Distortion again

[BlaydonBodger]

Ok Guys a bit of reassurance is needed.

I have now got the first part of the chassis tacked together but as I was about to move it forward on the build table to enable construction of the rear end I noticed it had curled up a bit at the back.

BR12 was now off the build table by 3-4 mm. Looking at the problem it seemed that the bend line was where BR5 joins BR3 and where BR6 joins BR4.

I decided that if I cut the join where TR2 meets with TR4 and U5 and do the same on the other side I would be able to get the chassis to lay flat again.

I cut through the welds and hey presto the back end dropped straight back on to the build table.

I am now left with a gap between TR2 and TR4 and likewise on the other side which I have bridged with weld. Pic is of the widest gap.

Do you think this was the right way to go?

[BlaydonBodger]

Here is the pic

[AshG]

should be ok but it may be worth investigating what is causing it.

when i built my chassis i fully welded parts as i went.

first i tacked then welded all the bottom rails fully so i had a solid base then worked upwards fully welding in stages.

i really dont agree with the idea of tacking the whole chassis up first then going around and welding it at the end as the pure nature of the welding process will cause stuff to move which will spread unnecessary stresses around all the tacks on the chassis.

westfield, caterham and many other manufactures make their chassis in pieces then join them all together as it allows you work to a tolerance on individual parts rather than a whole chassis.

[BlaydonBodger]

Thanks Ash,

It was flat until i put the diagonals in or at least thats when i noticed it and i think i would have noticed right away as i am paranoid about distortion.

We might be opening a debate up here again but I am with you as far as fully welding is concerned i think...

I am not an engineer but i have previously worked in a fabrication shop which made stuff for the mod and others. it was over 20 years ago but i am sure that stuff would be welded and distortion sorted as you went along. The benefit i see in tacking is it is easier to get apart again when a mistake has been made and that a couple of snots of weld hold the item in position nicely whilst you run a bead.

I am beginning to wonder if i should have followed your lead and my gut feeling. No doubt we will see when i fully weld it.

[flyerncle]

Mick,I put heavy tack welds on mine and then strapped it to a car jig and welded it,even then there was a very slight distortion but nothing to worry about.
The problems start if all is not well after welding all of it up and you need to alter it.

[BlaydonBodger]

I think what i will do when i begin to fully weld things is to tack the base to my build table (after i remove the wooden top) The frame is rather more substantial that 25mm box section the chassis iis made from. That should go some way to stopping things moving about.

Originally Posted by BlaydonBodger

I think what i will do when i begin to fully weld things is to tack the base to my build table (after i remove the wooden top) The frame is rather more substantial that 25mm box section the chassis iis made from. That should go some way to stopping things moving about.

It won't stop it moving, if it wants to go it will go once you release it from the table.

Weld sequence is the key to a chassis not twisting not clamping it up.

[Bonzo]

Originally Posted by AshG

first i tacked then welded all the bottom rails fully so i had a solid base then worked upwards fully welding in stages.

Same here Ash

When i laid out my chassis base rails, I could see that the unwelded joints sitting on the build table were likely to cause some distortion issues later on

Tacked my base up well, lifted it from the table & flipped it over to confirm it was all square, then fully welded the entire base section ( Linishing the bottom welds to allow it to sit flat on the table.

I won't go into detail of the rest of my fabrication process, It'll only start a whole bunch of arguments !!??

In the book it shows bottom section and the top rails and the front even the seat bulkhead if you make these as seperate things and fully weld making sure they don't twist then assemble it all.
I use to make the chassis's this way years ago and it's quicker easier and no distortion.


Clamping it won't stop it wanting to move if its been welded too hot in one place etc, it will pop once released.

[baz-r]

is the nature of the beast realy as your weld cools past its melting point it shrinks.

when i did my br's i tacked the bottom corners next to the wood all clamped down first then tacked the top corners, welded the sides of all tubes doing the same side one left one right, then the tops as sides put in my uprights corner tacked then tr's,d's and so on when i fliped it over to do the bottoms of the br's poped it back down after flating off and i had 1-2mm of gap if the back was clamped down after fully welding. i think was pritty good realy i was expecting more