Special combination boring head solves friction weld problems

The development of a special combination boring head that incorporates a trepanning and counterboring process by Mollart Engineering's production tooling team has enabled tool life to be increased by at least 50 per cent, created a more consistent roughing application in the bore of an aerospace component and enabled previous high specification inserts used in the cycle to be replaced with standard units at a quarter of the price.

The revised operational procedure that is now carried out at the Chessington, Surrey facility, on a forged aerospace turbine shaft that has a friction welded joint fusing the two ends of different aerospace alloy material tubes, overcomes the influence of a heavily distorted mushroom form that is created at the internal seam. The new tool design also solves machining problems created from any difference in the two alloy material specifications as well as countering any mismatch and eccentricity between the two joined tubes.

Prior to the new tooling being applied, the shock effect caused by a combination of interrupted and inconsistent depths-of-cut meant tool life was totally unpredictable despite the use of special carbide inserts. Said Mollart's technical director Chris Barker: "The material is very abrasive and also can have heavy scale generated from a previous heat treatment process. Despite having a very rigid Weiler E90 CNC lathe the boring tool was not robust enough to fully support the machining task even after considerable time was devoted to developing the process."

To overcome the problem, which involves two parts per week having different bore sizes to be processed as part of a long production lead time involving multiple operations, Mollart's tooling engineers developed two bespoke boring heads to cater for each version of the aerospace shaft.

The tubular tool solution combines a trepanning and counterbore action which uses three carbide guide pads set on the outside diameter of the tool just behind and forming quadrants in the holder with the single indexable cutting tool insert. As a result the cutting insert is fully supported in the component bore creating a very stable set up.

The two sizes of cutters are 55 mm and 52 mm diameter with a single insert having a 10 mm face width. This arrangement allows the insert to create the bore in a single pass using a trepanning action to start to remove the mushroom form created by the friction weld process. Following the engagement of the indexable insert with the excess material to be removed from the bore, the three carbide pads enter the machined portion to provide added support.