Tubing Shrinkage

So I'll start this new arrangement of forums with a simple question that I might already know the answer for. I'm mid process of a couple of filleted frames (photos at https://www.flickr.com/photos/731955...57658194060433) and have the same occurrence as with previous filleted frames. After the main frame has been filleted and I replace it into my jig for continuing with the rear triangle I find that the frame doesn't fit exactly. There is a slight shrinking between the head tube ends and the seat tube. the result is that I need to slightly shift the jig's reach to have the ST and BB shell seat as well as the HT ends seat fully on their contact points. My process has a small extension of the HT past the ultimate top and bottom of the finished HT length that gets cut off before reaming and facing the HT.

So what's going on?

My take is that the HT goes through a bowing due to the heat being added far more on the back side of the HT (where the TT and DT fillets are). As I have some HT extending above and below the virtual ends of the HT there is more length for any heat distortion to be "seen" by the jig's original settings. As mentioned my solution is to move the HT support (I use a HJ universal jig) back a touch to have the HT ends seat on their cones then jiggle the tube supports a touch to recontact the TT and DT. I use the BB and ST as my base line, in a manor od speaking.

Is this a acceptable way to deal with the after fillet changes?

Many thanks, Andy.

Two things generally cause movement- miter gaps and uneven heat. It sounds to me like you might be experiencing a bit of both.

I'd suggest paying tight attention to initial dry fit up to make sure the miters are tight and clocked (this will also help of course with general alignment).

If the miters are tight and you are still experiencing bowing or "shrinking" of the tubes you need to pay attention to pre-heat and brazing sequence/ heating cycles. Even though the tube connects at the back, that shouldn't be the only place getting the attention.

I have a different approach. Build the bicycle in one shot. The datum for your frame in many fixtures is the BB. The moment you touch that part with heat (THE MOMENT) it is no longer a trusted datum. If you put the part in the fixture once and when it is removed it is a complete frame you will have saved yourself the headache of trying to put a frame with distortion back into a fixture. Titanium has serious distortion and it pulls the joints so much tighter than when it started it would rarely end up where you began. Aluminum is even worse. Again I always build a complete frame because I trust my fixture and it always produces repeatable products. If you have a trustworthy fixture try building a complete frame. It will yield more repeatable results from my experience.

Thanks, your replies were some of what I expected to hear. I do add heat to the front side of the HT (and shell/ ST for those joints) as I finish off a fillet to try to compensate for the previous concentration of heat at the filleted joint side. Maybe not enough compensation.

Drew- your entire frame joining in one set up process is what I just asked about on the framebuilders@googlegroups.com list.

Since I don't do this stuff weekly I don't have the hundreds of repetitions to hone my skills and processes as quickly as I might dream of. It seems that every frame gets a slight tweak to the process. So I have developed a basic manor and have slight deviations from that. I do a bunch of practice torch work prior to each year's frame (or two) but then lay off during the retail wrenching and riding season.

Thanks for the insights. Andy.

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Originally Posted by Andrew R Stewart

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Is this a acceptable way to deal with the after fillet changes?

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Bicycle making by hand is an organic task at best; you have heat, skill sets, metal, interference fits, repetitive motion, mood, the actual tools used to hold things together at various times - I could go on. I guess the question is, why does it matter if the frame, after brazed, doesn't fit back into the fixture as easily as all the material did before the torch was lit?

The others above spoke to why things would or will move. I'll add that when I jig up a main triangle, I work it hard to ensure that no tubes are actually touching any others, despite touching the lugs that hold them. To look at them in the set-up process, the miters appear to be kissing the tube they're up against, but they don't. The brazing sequence, what I tack or pin, what's done in the fixture (about 1% of the heating) versus what's done combining a repeatable predictable heating cycle outside of it, these are grist for another discussion. But I can vouch for your concern that your not doing this with any particular frequency (unlike those who braze these up daily) is likely the biggest culprit here. But with care and practice, making the frame so that it fits right back in the blocks and holders it was birthed in is very possible indeed.

I'm replying as a dedicated lug guy, but believe the same as I write here applies to lugless/fillet brazed frames. Welding, well that's Drew's domain so I'd follow his cues and borrow heavily.

Just an additional data point. Very early on I used to tack everything except seatstays in the jig. I'd then pull from the jig, check alignment (on a table) and tweak where needed. Then I cut and tacked the seatstays *out of the jig*. After a while I realized that I was hardly doing anything at the pre seatstay tweaking stage and anything I was doing was likely being undone when I added the seatstays. So now everything goes into the jig and tacked all at once. I wouldn't do it any other way now but I do think if you have the time (and maybe not the confidence) then leaving the stays (seatstays only) off might not be such a bad thing.

But as Richard says, once it's out of the jig, it's out of the jig. I see no reason to try and put it back.

Steven