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Author Topic: G28.1 plasma touch off issue  (Read 7379 times)

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G28.1 plasma touch off issue
« on: January 10, 2017, 08:41:51 PM »
Im having an issue with the touch off at the beginning of each cut on my plasma table the G code generated by sheetcam

G28.1 Z0.12
G92 Z 0.0
G00 Z .0984
G92 Z 0.0
G00 Z .15
M03

The issue is that sometimes the plasma torch comes down and comes in contact with the material trips the switch on my floating Z head and everything does what it is supposed to, but sometimes it won't. Sometimes it stops 1/4" short of touching the material zeroes itself without ever touching the material, and goes through its process of moving offset etc. sometimes it's 1/2" up or more, but then the next cut it'll go all the way down trip the switch and everything is fine for a cut or two or ten. I've run the same program at least 20 times and it's random on which cut will work right and which one won't it's never the same, sometimes it only does it once out of the 25 cuts sometimes 4-5, but always does it at least once. It always registers when it has hit the top of the material and never crashes or tries to keep going past top of material so I know the switch is working and it's registering that the switch has been hit, and it goes through its process of zeroing, moving offset amount etc..

I had a problem with my computer ( bought machine used but working  I've never installed Mach before) had to get a new one and do a fresh install of Mach on new computer, everything was working fine on old computer so there's gotta be some little setting that I'm missing

Post processor that I'm using was set up by the guy who built the machine, and was able to upload it along with all settings and tools and rules he had for sheetcam when I fresh installed it too so I'm sure it's good, and he had it set to use a THC  but I haven't re-hooked up the THC yet, going through the forum here it's my understanding that you don't need THC hooked up for G28.1 homing to work, someone correct me if I'm wrong on this.

Offline Davek0974

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Re: G28.1 plasma touch off issue
« Reply #1 on: January 11, 2017, 02:24:17 AM »
Sounds like it might be an issue with the touch-off sensor or switch, i guess it has a floating head and uses the torch to sense plate height?

Could be a switch failing or full of plasma muck or a loose/broken wire.

The thing that makes me feel its not the PC is the random part.
Re: G28.1 plasma touch off issue
« Reply #2 on: January 11, 2017, 10:19:40 AM »
Thank you for the reply,

I thought about that too, but he switch never fails to work when the torch hits the material, I've run the program and watched the diagnostic tab and when the switch is hit the M3home light comes on along with digitize light and the corresponding pin light goes out I believe. When the torch does not go all the way down to the metal the lights never come on or do anything so the program is not being tricked into thinking that the switch has been hit, it just stops mid air and zeros itself. So Mach doesn't register that the M3home has been hit but ignores that fact.

Another issue which I think is related, it's my understanding that when you click machine coordinates and hit the Ref All Home button that the machine is supposed to go until it hits a switch if it has one other wise it'll zero where ever it's at

If I move the Z to let's say 4" above the table and hit the button sometimes it'll go down 1/2" sometimes 3/4" sometimes a full 2" and sometimes it barely moves. I have to hit the button multiple times to get it all the way to the metal but once it hits the metal and trips the switch it's good I can keep hitting the button and it'll never crash or keep trying to force the torch down.

Is there some parameter or setting that would not allow G28.1 to keep going until it trips the switch , or says only go this far and if no switch is hit zero anyway?

Offline Davek0974

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Re: G28.1 plasma touch off issue
« Reply #3 on: January 11, 2017, 03:24:38 PM »
Thats odd, when you use G28.1 it uses the home switch, when you use G31 it uses the Digitize or probe switch - neither use both ??

The G31 has got a travel limit parameter but not the G28.1 AFIK, just an axis and speed parameter at which it should move until the home switch is hit. G28.1 can be told to only move say 100mm regardless of the probe switch activating.
Re: G28.1 plasma touch off issue
« Reply #4 on: January 11, 2017, 05:03:39 PM »
Before I replaced the computer and everything I went into all the screens (or I thought I did) that Mach had and took pictures of everything the guy who had the machine before me had it set to and that's what he had and everything worked fine, he had Z home and Probe active, I disabled the probe and the digitize light quit coming on but I still have the same problem. But I ran a few different drawings and from different post processors in sheetcam and get the same result. I'm not sure if it did this before if it did I didn't notice, but I took a good look at the diagnostics tab at each run and every once in a while the torch will come down contact the metal and trip the switch but the M3home light doesn't come on, Mach didn't register that it hit the switch but the torch stopped, zeroed, went the offset amount, zeroed again everything just like it should, again that's random too never does it on the same cut and whether the cut before or after tripped the switch doesn't seem to have an effect on when it happens.

There's gotta something that I've set or haven't set or done that's causing this, where would G28.1 be told to only move a certain distance?

Offline Davek0974

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Re: G28.1 plasma touch off issue
« Reply #5 on: January 12, 2017, 10:43:23 AM »
I would make up a little g-code file, in it place just the homing routine so g28.1, zero, move to a height and so on.

Then place a flat surface on the machine, probe it by running the code and each time watch what occurs, i use this on my plasma to set the switch offset after maintenance/repair etc, it just does a probe / position each time i press start - no X / Y move at all.

I have the routine move to 1mm height each time and use a feeler gauge to verify it, if its off i adjust the switch offset and repeat.
Re: G28.1 plasma touch off issue
« Reply #6 on: January 12, 2017, 04:10:58 PM »
Ok I'll try that and see what happens

Offline Davek0974

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Re: G28.1 plasma touch off issue
« Reply #7 on: January 12, 2017, 04:21:07 PM »
Its just a useful way of setting/testing the touch-off system and also checking its repeatability, it should run over and over without fault

In ports and pins is the Probe input set as the same pin as Z home by any chance??
Re: G28.1 plasma touch off issue
« Reply #8 on: January 12, 2017, 04:47:28 PM »
It was but I disabled the probe input and left it as Z home only, but still same issue
Re: G28.1 plasma touch off issue
« Reply #9 on: January 26, 2021, 04:49:40 AM »
I have the same problem, have you found a solution?