Hi all. finally getting started on the chassis, have the bottom rails all tacked up. It's time to put on the H tubes. I remember some discussion a while back about fully welding joins before you add in other tubes, but can't find the thread.

do you think I should fully weld the places where the H tubes go and grind flat before I tack on the H tubes? or will this introduce a lot of distortion?

if I should fully weld, how best to minimise distortion? weld the top then flip over and fully weld the bottom as well?

yours as ever paranoid,

blueshift

Did'nt fully weld mine until I had the whole frame tacked together including the diagonals. It uses the inherent strength of the frame to keep things from getting all bendy twisty....I have an aircraft welding book that suggested this method.. It also says alternate welds semetrically side to side..working from one end to the other( front to back or back to front) "to reduce accumulating stresses"..anyone agree?...

Fletch,

I welded the base of the chassis and ground the welds flat before I put the uprights on. If you do get distortion you can bend it back by jumping on it strategically , something you won't be able to do after. I did work from side to side as per destructions though.

Some say yes, and some say no, and I tend to agree. But in this case, I would tack the whole chassis together, then come back and fully weld, side to side, front to back, alternating from here to there, just make sure you stick them all.

David

well the welder that has done mine said tack first then weld, apparently a good welder can combat the twisting as he or she is going by knowing where to go next.

A particular problem with the H tubes is where the joints in the lower rails are obscured by the H's.
The weld must penetrate not only the H and the first lower rail, but also the joint between the lower rails themselves. Two alternatives, throw in loads of amps, OK, but then you have to use the same heat setting and speed on the opposite side of the H tube to equalise the distortion. You must put in the second pass straight after the first while the weld is still hot and plastic. More that 60 seconds and the rails will be locked, and the corrective second pass will not pull the H back far enough.

essentially you have to make sure it is tacked square. I did mine by a mix of the two approaches, i did the base and jumped on it like pete, then tacked everything else on and then fully welded it. Then i tacked some more on that id forgotten, and then fully welded that bit. And so on several times. I found it to be a good technique, for making bananas. unfortunately, i wasnt trying to make a banana.

If your chassis is banana'd, don't worry, it will only add to you castor angle!

I agree with Peteff. Any joints that will be covered up by another tube should be welded and ground flush.

quote:

---

Originally posted by Mark Allanson
If your chassis is banana'd, don't worry, it will only add to you castor angle!

---

The banana comments made me laugh

I hadn't thought of jumping on it to make it level again. Nice one, I think I'll go with fully welding the bottom rails and some brute force.

Having trouble getting into the "well you could always use A BIG HAMMER" mentality. too much time spent with computers (that rarely respond well to such tactics)

Interesting learning about the thing of doing the other side of a tube quickly, thanks mark.. something to bear in mind.

cheers all

Blueshift,
Sorry for the late comment but as you already know I've been away on holiday.

I asked this question when I was at the same stage as you because I didn't want to cover up any weld sites either before seam welding them. I didn't find distortion to be a big problem to be honest as I had a very sturdy blockboard building surface which I had very carefully levelled beforehand and I used to clamp my chassis down to it using bits of wood across the chassis tubes and other bits of wood in the vertices to prevent any lateral movement. I did adopt the alternate sides approach when welding to counteract any stresses and I also used some pretty sturdy tack welds, usually two on each tube 25mm vertex to keep it more stable. This has tradeoffs when you need to disassemble anything but I felt it was worth it provided you don't apply too much heat during the tacking process of course.

I felt it was important to weld every joint to the best of my limited welding ability and even if that meant a little distortion then so be it. In the end when I finished my chassis it had a 1mm difference in the diagonal lengths of the entire chassis and a "two-hacksaw blade" banana distortion front to back. I think this is miraculous to be honest and I am well chuffed. Someone on this forum once said that mass produced cars are built to something like 3mm tolerances over the length of the car IIRC.

Don't lose too much sleep on this one as there are many more important challenges awaiting you further up the line...

By the way, I'm glad to see you've started building the chassis finally, you must be just about the only person on this forum who has gained "Senior Builder" status without actually building anything!

Cheers,
Craig.

[Edited on 9/7/2004 by craig1410]

Hehe. bah! and yes, good to be started. However, I have just made a cockup which I will start another thread for

quote:

---

Originally posted by craig1410
By the way, I'm glad to see you've started building the chassis finally, you must be just about the only person on this forum who has gained "Senior Builder" status without actually building anything!

Cheers,
Craig.

---

I anchored all the bottom rails down to the 1 inch thick MDF base using lots of bits of wood. Gave me a nice flat starting point b4 building upwards.

got it nice and flat now with the jumping technique, but it's slightly on the wonk diagonally. looks like the F tubes have curved in a bit with the heat from welding E as well.

trying to think of a way to bend it.. perhaps stand it on the floor outside the garage door on a block, tie the top to the top of the garage, and jump on the outside bit..

hmm

Blueshift,
Yes I noticed the bending effect you are talking about although I only noticed it when I put the top rails on. (J Rails I think they are - the longest ones in the build anyway). I didn't have any problems with my diagonals changing though.

Once again the non-perfectionists amongst us (like me) probably wouldn't worry too much. Having just said that, I used to be a bit of a perfectionist before I started the Locost build. I'm much better now though!

Seriously though, given the nice clean cuts that you have made on the tubes and the resulting (presumably) close fitting of the ends, and also the pretty decent welding you have demonstrated, you have to ask yourself whether you could improve on what you have already done before you even think about redo-ing anything. I think you'd be hard pressed to improve it myself.

Cheers,
Craig.

thanks, but you haven't seen the wonkiness. if I line up the AB square then F2 is at least a centimetre off it's mark :/