De Crown Welding Table

[anotherjoe]

I made this table top today, and just put a string line to it to find a 1/16(a big 1/6) inch crown. So I was wondering if there is any technique to use heat distortion as a correction, either by welding or with a torch? Also how could I avoid that in the future? Did I use the wrong sequence or was it just to much heat(not waiting enough time between welds). FYI that is 2x4x 3/16 tube.

 

[A_Pmech]

I'm guessing you made all the welds on one side, then flipped it over. This created the differential "pull" which crowned the long tubes.

No worries, it's a relatively easy fix with some practice. What you want to do is heat the top of the tube on the convex side of the curve near the center of curvature. Heat the full width of the tube top to a dull red heat and allow it to cool back to room temperature. Then, re-check. Keep this up until you get it flat.

How it works:

When you heat up the top side of the curved tube, the heated portion of the tube is constrained by the majority of the tube section. With nowhere to go, it yields in the axis of strength as yield strength is temperature sensitive. When the top of the tube cools down again, the heated portion "pulls" the rest of the section into line.

 

[koditten]

Might be easier to run a weld bead on the side that has the crown. The bead will contract as it cools and pull the tube straight. I do it all the time with trailer frames.

KO

 

[jimgood]

what about leaving it alone and using shims when you put the top on?

 

[Tinner]

A_Pmech has given good advice, the key word being practice.

Structural steel is often bowed from the mill. The specs for allowable bow are an eye opener. I had some 2X4X1/4 recently that bowed 3/4 over 20 feet. You just learn to deal with it. In that case the crown was about 7 feet from one end. We cut short pieces from that end. The worst pieces we straightened with a torch before welding.

 

[anotherjoe]

thanks for the tips, I did not check the steel before so its possible that each piece was crowned and I happened to orientate each side in the same direction. When using either the bead or torch technique how long should I wait between each weld or heating before repeating? So that I dont go to far and crown it the other way.

 

[anotherjoe]

FWIW I will be a building a lower frame for it to sit on out of 2x2 tube. So possibly I could also put the top on the frame upside down(it is crowned down) and use some ratchet straps to aid while heating, but chances are the 2x2 will give before the 2x4.

 

[A_Pmech]

When using either the bead or torch technique how long should I wait between each weld or heating before repeating? So that I dont go to far and crown it the other way.

This is a differential temperature game, so it doesn't matter so much what the temperature is, just that the top and bottom of the section are the SAME temperature when you take your measurements. This will happen naturally as the tube cools and conducts heat from the hot side of the section to the cold side of the section.

Since it's your first attempt at heat straightening, I'd let it cool down almost to room temperature before checking and trying again.

Remember that you have to visualize the bow and apply heat along the crown to remove it. Just randomly applying heat willy nilly will make a pretzel out of your weldment. If you do bow it the other way you can bring it back by working from the other side.

 

[rsanter]

What AP said, however I would reheat the welds with a torch to help relive the welding shrink stress you induced

Bob

 

[sberry]

I have literally built hundreds of benches. I imagine the welding a little like bolted connections and I don't weld all that stuff 100%. It would have had modest welds on the flat part of the tube. All the pieces would be clamped to the plate and the connections welded some.
The joints don't need to be strong, it needs to be stiff across the span.
99.5% of the general jobs do not need a flat bench, my benches are no where near flat.

 

[anotherjoe]

wow that was easy, took about 15 minutes of heating with a oxy torch

 

[dr_clyde]

wow that was easy, took about 15 minutes of heating with a oxy torch

Once you start using your torch as the precision instrument it can be, the results can be really amazing. Most guys see the torch and think of only cutting and simple heating, when its capable of so much more. Truly one of the most useful tools in a welding shop.

Flame straightening is one of the most useful skills a fabricator can have. Keep it up, it'll just get easier.