pulling bow out of 1/2" plate for table top

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I've got a piece of 1/2" steel that is 2' x 3' that I bought to make a bread-board style TIG table with (so I can use step clamps and other hold downs that will utilize a pattern of 1/2"-13 holes that I'm going to put in the plate).  It's been sitting for a while (like most of my projects do at many times during their lives), but tonight I finally dragged it out to check flatness on it.  It's bowed in both directions about 1/16" with both bows toward the same side of the plate (so it's basically a really shallow rectangular bowl).My initial though is to put some sort of significantly stiffer subframe underneath the plate to which the plate would get pulled down and welded to.  Looking at area moment of inertia as measure of bending stiffness, here's how some numbers shake out:plate, bending along 24" direction:  0.375 in.^4plate, bending along 36" direction:  0.25 in.^41/2" x 4", oriented vertically:  2.67 in.^41/2" x 3", oriented vertically:  1.125 in.^4Assuming I can make a shadow-box style subframe that is decently flat, I think it should be able to flatten out the plate decently since it (the subframe) will be many times stiffer than the plate.  Of course, this isn't taking 3D effects into account, which may be important since I was planning to keep the subframe close to the edge of the plate.So here are my questions:1)  Is flat bar the right stuff to be using for a subframe like this or should I be looking at using something else?2)  Would the 4" be overkill?  Just one strip of it is over 7 times stiffer than the plate is in its stiffer direction.3)  Should I add a center bar running down the 24" direction or 36" direction or both?4)  What's a good flatness to shoot for prior to blanchard grinding for something this size?Below is an underside shot of what I've got modeled so far.  Subframe (1/2" x 4" flat bar) and legs (2 x 2 x 1/4" box tubing) shown translucent.  The subframe would be welded to the tabletop (thinking 1" stich welds with 1" spacing) and then the legs would bolt to the subframe.  The legs would most likely be part of their own frame (instead of individual legs) and most likely incorporate some leveling feet and retractable casters... Attached Images
Reply:welding on on side of a plate will bow it towards that side (if I am remembering it right), so build it with the legs on the convex side and it might straighten itself out some. Your perimeter frame won't effect the middle much though. Maybe an X brace would warp it enough to do what you want, and stiffen up the table significantly too.another thing might be to build it with the legs welded securely to the framework and flared out, then pull them in and weld them under tension with the same frame structure near the bottom.drilling all the holes might have a slight warping effect too, and weaken the table top, so drill 'em first.I'd drill 'em concave down.
Reply:That is what I did with mine. Be sure to round over the edges first, as that might relieve some of the stresses from cutting the plate. Also, drilling all those holes might relieve some as well.
Reply:Good call on the heat effects and holes.  The X brace sounds like a good idea for the center.I was thinking about this some more today.  Without any holes, the plate weighs 123 pounds.  ~118 with holes.  The 1/2" x 4" shadowbox frame would add another 59.  So that's another 50%.  I was wondering if that 50% would be better added as thickness to the plate (i.e. just get a 3/4" plate and use this 1/2" for other stuff).  Is 3/4" plate going to be much flatter, generally speaking, than 1/2"?  I'd rather this table be an aid to getting other projects done than being too much of a project itself.  If I was going to shop around for 3/4" plate, what should I be looking for as far as a pre-blanchard-grinding flatness?
Reply:Found a local shop with a 42" Blanchard grinder.  Gonna see what they have to say.  Stay tuned...
Reply:They were trying to steer me towards 4140 plate ($$$).  I called my steel supplier this morning and got a piece of 1" A36 for less than a third of what the 4140 in 3/4" would have cost.  I know A36 isn't the greatest stuff, but this table won't be getting banged on or torched on, so hopefully my gamble will pay off and it will stay flat.  We'll see how flat the plate is when it comes in and after I put a bunch of holes in it.  I know another vendor with a big gantry mill - may have them fly cut it if it needs a little massaging...
