Pro­fes­sor Ham­mer

Street Rodder - - Contents -

Strate­gies for Min­i­miz­ing Dis­tor­tion

Q I’m boxing the frame on my ’31 Chevy. I have the 10-gauge plates cut and tacked in place. What is the best way to weld them with­out warp­ing the frame? Lar­ryMer­rill

ViatheIn­ter­net

A Well, the short story is that weld­ing nearly al­ways causes dis­tor­tion (warp­ing). When you make a weld, you are fus­ing red-hot metal to the base metal and there is a zone of metal next to the weld that gets heated at the same time. As this hot metal cools, it shrinks (pulls) the metal around it. If you take a bare C-chan­nel fram­erail and weld a boxing plate to it all of the weld­ing will be on one side of the rail, and the rail will cup an alarm­ing amount when the weld shrinks. For­tu­nately, there are strate­gies for lim­it­ing this dis­tor­tion.

Your best “ace in the hole” are the chas­sis cross­mem­bers. These go a long way to­ward keep­ing your frame from bow­ing as you weld in the boxing plates. If you are us­ing the stock cross­mem­bers, I’d rec­om­mend keep­ing them in place, and fit­ting the boxing plates around them. The shorter the un­sup­ported length of your frame is, the less dis­tor­tion you’ll get.

If you plan to build your own cross­mem­bers, I’d use an­other strat­egy. I’d lightly tack the boxing plates to the bare frame rails and then tack-weld the cross­mem­bers into place, join­ing all the frame el­e­ments into a rigid unit be­fore you do the fin­ish weld­ing. Of course, it’s im­por­tant to get every­thing plumb and square be­fore you start weld­ing. If you start with any twist in your frame, the fin­ish weld­ing will lock this in, cre­at­ing a big prob­lem.

Not all frames are the same, and usu­ally the newer the car, the more cross­mem­bers it will have. Model A Fords are par­tic­u­larly flimsy, with only three cross­mem­bers and very long sec­tions be­tween them. On a chas­sis this lim­ber it would be a good idea to tack in some tem­po­rary braces halfway be­tween the cross­mem­bers. These could be made from 1-inch square tub­ing, and I’d rec­om­mend one brace tacked to the top flange of the frame, and one on the bot­tom.

An­other im­por­tant con­sid­er­a­tion is the se­quence and length of your fin­ish welds. You’ll get the least amount of dis­tor­tion by mak­ing short welds, and stag­ger­ing their lo­ca­tion. For ex­am­ple, your first weld could be at the top cor­ner of the boxing plate at the rear cross­mem­ber. The next weld could be at the front cross­mem­ber on the bot­tom of the rail. Next, you would shift to the rail on the other side, and make sim­i­lar welds. I’d rec­om­mend mak­ing welds about 1½ inches long through­out the process.

If you fol­low this strat­egy, al­ter­nat­ing your welds from top to bot­tom, front to back, and one side of the frame to the other, you’ll keep the dis­tor­tion to a min­i­mum.

Pro­fes­sional shops use a heavy chas­sis jig to hold every­thing in align­ment for both the tack and fin­ish weld­ing, but you can get de­cent re­sults with­out such an elab­o­rate piece of equip­ment if you work slowly, and check for twist and square­ness fre­quently.

Af­ter all the weld­ing is com­pleted, you can check the frame with di­ag­o­nal mea­sure­ments to make sure noth­ing has gone out of square, and if you have a flat sur­face to mea­sure from, you can sup­port the chas­sis at a con­ve­nient height, and mea­sure up to iden­ti­cal points on the right and left side of your frame to check for twist­ing.

Note that con­crete floors are NOT flat, and can’t be re­lied on for ac­cu­rate mea­sur­ing.

If you don’t have a good way to check your frame for twist­ing, you could take it to a body shop with a frame ma­chine, and they can mea­sure it for you. They also have the abil­ity to cor­rect any dis­tor­tion that may have oc­curred dur­ing the weld­ing.

This is an ex­am­ple of a nicely stepped and boxed frame.

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