Machine Stand

[uncrichie]

I'm planning on welding up a stand out of square tubing for a machine weighing 300 lbs. Stand dimensions are 24"wide x 24"deep x 30" tall. will also have a 3/8" plate top. What size and thickness tubing should I use. Thanks. Uncrichie.

[enlpck]

Need more info about design (what bracing are you planning on, etc) 300lbs, even with the 3/8 top (which will be about 60lbs) isn't much weight. The load on the legs will be less than 100lbs per leg, and even 1", 0.065"wall tube will be more than fine for the compressive load. I presume that you arn't planning on just welding legs to the top plate (which would probably be fine here), and may be concerned about stiffness.


Presuming that you are making a frame to set the top on, and the frame is the same tubing as the legs, if you use 1", 0.065 wall tubing, the compression stress from the load on each leg will be 400PSI, which is about 1/2% of the allowable. This is about 2% of the buckling stress (actually less, maybe 0.1% of actual buckling stress, making crushing of the wall the issue, since I did the calculation presuming Euler buckling, which is for high slenderness ratio)

Deflection by side load, such as someone leaning against the stand, will be about 0.0025"/lb, so a side load at the top of 300lb (a really, really big guy leaning really, really hard) will deflect the top about 3/4", giving a max stress in the tubing of about 36KSI (roughly the yield for A500 tubing) at the top attachment point. Diagonal bracing to 6" below the top will reduce this to 30KSI.

Welding the legs directly to the top, with no other framing will be sufficient, using 1/8" fillet welds. (3/32 would be ok, as the throat would be about 1/16", but 1/8 provides a little margin)


If I were building such a stand, I would use 1-1/2", 0.125 wall tubing, put a frame on the top of the same tubing, weld the legs all around with 1/8 fillets where applicable and 1/8 groove on the outside, and call it a day. In fact, I have built similar stands using 1", 1/16 wall tubing and they are fine, if flexible.

[uncrichie]

Enlpck, thanks for the response. This won't be a tabletop design with 4 legs. It'll be a cube design. Top will be a square of tubing, same with bottom, then it'll have 4 legs between the two squares. After that is welded up I'll add the top. Probably add riser wheels to bottom. The kind that can be raised and the frame becomes stationary to the floor. This stand will hold a very small 325 lb mill/drill machine. Thats about it, nothing fancy. Uncrichie.

[enlpck]

I'd definitely go with 1-1/2" 1/16 or 3/32 wall tube, or 1" 1/16" or 3/32" wall with diagonals across the sides of the cube (one across each side is fine, corner to corner. No need for an X) to stiffen it. Might even do the diagonals with 1-1/2" tube, might not. For drilling, not an issue, but if hte machine will be used with milling cutters, the stiffness is a good thing. Overkill, but not major overkill. Small drill press (similar to a midrange modern benchtop type) at work is on a frame made of 2" leg, 3/16" angle, with diagonals. MAJOR overkill, but it was in the scrap pile (other option was 4"X1/4 angle....) It is actually built heavier than the frame for the 15HP Quincy compressor (five times the weight, 100000 times the vibration)

[uncrichie]

Thanks again, 1-1/2" tube it shall be. Kurt.

[fjk]

Enlpck, thanks for the response. This won't be a tabletop design with 4 legs. It'll be a cube design. Top will be a square of tubing, same with bottom, then it'll have 4 legs between the two squares. After that is welded up I'll add the top. Probably add riser wheels to bottom. The kind that can be raised and the frame becomes stationary to the floor. This stand will hold a very small 325 lb mill/drill machine. Thats about it, nothing fancy. Uncrichie.



Hi, you don't say where it will be, but if the floor isn't level
or smooth then you might want to add some levelling feet
to the base. I also would suggest that you not put the "bottom
square" exactly at the bottom of the stand. It wants to be up
3/4/5 inches to allow room for your toes to get underneath while
standing at the stand. Also, any little bump/unevenness/etc
in the floor anyplace where the horizontal members are will
make the thing wobble. (Apologies for wasting time if you've
already thought of this)

Also, what kind of riser wheels? Do you have a pointer to a web source?
I've been looking (on and off) for some time and have not found exactly
what I want. What you're doing sounds like what I am looking for.

Thanks
Frank

[uncrichie]

Frank, the wheels I'm using I picked up surplus. They have a permanent adjustable threaded post and pad. Along side is a castor wheel that also adjusts up and down. Then along side of that is a lever that will lift or lower the castor about 1/4"-3/8" just enough for movement. The initial adjustment between the post and the castor is critical to the release height. Its pretty neat. I hope they will hold the weight, they should.

The closest thing I have found available but does not use a lever just threads can be found at http://www.atlascasters.com/index.php/LCSX Looks like you block one then spin the castor down then move it and reverse process to make it stationary again.

If I find somehting like the surplus ones I'll add an additional address. Kurt.

[fjk]

Frank, the wheels I'm using I picked up surplus...

Thanks

Frank