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 are imaginative.

"The idea to simulate a Y-axis came to mind some years ago in our application technology," explains Dr. Marcus Queins, technical director of Starrag Technology 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 expertise 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."

Always perfectly aligned with the workpiece

The simulated Y-axis creates a multitude of possibilities for production managers. Drilling and cutting threads which do not point towards the centre of the table is suddenly possible with a Dörries vertical lathe. And it is also possible to machine grooves with axially-parallel, off-centre lateral surfaces. Though all new off-centre machining processes have a common denominator.

Dr Queins explains: "The coordinated rotational movements of the two C- and CY-round axes ensure that the tool is always correctly aligned with the workpiece."

Though it is not just these technical aspects - the simulated Y-axis has even more cards up its sleeve. Staff in Mönchengladbach analysed the benefits on two standard vertical lathes of type VCE 2800 and VC 3500 with a swing diameter of 2,800 mm and 3,500 mm. "The investment turned out to be around 30% less compared to a machine with an additional linear Y-axis," summarises sales manager Hubert Erz. "This saving increases with the size of the machine due to the increasing technical outlay associated with the additional linear axis."

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