Start, Accel, and Max Velocity...

[alank2]

Hi Everyone,

I'm just getting started with CNC. I've ordered a Microcarve and G540/381oz Steppers and don't have it yet, but am trying to get acquainted with the software. I plan on primarily cutting plastic enclosures with this setup to begin with.

I understand the concept of the feed rate with G code commands. I am assuming the axis settings of start and accel control how fast it gets up to the feed rate. What is the benefit to this as opposed to just starting at the feed rate? Is it easier on the bit, the material, or the motors to start slower and work up to speed?

Thanks,

Alan

[ger21]

What software are you using? The only one I know of with a Start speed is TurboCNC?
Unless the feedrate is very low, you can't just start at the feedrate, as the lawas of physics dictate the need to accelerate. Setting the start speed too high will result in the steppers stalling before they even start spinning.

[alank2]

Hi Gerry,

Yes, I thought I had mentioned TurboCNC, that is what I'm playing with. What are good starting and accelerating IPM values?

Thanks,

Alan

[ger21]

Every machine is different, and you'll need to use trial and error to find out what works best for you.

I couldn't tell you where to start, as it's probably been over 7 years since I've looked at TurboCNC.

[alank2]

Hi Gerry,

So does other software like EMC2/Mach3 just have build in settings they use? Are these not tweakable on them?

Thanks,

Alan

[ger21]

Mach3 doesn't use a Start Speed. Just acceleration and max velocity. Mach3's acceleration is in inches per second squared, while TCNC is in Hz, if I recall correctly?

[alank2]

Hi Gerry,

The default values in Mach3 for acceleration are 4 in/sec which seems really fast to me. That would be from 0 to 240 ipm in one second (assuming the feed rate was 240 ipm). Is this a good accel number for most steppers? I'm going to use some of the Keling 381oz ones with a G540/48V PS. Probably much of my cutting will be at 15-30 ipm, this means that full speed will be reached in 1/16" or 1/8"... Is that a slow enough start?

Second question - I've been playing around with TurboCNC and it says jitter is 10% around 40K steps, but it can go to 100K when I set the number high enough and watch it on a scope. The G540 says it can handle 200K. I will have a 12 tpi 2 start X/Y so 6*200*10=12K steps per inch. 100K is 500 IPM. Is this too fast to run the motors when just moving (G00)? Is there any downside to it as long as steps aren't lost?

Thanks,

Alan

[ger21]

The default values in Mach3 for acceleration are 4 in/sec which seems really fast to me. That would be from 0 to 240 ipm in one second

That's actually pretty slow. Most guys building rack and pinion machines use an accel of 30-40. I'm currently designing/building a machine that I hope to be able to have an accel of over 300. I really want to be able to have at least 200, so I'm designing in a large safety margin.

Second question - I've been playing around with TurboCNC and it says jitter is 10% around 40K steps, but it can go to 100K when I set the number high enough and watch it on a scope. The G540 says it can handle 200K. I will have a 12 tpi 2 start X/Y so 6*200*10=12K steps per inch. 100K is 500 IPM. Is this too fast to run the motors when just moving (G00)? Is there any downside to it as long as steps aren't lost?

500ip, would be 3000rpm at the screws. You're motors will run out of torque well before they get to 3000 rpm. The most you can realistically get out of steppers is usually somewhere between 800-1500 rpm. Most likely closer to 1000 rpm.