Folks, I would like to convert a universal mill to CNC for the specific purpose of low volume gear hobbing. The machining spindle to rotary table axis must have a very specific ratio, which is dependent on the number of teeth to be cut on the blank. Some people do this mechanically using drive shafts, gear reduction and "master gears" for the specific number of teeth needed.

Others use a good rotary encoder on the spindle holding the hob and feed the encoder output directly to the input of the motor drive unit for the rotary table holding the blank, with fixed ratio gearing to step down the speed some amount that then allows them to use a specific master gear in the chain where the teeth on the master gear match the teeth that will be cut on the blank. This is to avoid the complication of having a computer control the motion.

The next possibility is to use a servo motor on the rotary table which is synchronised to the speed of the machining spindle and the speed ratio between the 2 axis is controlled via a CNC controller. This needs to be high reliability, similar to how an electronic leadscrew works on a lathe. So most of the discussion I have seen has been around Linux CNC which is considered to be better than Mach or other windows based software. But in looking at Linux CNC and its supported hardware, they start with printer ports (long obsolete) and then go on to PCI card based solutions but these would not be compatible with a NUC format computer (I would not like to run a big old desktop computer in this day and age...)...

So Centroid appears to bypass the entire OS limitation by using a dedicated computer optimised for motion control with what could be considered a real time OS. I can see how this should be much more reliable than anything running on a PC or Mac.

What I do not know is how the sort of synchronised motion is done for gear hobbing, where one needs a defined ratio between the 2 axis to form the gear. Would this be done by a specific G code function ? On the universal mill itself, its only the spindle speed and rotary table motion that is critical. The table is angled to the alpha angle given on the hob (for a spur gear), the spindle rotated at cutting speed and the rotary table speed will be matched according to the programmed speed ratio. Feed of the cutter across the gear can be done with a power feed or it could be a cnc axis (feed rate) and the Z axis defines depth of cut, could be manual, or another CNC axis.

If anyone has any insight, it would be appreciated. My plan is to buy a new small universal mill, so that one has an accurate mechanical system ($17500 and up for the Knuth universal mills) and then add just what is needed for gear hobbing, which seems to be the rotary table with servo drive and all the electronics and code to make that work. If I add servo control to the 3 main axis, it would allow the mill to be used as a small CNC mill when the table is set at 90 degrees to the saddle. The table should be more than big enough for the parts I have in mind which would be aluminum castings. Volume no more than a couple hundred parts a year.