CNC Rotary Table with Standalone Controller for Gear Hobbing

Hello,
I already have a Maho MH500 and a cycloidal lathe retrofitted to LinuxCNC, so I do have some practical experience with LinuxCNC retrofits.Now a colleague has approached me with the following idea:
He wants to build a CNC rotary table for his Maho MH600 in order to hob gears (gear hobbing).The idea is a rotary table with an “electronic gearbox”, synchronized to the machine spindle of the Maho.Key requirements:

• Maximum rotary table speed: ~300 rpm
• Servo or stepper drive with a gearbox, approx. 1:3
• The control should run fully standalone
• Only spindle position and spindle speed are taken from the spindle encoder
• The rotary table is synchronized to the spindle via electronic gearing

Questions / design considerations:

• What is the best choice of encoder for this application?
  Probably an absolute encoder? or better incremental for a better dynamic?
• Servo motor will most likely be NEMA 42 size
• Encoder setup ideas:
  • Incremental encoder on the spindle
  • Encoder directly on the rotary table
  • Possibly an additional encoder on the motor
• How should the control and feedback loops be structured for this setup?

I would appreciate any recommendations regarding:

• encoder types and placement
• control loop architecture
• practical LinuxCNC / Mesa hardware solutions for this kind of standalone rotary table

Just a few thoughts.

If for professional production, take it with a lot of caution. I only toyed around for hobby use.

300 rpm is fairly quick for a rotary axis.
How fast can the hob run during the cut?
The machine can tilt the spindle, can it?
You do not really need the spindle position, under the condition that the spindle is brought up to speed, before you start the cut and not stopped before it is finished.
A simple incremental encoder on the spindle probably works fine. More than 100 counts per rev and it might already work.
I run the rotary open loop, as long as the motor can keep up with torque and speed, encoder on the rotary is "nice to have", but not essential.

You can even get away with a semi electronic gearing for straight gears this will get rid of the computer altogether.


There is (was) an instruction on the Linux Wiki where the gear hobbing was explained.

The spindle syncronisation is independent from x-y-z axis, as long as you do not. a) want to cut helical gears b) want to run hob shifting c) want to try hobbing wormgears with tangential hobbing.