Hi,
not sure what to make of the quill, I don't know how it works and the forces involved. The X and Y axes are a little more straight forward.
Lets assume that you wish to get close to the performance that I posted earlier, ie 10m/min rapids and 0.5g.
If the ballscrews have a pitch of 6mm, per your previous post, then to achieve 10m/min the ballscrews must rotate at
10,000/6 =1666 rpm.
There is your first hurdle, steppers lose torque badly with speed, and only very few of them can maintain any useful torque above 1000rpm.
You could use an increasing gear reduction but that would require even more torque from the steppers, I suspect its unlikely to happen.
Per my previously posted calculation you can get about 0.2g with 1Nm, so to get 0.5g will require 0.5/0.2= 2.5Nm. Note also that this makes no allowance
for friction nor ballscrew efficiency, so I would suggest that around 2.5Nm should be considered the minimum torque. Note that this torque is required
at top speed.
A very low inductance (1mH) stepper may retain 40-50% of its torque at 1000 rpm, which would suggest that the stepper needs at least 5-6Nm at low
speed. Steppers that have that sort of torque and low (sub 2mH) inductance are rear beasts. A more normal 34size stepper may well have adequate torque
at low speed but 34 size steppers have inductances of 4mH and more. Such a stepper is going to run out of 'puff' at 500-750rpm.
Side note: 1Nm= 130oz.in
2.5Nm=325oz.in
5Nm=650oz.in
All in all I think steppers may well provide enough torque at low speed but I suspect that they will fail to satisfy at speed. High speed operation
is always steppers Achilles Heel.
I think 34 size steppers of around 800oz.in and 4mh (or less is preferred) would work OK, but maybe fail to get to 10m/min, but more likely 5m/min.
The other alternative is servos. Servos come in a wide variety of specs, but the readily available brands likely to be affordable for hobbyists are:
200W 3000rpm =0.65Nm cont, 1.9Nm peak
400W 3000rpm = 1.27Nm cont, 3.81Nm peak
750W 3000rpm = 2.4Nm cont, 7.2Nm peak
If you used any of these three servos with a 2:1 reduction then you would get very close to your desired 10m/min rapids.
The 200W servo would, after the 2:1 reduction, have barely enough torque without dipping into overload all the time.
The 400W servo would after the 2:1 reduction would have plenty of torque, (1.27 x 2 =2.54Nm) without using overload.
The 750W servo has enough torque without reduction. Without reduction you could have rapids of 20m/min, alternately
with reduction you could still get near 10m/min but have a surfeit of torque for even higher accelerations.
So servos will certainly provide the best performance but they cost more than steppers, and steppers will do OK except
at high speed.
For my new build mill have bought three (one braked) 750W Delta B2 series servos. They cost (excluding the braked one) $460USD each, which
includes the drive and cables, plus shipping. I found that the 750w models were only $40 more than 400W ones, so I got the 750W's.
I notice that many US buyers favor Clearpath (US) over DMM (Canadian made in China) or Delta (Taiwanese made in China) but they are
more expensive, about $480USD for 350W. For various reasons, cost primary among them I went with Delta, almost twice the power for less money!
Ultimately your budget will determine what you do. I would recommend 750W servos if you can afford them.
Craig
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