Z axis setup for plasma - have I done it correct ?

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21 Apr 2021 12:20 - 21 Apr 2021 12:28 #206637 by beefy
Just installed a home switch at the top of may Z axis.

The Qtplasmac documentation did not say what values to set so I took a guess and did it this way.
If I've done it right then this may help others. Feel free to tell me if I screwed up.

I am calling top of slats machine zero for the Z axis.

So to get the value for the home switch activation position I placed the torch nozzle directly over the lowest level slat then manually wound the Z axis up until the home switch activated.

In PNCCONF I set the homing to back off the home switch until it's reset, so I manually wound the Z down until I heard the home switch reset.
I then measured the distance with a steel rule from top of slat to bottom of torch nozzle, and got 145mm. That is my home activation value in PNCCONF.

I put 143mm in PNCCONF for the final home position, so the Z would move down 2mm and come off the home switch after homing.

My top limit switch activates at 150mm above top of slats, so I made Z max limit 148 in PNCCONF so the Z soft stops 2mm before the limit switch.

I made Z min limit -2. This more than allowed for the travel distance to activate the floating Z switch, plus the deceleration distance at my touch off velocity/acceleration.

Tested the Z soft limits at rapid speed and it works like a charm. REALLY STARTING TO LIKE LINUXCNC :woohoo:

Now something came to mind while doing all this !!
I did this setting up with a Finecut nozzle in the torch. When I change to the other nozzles (40, 60, 80 amp) they are a completely different shape and I'm assuming will change the distance to the end of the torch nozzle. This will mess up all the above settings.

So I guess I'll have to reposition the torch in the holder each time I swap from Finecuts to standard nozzles and visa versa ?
Or is there another way to get around this ?

Keith
Last edit: 21 Apr 2021 12:28 by beefy.

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21 Apr 2021 22:59 #206733 by andypugh

I am calling top of slats machine zero for the Z axis.


I know less than nothing about Plasmac. (ie, I know things that are probably wrong) but typically life is easier if you set top of Z to zero, and then all valid Z positions are negative in machine space. Yes, this is counterintuitive, but you have to actively choose to see machine coordinates anyway, and some CAM packages (Fusion 360 for one) assume this and start every file with a G53 Z0. If that's your slats, that is where your head goes.

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21 Apr 2021 23:59 #206741 by tommylight

So I guess I'll have to reposition the torch in the holder each time I swap from Finecuts to standard nozzles and visa versa ?
Or is there another way to get around this ?

There is no need to reposition anything as probing works all the way to the top, granted the probe speed and height is set accordingly.
And QtPlasmaC will show the actual height from the top of the material while cutting and reverts back to machine values when not cutting. This way it does not matter how Z axis is set up as long as it homes and moves correctly.

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22 Apr 2021 00:59 - 22 Apr 2021 01:02 #206753 by beefy
Cheers Andy,

I might do what you said there and make the home switch position zero.

So I guess I'll have to reposition the torch in the holder each time I swap from Finecuts to standard nozzles and visa versa ?
Or is there another way to get around this ?

There is no need to reposition anything as probing works all the way to the top, granted the probe speed and height is set accordingly.
And QtPlasmaC will show the actual height from the top of the material while cutting and reverts back to machine values when not cutting. This way it does not matter how Z axis is set up as long as it homes and moves correctly.


Cheers Tom,

I get what you are saying there but probing is for setting your Z work offset and does not affect your Z_MIN_LIMIT soft stop position which is related to machine co-ordinates. In the Qtplasmac documentation it instructs you to set Z_MIN_LIMIT so the torch nozzle is just past bottom of slats. The distance past is the addition of float switch activation distance and the deceleration overrun distance. In my case that's only about 0.54mm but I just stuck 2mm in there. So the bottom of the torch nozzle will soft stop 2mm below top of slats.

OK lets say I change the nozzle and the standard nozzles are longer by 5mm. My soft stop will now stop Z travel at 7mm below top of slats.
That's not a big deal but imagine if I set up the Z as per the instructions but with the longer nozzle. When I change to the shorter nozzle my soft stop would now be 5mm less or 3mm ABOVE top of slats, so if I was cutting 1mm plate the torch would not reach it, and could not even touch off.

Hope all that made sense.

Keith
Last edit: 22 Apr 2021 01:02 by beefy.

