THC problem - Torch keeps lifting


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    Default THC problem - Torch keeps lifting

    Hi,

    I have a CNC Plasma machine which I'm refurbishing. It was never commissioned before, so the THC which was installed had not been tested yet. My main problem is that, when the THC is activated, the torch keeps lifting to the Max THC limit I've set, whether thats 10mm or 30mm. When THC is not activated, the machine cuts fine, keeping the torch at the set cut height. When active, the torch goes down, the floating head is activated, z is referenced in Mach3, it lifts to pierce height, torch activates, pierces, lowers to cut height, then as it cuts, the torch keeps lifting. It seems to be a signal issue. I've checked the wiring of the THC to Hypertherm Powermax 65 cutter, interface cable connections, etc. This particular THC uses its own voltage divider card connected to the Raw Arc Voltage of the Powermax 65.

    The display board of the THC shows a correct voltage. It starts at around 110 Volts, and as it lifts, voltage increases, as expected. So the voltage its reading seems to be correct. The adjuster knobs on the board also don't make any difference but turning them, up or down.
    Here is the link to the supplier: CNC Direct. - Mach Equipment
    The problem is that there is no documentation for the THC unit.

    I also know that this is not a Mach3 settings issue because I tested the machine with the hard drive and Mach3 of our other CNC machine with a working THC, the torch still kept lifting. My suspicion is that the problem could be a faulty voltage divider or THC board.

    But I want to ask if this is a problem someone has experienced before, with a solution I can try.

    System:
    - CNC Plasma
    - Mach3 with THC300 add-in
    - Floating head for sensing plate
    - Stepper motors with 203V Geckodrives.
    - Mach3 BOB from leeuwinga.nl

    Any feedback is greatly appreciated.

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    Default Re: THC problem - Torch keeps lifting

    The THC300 is a simple analog "window comparator" circuit. It takes a present voltage and has an over and under circuit with a dead zone of about 1.5V Any voltage over the knob setting cause the DOWN circui to trip and any voltafe under the preset cause the UP put to trip. About as simple as it gets. The UP and DOWN are hooked in MACH to the THC UP and THC DOWN INPUTS and act like an external jog command for Z even when X and Y are moving under code. Screen LEDs should show the UP/ down status. The external voltage divider card divides the raw arc volts by 7.3 :1 so you should be able to test the raw volts at the input tabs (Elctrode and Workclamp and then read that divided by 7.3 a the Divided volts output back to the THC300.

    If you have the THC300D its a digital version of the same basic circuit but it displays both the real Torch volts as it cuts and the Preset volts when the torch is off . It also has Up and DOWN LEDs on it besides having the UP and DOWN in MACH. So tunr off the THC button in MACH and do a test cut and watch the Torch Volts. Then turn it on and make sure the UP led is ON when it climbs.



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    I once faced a case in which the plasma cutter refused firing. Firstly a voltage divider was designed by using the calculations. But circuit didnt worked, the same process repeated thrice but it didn't worked. Did you checked the circuit using a separate DC supply? Faulty THC has less chances troubleshoot the divider first.



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    Default Re: THC problem - Torch keeps lifting

    Quote Originally Posted by Torchhead View Post
    The THC300 is a simple analog "window comparator" circuit. It takes a present voltage and has an over and under circuit with a dead zone of about 1.5V Any voltage over the knob setting cause the DOWN circui to trip and any voltafe under the preset cause the UP put to trip. About as simple as it gets. The UP and DOWN are hooked in MACH to the THC UP and THC DOWN INPUTS and act like an external jog command for Z even when X and Y are moving under code. Screen LEDs should show the UP/ down status. The external voltage divider card divides the raw arc volts by 7.3 :1 so you should be able to test the raw volts at the input tabs (Elctrode and Workclamp and then read that divided by 7.3 a the Divided volts output back to the THC300.

    If you have the THC300D its a digital version of the same basic circuit but it displays both the real Torch volts as it cuts and the Preset volts when the torch is off . It also has Up and DOWN LEDs on it besides having the UP and DOWN in MACH. So tunr off the THC button in MACH and do a test cut and watch the Torch Volts. Then turn it on and make sure the UP led is ON when it climbs.
    Thanks for the reply.

    The voltage reading on the THC display seems to be correct. It reads around 100V at start of cut, but then as it lifts to about 20mm above the plate, reads about 170V. The "UP" LED is on until it reaches MAX THC height. One thing I noticed was when I turned off the THC button in Mach3, the "UP" LED still stayed illuminated despite not lifting. Would the best be to measure the volts before and after the voltage divider card to see if its functioning correctly? But if it wasn't functioning properly, wouldn't the voltage reading on the THC diplay also be wrong, when, to my understanding, its giving a correct reading?



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    Default Re: THC problem - Torch keeps lifting

    The UP LED is triggered from the THCnot from MACH. Turning off the button in MACH is supposed to turn off MACH's THC logic (it simply ignores the inputs) so it should stop any UP motion. It sounds like thre is a problem with the THC300D if it is locked in UP mode. Since the display shows the correct voltage either the output logic on the THC300D is fried or the input is bad at the PC or the input polarity is wrong. Note the UP down indicators on the THC300D . If uP is on there then its a probelm in the THC300D. Measuring volts will yiled nothing in the way of an answer. You already know from the readout its getting volts and they are an accurate reflection of the actual torch volts .

    Bottom line is there is a problem with the THC300D and the unit is no longer manufactured nor serviced.



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THC problem - Torch keeps lifting

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