Understanding THC

[mechfx]

HI all, first time posting..... be gentle......

I have CNC Plasma table, running Mach3, ESS Smoothstepper and a recently fitted PROMA THC (which amazingly worked straight out of the box !)

my question is regarding setting z axis zero.

When I'm doing multiple start and stops, (eg cutting the letters of a word) in 3mm steel, the material warps all over the place and the THC correctly adjusts once each cut has started.

My problem is when the next letter starts cutting, the z axis cut height from the original GCode is used and as the material has warped (generally higher) the torch tip is touching the sheet and the arc will not strike, so the THC has no chance to adjust the height.

Is there a way to "update" the absolute zero by the latest correction of the THC so the plasma head is now at a clearance height to the material ?

Or is there some other way to make it work ? Have I missed something ?

cheers
Art

[sphurley]

What are you using to create the gcode? SheetCam has distance to touch offs setting so once it touches off (zeros) and moves past the distance you set it will touch off again.

[robertspark]

I would suggest writing some macrocode for Mach3 which zeros the z axis and sets it to the correct cut height for every cut, then add that within the M3 macro

something such as:

Code: Select all

Code "G31 Z-__ F__ "       'add in the z downward distance required eq 100mm / 4 inches, at the feedrate you desire
Code "G4 P1"                            'allow for pause between completing motion (sometimes helps in completing code / stabilizing machine motion)
Code "G92 Z___"                       'zero's the Z axis DRO, add floating head switch offset distance
Code "G1 Z___"                         'rise to pierce height, add height offset you require
Code "G4 P1"                            'allow for pause between completing motion (sometimes helps in completing code / stabilizing machine motion)
DoSpinCW()                              ' fire torch {if THC enabled, once ARCok is received you will need to drop to cut height and then begin X+Y motion}

'pause is presumed to be in seconds, may need to change to 1000 if machine / ESS is set to milliseconds

If you are running sheetcam you can do the same thing via the post processor but both will achieve the same goal, the pierce height are normally the same for most materials using the same torch.

If you have an ohmic touch off + floating head then you can modify the code to test for which one has been triggered and set the right offset. I normally do this via a normally open relay so that both the ohmic and floating head are wired back to the same G31 probe input.
At the start of the G31 routine, I close the relay, and test the input (via code) and if the input is triggered, I then release the relay and then use only the floating head input, otherwise I probe with the relay enabled until the G31 skip (probe) input is triggered at which point, I then break the relay signal and test the input, and then set the offset according to the ohmic or floating head input.

[tcaudle]

Is there a way to "update" the absolute zero by the latest correction of the THC so the plasma head is now at a clearance height to the material ?

Or is there some other way to make it work ? Have I missed something ?

First you have to have a method of "touch off" The Proper term is "Initial Height Sensing" (IHS). Then the rest is easy. As noted its built into the MACH3 posts in SheetCAM you can set the distance between touch offs in SheetCAM in the post. The value is RefDistance = ## where "" is in mm in the post. I use 500 mm as a default (about 20 inches of total XY travel) but you may find on thin material it needs to be less.

[mechfx]

Thanks for the suggestions, thats starting to make sense.

I have never seen another machine set up other than this one that I built

I dont have a floating head, but looks like I will have to make it operate that way

I am running Sheetcam, I will investigate this feature more.
I'll also have a look at an initial height sensing set up,

cheers, Art

[acourtjester]

To give you a a better understanding about the starting process for cutting THC and Floating head (Ohmic sensor) are 2 different operations.
Since most of the time the metal we cut is not exactly flat so a way to control the torch height is needed for both start and continuous cutting.
For the start after moving to the start position the top of the metal needs to be found so the correct pierce height is set (value set in SheetCam). A floating head assembly or ohmic sensor will do that see attached video (no arc used for this). It is better to have both on your table the floating head assembly as a safety.
Next is the THC this starts after the pierce and will control the cut height as the torch continues to finish the cutting operation by monitoring the arc voltage of the cutting arc. This is done with different vendors electronics from a complete original setup to an add on type.
https://www.youtube.com/watch?v=irhDCCx ... e=youtu.be

[mechfx]

excellent info, thanks, Art

[PlasmaGuy8]

So I am having a similar situation in cutting 16 gauge steel. I have a home made table that is 4'x8' with a water table. It uses a Hypertherm 45xd with floating head, THC is proma 150, (not my favorite but its what I have right now). I am running MACH 3 (tyinking of upgrading to MAch 4) I have large double filters with 1 micron filters and dessicant beads to keep air supply dry and clean. I use Hypertherms manual settings for cut speed, pierce height etc. When the Proma is dialed in with right voltage (90-92 volts for me) , it cuts beautifully when steel is flat. But if I cut out a detailed design and metal warps, the next pierce point the tip goes down, straightens the metal, touches off and raises up the recommended .15 in pierce height. But because the metal is warped higher than the pierce height it doesn't raise the torch high enough to be above the warped metal so when the arc starts it causes my consumables to burn up fast. Once the torch fires the Proma does a decent job pf controlling the height, but that darn pierce point is frustrating.
As I am understanding this thread, you are saying having both the Ohmic and floating head hooked up in series, helps override the warped metal problem? I assume when the ohmic tip touches the warped metal it supercedes the floating head readout and raises up to pierce height from their?

[djreiswig]

Ohmic will sense the metal without pushing it down. The floating head is a backup in case the ohmic misses. You could wire it as an estop to keep from breaking something. Some people have figured out how to make the floating head switch apply the switch offset and go to pierce height even if the ohmic misses.
It's more complicated to setup this way in Mach3. If you are mostly cutting clean metal you shouldn't have too many misses. I would just wire it as an estop.