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BenCooper

Depends on the jig, but most jigs won't heat up enough to deflect significantly - however tacking in the jig and brazing freehand has the advantage that you can reposition to reach all the joints easily.

Andrew R Stewart 

I'll add that tacking in the jig and fully joining out of the jig allows for an alignment check by a more precise tool, a surface plate. Jigs can be almost but not quite dead on straight very easily. Trusting a jig to hold what ever alignment it has and transferring that alignment to the fully welded/brazed frame is a big request. The uneven heating that often happens when fully joining the tubes while still in a jig (because of access limitations) will often create misalignment stresses which will only be realized when the frame is removed from the jig. Andy.

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unterhausen

my procedure is to tack and braze outside the fixture. There are a number of people that pin the lugs, but they also braze outside the fixture

Andrew R Stewart 

Yes, tacking on the top and bottom (or front and back for HT and ST) on the centerline of the frame while in the jig is a method many use for the reason you mention. Yet others do other. The Hirose vids show him doing some tacks on the sides. So in the end one figures out what works best for their needs. Andy.

JohnDThompson 

Brazing in the jig may also prevent the tubes from expanding with the heat and lock in stresses that could lead to trouble later on. Use the jig to hold things for tacking or pinning, then braze freehand.

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BenCooper

A tube wants to bend, but it can't because it's attached to other tubes. I see it sometimes when retrofitting S&S BTCs, cut the frame and the cut ends spring apart.

I'm not sure it's a problem as such (unless someone wants to retrofit BTCs of course), but better to avoid if you can.

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Andrew R Stewart 

Some builders see a frame built with a minimum of internal stress as one sign of "perfection". Richard has mentioned this before (and I hope I didn't just put words in his mouth). As long as the frame member is attached at both ends (not a fork blade) any stresses will be "locked in" by the member's inability to move due to the stress's force, the "lock' is the other end's being held in place. As Ben mentions only when the "lock" is opened, the tube is cut, will the evidence of "locked" stresses be shown.

Which brings me to ask a question to Ben and others who do S&S installs of preexisting frames- When the tubes are cut and there is some spring out how do you handle the coupler install and alignment issues? I have only installed couplers in premitered tubes before. After the frame is complete there's only a mm or two of spring out and the coupler seems to hold that amount straight well enough. Andy.

BenCooper

Depends how much of a spring there is. Most of the time it's not much, only a couple of mm, so not worth bothering with - it can easily ping into place whenever the customer assembles the bike. If it's too much for that then I cold-set the frame a bit. It doesn't need much force, the front triangle hasn't got much strength when it's separated.

When I'm fitting the BTCs, I align HT to ST, make sure the front triangle is still in line with the rear, and make sure the tubes (with the correct amount cut off) are bottomed out in the BTCs.

Andrew R Stewart 

Ben- Sounds about what I thought, thanks. Andy.

MassiveD

Surface plates make a ton of sense given that they are almost being given away. But the idea one can align a bike to that degree of accuracy is a farce on so many levels. So if you need to use something else, by all means take your best shot.

duanedr 

While it's not perfect, I've found a (roughly ) 3ft x 3ft slab of marble works well for a surface plate. Mine is 1.3 in thick and doesn't require a special stand or anything other than a spot on my bench. I picked it up for $40 at a local counter top outlet. One key attribute is that it had to fit into my car. Renting a truck and so on would make the cost unreasonable at the time. I have 3 frames now that are all as straight as I can measure. I don't use a jig, I use tube blocks on the marble slab to tack and align but braze in a stand to minimize inbuilt stresses.