Feature Tools & tooling Optimising precision and tool life
Machining turbocharger impellers presents many challenges – high precision is required for the axial bore, and the thin impeller blades must also be machined accurately and without distortion. Mapal, a specialist in high precision machining solutions, has collaborated closely with Doncasters, one of Europe’s leading suppliers of impellers, to meet these challenges
are thus very easily distorted. When it first set up its machining operations, Doncasters approached Mapal to supply tooling, as it knew of the com- pany’s expertise and experience in providing efficient and effective tool- ing for unusual and demanding appli- cations. While the tooling supplied by Mapal at that time offered excellent performance, Doncasters – like all manufacturing companies – was always looking for ways to reduce pro- duction costs.
PCD tooling for the turning operations is delivering decisive benefits for impeller supplier Doncasters
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n response to the growing demand from automotive customers for finish machined parts, just over 10 years ago, Doncasters set up a machining operation. At that time, as now, one of Doncasters’ major products was impellers for use in turbochargers, which when in operation rotate at speeds of up to 250,000rpm. This
means that exceptionally accurate machining is essential. For example, the permissible tolerance on the fin- ished bore used to connect the impeller on the turbocharger shaft is just six microns.
Machining the blade profile pre- sents a special challenge as it involves and intermittent cut on blades that are often less than a millimetre thick and
Mapal T: 01788 574700
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Marking method facilitates punching tool set up
unching tools made by Trumpf can be set up even more quickly and easily thanks to the EasyUse marking concept. It eliminates the measurement work otherwise needed when installing re-ground dies and the required shims. Correct installation of a reworked die in the tool cartridge requires that the die’s upper surface be at the original height. This is achieved by inserting shims underneath. In the past, the operator had to measure both the shim and the reworked die and that involved considerable time and effort. Located at the edge of the EasyUse die is a scale showing graduations of 0.1mm. When the die is reground, a part of the scale is also removed. In this way the operator can easily determine how much the die has been reground and which shim is required. If the line edging the scale can be seen only as of the third graduation, then the die has been ground down by 0.3mm. Restoring the original position requires a shim 0.3mm thick. The thickness of the shim is also encoded, by way of holes, thus eliminating all measuring work. The shims exhibit one hole per 0.1mm of material thickness. This means that a shim 0.3mm thick is identified by its three holes. The EasyUse, a remarkably simple optical system, eliminates the measurements required in the past when installing the dies for Trumpf punching
tools.Trumpf ships dies from the Classic System and the matching shims all with the optical markings and at no additional charge to customers. Trumpf
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When approached by another tool supplier with tools that apparently offered a lower cost per part, it agreed to use them. It quickly became appar- ent, however, that the results were less than satisfactory. While the new tools were, on occasion, capable of deliver- ing results of the required standard, it was not possible to achieve these results consistently. At this point, Doncasters decided to reinstate Mapal as a key tooling supplier and the companies have been working together ever since.
In order to address the need for con- tinual improvement, Mapal has recently been collaborating with and part funding Rattanachai Rattanakit, an MSc student from Birmingham University who is sponsored by Doncasters and who is investigating ways in which PCD tooling can be used to cut production costs. To date, this collaborative effort has allowed the introduction of PCD tool- ing for the turning operations, and this is delivering decisive benefits. These include reduced cutting pressure, which minimises the risk of distorting the delicate impeller blades, improved surface finish and extended tool life. Work is ongoing to finalise the design of PCD tools that will bring similar ben- efits to the bore machining operations, a task that is made more complicated by the very small diameter of the bore, which in most cases is 6mm or less. Good progress is being made and the most recent prototype PCD drills and reamers delivered by Mapal already have the potential to outperform their carbide equivalents and to return a very attractive cost per part. “Almost every supplier claims that they approach and treat their cus- tomers as partners,” says Brendan Haffner, technical manager of Doncasters’ Coventry Operations, “so it’s not surprising that such claims are often taken with a pinch of salt. Our experience shows, however, that in the case of Mapal, the claim to be a partner really is justified.”
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