Success Story

Off-centreline machining on Dörries vertical lathes

Complete machining thanks to a simulated Y-axis

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."

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 Director Hubert Erz. "This saving increases with the size of the machine due to the increasing technical outlay associated with the additional linear axis."

Complete machining thanks to a simulated Y-axis.

All in all, there are eight major plus points

  • 1. Lathe with extras: The operator is able to perform milling, drilling and thread-cutting procedures off-centre on a vertical lathe with reduced additional technical outlay.
  • 2. Lower costs: Investment is reduced by around 30% compared to a traditional machine with a third linear axis.
  • 3. Small footprint: The installation space is halved.
  • 4. Reduced outlay for setup: Costs associated with creating the foundations are reduced by around 40%.
  • 5. No change to construction: Unlike normally, there is no need to convert a single-column machine into a portal machining centre.
  • 6. Low energy consumption: Less mass is moved compared to traditional Y-axis.
  • 7. Retrofitting options: A conventional Dörries vertical lathe can also be subsequently upgraded to include the functionality of off-centreline machining.
  • 8. Shorter delivery time for new machines: Reduced to 12–13 months (rather than 14–15 months for a traditional lathe with additional Y-axis).

Inexpensive retrofitting

"We recently created the first reference machine for a customer, which is currently being constructed," remarks the Sales Director. "From speaking with potential customers, it is clear that there is a considerable level of interest in this solution. But this also extends to customers who already own a vertical lathe, or those looking to upgrade." Upgrading is a particularly appealing option for subcontractors, since the simulated Y-axis offers them an inexpensive and technically-straightforward expansion of their range of services. This additional option is suitable for companies in a range of sectors — from wind energy to oil and gas. Erz adds, "Our first customer will produce torque supports for wind power transmissions on the reference machine. This new option is yet another example of how Starrag stays true to its claim of 'Engineering precisely what you value'. And sometimes, less is more."

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