The Craftmans Shop > New from Old
Titivating A CNC Plasma Table
RodW:
--- Quote from: PekkaNF on March 30, 2017, 06:36:53 AM ---Thank you, I was wondering the rollercoaster drive on steel plate. Long time ago I was watching often even then old "optical" tracker of semiautomatic oxyburner to cut plates. They were not that simple to use and adjust. Sometimes the "seeker went wild" and run ofcourse torch blasting thick steel plate to art forms :doh:
Pekka
--- End quote ---
I think now most Oxy cutters use capacitive sensing between the material and a ring around the torch that never touches the plate.
awemawson:
Hypertherm (makers of arguably the best Plasma power sources) advocate the use of 'Ohmic Sensing' as being most accurate, but my concern was I will often be cutting plate that is slightly rusty so it might not work reliably. There are systems out there that use both 'Floating Z' and 'Ohmic Sensing' - the floating Z effectively being a back stop.
Certainly Ohmic Sensing is simpler to implement and it avoids the use of a secondary slider.
... talking of secondary sliders - I'd got the hole spacing wrong on the THK-10RM rails and drilled and tapped at 20 mm centres rather than the 25 mm they should be :bang:
However as it was all made on an alignment fixture I've been able to return the prototype to the CNC Mill in exactly the same place as before, and run a little diddy program to spot face, drill and tap the relevant four holes (and also modify the Autocad drawing accordingly !)
So my Faux Pas is recovered from without raising a sweat :lol:
I do love making things on fixtures - this particular one just clamps in the Kurt vice, and I've bored an accurate 20 mm hole dead on centre which becomes my datum point. If I need to remove the fixture and replace it all I have to do is lower my Heidenhain touch probe into the hole and tell it to find the centre - fast and accurate. The various parts bolt to this fixture using holes that are part of the workings.
It also means in this case that the various parts are held about 10 mm higher than the vice jaws, allowing profiling of the edges without risking cutters and jaws.
RodW:
I know the feeling. I have one hole to fill in with weld and retap. I got all mine laser cut but until I could actually check for fit I was nervous as hell with $1000 worth of parts....
--- Quote from: awemawson on March 30, 2017, 08:52:21 AM ---Hypertherm (makers of arguably the best Plasma power sources) advocate the use of 'Ohmic Sensing' as being most accurate, but my concern was I will often be cutting plate that is slightly rusty so it might not work reliably. There are systems out there that use both 'Floating Z' and 'Ohmic Sensing' - the floating Z effectively being a back stop.
Certainly Ohmic Sensing is simpler to implement and it avoids the use of a secondary slider.
--- End quote ---
I don't think you can get of that lightly. Everything I've read says you need a backup either sensing motor torque or a float switch for those Oh! sh1t moments when you don't get an ohmic connection. I'd like to experiment with measuring motor torque by sensing the current the stepper axis draws. Even if I had to use an Arduino as an intermediary. My Plasma has a wire for the material clamp on its CNC interface (via a 100k resistor) so I might try it later. I have drawn up a circuit that should work and lock everything out while there is an arc.
Anyway, here's a sketch of the circuit I came up with. Check it carefully.
It was based on something I found on the net and I added the loop through the torch on relay as an added safety feature.
awemawson:
So, next thing to do is cut one of the THK-10RM rails in half, as I only need a very short movement. So - first transfer the slider off one rail onto the other one so that it can be cut without contaminating the bearing.
Now received wisdom is that this size of slider will fall apart shedding ball bearings unless the rails are pressed well together end to end, and the bearing pushed over the join carefully
Well I've news for you - they shed bearings anyway :bang: Hundreds of them, and they're tiny :bang: And in my case they are magnetic and stick all over the place :bang:
Now in the last picture that minute bit of black plastic is in fact one of the track guides that reverse the balls so that they circulate. Even with tiny tweezers it was very reluctant to go back into it's recess in the end cap.
awemawson:
So now the slider is in bits I decided that I might as well go ahead and cut the rail, so in the unlikely event that I can re-assemble the bearings at least it can be done on the rail it will eventually occupy.
Cut with a 1 mm abrasive disk on a 115 mm angle grinder held in a 'chop saw stand' (Thanks Spurry for the suggestion :thumbup:) I then tidied the ends up on the Clarkson T&C grinder
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