Complete machining in a single clamping operation on a vertical lathe — without an additional linear Y-axis? There is no denying that reports from Starrag experts on converting a Dörries vertical lathe into a machining centre using double C-axis interpolation sound a little fantastical.

"The idea to simulate a Y-axis came to mind some years ago in our application technology," explains Dr-Ing. Marcus Queins, Technical Director of Starrag Technology GmbH at Mönchengladbach. "We've now made it a reality in a project for a customer from the wind energy sector (gear manufacturer)."
 

Electronically-controlled interplay

Ordinarily, all machining processes on a lathe are oriented towards the centre of rotation; only two infeed axes - the X-axis and Y-axis - are used. If machining is also required in the Y-direction, an additional third linear axis, the Y-axis, is usually necessary. The alternative to this involves rotating two C-axes which are synchronised with one another. This is achieved thanks to the electronically-controlled interplay of the rotary table (C-axis) with an axially-parallel CY-axis (angle head with an NC-axis rotating around the Z-axis).

This is where double-C-axis interpolation comes into play. The linear Y-axis is thus achieved using simultaneous interpolation of the round table C-axis and the CY-axis. It is certainly not fitting a square peg into a round hole, but the method does have a sense of mathematical wizardry about it. Dr Queins explains: "The intelligent interplay of the C- and CY-axes transforms two circular movements into one linear motion. When combined with the X- and Z-axes, a lateral surface on a workpiece, for instance, can be milled off-centre thanks to four-axis interpolation."