Jarpmann

I TIG welded a bike frame up and it turns out my jig was not terribly accurate. I set up my frame on a granite table, and clamped the bottom bracket to the top of the table. measuring from the granite table up to two points on the head tube, my head tube is off by roughly .037"[just under 1MM](measured at both ends of a 190MM HT). my seat tube is off by .077"[ just under 2 mm](measured close to the BB and right around the TT and ST junction. this is on a 58 cm frame.

my questions are, what do you measure when you build a frame, and what are your acceptable tolerances?

ftwelder

Everyone has their own method and reasoning I am sure. I will tell you my order of business.
1) face BB, ream seat tube, face and ream head tube.
2) clamp BB to whipping post and level seat tube.
3) evaluate and address any twist in relation between head tube center line. I put a bar in the head tube that extends to where the front axle is located and take measurements at top of head tube and axle. I try to get the seat tube and head tube as close to perfect regarding twist. I then "fix" the head tube in this position with the head tube center line parallel and at the same elevation at seat tube.
4) Raise or lower the drops to match, check drop slots and face relation. Any amount you are off here is going to be area of greater concern. I also try to get this close to perfect.

Much of what is assumed to be "tolerances" is really not so much for me. You have the weight and fit of the tools, tube straightness, and the likelihood that you are merely distorting the BB shell with much of your effort all effecting the outcome. Even the best facing tools leave a fraction of a MM that ends up driving the seat tube out over 1mm at the top.

I learned a few tricks from a guy who said " when you ride,the seat tube is always vertical, keep that in mind".

I would say you are "out of spec" on the twist and fine on the height of the HT. I think that will change when you sort the seat tube however.

tuz

Good stuff ftwelder.

So yes you have to make sure the BB is faced, but even the facing isn't perfect, and I read you can distort the shell slightly by clamping it too hard. 77 thou runoff at 58 cm isn't too bad, that's 0.2 degree off. At the saddle the runoff is bigger, but the butt-saddle contact region is quite large and one moves around.

Now your HT twist is a bit large I'd say. 37 thou on 7.5" means that the contact patch of the front wheel will be off by about 3.5 mm from the frame's centreline. The contact patch is probably around 2 cm wide for a road tire.

Btw I find HT twist quite difficult to avoid. I usually put my TT last so I'm thinking that having an accurately mitred TT tacked in place while brazing the DT/HT to BB would help? Or it's all in the brazing/welding technique.

Also I would not trust a fixture to give good alignment. It's better to tack in it, check for alignment, weld/braze outside the jig and check again.

Dave Kirk 

Alignment tolerances and standards are for the most part left up to the builder. I can tell you that cheap bikes built by the big Three aim for +/- 3mm or .120" and they still, for the most part, go straight enough to be serviceable.

I aim for a +/- .005" in having the bike all in plane and less than .005" of head tube twist over it's length.

The very important thing here is that the BB be faced well so that the faces are parallel within .001". This is perfectly doable with Park hand tools if care is taken. Then you need an accurate and repeatable set up to hold this square BB. If you can clamp the bike down and take a reading and then unclamp it and set it all back up again and get the same reading you are good to go. If you get a different reading on second clamping then you will forever chase your tail.

I'm not at all a 'more tooling is better' kind of guy and think lots of good stuff can happen without real tooling but good alignment is very difficult without good tools.

I hope that helps at least a little bit.

Dave

Jarpmann

thanks guys for all the very helpful responses. lots of good info here. two setup's yielded very similar measurements, so i feel pretty good about the measurements. i chucked up the BB in a lathe prior to welding, and the faces should have been very close to parallel. not the best practice but i cant really afford a BB facer at this point. i suppose that i could use a bike shop after the fact.

Can you all offer some incite into which tubes you tack in first? when do you complete weld a joint, and when do you measure?

Thanks

Dave Kirk 

If you faced the BB before welding the chances of the faces being anywhere close to parallel after welding are very low. The welding distortion will see to that.

Do you have some good calipers that you can measure the width of the BB shell with? If so do it in 4 places around the shell and see what you get. If it's more than a few thou off then your alignment will suffer in a rather large way.

All the best,

dave