The final Z axis design.

It worked out to be 1265 pulses per mm when driving the servo with a 2.6: 1 ratio on the timing belt pulleys. I opted for the strange number of 2.6: 1 because that was the highest ratio with the appropriate kilo-watt load rating that would not have the big pulley hanging out past the column of the mill.

 

At this point I was still really nervous, because I knew how stiff the handle was to turn, and I was not sure if the 2.6:1 ratio would give me enough torque to raise and lower the head.

 

I did a crude test and connected just the naked dc servo motor (without encoders, controllers, breakout boards… all the other complicated stuff) to a cordless drill battery to try and see if  the motor would have enough grunt to raise and lower the head. I knew that I had to be quick when connecting the motor to the battery contacts, because I didn’t want to crash the head into the table on the way down, or drive it up off the top of the column when raising it!

 

The motor could easily lower the head, but did not have enough power to raise the head. It sort of went half a turn, stalled and I gave it a tap, then it moved another turn or so, but was not rocketing along like I had hoped. The drill battery could provide enough torque to just overcome the frictional forces, but not enough to provide rapid acceleration. At least I could approximately calculate the friction now to gauge if I had used the wrong motor size, or pulley ratio.

 

If I loosened the gib strips, the head would lower rapidly and raise slowly, but I was not convinced that I had purchased a big enough servo motor. The gibs would have to be tight when the mill was actually running to stop the head from flopping around on the column, otherwise if I tried to cut squares they would become ‘squircles.’

 

I measured the current coming out of the 14V cordless drill battery, with a multi-meter, and it was 10.1 Amps. The servo drive could pump out 20 Amps at 72 Volts (with my power supply anyway)  so I should have enough margin for error– I hoped. 10.1 Amps x 28.125 oz-inches per amp = 284 oz-inches of motor torque to overcome friction and gravity when raising the head with a 2.6:1 pulley, approximately 740 oz-inches on the big pulley’s shaft… hopefully the remaining torque when the servo is connected properly to the servo controller will be enough.

 

At this stage the electrical cabinet was still a work in progress, but after many beers and a few more late nights in the shed I got the electrical cabinet working and connected the Z axis to it… Time for a very nerve-racking test.

 

Grae-Tech 

This is some footage of me up late, playing with the manual pulse generator (jog wheel) after just getting the Z axis to move!!! This was a big moment of triumph for me, as I now knew that  IT WAS GOING TO WORK!!!

 

This was a major milestone because it proved that the software, interface hardware, electronics and electrical designs were all working together nicely, and my mechanical designs worked well enough to support the servo, with enough torque to raise and lower the head. What a relief.