"A" axis on cnc machines ...

[Thomas Wayne]

We make close to 5000 taper passes every week, all done with steppers spinning the parts.

[...]

I would never have thought it a suitable application for a stepper motor, and my understanding of the internal workings of steppers seems to contraindicate the idea of getting a smooth cut with such use.

However, experience often trumps theory - assuming the "experience" is documented. Your report is good enough for me... but I'll still use my existing DC motors to turn my parts. That said, any future "lathe" machines will be using Clearpath motors for sure - they literally are the best of both worlds.

Which brings to another point - I've got the "hard-stop" homing for these Clearpath motors figured out, and it is way cool.

TW

 

[waynewrc]

I have 2 machines setup like Royce described and they work great for me. They both use pulleys with a ratio of 1.8 to 1.

 

[RBC]

I would never have thought it a suitable application for a stepper motor, and my understanding of the internal workings of steppers seems to contraindicate the idea of getting a smooth cut with such use.

However, experience often trumps theory - assuming the "experience" is documented. Your report is good enough for me... but I'll still use my existing DC motors to turn my parts. That said, any future "lathe" machines will be using Clearpath motors for sure - they literally are the best of both worlds.

Which brings to another point - I've got the "hard-stop" homing for these Clearpath motors figured out, and it is way cool.

TW

Thomas

Thanks for your post! I appreciate it very much.

When we bought our first turning machine, one of the first of the newer style Bludworth 4 blade saw machines, I asked Leonard to not install his normal motors for spinning the 4 lathe spindles. I told him that I would install a single stepper motor and belt drive all the spindles from that. Leonard was hesitant, but after a little discussion he agreed.

The Gecko drives we use have 10 times micro stepping and mid band resonance reduction. At 100 rpm, the stepper use 2000 steps per revolution or a little over 3300 steps per second. The rotation is actually very smooth.

Don't get me wrong, DC motors would actually a better choice if it weren't for a few things. First, I already had an extra drive on the control and an extra stepper motor. Also, I really wanted to be able to have the program control the rotational speed. It was just a good fit for the situation and it has continued to work very well. Of course, the Clearpath motors will do even better.

Speaking of Clearpath, that's great news that you worked out a good homing routine using the hard stop. I'd love to hear more about it, if it's something you feel comfortable sharing.


Royce

 

[Thomas Wayne]

Thomas

[...]

Speaking of Clearpath, that's great news that you worked out a good homing routine using the hard stop. I'd love to hear more about it, if it's something you feel comfortable sharing.


Royce

For the linear table I was experimenting with, I got the most accuracy with a steel hard stop - simply a small piece machined from square stock and mounted to the aluminum bearing block at the motor end of the ballscrew. I got repeatability of less than one thou over many, many tests. The way I tested it was pretty easy. I ran it up against the hard stop until the Clearpath disabled itself, then I zeroed the axis readout. I re-enabled the motor and ran it some distance from the stop, then used the MDI function to sent it back to "zero". When it got there it remained enabled. Finally, I used the MDI function again to try to move it one more thousandth toward the stop, at which point it disabled itself due to tracking error. I repeated that test at least 100 times, partially because I wanted a reasonably thorough test, and also because it's kind of cool to watch.

The fun part came with the homing sequence itself. In order to re-enable the Clearpath motor you have to de-assert the enable circuit to the motor (even though it has already disabled itself due to hard stop). Then when you re-enable it you can jog it away from the stop at will. I didn't want to use the e-stop for this purpose because then I would have to reset it in the software as well. So I wired in an NC momentary button switch in-line on the outboard side of the e-stop's enable line. The e-stop has a second switch block that sends the e-stop pin on the BoB to a high state, thereby tripping the software reset in Mach3, but that second circuit does NOT run through the momentary button switch.

SO, when the e-stop switch is triggered, both the enable circuit and the software reset are tripped, but when just the momentary switch is triggered the enable circuit (only) is cycled off (and then back on upon release). Therefore, when the motor is homed against a hard stop it disables itself and the operator simply triggers the momentary button to bring the motor back on line. Easy peasy.

TW