welding aluminum

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Making table @ 22" wide x 9' long x x 29"high with fabricated 2 1/2" x 2 1/2"  x 1" thick angle legs(to be butted and welded after cutting from plate) Will mill out top of legs so we are welding similar thicknesses to the 1/4"  or 1/8" wall rect tube ( 1 3/4" x ?)forming top frame to be welded to legs flush to outside. Final top thickness after skinning 1 7/8" - to max 2"Going to skin top with 1/8" or thicker sheet/plate  flush to outside of mitered frame mentioned above. All will be flush on  outside.Was debating weather to glue skin to top with toughened epoxy after welding legs then sand flush or to weld. Product will have to have automotive grade sprayed finish.If welding  I was concerned with heat and proper thickness  and alloy of skin to avoid panning, paintability or other issues plus that is 24 feet of weld by MIG or TIG.Will inlay MDF(fiberboard) inside of frame up against underside of alum skin to eliminate deflection and "drumming" if using skin of 3/16 or less. Any suggestions on materials, thickness of top skin and techniques.ThanksNorm
Reply:Norm. Sounds like you have just designed the worlds most complicated welding table. Unless you have to have this table for a special project I would suggest the basic thick as you can afford steel top with steel legs.
Reply:Steel has the added advantage you can tack stuff right to the table and tack weld fixtures to bend or align things.DennisThermal Arc 185-TSWMillermatic Challenger 172VictorO/AAtlas Craftsman 12 by 24 LatheEsab PCM-875Wholesalem Tool Mill-Drill
Reply:Keep in mind the thermal expansion of aluminum.    That 9' top can grow up to 3/8" in length as it's heated .   I'd suggest starting in the middle on both sides and work your way to the ends, alternating sides as you go.  Try to keep a symmetrical pattern for the welds so you can control how the top expands.Take your time and allow the top to cool sufficiently as you go.   If you try to weld the entire top without stopping, the built up heat will most likely cause it to distort.Also, I'd suggest tacking the top to the frame with 1/2" or less welds, no more more than 4" apart around the entire perimeter before final welding.   This will help lock down the top so it behaves better during final welding.If you're going to grind the welds smooth, I'd go with MIG to keep heat down.There are no small projects
Reply:I would Like to reinforce Imaginer suggestions. aluminum likes to distort allot it will expand and shrink. You just have to really take your time and not rush it and it will be fine.  But when you do grind it I would suggest a light sanding disc with a dual action sander to keep from gougeing the material to deep which will show through the paint. It takes a bit longer but it's worth it for a better finish