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  Author Topic: Question on Plasma THC control to users of this facility (Read 8378 times)
  « Reply #30 on: November 27, 2016, 09:33:46 AM »
  Moray? is that correct. 2003'ish is that correct?
  It is a transformer based psu rather than switched it is a 24 Volt 12 amp Lamda PSU I had spare. The blowing of the chips was when I was arcing up or, more when I can to the end of the cut I had an arc away from the cut edge and a dwell time to the plasma cutter was trying to arc into thin air and the HF would blow an axis. Put it this way I have spare units and chips now sat on my shelves just in case.
  Robert, noted on the transformers if ever needed.
  Not quite sure what you mean by how am I doing the THC, or perhaps I do I have not mentioned what I am using have I
  I will copy what I have on another thread again in Mig welding
  And as there some other like minded souls on here I thought I would share my thoughts so they can be picked over and comments raised.
  When playing with my cut40, so I will assume very similar to cut50 etc. I found then when not cutting, i.e. the torch switch is off, the voltage at the terminals sits around -24 Volts ground to torch.
  When cutting, the voltage can vary depending I guess on height, nozzle, speed etc on mine from around -85 Volts to -110 volts. The manual says it can go to 250 volts and I assume that is when it is trying to cut but the arc is broken and lastly on mine the manual says the rated voltage is 100 Volts.
  So I read from that that I really want to keep the cutting voltage at 100 Volts, and that from what I have measured and assume, if I get less then 75 Volts I am probably driving the nozzle into the work piece and if the voltage was to go higher then say 125 volts I run the risk of breaking the arc.
  I hope in my assumptions are OK so far ?
  If I have read it correctly, THC controllers seem to provide 3 outputs: THC ON or ARC OK, move up, move down.
  From my readings and figures above, I would say that if I can measure a voltage across the terminals of between 75 to 130 Volts, it must mean that the ARC is OK, in that it has an arc between torch and metal.
  Then further to THC on, if the voltage goes above 101 Volts move the torch down and if the voltage goes above 99 Volts move the torch up. Guessing at a level of hysteresis here.
  I would not like to be messing with anything that can reach over 100 Volts DC let alone the no load level of 250 Volts DC. So the output will get tapped with a resistive divider. To get me down to a respectable level I will probably consider a 10 to 1 divider, so 250 volts becomes 25 volts 110 to 10 etc.
  Bring the voltage out of the plasma unit to a 4 way voltage comparator with relay outputs and set it up to provide THC on at a range of voltages between 7.5 Volts and 12.5 Volts.
  Down at 10.1 Volts and up at 9.9 Volts.
  I have a board from Ebay that looks like it may do the task
  This is from the Ebay add.
  and cost less tan £10 delivered, has 4 inputs and drives relay outputs which means I should be able to keep things isolated.
  It has a common ground between all inputs, has a 12 Volt input which provides an internal voltage of 31 Volts for the devices so can measure up to 30 volts.
  Initial tests suggest it could work, but the idea is open to your thoughts.
  So that above is what I looked at doing and I am part way through. The psu for the comparator is a plugtop 12 volt unit that is fully isolated, from my junk box, the board was less than £10 and I have just bought a plastic box to fit it in so I do not short out on my welding bench, wires and most of the other parts I have.
  If it does not work then less than £20 wasted, just time.
  If it helps make the cut40 and CNC that little bit better because I have a bit more control and I am not dragging the tip all the time then that will be a reward.
  « Reply #31 on: November 27, 2016, 06:29:59 PM »
  « Reply #32 on: November 27, 2016, 06:56:19 PM »
  I have some very simple filtering in there at the moment just to really stop HF, but yes I think you are correct in that I need to slow the readings from the arc voltage.
  I am considering just a simple Resister/Capacitor time delay/filter 5K Ohm and 20uF just to give around 100ms.
  I am also wondering about pierce delay as I believe from reading posts that I will loose the pierce delay in Sheetcam as soon as the THCON signal is present.
  I think I need to change config/ports and pins/spindle set up/CW Delay spin Up to be my pierce delay, but time will tell I guess.
  I have posted a bit more on the Mig welding forum as I have modified a few tips and test run the THC and Mach3, just doing another post processor now and a cad file to cut out three 15mm by 300mm slots in sheet for tomorrow.
  I will either have some success or smash things up with a hammer!
  « Last Edit: November 27, 2016, 06:58:36 PM by AdrianH »
  « Reply #33 on: November 28, 2016, 03:25:06 AM »
  Make sure you use the proper hammer!
  As far as you have gotten you wouldn't want to make a mistake now
  « Reply #34 on: November 28, 2016, 10:00:35 AM »
  Quote from: Davek0974 on November 24, 2016, 04:03:43 PM
  Well you were correct on this I was definitely wrong, I could only start the arc when tip grounded with a drag tip and then move up, The tips I modified to remove the castelations would not work, not sure if I restricted the hole a bit with the lathe work, so until I get a Plasma with pilot arc, that is the way it will have to be.
  There was a method I tried once that grounded the tip through high wattage resisters, may need to try that again to see if it makes a sort off pilot arc.
  What it would do to the arc volts is anyone guess.
  « Reply #35 on: November 28, 2016, 12:28:01 PM »
  « Reply #36 on: November 28, 2016, 12:52:00 PM »
  « Reply #37 on: December 04, 2016, 02:57:41 PM »
  I thought I would add a quick update.
  I have got Probing to work on my home made rig, using the torch position so I have no X and Y offsets to deal with only Z height offset.
  I am very new to this so I have questions on my Macro as follows
  So mainly using what I see in the Macro guide and I can understand, one question is do I need to have the following after the GottSafeZ
  Should I use SetOemDRO(802,-1.4) rather than
  Do I need that last Sleep there.
  It all works and in my post processor to generate G-Code I have an opening option set of G21 and G90 which sets milli-meters and machine co-ordinates if that makes a difference to this.
  Tomorrow is trying to get it all working together.
  Perhaps I should say using a micro-switch on a parallel port pin pulled up with a low'ish value 2K2 Ohm resistor, when the probe is active it opens the switch and allows the pin to get pulled to 5 Volts, seems safer then relying on a closing contact to always work.
  « Last Edit: December 04, 2016, 03:05:31 PM by AdrianH »
  « Reply #38 on: December 04, 2016, 03:04:27 PM »
  I'd suggest using sleep 100, as macros run at 10hz, 100msec.
  G92 z-*********x is correct.
  If you have a motion command, yes you need to use while is moving to let the motion complete before the next item.
  « Reply #39 on: December 04, 2016, 03:25:59 PM »
  Hi Robert thanks for the information I assumed the 100 was just arbitrary I will put it back to 100 from 80.
  I was not sure about sleep after the Code ("G92 Z-1.4") as it is not a motion command just a change to the DRO as such?