CitationDownload as .RIS
Emerald Group Publishing Limited
Copyright © 2001, MCB UP Limited
Easier five-axis machining for TTL
Easier five-axis machining for TTL
Keywords Traub Heckert, TTL, Machining
The business of five-axis machining is said to be very brisk on the Aylesbury doorstep of Toolroom Technology Ltd, better known as TTL, which was set up some 12 years ago to exploit a market niche in support of 3-D machining based manufacturing solutions. Founded by managing director Ben Dix, it now has a long list of system development under its belt for the manufacture of anything from golf balls to turbine blades. However, aerospace related applications tend to predominate due to the technology focus of that business.
As the complexity of end-user challenges has increased, so the need for more precise multi-axis manipulation of cutter paths has arisen. This in turn, we understand, has led TTL to invest in two Hermle C800U five-axis machining centres (Plate 1) supplied by Traub Heckert of Brackley.
Plate 1 Two Hermle C800U five-axis machining centres are key to 3-D machining at TTL
The nature of the subcontract work undertaken at TTL tends to be highly specialised and mostly having a very high value added content. Batch quantities are small, tending to range between one and four and materials are predominantly titanium or nickel alloys. Accuracy requirements demand repeatability in the sub-10 micron arena while most clients also demand a very fast turn-around.
Ben Dix explains:
We tend to undertake the sort of assignment that other companies either do not have the facilities or in-house expertise. The subcontract side of our business, for instance, requires us to perform a substantial amount of machining of "difficult" materials for Formula 1 car and engine builders and our main area of expertise has become the development of software-dependent machining processes for use by third parties. Much of this work relates to gas turbine blade machining for which a high level of expertise in five-axis machining is essential.
A key development by TTL, we are told, has been the successful configuration of "adaptive manufacturing" software for machining similar, but not identical, components. This involves detailed probing of each workpiece to establish its exact parameters, from which the data are then fed into an on-line programming system to generate a bespoke CNC part program for that component.
One highly successful demonstration of this capability is the engraving of peoples' signatures or company logos onto the shell of a raw egg using one of the Hermle machining centres installed at Aylesbury. A similar, but more rewarding, exercise is the use of the same equipment to re-shape the end of a turbine compressor blade following the rebuilding of the worn portion by weld. In this case, TTL's adaptive software is used to program the Hermle to produce the shape by developing a best-fit shape to blend with the original blade form which tends to distort in service due to the high temperatures and pressures that it encounters.
"Around 30 million aero engine turbine blades are reconditioned this way every year", comments Ben Dix, "but in most cases the blending is a manual process carried out with a bench grinder. Our machining capability produces a far more consistent end product and also removes any health risk associated with manual grinding".
To develop the process, TTL needed its own five-axis machining facilities. Although the company has carried out a lot of five-axis development work, it was always reliant on access to clients' machines for trials. Ben Dix explained:
Really for the blade reconditioning project we needed our own facilities as many of the clients are based overseas. That ultimately led to our scouring the market for the best machine and the installation of the first Hermle C800U. At the time of purchase, we anticipated having spare capacity for subcontract work but since installation, the amount of five-axis related development work has increased to the point where we really needed the second machine just to fulfill our subcontract commitments.
Although the company had no five-axis facilities of its own, paradoxically it had a great deal of five-axis machining experience to draw upon when selecting a machine. The easy option would have been to purchase a three-axis vertical machining centre and equip it with a two-axis CNC indexing device for a three plus two configuration. This was rejected for the very reason that a setup of this kind can demand considerable axis movement to achieve a relatively small cutter path increment.
Maintains Ben Dix:
One of the basics applicable to successful five-axis machining is to keep everything as simple as possible which is a major advantage of the Hermle machine design. The below centre line swivelling trunnion table configuration keeps the set up very rigid and allows full machining access to all sides of the workpiece while minimising the number and complexity of axis movements. Similarly, all 800mm by 600mm by 500mm linear axis movements are kept behind the back wall of the machining area.
He goes on to describe how the M 128 function within the Heidenhain TNC 430 contouring control system provides programmable control of the cutter contact speed. "This feature allows far more enhanced control of the cutting process which results in smooth cutter trajectories. It is a major contributor to the success of demonstrations such as the machining of the eggshell".
The construction of the Hermle C800U is praiseworthy maintains Ben Dix, from the stability provided by the granite bed to the use of aluminium crush washers to support the spindle. "These provide very effective collision protection for the spindle and have saved us from a major repair bill on at least two occasions", he admitted.
Both machines are fitted with 15kW, 15,000 revs/min spindles, which provide a useable torque over a wide range of cutting speeds. HSK short taper spindle tooling is used and TTL has recently standardised on shrink-fit tool clamping. This not only reduces the cost of toolholders but the narrower tool shank offers improved machining access on blisk and engine port work.
Due to the suitability of the machine design, the turbine blade refurbishment project has led to a number of overseas sales of identical Hermle C800Us and Ben Dix qualifies that TTL's subcontract activities have gained significant benefits from availability of the machines. "They can also be used for three-axis cutting on those occasions when the full five-axis capability is not required", he maintained.
Details are available from: Traub Heckert UK Ltd. Tel: +44 (0)1280 705482; Web site: www.traub-uk.com