Is cutting speed limited to Z-axis speed

[Beefy]

I have bought the MP3000 with digital torch height control from CandCNC. I built my floating Z-axis to have a max. speed of about 150 ipm (5mm leadscrew). As far as I understand Mach limits the speed to all axis to that of the slowest axis. Does this mean that my X & Y axis will only run at a max of 150 imp too.

Thanks,

Keith.

[millman52]

No Keith, Your table X&Y will still run as normal. The MP 3000 cuts the rate of your Z axis to a percentage of it's full operational speed while the plasma machine is cutting.

When the machine rapids from part to part all 3 axis will run at full speed.

The THC slows the Z axis while under cut conditions to help prevent it from pulsing up & down (overshooting & then correcting itself).

[Beefy]

Thanks for the reply Millman-52

Still trying to get my head around this co-ordinated move as it applies to the MP-3000 THC plugin for Mach3. The documentation says that during a co-ordinated move any axis can only move as fast as the slowest axis. I would guess that with 3D cutting this would certainly apply to X,Y and Z. Is it different with the plasma control. Do X and Y operated independantly of the Z-axis, seeing as the Z is in no way co-ordinatd with them. And so my X and Y could be cutting at 400 inches per minute while the Z-axis THC had a limit of 150 ipm (that's just for explanation, I've noted what you said about the MP3000 THC plugin reducing the actually Z-axis speed).

Thanks,

Keith.

[millman52]

Thanks for the reply Millman-52

Still trying to get my head around this co-ordinated move as it applies to the MP-3000 THC plugin for Mach3. The documentation says that during a co-ordinated move any axis can only move as fast as the slowest axis. I would guess that with 3D cutting this would certainly apply to X,Y and Z. Is it different with the plasma control. Do X and Y operated independantly of the Z-axis, seeing as the Z is in no way co-ordinatd with them. And so my X and Y could be cutting at 400 inches per minute while the Z-axis THC had a limit of 150 ipm (that's just for explanation, I've noted what you said about the MP3000 THC plugin reducing the actually Z-axis speed).

Thanks,

Keith.

Yes I'm sure the coordinated move is 3D. At least that's what it sounds like to me. From your above statement I think you have it right.

My Z axis for my Oxy will only muster about 60 IPM I have it on a 18 TPI lead screw so when I jog it from the keyboard while under cut conditions It isn't so rapid I can't manually keep up with it.

Your X & y must run at the same speed or A circle will be an oval etc.

[Torchhead]

The coordinated toolpath in MACH is so the tool will get to a certain point in space. Since plasma is 2d cutting only the XY is coordinated. Under THC control the Z runs independent. That means you can cut at full XY velocity ipm with a Z that will only do 60 IPM (less under THC Control). The only time the Z speed is part of the equation is when it is part of a 3D cut and the Z move is on the same line as X & Y in the Code.

If you not the G-code for plasma the Z moves in the code (for pierce height and inital cut height) are on a separate line and there are no Z moves until the end of the cut.

The only time the Z speed becomes a factor is if the material has a severe sharp slope. At say 45 deg your Z needs to be able to move at the same speed as your X & Y are cutting of it can't pull up fast enough. Because there is no acceleration applied to Z moves under THC the actual velocity has to be reduced to prevent the Z from stalling (lost steps). That rate of reductiois set in MACH by the THC RATE setting. Default is normally 30%. So if your Z max velocity is 100 IPM the THC speed is 30 IPM. That works fine on a gradual slope but would not lift fast enough to cut something like corrugated metal at XY speeds of 100 IPM.

The dynamics of the Z do effect the response of the THC cutting process. Make the Z too course (low resolution) and you get overshoot and head "bounce" (oscillation). If the resolution is too high you end up with low speeds that can effect response times, The best rate is from 150 to 300 IPM. Since steppers will spin about 800 RPM upi can get there with a 8 TPI 2 start (effective 4 TPI) and get close to 200 and around 65 IPM in THC mode. If you have a servo system with it's faster RPM your speeds can be higher.

The moves in THC are typically in the .010 range or less. Depends on the sensitivity and accuracy of your THC electronics. Typical 1 v span accuracy yields move of about .015 for a volt of change. With systems that have closer tracking the moves can be smaller.

TOM Caudle
www.CandCNC.com