corrugated roof panel cutting suggestions.

[trucutcnc]

For every machine using CandCNC's THC correctly there are 10 of them out there that dont. Its not their fault its an open source collection of hardware with a ton of misunderstood info out there on it. Just like Mach 3 an open source software with millions of options.

If only 1 out of 10 works correctly, then something is wrong. That's the beauty of our THC. There are no secret procedures to make is work correctly. Just solid engineering. It is an open source collection of hardware, but with support from all hardware suppliers, it shouldn't be difficult to get it working properly.

[trucutcnc]

The statement about Mach3 THC always having bounce to some degree should not be true with the Candcnc system, which has a SPAN setting in addition to the THC RATE setting. The span setting is a dead band area where the THC makes no adjustment. Granted this dead band area gives a voltage range where the torch voltage can change without any z height corrections. Overshoot (bouncing) in the Candcnc system only occurs if the THC RATE setting gives a z velocity which makes the z drive right through the dead band zone to the "other side", then it all reverses and happens again. Or likewise if the SPAN setting is so narrow that even a small THC RATE setting is not low enough to prevent this overshoot through the dead band.

The statement that Mach3 THC always has some bouncing leads me to guess that Mach3 on it's own doesn't have such a SPAN setting, and I can see this making it susceptible to overshoot and bouncing if there is no accel & decel as the torch voltage is approaching.

The Candcnc system is a plug-in for Mach3 so it customizes Mach3 for the plasma process. Seeing as Candcnc is being mentioned in this thread, we need to be careful that any of Mach3 shortcomings are not equated to the Candcnc system if it's not the case.

Keith.

All THC's I've ever seen using Mach THC inputs have a dead zone. The dead zone does help to reduce bounce but it also desensitizes the THC. The dead zone is programmed in the THC externally, not in Mach. CandCNC uses a plugin mostly because of it's port multiplexing. A lot of it's other functions could be done without a plugin. The lack of accel/decel is a Mach shortcoming. It has nothing to do with CandCNC, but it does affect any THC that uses the Mach THC inputs. Optimum performance is a balancing act between THC Feed Rate and the dead zone.

PID controlled THC's need a very small dead zone or a good filter just to smooth out any unwanted motion due to electrical noise.

[jimcolt]

First, while many plasma cutting THC systems can control height accurately enough to cut corrugated metal, none of them were designed specifically to do that. The function of the height control is to locate the surface of the plate, adjust to accurate pierce height, when the pierce is complete index down to cut height, then during steady state cutting to control realt ime height by using arc voltage as the feedback loop.

With any height control system you need to have the parameters dialed in just right if you plan to use it for something like corrugated metal. The z axis speed must be optimized, acceleration must be optimized, and there can be no mechanical backlash. Each different z axis drive is a little different with different payload, different loads from the torch leads and different gearing and motor sizing. Steppers are perfectly adequate for z axis....and are even used on $10,000 industrial precision height controls.

I have cut a few pieces of corrugated in my years. The best rule of thumb is to use the lowest amperage consumables and slow the x and y cutting process down as low as possible. I would rather have a little easy to remove low speed dross on my part....than have a broken torch. If the x and y speeds are reasonable then the z axis will have a much better chance of controlling height.

I have worked with users that cut corrugated metal with CandCNC height controls, PlasmaCam height controls, and all kinds of industrial (expensive) height controls. It requires a little experimentation and practice, and I would not count on it as an every day type of production job as it can be difficult.

Jim Colt Hypertherm

[beefy]

[quote="trucutcnc"]All THC's I've ever seen using Mach THC inputs have a dead zone. The dead zone does help to reduce bounce but it also desensitizes the THC.[/
PID controlled quote]

The dead zone / THC Rate settings when done correctly will STOP bounce, not just help to reduce it. So long as the z corrects the voltage and enters into the dead zone without overshooting to the opposite side, there's no condition to cause bounce, the THC just stops adjusting. The big question with systems using a span / dead-band area is what does this span setting equate to in terms of height variation where no z corrections are made, and is that small distance going to give any noticeable effect on the quality of the cut.

Fitted with servo the Candcnc system has the speed to cut corrugated sheet (Candcnc info, not from me). One thing that needs to be considered with any system cutting corrugated is that the THC is serving no other purpose than to keep the torch off the plate surface by the set amount. On the inclines / declines however this height is no longer the optimum for cut quality because the surface speed has changed. I beleive most plasma systems (unlike 3D milling) only interpolate the X & Y axis speeds to maintain a set feedrate on a horizontal plane . So with the z axis pulling the torch up quite fast to climb the incline the linear feedrate has actually increased. Optimal cut is the combination of feedrate, nozzle height and a few other goodies, so now that ideal combination has gone out the window. Jims suggestion of using the lowest speed possible actually helps reduce this negative side effect, which NO height control system can correct unless the system uses 3D interpolation together with THC to maintain the set linear speed.

I don't know about other systems but in the Candcnc system the Z position is always tracked even under THC. I'm not sure if this is unique to Candcnc or is a Mach3 thing.

Keith.

[trucutcnc]

All THC's I've ever seen using Mach THC inputs have a dead zone. The dead zone does help to reduce bounce but it also desensitizes the THC.

