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Tolerances on a cnc plasma table with plasma cutter?



brucer
01-25-2011, 03:47 AM
what kind of tolerances can you get on a cnc plasma table using a plasma cutter?


if i were to draw up a print of a part and the part had 1/2" bolt holes in it, would i just draw the holes at .530 diameter or would .562 be better? would the plasma torch be able to cut the .530 or 562 diameter holes good enough without me having to redrill the hole?

would a plasma get within .015 repeated? say if i needed to ream holes on location, could the cnc plasma get me close enough to where i could just ream the holes.. for instance if i needed to ream a hole .880 could i draw the holes at .865dia and the plasma put it on location and close enough to size i could just ream it, or would i be better off just machining the holes?

also:
if i were wanting to cut a 2x2 square pocket in a 5/8 plate and wanting to stick a piece of 2x2x1/4" wall tubing in the 2x2 pocket how much clearance should i figure in the drawing? would .015 per side be enough?

thanks
brucer

Gadget
01-25-2011, 08:45 AM
Brucer,
I've found the thicker the metal the more taper you will get when cutting circles or holes. The taper will be in the same direction (say -X axis direction) around the hole so one side will have the the metal taper away at the bottom and the other side (on X axis) will have more metal at the bottom kind of like this / / so the hole won't be cut perpendicular to the metal. For thinner pieces it isn't a problelm or not as much. The program you use to generate the G Code should have an offset function to allow for the plasma kerf. You can usually tweak this to get a very accurate cut but will have to test the kerf to dial it in. That will change slightly depending on the condition of the plasma tip at the time too. I've cut a 1.5" circle in .085 aluminum and measured it at 1.499. Recently I cut 3/8" bolt holes in 1/4" steel and had to drill them out due to taper.
Hope this helps.

KHK
01-25-2011, 10:55 AM
I agree with Gadget.
In addition;
If you want accurate cuts start with new consumables, If you are cutting many holes you may have to change the consumables frequently. The more you use the torch, the kerf will get larger causing tapered holes.

Kel Fab Creations
01-26-2011, 01:10 PM
for holes that need to be perfect i draw it .050 small then drill it after. when you cut to size and find it's small then try to drill it can be really hard on the bits. if you undersize it .050 or more it's easier to drill after.

samsberr
01-27-2011, 12:54 AM
Good to know...
Why reinvent the wheel.
Thanks for the tips.

brucer
01-27-2011, 06:15 PM
wish you guys with the cnc plasma's were closer...

had a quote to cut two 5/8 thick plates 24x34 triangle with a triangle cut out of the center leaving a 3 inch wide frame for $150, i think thats pretty steep, the steel was gonna cost $180... that was with me supplying a complete .dwg...

when i get into a better financial situation i will find a way to build or buy a cnc table... i'm pretty sure i could make some money with one.. i have a lead on some servos and a controller of a machine that was retro fitted with a new controller, might see what i can get them for just to have them..

matteh99
01-27-2011, 07:10 PM
for holes that need to be perfect i draw it .050 small then drill it after. when you cut to size and find it's small then try to drill it can be really hard on the bits. if you undersize it .050 or more it's easier to drill after.

I have used a tapered reamer to get messed up hole round again. Drilling out an oval weirdly shaped hole with a drill can be a royal pain. Once it is round you can then re drill it or leave it tapered.