VMC 100 Z Axis problems.


Hi all.
I have converted my VMC to Mach3. I have put on new stepper motors type 34HS38 3008S (basically a normal stepper with 1000 oz holding torque). I am not saying this was the correct choice but I had them available to me and NEMA 34 just about exactly fits the old motor bolting.
Anyway - system up and running. X and Y slides are excellent.
But I cannot get the Z axis to any kind of repeatable measurements.
Using my PC (or the axis setting wizard in Mach 3) I get something between 2.7 and 3.02 mm each turn.
I have traced the problem I think to the Z azis ballscrew fitting at the top. My Z axis ballscrew had been a bit damaged in a former life so is very hard to get a nut to screw on it any longer. I have used grub screws to hold it fast.
My question is:
Has anyone replaced the Ballscrew and nut for the Z axis, and if so what did you use, how did it work out and where did you get the replacement from.
Also is it worth persisting with the original pulleys and belts - they are all quite worn. If not what is a good replacement? T5 belts and pulleys?
Pushing my luck here a bit, the stepper motor driver gives me all kinds of options of microsteps per step, from 1 to 256. I read somewhere that 32 micro steps per step was fine, and this is what I have used on X and Y (and Z), but does anyone have any views on this?

many thanks & regards




My experience of modifying a Emco PC50 mill was that using microsteps any more than 8 for the Z axis led to it loosing steps and position.
The reason is that as you increase the microstep count the Stepper Motor Torque drops off significantly. 
Try reducing your Microstep count  before ripping the mechanicals apart.
A counter balance arrangement on the Z axis to equalize the vertical up and down load on the motor is another approach, although a little bit more complicated !



Hi Steve.

Thank you for this. I finally got out to work on the machine. 8 microsteps 640 steps per mm gave an exact result with no missing steps.
I still need to help ensure the gear pully does not rise (actually the ballscrew is being pulled down), but I have a plan for this by revising the bolting plate of the motor and putting a "top down" screwed nut onto a thrust washer on the pully. (The thread on the actual ballscrew top which should hold it is all mashed up!).

Still - good progress being made.