Reply:Just wondering, but could it have been put to a perimeter fram using counter suck bolts and lock nuts . From an x brace underneath and centered attach a thjreaded rod and a tensioning bolt to pull or push , or maybe a jack . Just a thought and maybe not a good one. I was thinking maybe that could compensate for future warping . MLM
Reply:That's a good idea.  I like having stuff modular / adjustable, especially if you're not sure if it's going to work like you planned it.  Plus doing everything with threaded fasteners, you avoid the heat input of welding the plate to the subframe...
Reply:Got my hernia-on-a-pallet today after a bit of delay due to my steel supplier losing my ticket during their physical inventory and then having to re-enter it.  It was nicely banded on there, so I was able to tip the whole mess down out of the back of my truck and onto the ground.  Dragged the plate off onto two trash can dollies, hit the top edges with a file, wiped her down with lube, and then checked flatness.  No discernable bow in the 2 foot direction.  In the 3 foot direction, it's about 0.040".  Time to start marking, drilling, and tapping... Attached Images
Reply:A lot of people ask alot about this, for thinner plate and smaller pieces it's generally more efficient to just buy another plate and check out the true at the supplier.I dont worry too much about my welding table as it's 2 inches thick and flat enough for what I do. the one thing to watch out for though is no matter how flat your table is there will still be stresses from welding that will pull everything out of square if you don't follow good practice.there are plenty of excellent welders that can make a box or table that is perfectly square without any suitable surface available.  In fact you could have everyhing straight as an arrow until you loosen all your clamps and then it springs into the shape it wants.while tolerance stacking is aways the enemy I have been seeing a trend where the trueness of a welding table has become more bragging rights than anything else.  It should be flat to a straight edge but it need not be a reference plate. If you do have a bent plate, look up flame straightening.  It's more an art than a science but there is plenty available and information and once you have it down it comes naturally.
Reply:You could do it the way they use to flatten out plates after explosion welding...  Heat treat, and a 3 million tonn press!  :PYou didn't build that.'85 Miller AEAD-200LE
Reply:Got it flipped over, removed bottom side dross, and got it up on a better platform (the bottom of an old rubbermaid cart):
Reply:I get the bow out of my tables by putting the legs on the convex side, weld them on, then pull the legs together until the plate is flat. Then wrap a foot rail around.
Reply:216 holes marked, center punched, and started with a #2 center drill:Last edited by dbotos; 10-21-2012 at 12:09 AM.
Reply:Originally Posted by dbotosGot my hernia-on-a-pallet today after a bit of delay due to my steel supplier losing my ticket during their physical inventory and then having to re-enter it.  It was nicely banded on there, so I was able to tip the whole mess down out of the back of my truck and onto the ground.  Dragged the plate off onto two trash can dollies, hit the top edges with a file, wiped her down with lube, and then checked flatness.  No discernable bow in the 2 foot direction.  In the 3 foot direction, it's about 0.040".  Time to start marking, drilling, and tapping...
Reply:$169, delivered to where I work.
Reply:how much of that was delivery?miller syncrowave 250hobart handler 140home made 400 amp engine driven in progress...
Reply:Dunno, but it saved me 75 miles of driving and a couple hours off work.
Reply:What did you use to start all your holes?
Reply:Originally Posted by Unfinished_ProjectsWhat did you use to start all your holes?
Reply:Marked everything with a 36" stainless ruler and a mix of ultra fine point sharpie and carbide scribe.  Center punched 'em all with an auto center punch and then drilled with a #2 center drill in a 3/8" corded hand drill.  The tap drill holes (0.422" diameter) are going to be done with a mag drill.  Hopefully I will get started on that this weekend...
Reply:Didn't get to any of the drilling / tapping this weekend, but I did get the plate up higher so I can actually start using it as a table and it will make it more ergonomic to drill and tap the holes as I do those.  The 2x4s were freebies and the casters were some I had gotten at Harbor Freight one time when they were on sale for some ridiculously low price.  The unpainted 2x4s are sacrificial so I can drill and tap through the plate and not worry about going into the frame.  The orange portion is 30" W x 18" D x 30" H (not including casters).  Working height should be just about right once the sacrificial 2x4s come out...