weeks to set-up and the TAKT time (longest operation) would have been about 30 min (Plate 2).
Plate 2 Close-up of the nickel alloy nozzle guide vane and the grinding wheel
However, on the Makino Viper grinding centre, which was available for use immediately after the program had been written, the vane was machined complete in two clampings. It took 26min to machine the first side and 20 min for the second, with just an hour to change over. Moreover, only one operator was needed.
The second new component produced on the machine was a shroudless turbine blade for a new customer in the US, which placed the order as a direct result of learning from the Teleflex Aerospace sales team that the company was using Viper grinding and offering significant price reductions.
The blade is made from a nickel-based, high temperature alloy casting. The fir tree root form is ground in a single hit on the Makino, including everything under the platform, i.e. the leading and trailing edge rhomboids, lock plate grooves, root end face and key peg, and the under-neck area.
Set-up time for the part was 2 days, including offline programming using Unigraphics CADCAM, but the next time the job is put on the machine for a repeat batch, it will only take an hour to set. The single machining operation took less than 15 min.
Mr Chadwick contrasts this process with the conventional approach, which would involve six or seven machines, taking 3-4 days to set. The TAKT time would be longer at 20 min. In addition, it would tie up three operators rather than one. He continued, “Both new jobs, and a couple of others we have recently taken orders for, have been won as a direct result of offering customers lower prices. I don't think we would have been awarded the contracts otherwise. We are quoting a lot of other work at the moment and are hopeful of receiving a regular supply of new orders. The whole Viper project is the cornerstone of an initiative started by our Managing Director, Chris Turner, to grow the business here by 30 per cent in 18 months, starting from October 2003. In addition to increasing our aerospace work, our goal for 2004 is to manufacture in-house our medical division's components that are currently sub-contracted out. We have an option on a second Makino A55 Viper machine, which we hope will be installed this summer”.
The company's vision is to set-up a two- machine, conveyor-fed cell with robot loading, five-axis deburring, washing and shop floor inspection by a co-ordinate measuring machine (CMM). If things go well, several such cells will be created and older production plant sold off.
During the evaluation phase, the Teleflex Aerospace team in Dunnockshaw worked closely with one of its major customers, Rolls-Royce, in particular the new “high pressure” facility in Derby where Makino A55s have been Viper grinding turbine and compressor parts for some years. It was clear that adoption of this new technology was the way to go.
So in May 2003, Teleflex Aerospace took the plunge and ordered a Makino A55 five-axis, horizontal machining centre configured for Viper grinding. One modification was specified, namely offsetting of the rotary A-axis by 60 mm from the centreline of the rotary B-axis, allowing larger workpieces to be machined.
Aluminium oxide grinding wheels of 280 mm diameter from Tyrolit are used in the main, although smaller diameter, CBN wheels are being evaluated for machining tighter features. Up to 34 wheels are exchanged automatically from the 72-position magazine, which also contains metalcutting tools. The guide vane, for example, requires drilling as well as grinding. Similarly, the under-neck area on the shroudless turbine blade is ground and milled in automatic cycle.
Continuous dressing of the grinding wheel during production is available as an option, but Mr Chadwick feels that high volume production is needed to justify the extra cost. At Dunnockshaw, the wheels are preset off the machine to minimise the amount of subsequent form dressing required when the wheel periodically visits Tyrolit diamond rolls mounted on the rotary table beneath the workpiece.
Repeatability has improved with Viper grinding due to fewer clampings. Mr Chadwick said that as there are no longer repeated set-ups on different machines, cumulative error does not use up the drawing tolerance, so it is rare for a part to be out of tolerance. Scrap is therefore reduced; indeed, 100 per cent CMM inspection of the first 160 nozzle guide vanes showed that there were no rejects or even any “concessions” (parts out of tolerance, but still acceptable).
Concluded Mr Chadwick, “Machining always used to be the bottleneck here, but no longer. If anything it tends to be delivery of the castings, especially when they are 'free issue', as we do not control their supply. I can see the speed and accuracy of Viper grinding transforming this business over the next few years.”
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