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I helped with building CNC plasma cutter as well: http://www.boot.lv/forums/index.php?/topic/133809-cnc-plazmas-griezejs/ http://www.boot.lv/forums/index.php?/t

by fest 10y ago

I helped with building CNC plasma cutter as well: http://www.boot.lv/forums/index.php?/topic/133809-cnc-plazmas-griezejs/ http://www.boot.lv/forums/index.php?/topic/133809-cnc-plazma... (beware of the latvian language but has nice pics). How did you solve the torch height control? We ended up using "digital torch height controller" - basically measure the arc voltage, compare to fixed preset and move Z axis up-down accordingly. This is a stupidly simple approach but requires complicated lookup tables of voltage vs height vs feedrate vs material and I'm thinking about how to improve it.

jacquesm 10y ago
That's exactly how I did it, but I built the controller myself. The arc voltage is directly proportionate to the height above the workpiece so you don't need much in terms of lookups or feedrate, all you want to do is to keep the voltage constant, so if the voltage is too high you need to get closer to the material (beware of strikes!) and if it is too low you need to back off (fast!, sometimes the material is warping towards the torch very rapidly, especially when the material you're cutting out of is thin). Where in Latvia are you? I go there with some regularity, I'd love to come visit. Mail: jacques@mattheij.com Tip: you could lose quite a bit of weight on that gantry but turning it into a lattice. http://pics.camarades.com/v/jacques/renewables/windmill/09_sep_final.jpg.html http://pics.camarades.com/v/jacques/renewables/windmill/09_s... Weight in a plasmacutter gantry is killing because it reduces the speed with which you can change the direction of motion while cornering in detailed work. That in turn causes you to have to slow down which will burn away fine detail so any weightloss on the gantry should be pursued. Ideally you want constant velocity relative to the workpiece while cutting. This also helps in maintaining precision because the cone shape that the flame has will cause it to cut thinner and thicker depending on the stand-off distance.
- fest 10y ago
  IIRC, the arc voltage was proportional to height only at constant velocity. I guess it was also a function of how much material is below the arc at the instant. I was working on a controller which was supposed to get it's setpoint by measuring voltage during first few seconds of a cut, but the project turned out to be outside of my EE skills at the time (isolating analog signals, high voltage resistive dividers that didn't blow up because I had no low-voltage feedback signal from plasma source etc.) About the need for rapid accelerations- that too was one of the things we learned by building this machine. IMO it should be stressed more in discussions about plasma cutters. In this case the blue linear slide actuator was off-the-shelf part and modifying it would require to cover the belt and linear guides inside with something else and was thought to be too much of an effort.
  - jacquesm 10y ago
    You can usually tap a point in the plasma cutter power unit that will tell you exactly what the arc voltage is, almost any slightly larger plasma cutter power supply will have a point that's convenient. You'll still need to use an opto-coupler, the way I built mine was the setpoint and comparator lived on one side of the divide, then two input bits, one for 0.1 V below the setpoint or lower, -0.1 to 0.1 was both inputs off and one for 0.1 V or higher, the setpoint was scaled so that it would have a nice range mapped to the inputs of two op-amps doing the comparison. The outputs of those op-amps drove the opto couplers and the software would track the 'Z' stepper up, down or hold depending on those two inputs. Super simple really.

by fest 10y ago

jacquesm 10y ago

That's exactly how I did it, but I built the controller myself. The arc voltage is directly proportionate to the height above the workpiece so you don't need much in terms of lookups or feedrate, all you want to do is to keep the voltage constant, so if the voltage is too high you need to get closer to the material (beware of strikes!) and if it is too low you need to back off (fast!, sometimes the material is warping towards the torch very rapidly, especially when the material you're cutting out of is thin). Where in Latvia are you? I go there with some regularity, I'd love to come visit. Mail: jacques@mattheij.com Tip: you could lose quite a bit of weight on that gantry but turning it into a lattice. http://pics.camarades.com/v/jacques/renewables/windmill/09_sep_final.jpg.html http://pics.camarades.com/v/jacques/renewables/windmill/09_s... Weight in a plasmacutter gantry is killing because it reduces the speed with which you can change the direction of motion while cornering in detailed work. That in turn causes you to have to slow down which will burn away fine detail so any weightloss on the gantry should be pursued. Ideally you want constant velocity relative to the workpiece while cutting. This also helps in maintaining precision because the cone shape that the flame has will cause it to cut thinner and thicker depending on the stand-off distance.

fest 10y ago
IIRC, the arc voltage was proportional to height only at constant velocity. I guess it was also a function of how much material is below the arc at the instant. I was working on a controller which was supposed to get it's setpoint by measuring voltage during first few seconds of a cut, but the project turned out to be outside of my EE skills at the time (isolating analog signals, high voltage resistive dividers that didn't blow up because I had no low-voltage feedback signal from plasma source etc.) About the need for rapid accelerations- that too was one of the things we learned by building this machine. IMO it should be stressed more in discussions about plasma cutters. In this case the blue linear slide actuator was off-the-shelf part and modifying it would require to cover the belt and linear guides inside with something else and was thought to be too much of an effort.
- jacquesm 10y ago
  You can usually tap a point in the plasma cutter power unit that will tell you exactly what the arc voltage is, almost any slightly larger plasma cutter power supply will have a point that's convenient. You'll still need to use an opto-coupler, the way I built mine was the setpoint and comparator lived on one side of the divide, then two input bits, one for 0.1 V below the setpoint or lower, -0.1 to 0.1 was both inputs off and one for 0.1 V or higher, the setpoint was scaled so that it would have a nice range mapped to the inputs of two op-amps doing the comparison. The outputs of those op-amps drove the opto couplers and the software would track the 'Z' stepper up, down or hold depending on those two inputs. Super simple really.