Wondering if there is a (free) way to get a mitering template ala tube notcher for mitering the DT at the BB that plots out the curves needed for both the BB and ST junctions simultaneously? Or do you just use the template for the DT/BB junction then eyeball the additional filing needed to fit the ST?

This is for my first foray into fillet brazing, so I can't just scribe the tube inside a lugged bottom bracket as I am used to doing.

Thanks!

the free app at Nova doesn't do this? The bikecad template generator gives you the cutout for the st/dt miter, but for me it's actually pretty easy to fake it.

novacycles has a tube notcher program built into their web site, it's good!

Right, but unless I am missing something there is no way to get it to calculate a double miter for you?

If I'm hearing you, you're essentially hoping to recreate the tube w/ miters in paper?
Just lay something like a small piece of aluminum angle along the tube to mark a center line w/ a fine sharpie. The patterns have the 90* markings, so you should be able to line them up in phase. Play around with the tube size because I've found that at the tube-size, the patterns are a bit to short to go all the way around. Try an extra .1-.2mm.

The bicycle geometry 101 (https://www.os2.dhs.org/~john/Bicycle_Geometry/) spreadsheet has the compound mitre profile in the last few sheets. I never figured out how to use the rest of that document sadly.

What I do. I have a cad that gives me the length of the overlap between the tubes at the BB. I did the math at some point but I can't remember how hard is was to derive. Anyway once I have that length I scribe it from the BB mitre on the tube centreline, cut a v with the hacksaw, and scoop the mitre out with the proper half-round file. I set the tube in the vise at the proper angle and file horizontally.

That spreadsheet is gold. Exactly what I needed, thanks! How have I never run across that?!

While I work with Bamboo rather than steel, I still do all the mitering by hand with no template or lines drawn on. I roughly file two angled faces where the notches will go and then hollow them with a half round rasp and then finish with a sandpaper wrapped tube the same size as a bb shell. Do that with both tubes and then miter the second notch where the tubes will cross. Use the same technique to form the second miter by using a sandpaper(emery) wrapped tube equaling the corresponding tube dimensions.
It's not that hard to do precise work this way.
It's all fine working out everything on paper and doing elaborate diagrams but ultimately it comes down to how you execute things.

I just insert the tubes into an investment cast BB shell and run a carbide scribe around the inside to mark the miters.

https://www.ozhpv.org.au/shed/tubemiter.htm

Been a long time since I loaded it, but tubemiter is the program I use. To get the overlap, I just eyeball it.

I would add that I'm not a big fan of the printed profiles. While they are accurate, once stuck on the tube they only give out a rough outline, just like a lug does. So I keep some lugs around just to scribe the mitres.

Thanks all for the thoughts.

One reason I wanted to use a printed template is that I am building my first fillet frame without using a jig. I am counting 100% on exact miters to get things lined up properly. Scribing with lugs would be ideal, but I don't have lugs!

All this being said.....

I used the template spit out by that spreadsheet above yesterday and it was pretty much spot on with just a tiny amount of fine tuning. Finished up my front triangle and it came out surprisingly straight considering I have all manner of wacky sh$t going on to hold the tubes together for me (random chunks of angle iron, c-clamps, cable and weights, etc.) for brazing.

Thanks again!

Doesn't matter. The miters are the same. I have a cast shell I keep around for exactly this purpose.