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Thread: THC initial pierce height

  1. #1
    Registered Karl_T's Avatar
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    THC initial pierce height

    I've been reading every thread I can find on how torch height controls work... Lots of stuff on the zone if you got time to sort the wheat from the chaff.

    I'm not seeing information on setting the unit's unit height before you turn the torch on to pierce. From what I've read it looks like the torch should be a bit higher for pierce (how much?) and I don't see how the equipment determines how to do this. I'm guessing you need an additional sensor. What is used?

    Karl

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  2. #2
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    General setting is twice the cut height. One way it is figured out is a floating touch head. When the head is all the way down the switch is engaged when it floats up x amount. It then pulls the head up that x amount when it touches off.
    Now the tip of the torch is on top of the metal. The program now raises the head to twice the cut height.

    Now you have to have a program that creates the touch off code or manually put it in to your gcode for every pierce.

    I hope this makes since.



  3. #3
    That code can be generated automatically using the correct POST in SheetCAM. It also allows for touch-off frequency so you can set it so it does not do it before every pierce. That can save time on a job that has lots of close pierces or the material is thincker and flat. The pierce is made higher because if you could watch the action in slow motion the metal starts to melt and forms a hollow and the molten metal "splashes" out in a upward cone shape. The further the end of the nozzle is from the splash the less like it will clog and instantly self destruct. It's kinda like blowing a hole in a wall with a bazooka: Stand too close to the operation and you risk injusry form the debris!

    When you look at all the costs involved in cutting, the pre-mature failure of consumables ranks higher than electricity or even possibly labor. Replace 3 or 4 sets an hour and you exceed the other costs of operation.

    There are several ways to do the IHS (Initial Height Sensing) each has pro's and cons.

    Here is a sample of the code that would be used in MACH for a touch-off with comments in brackets:

    G00 X12.345Y12.345 [move to next pierce point]
    G28.1 Z.5 [Move to Z home starting at homing speed (set in MACH0 at .5 in height]
    G92 Z0 [Set Z DRO to Zero when it stops after the reference (home) move]
    G00 Z #.### [lift the head the "switch offset" value (overtravel) programmed in the POST]
    G92 Z0 [rezero the Z DRO. At this point the tip of the torch should be just a the material surface]
    G00 Z0.125 [Raise to pierce height...parameter from the tool table in SheetCAM]
    M03 [fire the torch...wait for ARC OKAY Signal to release movement]
    G01 Z0.063 F40 [Plunge the torch form pierce height to initial cut height at the called "plunge rate" (from tool table in SheetCAM)
    G01 X#.###Y#.### [move XY]
    etc,etc


    With the torch able to "float" up (like a shock on a 58 Buick Roadmaster), when it moves towards the material and the tip hits the top of the material, it shoves the torch (and holder) up and trips the switch. The distance it has to move between first striking the plate and striking the switch is overtravel (switch offset) and remains the same as a constant. That value then is put into the POST as a constant (variable) that is used to raise the Z the correct amount each time (4th step above)

    All this happens pretty fast because the distances are small.

    SheetCAM has the unique ability to do calculations in the POST so it can measure the total distance of the XY moves and keep a running total in a variable and compare it to a preset (REFDISTANCE) and stick a touch off in before the next pierce.

    The moves in plasma cutting are more complex that doing other 2 or 2.5 d work because the Z of the complex IHS and pierce moves coupled with the fact the Z zero changes with heat distortion.


    TOM CAUDLE
    www.CandCNC.com



  4. #4
    Registered Karl_T's Avatar
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    Quote Originally Posted by Torchhead View Post
    ...


    With the torch able to "float" up (like a shock on a 58 Buick Roadmaster), when it moves towards the material and the tip hits the top of the material, it shoves the torch (and holder) up and trips the switch. The distance it has to move between first striking the plate and striking the switch is overtravel (switch offset) and remains the same as a constant. That value then is put into the POST as a constant (variable) that is used to raise the Z the correct amount each time (4th step above)

    ...


    TOM CAUDLE
    www.CandCNC.com
    Thanks for the very informative post. I'm designing the torch holder and lifting assembly and didn't allow for float and a limit switch. Its best find out out before you cut metal but this is going to take some serious rethinking.

    Karl



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    Karl,

    There are a variety of Torch Height Control (THC) systems from different manufacturers....using some different technology in regards to the way they are programmed as well as the way Initial Height Sensing (IHS) is achieved.

    Speaking for the THC systems produced by Hypertherm...initial height has two methods of finding the top surface of the plate. When the plasma start signal is issued by the CNC control...the torch z axis indexes towards the plate....if the surface of the plate is conductive (no primer, above water, not a lot of surface corrosion) the contact between the shield (frontmost part of the plasma torch) and the plate is sensed ohmically (electrically)......if the plate surface is not conductive....the the THC automatically switches to it's backup system which senses an increase in current on the z axis drive which is caused by bumping the plate.

    At this point, the z axis reverses, and after losing ohmic contact from the plate (important if you are cutting thin material that may deflect during IHS sensing) and (using encoder feedback) acurately measures the distance away from the plate for accurate pierce height. Pierce height recommendations come from the plasma system manufacturer for each thicknes, type of material and power level....and are very important to provide the best nozzle and shield life.

    Once the pierce height is accurately established...the THC sends a start signal to fire the plasma torch.....the pierce delay timer then activates (this can be a THC function or a CNC/part program function) which allows the (torch manufacturer recommended) pierce through the plate to be completed before any x, y or z movement occurs. As soon as the pierce is complete...the z rapidly indexes down to the recomended cut height, and the cut path contour (x and y) motion is allowed.

    Another nice function of the ohmic contact is....during steady state cutting if the torch shield contacts the plate for any reason (thermal stress in the plate..tipped up previously cut parts, etc.) the cut will continue and the torch will index up rapidly to the proper cut height...then will resume arc voltage sensing to control height.

    Depending on which THC, and which type of cnc control it is interfaced to....the settings for the THC can be done with a manual switch type interface box....or can be done through the cnc/part program on more sophisticated systems.

    Jim Colt Hypertherm



  6. #6
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    Quote Originally Posted by jimcolt View Post
    Karl,

    Speaking for the THC systems produced by Hypertherm...initial height has two methods of finding the top surface of the plate.

    Jim Colt Hypertherm
    You make me GWE (green with envy) I bought a cybermation table with an Innerlogic plasma at an auction. I'm working on replacing the control and adding THC. I bet a hypertherm is in my future.

    Thanks for all the help.

    Karl



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    I'm willing to bet Torchead (Tom Caudle) has been involved with retrofitting old Cybermation machines on more than one occasion.....he may have some good equipment packages that can make those machines work like new again. There are thousands of Cybermation CNC machines that were sold to HVAC contractors for plasma cutting galvanized duct fittings.....not high tolerance cutting....but the mechanics of the machines may be a good base for a retrofit type machine.

    Many have upgraded these machines with Hypertherm CNC controls, torch height controls and plasma as well......although that package can get a bit pricey...it can turn the machine into an industrial quality cutting system.

    Jim Colt



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