Mach THC - full external Z control vs Mach Z control

[beefy]

I'm on the design path again, this time to build a plasma torch height controller for Mach.

I'm going to start with letting Mach do the Z movements so my THC will simply monitor the torch volts and send the up/down signals to Machs input pins.

I've heard a certain manufacturer of a THC say Machs built in THC was too slow (???? no more details). His model fully controls the Z motor, leaving Mach to just do the XY moves during the cut.

Does anyone have any actual experience with one verses the other. What shortcomings would a THC have when it used Mach to do the Z moves. Has anyone came across this way being too slow as just mentioned.

I know Machs THC does not have acceleration / deceleration on the Z moves, and this can limit the speed at which a stepper can operate. Try to make a stepper go a certain speed instantaniously and it stalls / loses steps. Would the closed loop of a servo motor help in this regard because it can catch up to the steps commanded.

Cheers,

Keith.

[East German]

Hallo

I have the Proma THC directly in Mach3 is not to slow.

You can adjust the THC speed from the Z Axis Motor in Mach3.

Works fine.

Regards Peter

[BTA Plasma]

If Mach has to take in a signal, run it through a calculation and send out a motor step and direction the latency may be too great for ultra fast response times. There are ways to bend the rules to get a cut to work but in general you will be stuck using Mach 3 to control the Z axis for THC. It may work fine up to 120ipm on a small angled slope. If it gets beyond that you need to speed up the Z axis motor speed. If the speed is too great the Z axis will oscillate and show as a saw tooth cut in the cut path. If its too slow well then you have a collision. To operate a CNC plasma at BOOK speed, meaning 300+ IPM without a ramp up and ramp down of air pressure and amperage while the machine is changing velocity for trajectory changes you need to overcome the latency issue. Very few THCs in the world can do that and those that can were well fought to get there. CandCNC's MarkIV is one of them.

But you also have a sample rate to figure in. If your using logic outside Mach you will always need to change your acceleration rate for different materials or you will be too slow on thin materials again. You also have to consider cornering when the amperage and air pressure are static but the speed halted or slowed. The THC will drive the plasma torch into the material unless you have a logic control for that. The more complex the logic that greater the latency and you may not see it but its there. One of the greatest challenges I see in creating a THC is the large amount of changing dynamics that must be taken into account while cutting even simple shapes. If it were only strait lines you may only have 10 things to think about.

[beefy]

Hallo

I have the Proma THC directly in Mach3 is not to slow.

You can adjust the THC speed from the Z Axis Motor in Mach3.

Works fine.

Regards Peter

Cheers Peter,

I seen a guy on Youtube with that one and he has his "Correction Speed" in Mach at only 5%. He reckons the z axis jumps around above that setting. I've seen the manual for that THC but don't seem to see a setting for a dead band like I have with my Candcnc system. If it has no dead band it will cause oscillation quite easily I would imagine. I've noticed quite a few THCs have an adjustable dead band setting.

Keith

[beefy]

If Mach has to take in a signal, run it through a calculation and send out a motor step and direction the latency may be too great for ultra fast response times. There are ways to bend the rules to get a cut to work but in general you will be stuck using Mach 3 to control the Z axis for THC. It may work fine up to 120ipm on a small angled slope. If it gets beyond that you need to speed up the Z axis motor speed. If the speed is too great the Z axis will oscillate and show as a saw tooth cut in the cut path. If its too slow well then you have a collision. To operate a CNC plasma at BOOK speed, meaning 300+ IPM without a ramp up and ramp down of air pressure and amperage while the machine is changing velocity for trajectory changes you need to overcome the latency issue. Very few THCs in the world can do that and those that can were well fought to get there. CandCNC's MarkIV is one of them.

But you also have a sample rate to figure in. If your using logic outside Mach you will always need to change your acceleration rate for different materials or you will be too slow on thin materials again. You also have to consider cornering when the amperage and air pressure are static but the speed halted or slowed. The THC will drive the plasma torch into the material unless you have a logic control for that. The more complex the logic that greater the latency and you may not see it but its there. One of the greatest challenges I see in creating a THC is the large amount of changing dynamics that must be taken into account while cutting even simple shapes. If it were only strait lines you may only have 10 things to think about.

