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Mch3 plasma THC min and max question
« on: June 27, 2010, 04:44:32 AM »
Hi,

We've built a plasmacutter recently and have some problems with the THC.
The problem is that however we set the THC min and max between values, e.g. 0 .. 3mm,
but Mach allows it to go under the minimum value.

Especially when the tourch goes on, the arc Voltage gets high for some subseconds till the material is cut through,
at that point Mach contols the Z down, because the THC down input gets active. This would be OK, if it
checks the THC min value and would not allow the Z to go lower than 0 (THC min), but it controls the Z into negative and
pushes the tourch into the material which is not so good.

We've somehow overcame the problem with generating the G-code with some delay after the tourch activation and only after the delay
switching on the THC. This solves the problem, because the arc Voltage normalise before the THC goes on, but I'm still confused why Mach
allows the Z to go lower than the set THC min value?

Video of the machine first testings: http://www.cncdrive.com/MOV00355.3GP

I have another question which is not really required for the machine, but interests me: How to set a negative value into the THC min/max. fields?
I mean Mach does not accept a "-" (minus) character to be written into these fields, however the basic screen includes a negative value on first startup.
So, I'm wondering why I cannot write a negative value into the THC min and max. fields?

All ideas are appritiated!

Cheers,
         Balazs

Offline stirling

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Re: Mch3 plasma THC min and max question
« Reply #1 on: June 27, 2010, 07:58:31 AM »
You don't say which THC you're using but I use the THC300 and it's a bit of a black art to get it to behave and I find myself doing things just like you've done. That said any THC shouldn't command Mach to go lower than whatever you've set your cut height to be as long as you've got your reference voltage set correctly.

Re: your -ve THC min: Again you haven't said what screenset you're using but I'm guessing it's the "default" mach plasma set. If so then you're quite right it doesn't allow -ve numbers - no conspiracy - just cockup. You'll need to get the screen editor of your choice out and change the field to %+1.3f. (note the plus sign)

Cheers

Ian
Re: Mch3 plasma THC min and max question
« Reply #2 on: June 27, 2010, 08:21:25 AM »
Ian,

Thanks a lot for the advices.

I'm using my own built THC electronics and the original Mach3 plasma screenset, I've made only some minor editing in the original screen, just added some buttons
and wrote some VB script behind them, nothing serious, just some things which made work easier...

Anyway a THC electronics seems quite simple to make, it is only some resistors, caps, comparator and optocouplers wired up correctly.  ;D

I don't have any problems with the THC electronics, it works as it should work, I only have problem with Mach THC handling, I mean the problem is that it allows
THC to go under the set minimum value, however it should not do this.

An interesting thing is that when emulating the THC (with keyboard keys) then it works well, it does not allow the Z to go under min. THC value,
but when the keys changed to LPT inputs then it works unwell.

How we overcame the problem is we've adjusted a Sheetcam postprocessor to only switch THC on after the torch comes on plus some delay
and this solved the problem, but I'm still curious why Mach allows the Z to go under 0 when THC min set to 0.

So, I feel that some logic inside Mach is faulty......

About your screenset idea, I see and I will edit the field as adviced and thanks a lot for the idea.  :)

Cheers,
          Balazs

Offline stirling

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Re: Mch3 plasma THC min and max question
« Reply #3 on: June 28, 2010, 07:29:57 AM »
I mean the problem is that it allows
THC to go under the set minimum value, however it should not do this.

An interesting thing is that when emulating the THC (with keyboard keys) then it works well, it does not allow the Z to go under min. THC value,
but when the keys changed to LPT inputs then it works unwell.

Balazs - just double checked on my system and it works fine emulated AND for real.

Cheers

Ian
Re: Mch3 plasma THC min and max question
« Reply #4 on: June 28, 2010, 07:56:50 AM »
Ian,

Thanks for helping me. :)

I write down shortly how I tested things and you will see that this is sure Mach fault/bug:

I've tried to first emulate the THC with keyboard and THC worked in THC min .. max. range, so that was OK.

Executed a program and before movement Mach wait for the Tourch on signal, that is also OK.
Then I pressed torch on key (emulated) then motion started which is again good.
If then I pressed THC down, Z moved down, if I pressed THC up, Z moved up, but it did not move out of range, so everything OK when emulating.

Then I go to check it with the LPT port instead of emulation, put the THC up and down and also the tourch on signals to different LPT port inputs.
The machine and plasma is now not connected at all, everything is on desk connected to a breakout board and buttons connected for the signals.

Now I executed the same program as before.
Program start.... before motion Mach waits for tourch on signal which is OK.
I press the torch on LPT pin and motion starts.
Now I press the THC down input and Mach pushes the Z axis down endless without limit, and this is where the problem is  :-\
Interesting is that for THC up it not allows the Z to go above the max. limit.

It works the same way on the machine setup too, however now it not pushes the Z down because g-code now includes a THC off and torch on and delay and THC on sequence. In this way the Arc Voltage gets in range before THC control switched on, so now everything works well, except that I feel this unsafe and wondering why Mach allows the Z to go under THC minimum.  :-\

Maybe you can tell me the version of Mach you are using? I will try to downlod and install it and will test it, maybe I have a version which have a bug,
however I've tried 2 versions, but maybe.....

Again thanks for your time with help Ian!

Cheers,
         Balazs

Offline stirling

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Re: Mch3 plasma THC min and max question
« Reply #5 on: June 28, 2010, 10:59:48 AM »
Version R3.042.020
Re: Mch3 plasma THC min and max question
« Reply #6 on: July 01, 2010, 07:13:43 AM »
Thanks Ian,

I tried different versions, so I will try this too and will report if it works at me.

Anyway now the machine is working and cutting steel 8 hours a day already, so there is no urgency, but
I'm curious to have the better solution, so I will test the R3.042.020 version. :)

Cheers,
          Balazs