Whats the best way to do this to improve flatness for a diy fixture table. The next step will be to order the fireball diy hole guide.
The top is 54” x 36” x 3/4” and it was flame cut to size so its not 100% perfect but its pretty good considering
Flame straightening, or make a frame that can bend and flex the plate into position. 3/4 plate is strong so it’s going to need to be a stout frame if that’s the route you decide. Or build the table then have it milled or Blanchard ground.
@Fireball_Jason Flame straightening is an art I haven’t mastered. The 2 x 2 x .120 tubing base might be sufficient for this except that I also built it so that the plate over hangs by 6” on the short sides and 7” on the long sides.
I do also have some 2 x 4 x .250 wall and some monster (7700 ish lb) bessey clamps that I might be able to use to take the crown out I could probably also use some heat to help it out.
Ideally I’d like to not have to pull the plate off to take it somewhere to get it machined/ground as it’s a bit of an effort to move 413 lbs of plate around without help or some sort of mechanical lifting device.
So I can bow it back the opposite way of the crown so that I can slide a piece of 1/4” under my level, but when I loosen the clamps off it goes back to being crowned in the center. The crown is also worse at one end of the plate then the other. I’m guessing it was cut towards where the crown is worse.
I’m thinking I should try some heat on the backside as I flex the crown out as it may relieve the residual stress from being flame cut.
WOW! Three quarter inch plate! At 3 x 4.5 feet, once you have it flat there isn’t much that’s going to either flex or bend it. As much of a bitch it will/would be to haul it somewhere I think the machined or ground surface will / would be a worthwhile investment for years to come.
Imagine squaring a 36 inch leg off of each corner of your table and having them all parallel all the way up.
I’d recommend watching the video Jason put up about weld distortion. It’s been a million years since my Mechanical Metallurgy class but heating the concave side may excite the Fe crystals on that side from BCC to FCC (or vise versa, It has been a long time) The upshot is the hot state gets molecularly larger and will TEND to expand the concave side. Upon cooling , not all of the modified crystal structure goes back and you get a flatter plate.
In short, you might want to try heat to take out some arc BEFORE MACHINING to reduce the amount of metal to be removed to get flatness. But it is wizardry.
I built a 5x8 table with a .75 thick top and I made it with studs that mount it to the frame. That allowed me to push and pull at every location. This worked well for me, but the holes were are ready in the plate on 2” centers. I’m sure this made it much easier to make it move. Plus mine was flatter to start.
I definitely want to add threaded holes for leveling grub screws like the fireball plates as this will definitely help with making sure the plate is flat
Despite it being .75” thick i can bend it with relative ease with the large clamps that i have.
Ive gotten the worst of the crowning out, but it was also along the edge of the plate shown in the posted picture but now i have to get the rest out and ill need to make a foot to push it out after adding heat to the plate. I’ll probably need another 3’ chunk of 2” x 4” x .25” wall tube, some heavy plate, some acme rod and a few nuts
You can flex it with those clamps, but if you were bending it the plate would assume the new form. It’s also worth considering (as another commenter related) that removing the material from the holes may relieve some heretofore balanced stresses and re-warp a mostly flattened plate.
I’d recommend:
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sorta flattening the plate before drilling (gets the holes drilled perpendicular to the plate surface).
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drilling the holes to see how flat the result is.
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re-flattening the drilled plate as necessary / possible- including stress-relief
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sending the plate out to be Blanchard ground (if it’s mostly flat) or GENTLY fly cut if a bit more material needs to be removed to achieve flatness. IT IS CRITICAL THAT THE PLATE NOT BE CLAMPED FLAT FOR GRINDING OR MACHINING. Let the machine remove a minimum of de-stressed metal to flatten the top surface.
These recommendations are of course subject to modification by flame-straightening wizards or experienced machinists neither of which I can claim to be.
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No i was able to bend it a easy enough but not enough for my needs as it does like you say flex a ton. Once i started using sufficient heat and clamped/flexed it it made my job easier.
Either way i think I’m going to have to add wings to the table structure I’ve already built as I’m sure it’ll probably still take some flexing to get close to that .0005” of flatness.
That 0.0005 my friend, is a lofty goal. I am impressed at the goal, I will no doubt be impressed by your efforts, and even if you have to settle for less flat it will be an impressive table!
I think i dropped an extra 0 in there. The thinnest feeler gauge I have is 0.0015”. I dont think I’ll get 5 ten thousands of an inch but diagonally across it, I’m below .005” for most of it. With it only clamped in four spots currently. Between 0 and 90 theres more variation across the table some of that will come out with grub screws and adding wings from the primary 2 x 2 box structure of the table. The main frame is 24” x 48” with 2 x 1 ribbing on 10” centers. I’ll probably add 5” to each end of 2” square tube then just use 2 x 1 for the rest of the wings.
Perhaps @Fireball_Jason will chime in and give his thoughts on what reasonable for a non machined plate table top 
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OK, the extra zero makes sense! I’d love to hear from Jason on that. Maybe he could sponsor a contest for who can achieve the flattest home-built table…
As an aside, for plate drilling, I came up with a method and tool design I’d like to get Jason’s opinion on. It might reduce some of the issues with hole positions but might introduce some others I haven’t forseen. I won’t be building a table but I’m pleased to brainstorm with anyone who is.
Flame straightening is the way to go. Also be aware that drilling holes in it will cause changes as well.
Yes because drilling holes relieves stress in the plate