Jcb plasma table

14 May 2024 16:28 #300584 by MrFluffy
Jcb plasma table was created by MrFluffy
Hi all, its been awhile sorry, not had much time for anything of late and I've just been quietly using my linuxcnc converted bridgeport, but it seems I finally will get to play again.
I decided I have need for a cnc plasma table, so I started to put together a list and get parts.
So far I have:
Usual old PC for control,
Mesa 7i76e for control,
Mesa THC-5,
3x 3Nm closed loop steppers with drivers + psu's, the closed loop function is handled inside the stepper driver so they look like normal open loop steppers to the mesa, I hope they dont get affected by interference...
A 200mm travel z axis
Various limit switches
4 HGR20 rails with trucks and ball screws for x & y
P80 plasma torch suitable for z axis mounting

For a table, I have a used very old JCB backhoe chassis that had become damaged in a roll over incident and had to be cut up :)
I rotated the backhoe chassis and started to cut off extra bits, and put it on heavy casters to roll about already, and am in the process of cleaning it up to mount the rails directly onto it via tapped holes then I'll shim the rail->chassis where needed. 
A chassis for a digger weighing 8t is going to be plenty rigid & I can move it round with the fork truck if needed without worrying about it distorting. I'll mount the control electronics inside the section of the frame at the front that used to form the machine's diesel tank, so it will be good for sheilding + heat dissapation.
I'm posting this because I'm a bit hazy on the details how to connect a THC to the plasma unit.
I have a mesa THC-5 ordered, but I'd like to use my Stahlwerk Cut60 because its 380v tri-phase and I have a amp limit to run the table + compressor.  Someone from Stahlwerk told me when I enquired that it was scratch start and I'd need to buy a later model to get HF start, but if you hold the tip near the work you can see a small spark jump before it establishes the plasma and it doesnt need to touch to establish so perhaps something was lost in translation.
I read that the stahlwerk plasma cutters need modification to get some of the signals out for cnc use, my search is failing me for finding more info on what's specifically needed to be done inside this plasma cutter itself, any links?

Also I have only one ballscrew for the table long axis (2m), is moving the gantry axis from one side only going to be ok? I'm a little worried that rapid moves will cause it to go out of square. I dont mind slowing it down a little if that can help eliminate this, I dont care if I have to wait a bit longer for parts.

Looking forward to finally making progress on making something again, and your comments.

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14 May 2024 16:56 #300586 by tommylight
Replied by tommylight on topic Jcb plasma table
Since you did not ask, i would say no ballscrews on plasma machines, except Z axis.
Use belts or rack/pinion preferably helical type as it makes less noise and less vibrations.

Also I have only one ballscrew for the table long axis (2m), is moving the gantry axis from one side only going to be ok?

Again, no for more than one reason in this case, to long so it will probably start whipping at 100-150 RPM, and one side only very, very slow speed, like snail pace, otherwise it will bend and rake a lot. Square carriages have wipers on them causing friction and requiring quite some force to move.
As for Stahlwerk, fire the torch in mid air, if it keeps arcing = all good, otherwise no good, although with a bit of hal wizardry it can be probably used but not reliably.
THCAD will need 2 of 1MOhm or 2MOhm resistors and a capacitor of 0.1uF/350V if you do not want to open the plasma and wire stuff inside.
Do this at your own risk, it is not advised to wire it to output terminals. Despite this, me and several others have it wired that way without issues.
Torch on should be on a connector so use that, and no need for ARC-OK signal as it can be derived from the arc voltage.

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16 May 2024 13:58 - 16 May 2024 13:59 #300746 by MrFluffy
Replied by MrFluffy on topic Jcb plasma table
I just realized I used to post under a different username before, but no big issue.
I originally went with ballscrews because I wanted it to double as a large format router for aluminium pieces, because it would have had more travel than my original bridgeport interact. However since I bought the parts, I've got a larger cnc machining centre too when I eventually get it commisioned properly.

I did some research about converting the screws to rotating nut and buying a second ballscrew of the same pitch for the long axis, but the pitch of the ballscrews is too fine so the nut speed would have to be so fast at plasma speed the screws would destroy themselves in a few months.
So, I've ordered some budget helical racks & pinions for the X&Y, and when they arrive the build can continue & I'll use the ballscrews on another project later on.

In the meantime I'll try the plasma as per your post.
Last edit: 16 May 2024 13:59 by MrFluffy. Reason: spelt some words the french way...

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