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Learning how to build Lotus Seven replicas...together!
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PostPosted: April 12, 2018, 9:51 pm 
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Joined: April 12, 2018, 8:10 pm
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Location: Durham - NC
Just getting thoughts or advice from people who've already done it - when to weld the frame? I've tacked most of the frame, welding only sections where intersecting frame members would be covered and not welded.

Thoughts were to clamp it down, weld what I can on the table to reduce flexing and twisting, then flip the frame and weld underneath clamping it where I can.

I think I read here that people were adding suspensions points/tabs before welding the frame? I would have thought after, in case there was any twisting? But this is my first frame, not built on a complete metal jig so buggered if I know.. :-?

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PostPosted: April 13, 2018, 7:25 am 
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Joined: September 22, 2005, 8:12 am
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Location: 4AGE in S.E. Michigan
If you frame is tacked together then it will hold the dimensional alignment. I would recommend that you weld the frame in the free-state and on a rotisserie once the frame is completely tack welded together. If weld in a systematic order there will not be any issues. I would wait to install suspension brackets, because it will give you better access to the other welding areas, but it should not make a big difference. With the complete frame has been tack welded together, you are ready for the final welding. One should try to balance all of your welding to minimize distortion. Weld the outside of the joint then move to the inside 180* opposite and then weld that side of the joint. Next move to the opposite side of the frame and do the same process on the other side "mirror side" so the weld distortion is balance from side to side.
Never completely weld all of the tubing joints on one side, top, bottom, of the frame at one time and than flip it over and do the other side. Your frame will end up looking like a banana. Even if the frame is clamped down, you are building the frame with internal stresses. As an example, production frames are always welded in a balanced pattern [Two welders or two robots on opposite sides]. Just take time to think how the weld will shrink and draw the tube to the side that the weld is place on. You probably already have half of a rotisserie "your engine stand" just mount the other end of the frame to the work bench, in my case I used the hydraulic arbor press as the other end of the stand. The frame final welding was completed in one morning on a rotisserie and with less than 1mm of distortion. DaveW


Last edited by davew on April 13, 2018, 7:28 pm, edited 1 time in total.

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PostPosted: April 13, 2018, 8:00 am 
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Joined: April 12, 2018, 8:10 pm
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Thanks Dave, good advice there, there's a few pointers that came back to me when you mentioned not welding the ones side in one go, I remembered the alternating sides but was stol thinking in a 2D realm rather than 3D.

Just picked up an engine stand last week, I'll figure out the end, I've some spare metal laying around, sure I can come up with something.

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PostPosted: April 13, 2018, 8:16 am 
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There is no need for firmly clamp the frame down if is is properly braced. You should be sure that all of your diagonal bracing is there before you start welding as that is what keeps your frame from twisting when it is completed and it will help keep your frame from twisting during welding. Clamping the frame to a jig only helps warping if that jig prestresses the joint in the opposite direction you expect the warping to occur during welding. I would also recommend adding temporary bracing across any areas that do not have diagonal bracing when completed like across the cockpit and engine bay. These can be removed after welding is completed Also whenever possible welding a pair of joints in an order such as driver side top passenger side top then go 180° and weld driver side bottom passenger side bottom. I also when deciding which of the 4 sides to start with usually pick the side with the smallest gap as that provides the least cross sectional area to the weld and will have less shrinkage and pulling. If you have one side of a weld touching and the other side with a gap and you weld that larger gap first it will pull the weld more. If you weld the side that is touching first that will help resist the pulling caused by the side with the gap.

I welded my frame on saw horses using these techniques I started on the back and worked my way to the front then I flipped the frame 180 degrees and started from the back again, then flipped it 90° and started the process again. I ended up with less than 1/16 off in all directions.


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PostPosted: April 13, 2018, 10:59 am 
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More great advice, I knew I'd love this site. My only experience has been jig builds, so its great to get advice on "free-welding", a term I just made up...

What technique did you use to measure distortion of the frame? My table is fairly flat but it's still wood vs metal...

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http://www.locostusa.com/forums/viewtopic.php?f=35&t=19352


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PostPosted: April 13, 2018, 2:13 pm 
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Location: ontario
If or when I build a third seven I will weld the frame ASAP. On my current build I think I made the mistake to keep full welding too late, being happy and anxious to see the whole project take shape. I ended up with hundreds of welds to complete, missed a few, had to go back, ...If I had to do it again I would complete all the welds on the chassis including the suspension brackets as soon as all the tubes are tacked. The plywood assembly bench as adopted by the Seven builder fraternity is a nice way to position the tubes for tack welding. It is not however stiff enough to keep the chassis from warping. Some builders have recommended a sequential approach to welding ( do not weld one side and then move to the other side) they are right. Once the chassis is partially welded (about 50% full welds) distributed throughout the whole chassis, check all horizontal tubes visually or with a bubble level (or a laser level) to make sure that they are all strictly parallel. Naked eye is pretty good at spotting discrepancies. On a BOOK build the tubes in question are: lc, q, p,o. On my build I found that the equivalent to lc and o had developed a 2 to 5 degree deviation. I had no choice but grinding off some of the welds on vertical tubes to force these two members into perfect parallelism. Hope this is of some use. Good luck to you :cheers:


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PostPosted: April 13, 2018, 7:42 pm 
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Joined: July 4, 2006, 5:40 pm
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I welded my frame once all the tubes were in place, but before any brackets or panels. I was careful to move around the frame a lot while welding, but welded all the top joints first. I didn't get any lateral of longitudinal distortion, but the frame bowed about 1/4" front to back. I flipped the frame over and welded all the bottom joints and the frame came back to within about 1/16"-1/8" of straight. It's not something you can tell by looking at the car, but next time I would flip the frame over every 20-30 welds, just to even it out.


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PostPosted: April 13, 2018, 8:31 pm 
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i tacked up my frame and removed it from the table to fully weld it and proceeded to drop it!

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PostPosted: April 13, 2018, 9:36 pm 
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When did I weld my frame? Let's see, it was about 2007 :headbang:

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