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22 Apr 2021 07:05 #206768 by rodw
Keith, this whole Z axis thing did my head in when I first started and it took a couple of years before I finally understood it. I would follow Andy's advice where the top of travel is Z 0 and moving down from there makes the DRO goes into negative values.

Then I would go to the bottom of travel and touch off the axis to 0. There is a button in all the GUI's to do this but you can also do it by typing G54 Z0 on the MDI window. So now the DRO will report sensible values so +ve numbers are above the slats.

Linuxcnc will remember that value so I never bother to touch off the Z again. When I do, I know I have 86mm of travel so I will sometimes go to top of travel and touch off to 86mm (eg G54 Z 86) which is easier if there is some material in the road stopping you go get to bottom of travel.

As far as setting up your torch, I would allow your shortest tip reach the slats (or a bit below them). The probing touch off will set the correct height as Tommy says. That makes it much easier to change consumables

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22 Apr 2021 07:38 - 22 Apr 2021 07:41 #206771 by beefy
OK thanks Rod, understood.

With Mach3 and UCCNC I always set my Z axis work offset (G54) to zero with a touch off on top of material. Only had a touch off switch with those systems.

And I'll do exactly what you say for the MIN soft limit concern and use the shortest nozzle to set up the Z. I'm guessing the Finecut nozzle is the shorter but I'd better check.

Later on I might even make some sort of height positioning jig for the torch so whatever nozzle I use I can quickly set the height of the torch tip to exactly the same level each time. Just don't like the idea of the soft limits allowing the nozzle to go too much down past top of slats. I have no breakaway yet and my machine torch cost me $1100

Keith
Last edit: 22 Apr 2021 07:41 by beefy.

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22 Apr 2021 11:06 #206779 by bevins
I am just finishing up a plasma with QTPlasmaC. I puit my home switches to the top of the Z axis and the qtplasmac docs tell you to go below the slats and set your Z0 which is default G54. Then home again. After that I have not touched my Z0 and I have cut alot of stuf practicing and never have to revisit. I am sure there will come a day that I have to but for now it works great.

Just my 2 cents....

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22 Apr 2021 11:49 - 22 Apr 2021 11:50 #206782 by beefy

I am just finishing up a plasma with QTPlasmaC. I puit my home switches to the top of the Z axis and the qtplasmac docs tell you to go below the slats and set your Z0 which is default G54. Then home again. After that I have not touched my Z0 and I have cut alot of stuf practicing and never have to revisit. I am sure there will come a day that I have to but for now it works great.

Just my 2 cents....


Well the docs do not actually tell you to "go below the top of slats". They tell you to:
jog the Z axis down until it stops at the Z axis MINIMUM_LIMIT then click the 0 next to the Z axis DRO to Touch Off with the Z axis selected to set the Z axis at zero offset

So it appears we are setting WORK CO-ORDINATES (G54) origin zero at the MIN soft limit position, which happens to be just below top of slats because that's the way we set up the Z axis in PNCCONF (or whichever one) based on "4.3 Recommended Settings" in the documentation.

So what I said earlier would still apply. If you set up the Z with the longest nozzle, went through the initial configuration etc, then later on changed to a nozzle that was significantly shorter, the possibility exists that the torch could not get down far enough (due to MIN soft limit) to reach a thin plate. Perhaps this has never happened because the difference in length between the shortest and longest nozzles has never been enough for this situation to happen. And of course the answer is to set up with the shortest nozzle, as Rod suggested.

The one thing I am confused about is why we home the machine a 2nd time after setting the DRO to zero in step 3. in section "4.6 Initial Setup".

Keith
Last edit: 22 Apr 2021 11:50 by beefy.

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22 Apr 2021 11:59 #206785 by phillc54

The one thing I am confused about is why we home the machine a 2nd time after setting the DRO to zero in step 3. in section "4.6 Initial Setup".

That is a leftover from the days before we added a G0Z(just_below_the_max_limit) move in the GCode filter. It was easier to get folk to home rather explain that they need to jog back up to 5~10mm below the Z maximum limit. It it not necessary now but it sure doesn't hurt.

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22 Apr 2021 12:10 #206787 by beefy
Thank you Phill.

I hope I'm not irritating you with all these queries. I tend to have a thirst for understanding things a little too deeply at times. I could literally quit work and spent a lot of my time learning all this stuff. It's a bit of a drug that hooks me.

Keith

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