The dead zone / THC Rate settings when done correctly will STOP bounce, not just help to reduce it. So long as the z corrects the voltage and enters into the dead zone without overshooting to the opposite side, there's no condition to cause bounce, the THC just stops adjusting. The big question with systems using a span / dead-band area is what does this span setting equate to in terms of height variation where no z corrections are made, and is that small distance going to give any noticeable effect on the quality of the cut.

Fitted with servo the Candcnc system has the speed to cut corrugated sheet (Candcnc info, not from me). One thing that needs to be considered with any system cutting corrugated is that the THC is serving no other purpose than to keep the torch off the plate surface by the set amount. On the inclines / declines however this height is no longer the optimum for cut quality because the surface speed has changed. I beleive most plasma systems (unlike 3D milling) only interpolate the X & Y axis speeds to maintain a set feedrate on a horizontal plane . So with the z axis pulling the torch up quite fast to climb the incline the linear feedrate has actually increased. Optimal cut is the combination of feedrate, nozzle height and a few other goodies, so now that ideal combination has gone out the window. Jims suggestion of using the lowest speed possible actually helps reduce this negative side effect, which NO height control system can correct unless the system uses 3D interpolation together with THC to maintain the set linear speed.

I don't know about other systems but in the Candcnc system the Z position is always tracked even under THC. I'm not sure if this is unique to Candcnc or is a Mach3 thing.

Keith.

The deadzone will elimate bounce, but at the cost of THC overall performance. As you mentioned, there will be a zone where TOM height will change and no correction will be made. Whether it's enough to see it in the cut depends on the settings, but it will always be there whether you notice it or not.

Because the THC UP/Down inputs in Mach3 are on/off inputs, Z corrections are made at a set rate, which is set by THC Feedrate, with no accel/decel applied. The feedrate of the Z axis doesn't change. A correction of .01" and a correction of 1" are made at exactly the same speed. I'm not sure if a rapidly climbing Z under Mach3 THC control would slow the X and Y. CV would reduce X and Y in regards to Z during a normal 3D milling move. A THC that controls the Z externally from Mach3 does not have that issue. X and Y operate independent from Z. All the THC has to do is maintain the Z feedrate at a speed adequate to maintain the proper offset at the current XY feedrate. THE THC's job during the cut is to maintain the correct offset regardless of how fast or slow the X and Y are moving. The THC doesn't need to know how fast the X and Y are moving, it just needs to respond fast enough to correct for any changes at any given speed.

The purpose of a PID controller is to adjust the Z feedrate so it is proportional to the difference between the process variable (tipV) and the set point (SetV). If the difference between PV and SP is 1/4 volt then the Z only needs to respond at a very slow speed. If it were to respond at a fast speed, it would overshoot. Depending on the dead zone setting, a Mach3 THC would probably not even respond to 1/4 volt. If the difference is say 10 volts, then the Z needs to respond very fast to catch up, but as PV approaches SP it needs to slow down otherwise PV will overshoot SP.

I'm hoping Brian takes a look at the THC feature in Mach4. Adding a PID loop to control Z in THC mode would not be difficult, but it would have to be done at the kernel level. All you would need is an analog input for arc voltage which can easily be done with a simple modbus device.

I don't know about other systems but in the Candcnc system the Z position is always tracked even under THC. I'm not sure if this is unique to Candcnc or is a Mach3 thing.

Z is tracked even under THC control because it is Mach3 doing the motion control, not the THC. The THC just signals Mach3 to go up or down through the THC UP and THC Down inputs, and Mach3 handles the motion control. External THC's control the Z axis directly. Mach3 is out of the loop. Our TruTrac Pro is a hybrid system. With Mach3 in a paused state, it has control of the Z. While the torch is cutting, the THC controls the Z motor drive directly.

[beefy]

This might be getting confusing to some readers. I've had experience with PID controls on generator sets, furnace temperature controllers, and electro-mechanical governors, so I'm OK with them. Maybe a consideration summary can help users decide which one they need:

1) if the dead band voltage is not enough to give a variation in the cut that is noticeable then is that good enough ?. If a variation in cut is there but is not visible to the naked eye then where is the need for further perfection. If further perfection is needed then PID seems the way to go. However it also needs to be considered that real time tracking of z axis postion is lost with an external THC controller.

2) ideal cut conditions are lost when climbing a steep incline (or going down a steep slope) regardless of which THC is used. Under these conditions linear speed along the plate surface INCREASES. The X & Y speeds remain the same on a horizontal plane but the fast change of the Z adds to this linear speed along the plate thereby taking away the ideal feedrate at the plate surface. The THC will still maintain the set torch voltage but the torch to plate distance will change because of the surface speed change.

3) a Mach3 / Candcnc system fitted with a servo sounds like it has good speed capabilities. Under what conditions will this NOT be enough, and the fast precise control of PID is necessary. Of course 1) above also needs to be considered. A sudden change of 10 volts could be when passing over a void. If the z speed is capable of reacting to this very fast then it could slam the torch down onto the plate. Candcnc has a TIP SAVER feature that locks the z when a sudden increase in voltage is encountered. I don't know if other systems also have this feature.

Because I've dealt with PID controlled machinery and understand it's phenomenal advantage to maintaining a precise control of a setpoint which can fluctuated wildly, I'm actually interested in your PID THC system but need to know exactly when the benefits come into play for basic air plasma systems, and when a Mach3/Candcnc system will be good enough.

It would be good if anyone could chip in to say under what conditions they could not get optimal (or satisfactory) torch height control with a Mach3 or Candcnc system.

Keith.