Frame steel tubing choice for 5'x5' plasma table

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07 Jan 2022 16:13 #231032 by dlv
Hi all.  A question to the collective wisdom & experience here on plasma tables,

I'm in the early stages of designing a 5'x5' plasma machine.   Actually I've bought most of the electronics and CNC components.  CNC components are all alum extrusion (40x80 "C rail"), Nema 23's & leadscrew, etc. so relatively light.  

I'm dithering on what steel to use for my table frame.  I'm not well versed in steel strength, I'm not a structural engineer.

This will be a hobby shop table.  Not a full time production shop.  Thickest steel I'll probably ever cut on this table will be 1/2" AR500 plate.   So figure around 700lbs for 1/2 sheet.  I may build an extension for thinner full sheets but I don't really have the ability to manage a full sheet of 1/2" steel.  I expect most of my cutting to be 1/4" or less but I don't want the thing collapsing if I ever put the big stuff on it!

This will be a downdraft machine so no water table.

Steel is stupid expensive here in the USA (probably everywhere) right now.  So I don't want to over-build too much (my usual approach is over engineer) and drive the cost up needlessly.   

I'm considering 2"x2"x1/8" box tube.   That runs about $10.60 a foot here.   I'm going to need about 50' if I do the entire table in the same steel in the current design.  

Things I'm wondering about:

Do folks think 2x2 1/8" box is strong enough for this design?
Could I go even lighter?  (say 0.90 wall tube?). Possibly for the legs and especially the bottom cross braces?
Do I need to go even bigger?  Maybe 2"x3" around the top frame?
Any other advice before I buy a lot of really expensive steel?

Many thanks in advance!

Here's roughly what I'm looking at building (sheetmetal for enclosed sides/bottom removed):
Note: I do plan gussets in all corners.

 
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07 Jan 2022 18:16 #231039 by tommylight
Leadscrew will not do for plasma, to slow short of having something with 15 to 25mm of move per revolution, and they whip a lot when rotated at any usable plasma speed.
Get belts with reductions, 10mm wide belts will do just fine for that size.
Keep the work table separated from the machine frame, 50x50mm is OK for that size, although i would use a bit bigger only on the two sides where the rails are mounted, helps with dampening vibrations from stepper motors.
Use alu section for gantry only, that needs to be lite weight, the rest is heavier=better.
forum.linuxcnc.org/plasma-laser/35349-cn...fo-for-building-them
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10 Jan 2022 12:33 #231299 by dlv
I scored a deal on some 2"x3"x1/4" steel tube.  Barely used and cheaper than the 2x2 1/8" stuff I was pricing new.   This should be more than strong enough!

 
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10 Jan 2022 13:54 #231306 by robertspark
strength is not normally an issue with plasma as it normally is best to be lightweight and fast moving (no cutting forces).

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10 Jan 2022 16:08 #231315 by dlv
Sorry Robert, I may have created confusion. This question is primarily about the table frame itself. No moving parts here. The CNC machine is separate and thank you yes, I'm going as light on the moving gantry as I can make it. :)

I wanted to be sure I choose table construction so I won't have any issues with handling the weight of the material I might have on it. Particularly say when wrestling 600+ lbs of steel plate into position.

I'm just a hobby shop so I'll have just a 2-ton shop hoist (Harbor Freight) to load material. No forklifts etc. here sadly. So 1/2 sheet is about as heavy as I think I'll get. This tube should handle that and them some I'd think.

Cheers,

-Dave

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10 Jan 2022 16:26 - 10 Jan 2022 16:27 #231318 by robertspark
if you have a sketch design / layout then I'm sure someone will offer advice on any bracing etc and an optimum layout.

Are you intending to use a water table / ventilated extract table?

(  head slap moment..... just seen the picture posted above!)   

what thickness of material are you intending to cut?

for a 5'x5' table I presume you are intending to cut 1" thick material of size ~4'x4' which is impressive.

I'd suggest that you probably want to think about your slat design at that sort of weight.
Last edit: 10 Jan 2022 16:27 by robertspark.
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10 Jan 2022 21:16 #231346 by dlv
Yeah downdraft.

Nah, 1/2" AR500 is about the thickest I'll cut. My machine is 60A and rated at 5/8 max for a clean cut.

I based my weight off of 5'x5' 1/2" mild steel which goes about 25lbs per ft2. That's where I got 625lbs. My actual sheet size will be more like 4'x5' due to 50" travel on my X axis. I'll be open in the Y direction so I might build an extension for index cutting 4'x8' full sheet but that'll be thinner stuff if I do. I won't have the capability to load full sheets of heavy stuff.

Great point on the slat design. I haven't given it much thought and you're right about giving that some thought. I was going to go with 2" wide slats and 2" apart but no matter what I'll definitely have to put some kind of center bracing down the middle at a minimum.

I'll get a better mock up made of that to post for feedback.

Thanks!

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13 Jan 2022 16:03 - 13 Jan 2022 16:12 #231674 by Blackslacks
Here.s a pic of my 4x4 I built a couple years ago.  The legs and top perimeter is 2 x 3 x 3/16.  The bracing is 1-1/4 x 1/8. I haven't done it, but I'd have no hesitation loading this with 1/2".  The water pan sits on top of the frame between the extrusions.
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