Weiss VM32 Mill Conversion

[picclock]

As requested my take and decision making on :-
Controllers,
I have chosen to go with a standalone option, the DDCSV 2V1. This is a 4 axis controller integrated into a single unit which runs Gcode. It incorporates a colour screen and keypad, providing the signals for the stepper drivers, and inputs for the limit/homing switches. The advantage of this approach is cost, simplicity, reliability and mechanical robustness, as its all solid state. The gcode is written to a USB stick and plugged into the controller. There are no software licence fees or internet connection needed for licence verification, no spinning hard disks to fail or corrupt, no fans to fail, no external interface compatability issues . Additionally the performance of the box is outstanding with stepper rates on all axis up to 250KHz and pendant support thrown in. In size terms the box is about the same size as two cd rom drives ~ 160x100x50mm. It requires an 18 - 32V supply which can be derived from a dedicated supply or the stepper psu. Power requirements are very modest at up to 0.5A max (12 watts or so). Inputs are opto isolated, and it incorporates a VSO for motor speed control. I have no financial or other interests in this company, just a user.

Motors and ballscrews
How much force do I need ? How quickly do I want to go ?.
Mill cutting forces* for metal under maximum load measured at ~ 50-60kgs, with smaller cutters around ~20kgs or less.
How fast can the motor turn while producing this force?. Stepper motor specifications generally only quote the holding torque - useful only if the motor is not turning. Getting a torque curve for the motor you are going to use is paramount - if you cant get one don’t buy it !!. Hybrid stepper, bipolar 4 wire versions are generally best. A torque curve for the popular longs motor 23HS9442 indicated I could get 1Nm (140 ozin) at 450 rpm.

Using a 5mm(0.2”) pitch ballscrew with this calculator :-
http://www.cncroutersource.com/linea...alculator.html
indicates a useable force of 109Kg whilst cutting at a speed of 2.3M/min (90 inches/min), more than enough for my requirements. Smaller pitch = less speed more force, larger pitch more speed less force.
The speed of a stepper motor is limited by the driver voltage and its inductance. The inductance resists the change of current which produces the magnetic force. But its the magnetic force which produces the torque. So low inductance = fast motor, low torque. High inductance = slow motor, high torque. The maximum voltage for a motor is reckoned to be 32*sqrt(inductance in mH)**. For my 3.8mH choice this gives 63V. DM860 drivers meet this and seem to perform well without fan cooling.

https://www.hobbytronics.co.za/Conte...ALENZ_rev1.xls
https://www.allaboutcircuits.com/too...0motor%20info/

Tests on my Y axis shown below. 1st picture shows controller, driver, motor and steel cable attached to scales. Bathroom scales indicate 130kgs @ 0 (having gone round once) and a further 25kgs before they hit the stop at 155kgs. Calculations indicate that with 4.2A max force should be over 200kgs.

Best Regards

picclock

* the cutting forces I estimated are simply using the same bathroom scale method and remembering how much force I had to apply to the axis wheel. At the high load figure with a big cutter, deep cut in steel the machine was overloaded - so just my estimate, marginally better than no idea :-).

**Although this formula is widely quoted I cannot find any derivation of it. IMHO the voltage should be limited only by the breakdown voltage of the windings.