Thanks BTA,

Sheetcams path rules will take care of turning off THC for corners, etc. My gantry is built very badly at this point so I'm never going to be able to cut at speeds that would require full independent control of Z for torch height control. So for now looks like a very basic THC which sends UP/DOWN to Mach will get me by.
Interesting what you mentioned about different settings for different materials. I was thinking (down the track) to make a PID controlled Z for THC. I can imagine having cut profiles stored in memory would be a huge asset in such a THC. Once the PID settings are tuned for certain materials / feedrates / etc, they could be stored and called up later on. I believe the Neuron THC stores cut profiles. But like I say, I'm a long way at present from designing that system, it may be something I gravitate to later on.
To be honest I see the answer to a lot of these high speed THC issues is to have a tiny cutting torch which operates on low amps and has a very narrow kerf. Jim Colt has said a few times we can experiment with reducing the cut current well below the nozzles rating and trying to find settings that still give a good cut, still have to play with that one. A table also needs to be built to handle super fast speeds, acceleration, & direction changes. The cheaper / simpler solution seems to be negating the need for high speed in the first place.

Keith.

[BTA Plasma]

You could bubble chart the whole thing out and find solutions along the way. I think PID is good. To obtain the best results you need to dial down air pressure and amperage. But we are talking about a beam of energy that only has so much velocity and it can change directions from the top of the material to the trailing edge underneath. The higher amperage energy dense beams work super well on lighter industrial units. But they do not have the ability to do any ramping when coupled outside the parent controller. What most folks want is a THC that can handle cuts in 18ga steel at book speed and at least 45 amps. There are solutions out there but that leaves the question ...would you do this for yourself or as an aftermarket THC?

[beefy]

You could bubble chart the whole thing out and find solutions along the way. I think PID is good. To obtain the best results you need to dial down air pressure and amperage. But we are talking about a beam of energy that only has so much velocity and it can change directions from the top of the material to the trailing edge underneath. The higher amperage energy dense beams work super well on lighter industrial units. But they do not have the ability to do any ramping when coupled outside the parent controller. What most folks want is a THC that can handle cuts in 18ga steel at book speed and at least 45 amps. There are solutions out there but that leaves the question ...would you do this for yourself or as an aftermarket THC?

It's crossed my mind about turning down amps and airflow to reduce cut speed, and how that may reduce arc density. Arc density is always something that's mentioned and that method is certainly reducing it. Like I said I haven't tried it yet but my guess is a tiny nozzle on low (rated) amps is going to do a much better cut than a bigger nozzle on reduced amps and air flow.

Ha ha, good question (the last one).

Economic sense wise I should just go and buy an off the shelf unit and be up and running. I guess I quite enjoy the electronic design side of things and the kick I get when I design something from scratch and it works. I made that hole centre marking board and it's in full time use on my plasma table now, and it's extremely useful. That sort of personal success leads you to waste more time designing other goodies. Actually there is some practical reasoning for building a basic external THC while I have my Candcnc MP3000. It cannot do THC on/off during motion without a motion pause. With an external THC controller sending up/down signals to Mach, I can use Sheetcams path rules and gcode snippets to disable the THC during a cut, and have no pause in motion. Doesn't hurt to keep up my electronics and microcontroller programming abilities for the moment too, they're a useful tool.

Cheers,

Keith.

[tcaudle]

[quote]It cannot do THC on/off during motion without a motion pause. With an external THC controller sending up/down signals to Mach, I can use Sheetcams path rules and gcode snippets to disable the THC during a cut, and have no pause in motion. Doesn't hurt to keep up my electronics and microcontroller programming abilities for the moment too, they're a useful tool.{/quote]

You list the MP3000 but not the DTHC model.

Actually you can turn off the DTHCII with no motion pause. With the DTHCII you can simply use the S10 command in the code snip provided you are running the updated plugins.
The DTHCIV supports S10 as well and will have instant torch off (before end) in about 72 hours via a custom S code. Either way will work with SheetCAM Code snips

There are some ways to use the M11/M10 to trigger the THC (in MACH ) ON/OFF. Those codes call an output instantly with no code pause and are typically used for laser on/off) I would think you can trigger the THC ON/OFF via an OEM Trigger because THC ON/OFF has an OEM number you can call. I have not done this but some others have..

[beefy]

Tom,

my DTHC is an "old" one, version 14 I think. The features like pauseless THC on/off came AFTER I bought my package.

Checked out the prices of the DTHC-IV. They are twice the price of the MP3000 on its own so I think I'll just wait until I can buy a complete package. Made some good money in Feb/Mar then had 6 weeks no income so I'm a bit skint at the moment.

Keith.