Distance Traveled Per Motor Revolution

Stepper motors suffer from resonance and a direct drive pinion is likely to mean that the motor is operating under unfavourable conditions. Ideally, for plasma machines a distance of around 15-25mm per motor revolution is considered ideal but even around 30mm per revolutions is still acceptable. A 5mm pitch ball screw with a 3:1 or 5:1 reduction drive is ideal for the Z axis.

1. Where did these numbers come from?
2. Realistically, all of those numbers are possible. 2:1 would put me at 30mm, 3:1 would put me at 20mm per rev, 4:1 would put me right at 15mm. Is there really a huge difference between 15mm and 20mm? Where should I shoot to land realistically
3. I know that reduction is going to slow down my rapids. One reason I wonder all of this is because it would seem that the number would really be driven by max RPM of the stepper, and it would also seem that the resonance would be different for different steppers, but maybe that's incorrect?
4. I think this is the last one... What does this do for acceleration? I think I read somewhere that ideal acceleration for Plasma would be 50 in/sec^2 (or maybe that's 50 in/min^2?)... When you reduce the axis, does that cut the acceleration by the same amount? In a 3:1 reduction system, would I have to set my acceleration to 150 in order to get the desired 50?

tommylight wrote:

I'll put all of that in a single question:
What do you want to cut with it?

snowgoer540 wrote:

Everything and anything that comes my way. Signs, brackets, round holes, etc. The real answer is no matter what it is, I want a quality cut.

rodw wrote:

snowgoer540 wrote:

Forgive me if this is covered in another thread I couldn't find...

I currently have a direct drive R&P system on my X and Y axis'. The D.P. of the spur gear is .75", which makes my distance traveled per revolution ≈2.37" (≈59.8mm).

I didn't know about the plasma primer before I built my table , but now that I do, and I see the issues associated with resonance and cutting around curves, etc. I want to add a reduction system. I know that the Plasma Primer says:

Stepper motors suffer from resonance and a direct drive pinion is likely to mean that the motor is operating under unfavourable conditions. Ideally, for plasma machines a distance of around 15-25mm per motor revolution is considered ideal but even around 30mm per revolutions is still acceptable. A 5mm pitch ball screw with a 3:1 or 5:1 reduction drive is ideal for the Z axis.

My questions are as follows:

1. Where did these numbers come from?
2. Realistically, all of those numbers are possible. 2:1 would put me at 30mm, 3:1 would put me at 20mm per rev, 4:1 would put me right at 15mm. Is there really a huge difference between 15mm and 20mm? Where should I shoot to land realistically
3. I know that reduction is going to slow down my rapids. One reason I wonder all of this is because it would seem that the number would really be driven by max RPM of the stepper, and it would also seem that the resonance would be different for different steppers, but maybe that's incorrect?
4. I think this is the last one... What does this do for acceleration? I think I read somewhere that ideal acceleration for Plasma would be 50 in/sec^2 (or maybe that's 50 in/min^2?)... When you reduce the axis, does that cut the acceleration by the same amount? In a 3:1 reduction system, would I have to set my acceleration to 150 in order to get the desired 50?

I think that's it for now. If you think of anything I didn't think of, I'd be happy to learn more!

1. It was me! (complete with typo referring to the Z) as I wrote it. And Tommy cos he taught me forum.linuxcnc.org/30-cnc-machines/31509...-loaded-pinion-drive Plus Tom Caudle from Candcnc who often mentioned 1" per rev. You can see from the link I made the same mistake as you but fixed it before I built my table (Thanks Tom!). I settled on 3:1 and 30mm/rev becasue thats what I had in my shed!

2. Its actually a lot more complex to be able to make a recommendation. Everything about steppers is predictable. You could try this approach www.machinedesign.com/motors-drives/arti...rs-for-linear-motion but I think it has some bugs in the maths. With a complex model we found the two key parameters were reduction drive and pinion diameter. You can't really lump them together into a distance per rev. And yes, even resonance RPM can be predicted.

3. Yes its quite a balancing act between rapids and designing for maximum cut speed. (We used 10240 mm/min) based on one manufacturer's cut charts. Basically I designed for our cut speed but then looked at the torque chart for a given stepper to see if it had enough left in the tank to hit our target rapids. say 30 m/min (at least 25 m/min). There is a high probability your motor selection is wrong (cos we could not find many motors that hit our target performance)

4. The model cited does not really work on target acceleration, but on an acceleration profile. But the time taken to get to speed sets the acceleration.... More reduction give more torque to overcome inertia so acelleration gets better with bigger reductions.

so 1 G = 9.81 m/sec/sec. Jim Colt from Hypertherm often mentions 0.3 G as a target but when I reviewed high end machines, they were doing up to 0.5G to 0.7G. We settled on a target of 0.5 G (Roughly 5 m/sec/sec) becasue we did not want to be just mediocre. So with a 3:1 reduction, we exceeded the model and achieved 8 m/sec/sec. (0.8G) But with a 5:1 reduction (different motors), we could not get past 5 m/sec/sec 0.5G. Rapids we settled on 36 m/min.

But this was with high end stepper motors and high end stepper drivers (Tommy's beloved Lams) and a 64 volt AC power supply (about 90 volts to the motors). All in all about an AUD $3k upgrade.

Send me a message here www.vmn.com.au/contact-us and If I get your motor specs and axis weights, I might be able to advise further. Sorry but I can't share the model here due to an NDA.