Bottom bracket down tube miter

[mattypichu]

Been lurking and reading, learning and practicing. Now I have a question that I haven't found an answer to. Is there a reliable technique that is not machine based or computerized to create the double miter that results at the bb dt st junction? My method of joining is TIG, so I don't have the benefit of a lug to guide me. So far I have been file file file, check, file file file, check. What is the traditional method?

[Andrew R Stewart]

You named the traditional method. File. You could make a template from .125" larger tubing having a .058" wall that has a common joint geometry to act as a start (like a lug). Andy.

[Michael Gordon]

The spreadsheet on the following link will let you create a template for a compound miter. I'm building a lugged bike, but I used this as the template for my compound miter instead of tracing the inside of the lug. It was dead on!

http://www.os2.dhs.org/~john/Bicycle_Geometry/

Mike Gordon
Highland Park, IL

[afwalker]

Novacycle supply has a java based tube notcher page that will print a miter template for you,
Miter Your Tubes with Tube Notcher ! :: Nova Cycles Supply Inc.
And should you ever go computerized, bikecad can do templates for you also. Great program even can tell you the mm's the downtube-seat tube notch will be,
Miter Templates | www.bikecad.ca
cheers
andy walker
walker bicycle co
Walker Bicycle Company | | Walker Bicycle Company
Flickr: afwalker's Photostream

[mattypichu]

Thanks for the links, for the time being I'll just keep hoeing the row I'm in. I do have an older Atlas/Craftsman lathe that I am working on getting set up to do this process. It seems from what I can gather that most builders that are mechanized use a mill for this. Is there a reason that the lathe is not being utilized? I may be answering my own question, but is it because the cross feed of the lathe is not as stiff as the bench of the mill? If so, is the result really inferior?
Andy, thanks for the template lug idea, I'll keep that in my pocket for later.

[Amaro Bikes]

Might not be very helpful as I do not like templates or machines, but here is how I go (Tig procedure as you)
-miter ST-BB, but don't weld/tack it. Place the ST-BB in position and mark a circle around the joining line (where the tig bead would go).
-Take the ST away from BB
-Miter the DT-BB area, but only the "axle" part, so forget about ST copound area and focus on getting a perfectlt aligned DT-BB-HT miter. Once the DT-BB area is done, complete it with the DT-HT end. This will leave you with a DT placed perfectly aligned in BB-HT line.
-With the DT resting in place (BB-HT) and using the previously marked circle in the BB shell, you can mark the two intersection points on the DT, so you know how wide you might need to go to "avoid" the ST
-According to your frame drawing, you should also know the distance from miter to miter along top of DT, so you can make a third mark on the DT as how "deep" this compound miter should go. Now you should have a three point reference, two for "wide" and one for "deep"
-Put ST back in place, tack to BB shell
-Weld the ST-BB
-Place ST-BB piece back in place and start with the compound mitering process, use the "three point" signals as reference, but never as "holy truth".
-File, check, file, check, and you should end up with a nice compound miter that mantains the whole alignment of previous miters.

I hope it makes some sense, I'll try to make some pictures of it if you need visual reference.

And even if it sounds to long and complicated, once you got a bit of practice, this "traditional" method is not that bad at all and you can do it fast enough to be time/cost effective. Obviously nothing compared with the inmediate result of machinery, but I do still prefer human machinery.

Cheers

[mattypichu]

Amaro, that is exactly the process I was looking for. Thank you. If there are any other variations on this theme that anyone could share, I'm not full yet.

[afwalker]

I have really enjoyed the benefits of my home made main tube mitering jig that I made for a Nichol's horizontal mill with a rotary table and 8020 extrusions.
I guess it's so easy to do the miters now, they all come out perfect, the self centering vise is the key.
There's kind of this yin yang thing hand done vs machinery. I've do both and the machinery sure is appreciated especially when I made the some of the jigs myself:)
10618594463_f3b3164510_z.jpg9317648382_81011fe759_m.jpg10631125245_68364d280f_m.jpg9012312094_85a53c8978_m.jpg11076724684_7a5a7de6fd_m.jpg
cheers
andy walker
walker bicycle co
Walker Bicycle Company | | Walker Bicycle Company
Flickr: afwalker's Photostream