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How does CNC _actually_ work?

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mc:

--- Quote from: PK on April 14, 2017, 05:42:53 PM ---
--- Quote ---Something like Mach 3 which doesn't have any kind of built in non-linear kinematics,
--- End quote ---
I've used mach3 to control a mates scara arm with a grab on the end. You can key in formula that compute output positions for real axii (usually the rotary ones) based on X and Y.
Might have a pic somewhere....

--- End quote ---

I thought Mach 3 had limitations on what it could do?
It could be I'm mixing it up with some other feature (geometric corrections?), as it's not something I've ever paid much attention to.

Regardless, what the OP wants to do is achievable.

PK:

--- Quote from: mc on April 14, 2017, 05:53:30 PM ---I thought Mach 3 had limitations on what it could do?

--- End quote ---
I admit it was some time ago, so I just went out to the shed and checked.

Function Cfgs->Formulas, whilst notionally for screw pitch error correction and bed leveling, allows you to implement arbitrary kinematics..

LinuxCNC can do it too. We can (but don't) program this in an XYZAB space.



--- Quote ---Regardless, what the OP wants to do is achievable.
--- End quote ---
Agreed.

RodW:
You should read this LinuxCNC page on kinematics.
http://linuxcnc.org/docs/html/motion/kinematics.html

There are some simulators in the default Linuxcnc installation  for various robot arms etc that allow you to command a 3d drrawing of the machine on screen but i have not played with them.
As far as gravity and a vertical plasma table  all you need to do is to tab your parts which  is supported in sheetcam.

AdeV:
Many thanks for all the replies! Seems others were waaay ahead of me - I had no idea that there were controllers out there with reprogrammable kinematics. Cool! Now all I need to do is work out the formulae.... that should take me most of a day at least :D

Andrew - yes, gravity... I've been bitten by it more than once! As Rod says, leave tabs to keep the piece in place & free it with an angle grinder is one option. Problem is, even with a vertical table, it's going to take a fair amount of room; fume extraction is difficult, there's a very real danger of setting fire to the building (OK, you'd put a backplate behind it to stop the sparks...), and I'm still not sure I've actually got room for it.

I see my "robot arm" (TBH that's being generous to it) being something that stands unobtrusively out of the way when not being used, then gets dragged into the middle of the room, attached to a bench & used once in a while. I'll still need some kind of bed to hold the metal while it's being cut; but this doesn't need to be anything like as fancy as yours. I'm thinking a lightweight ali box with a few curved slats to hold the sheet being cut, and some way of clamping it down (magnets?) whilst its being cut.

Maybe I've been watching too many of those "Make it Extreme" videos...  :lol:

awemawson:
Beware of magnetic hold downs, they deflect the plasma jet so it doesn't cut where you want it to.

A while back someone developed a device with two axis's that clamped to the plate of steel being cut, and was CNC controlling the movement of an oxy-acetylene torch. Basically just two beams mutually at right angles with an appropriate carriage.OK it had relatively small operating envelope, but handy on site never the less. I seem to remember that they were hugely expensive.  No reason it couldn't carry a plasma torch.

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