Steel selection for rigid table

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Wonder if you folks could give me a hand. Spent a bit of time on onlinemetals just to get an rough idea of the cost for steel for an upcoming project. I'm not sure what I should get for square steel tubing. dimensions, wall thickness, 304, 316, stainless, etc.The project is a large homemade CNC router and Plasma cutter table. The outside dimensions are going to be either 5'x10' or 4'x8'.My current machine is built out of extruded aluminum, and just isn't rigid enough to get good accuracy, and its cutting envelops is only 2'x3'.  I can get +/- 8 thousandths or so in a 2'x2' area depending on how slow I run the machine which is OK.On to the steel:1) dimensions: 2x2 or 3x3 square tubing2) wall thickness .125, .1875?3) type of steel?? most clueless about this part. Most likely Ill have a decent welder by then (eye on a Miller 211) so I'll probably be MIG welding parts of it.ESAB Heliarc 252
Reply:3x3x.125 for the framing should be good.  regular hot rolled tubing, nothing fancy.  sturdy and cheap.  what made you use Aluminum for the current set up?  is the current set up also about 5'x10'?  what size aluminum do you have now?
Reply:In general taller/wider (depending on which axis you want the stiffness in) sections will always be stiffer than narrower ones of the same thickness. Even dropping down the thickness a bit the larger tube/sections will usually win. It's the same principal as a truss, and why a truss can use lighter members than a solid beam that will hold the same load, for example I bar roof trusses vs I beams..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Originally Posted by oxy moron3x3x.125 for the framing should be good.  regular hot rolled tubing, nothing fancy.  sturdy and cheap.  what made you use Aluminum for the current set up?  is the current set up also about 5'x10'?  what size aluminum do you have now?
Reply:Go 3"x3"x0.188" or 0.250"I just built a similar machine using 3x3x.125, I like the size but wish I had more meat to tap for bolt holes.Why are you considering stainless?BTW Onlinemetals prices should be about 175% of a local steel supplier.Last edited by fortyonethirty; 07-02-2010 at 06:03 PM.
Reply:I wouldn't even consider stainless for this.  It's rediculusly expensive and has no increased strength value over steel.  Unless you really need the corrosion resistance of stainless and are willing to pay the huge $$ premium that goes with buying stainless of that size, make it from steel and paint it and it will be fine.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Before you get all nervous and excited about exactly what steel topurchase and where to get it the cheapest, there's some other littlepesky things you may wish to consider before the fact-rather than after:1-finished parameters?-flat? within what tolerance?2-for plasma cutting considering a water table?3-design of the weldment-specifically to control weld distortion.Steel as supplied is not dead flat, square, or true.Consider using adjustable or shim-able contact point of the gantry/etc.on this framing for flnal leveling out over the entire work area.4-The design of this table needs to be thought out. Look at otheravailable tables for some ideas.Blackbird
Reply:Originally Posted by dave powelsonBefore you get all nervous and excited about exactly what steel topurchase and where to get it the cheapest, there's some other littlepesky things you may wish to consider before the fact-rather than after:1-finished parameters?-flat? within what tolerance?2-for plasma cutting considering a water table?3-design of the weldment-specifically to control weld distortion.Steel as supplied is not dead flat, square, or true.Consider using adjustable or shim-able contact point of the gantry/etc.on this framing for flnal leveling out over the entire work area.4-The design of this table needs to be thought out. Look at otheravailable tables for some ideas.
Reply:Originally Posted by DesertRider33I wouldn't even consider stainless for this.  It's rediculusly expensive and has no increased strength value over steel.  Unless you really need the corrosion resistance of stainless and are willing to pay the huge $$ premium that goes with buying stainless of that size, make it from steel and paint it and it will be fine.