chassis welding: how to control weld shrink?

[Waldi]

Hello,

I am in the process of a body-off restauration of my 1970 CP TR6, and the chassis is not too bad overall (it has all fatigue cracks in the known places and some minor corrosion here and there, but it is severely corroded at the T-shirt area, bottom side only.

The chassis is gritblasted, and the rotten sections are removed (see 1st picture).

I measured the chassis and compared it with the chassis dimensions table from the Brown Bible; it is remarkable how staight a chassis can be after 40 years, all dimensions are within +/1-3 mm, so I assume acceptable.

During welding (small sections, working cross-pattern), I noticed the chassis was deflecting from the accumulative weld shrink, so I stopped welding, and with hammering out the welds (peening), I was able to correct some of it.

I have now supported the chassis below the repair area (see 2nd picture), and with some nylon straps I can pre-stress the chassis and I was able to correct the remaining deflection rather easily, going in the plastic range, so the chassis is straight again.

Now my question:
Since the upper side will not be welded (it is not corroded) the chassis will deflect, probably to unacceptable levels if I don't control the weld shrink, so I want to pre-stress the chassis.

Does anyone know how much is needed?

I realize the amount of weld shrinkage is depending on weld speed, sequence, weld size etc, so a fixed number cannot be given.

 

Thanks for your help,

Waldi

[boxofbits]

Hi Waldi

 

Bit of a difficult one to answer without seeing it, but I assume it is cumulative heat which is causing the distortion?

 

If that is the case, when I welded a Mini scuttle a few months back I had a similar heat issue. I used some iron body work dollies and wet rag to absorb heat and stop the heat spreading. Also, are you able to spot weld the chassis flanges instead as this should not cause any shrinkage.

 

Not sure if this will help with your problem.

 

Regards

 

Kevin

[Waldi]

Kevin,

thanks. I agree with you on the spotwelding, but I don't have one so I use a good MAG-welder (Lincoln), and from a strengh perspective it is also preferred to have full penetration welds (FPW).

I can do cooling with wet rags, would it help / be sufficient?
I aim to minimize weld size (and related heat input).

Anyone did succesfully pre-stress before for a similar weld job on the T-shirt area?

[boxofbits]

I do know that heat absorbing iron dollies as I mentioned do work, or anything like it for that matter,and though not state of the art technology, wet rags do also!

 

Only problem is they start to steam a bit when getting hot! Perhaps also only a couple of welds at a time and leave to cool for a couple of mins.

 

Good luck.

 

Kevin

[bob-menhennett]

Waldi

I'd being placing the " tack " welds evenly along the whole length of the metal to be welded , darting about the perimeter to spread the heat build up. I'd then

do the same with the short runs of continuous weld , i.e. darting about so that the heat didn't build too much in one area , giving it chance to cool down.

Bob

[David James]

Hi Waldi

The use of heating and wet rags cooling is extremely effective. Never fails to amaze me.

From my work experience the Fabrication Dept had to resort to straightening heat distorted components through welding..

I believe it can also be successfully used to produce intentional curvature in chassis components.

I would not heat the material beyond a dull red at the most though.

Just try laying of a load weld on one side of a 50mm box section. Watch the tube bend.

Then heat and cool the opposite side to straighten!

All the best

David

[Waldi]

Kevin, Bob and David,

Thanks for your advice. So if I understand correct I have to:

1) cool the steel close to the weld during welding to minimize deformation

2) can correct further by heating the opposite side (top side in my case) and then rapidly cool.

 

I did tack weld first every 25 mm or so and then did small weld runs, but that was not enough.

[stuart]

This is why you will often see a supposed restored car wit the whole of the rear chassis leg below the back panel! Its why Colin CTM who make new chassis have a huge jig made from very heavy steel for assembling theirs in.When you do your short welds think about which way the distortion goes and do the opposite weld the other way to cancel it, Lumps of iron or copper used as heat sinks always help as does a squirt from a plant mist gun.

Stuart.

[Waldi]

Thanks Stuart,

I will prestress a bit and weld as you propose with a lot of inttermediate cooling, as also proposed by the others.

I will post the result, good or bad.

What a great forum!

Waldi

[GT6M]

Can buy heat sink putty.

roll it up like a bigg sosidge, then press it onto panel.

helps alott if its bung,d int fridge fust too.

 

http://www.eastwood.com/coldshield-thermal-paste.html

 

 

M

[ed_h]

Waldi--

 

I just replaced the body back onto my frame which had considerable welding done on it to fix rust. I did try to minimize heat distortion by doing intermittent welding with wet rag cooling in between. I clamped angle iron as strong backs across the frame members to keep them aligned and flat, but did no pre- or post-stressing.

 

http://bullfire.net/TR6/TR6-29/TR6-29.html

 

Though the body isn't completely fastened down yet, the rear frame horns hang about 3/4 to one inch below the lower edge of the rear valance, which appears to be about right. Pre-stressing might be a good precaution, but don't overdo it.

 

Ed

[Waldi]

Ed and M (as in 007?),

thanks for your input.

Today I welded the insertplates in my chassis (bottom side) and then after checking if there was weld shrink also the T-shirt, using some pre-stress and a lot of intermittent watercooling with a small sponge (yes, it's missing from the kitchen now ).

I welded short lengths (max. 20-30 mm) at a time, going in a cross pattern, it was more easy than I anticipated.

I have not checked all the chassis dimensions yet, but the most critical ones for this repair like the rear legs vertical position did not show any measureable deflection/deformation.

To make measuring easier, I used 0.010 mm black dynema fishing line which I ran parallel to the flat section of the chassis, all the way from fromt to rear, this was my reference line and made measurements very easy.

Thank you all for responding, it made all the difference for this job.

Waldi

Edited May 1, 2016 by Waldi

[Alec Pringle]

" Its why Colin CTM who make new chassis have a huge jig made from very heavy steel for assembling theirs in. "

 

That's something of an understatement . . . . . . Colin's jig looks like its made from spare component that went over the dockyard wall from the Titanic build . . . . .

 

But then, that's what's required for a proper job.

 

Cheers

 

Alec