Thunderstang

Hello everyone,

I'm planning a new frame. I wonder if there are any disadvantages with a "hollowed out joint" of down tube (xcr 35mm) and bottom bracket (T47).

unterhausen

I'm not sure what you are talking about. Do you mean a joint that's not fully connected so cables can exit the down tube or something else? Can you point at any examples?

dsaul

I usually mill out a 3/4" vent hole for the downtube/BB joint. That is plenty of space to pass cables through. Theoretically, making the hole in the bottom bracket match the ID of the downtube shouldn't be an issue. In practice, its a lot more difficult to achieve the perfect placement of the down tube over the hole in the BB and weld it.

Thunderstang

Bottom brackets (39x68mm) are available with a seat hole (28.6 or 31.8mm).
I have used these brackets for (lightly loaded) children's bicycles. I used a T47 bracket for my own bike with the down tube in the bracket. Always with fillet brazing.
The new frame is for my son who has more weight (80kg) and power.
The thickness of T47 is 2.5 mm outside(2x 12.5 mm) and 1.6 mm thick in the middle (43mm).
For me it is easier to drill a hole than to place the miters exactly perpendicular and to keep the exact length.
It is also the case that the rear brake cable runs through the downtube, bottom bracket and left chainstay.
Question is : how should I imagine shell distortion : less stiff, deformation, breakage?

unterhausen

Okay, that's an interesting question. I don't have an authoritative answer. However, I have heard of people that had failures from vent holes that were too big. If you do this for both the down tube and seat tube that's a lot of material missing.

Andrew R Stewart 

Years ago I had the idea of doing what I think the OP is talking about. I hole sawed a socketless BB shell for the ST and DT. While I was setting up to saw the chain stay holes I began to question my plan. There's already some shell face deformation due to heat induced shrinkage with shells, of either filleted or lugged designs. When I saw the huge openings and thought about the tube's lack of stiffness, across the tube section, I suspected this plan would result in even more distortion after brazing. So I stopped my plan. I kept that shell as a "paperweight" for a long time. Now I can't find it so suspect I tossed it out. Andy

unterhausen

I feel like mitering isn't that difficult. Get some practice material and have at it and your miters should be okay pretty quickly. Get the biggest diameter half round file you can find.

Improving your miters would be much preferable to experimenting with the longevity of big holes in a bb shell

Thunderstang

Thanks for the answers ; it helped me (no holes in bb).

Martin

Doug Fattic 

Keep in mind that you do need a vent hole somewhere in your down tube area to let hot air escape when welding up your down tube. It can be at your head tube or BB or pre-drill your water bottle boss holes. Like others I wouldn't drill big holes in the BB shell but a small one works as a vent hole.

guy153

OK I see what you're suggesting. Drill out the BB to the OD of the DT and then just slot it in. In theory you lose a bit of stiffness but if you're worried you could get that back by welding (or brazing) a cap onto the end of the DT first. As Doug says make sure you do have some vent holes (bottle holes, or drill a small hole in the cap).

smontanaro 

This. When taking Doug's class, all my miters were done on a milling machine except for one practice miter that he had me do by hand. It wasn't difficult, just slower than using automation.

bulgie 

I've told this story here before, skip if you've heard it!

When I worked elbow-to-elbow with Glenn Erickson at R+E Cycles in the early '80s, we filed all the miters at first. Naturally we got good and fast at it, with two hacksaw cuts to start that got us so close there wasn't much filing to do. We had angle templates precision cut out of fairly stout sheetmetal, at 1 degree increments, in the normal range of bike frame joints so it wasn't too many, and we learned to interpolate the in-between angles reliably to about 2 tenths, plenty good enough for bike frames. Closer tolerances were possible with more time spent, but you have to stop somewhere.

We made a lot of lugless frames too, so this is not just for lugs that would hide poor mitering. We were a bit fanatical about the miters even for lugs, beyond what was strictly necessary. With a little care we could get them "water-tight".

We clamped tube blocks to both ends of a tube before beginning, which ensured the two ends of DTs and TTs were clocked to each other. A trammel to mark out the lengths. We had half-round files in the sizes that cut the correct radius for each joint. Then the miter shape takes care of itself, and you only have to mind the angle and the side-to-side. (I could never understand the need for those templates printed on paper and wrapped around, but maybe those are good for a beginner who doesn't know what a proper miter looks like.)

Then we got a cute little Emco (made in Austria — not Enco) milling machine, and I quickly took to it and started mitering with holesaws, but Glenn preferred to stick with filing, said it was faster. I didn't believe him, so we raced. He won! Only by a little, seconds if I remember right, and my miters were just slightly more perfect, so I like to think of it as a tie. But his miters were beyond "good enough", and he was indeed faster, due to the setup time on the mill. I can't remember if he ever switched over to using the mill, in the remaining few years we worked together before he left R+E to go off on his own. I think not, I think he stuck to his files.

That was for one-off custom frames. The mill would win if you were making batches of production frames, but Glenn never did that.

So my advice for an amateur/hobbyist FB is practice with the hacksaw and files, and avoid buying a big expensive machine to do what you can do with your hands, and just sits there doing nothing most days.

Said the guy who just bought a mill! Eek, what have I done, now I have to get it downstairs and shoe-horn it into my tiny basement shop. It's an Enco (made in Taiwan, couldn't afford an Emco), the dreaded round-column mill-drill, RF-30. But it's heavy and powerful enough to take some biggish cuts in steel once you get it dialled in. It was cheap enough I couldn't resist, and has had almost zero use, it's like a time capsule (OK it's only ~20 years old, hardly an antique.) It came pretty well tooled up too. "Some disassembly required" to get it down the stairs, I am not a heavy equipment rigger. I've instructed Laurie to dial 9-1 and have her finger poised over the next "1" as I winch the heaviest pieces down... I did it with my lathe though, so I'm familiar with the technique.

Why did I get a mill? I dunno, just always wanted one. Hobbyists don't need to justify their hobby to anyone else, IMHO. I ran a Bridgeport at work years ago and a smaller knee-mill for a decade before that, and I've missed having one ever since. But there's just no getting a knee mill down my stairs, nor a place to put one. Tiny house, no garage.

Oh I did consider a Clausing 8530, a knee mill half (or less) the weight of a Bridgeport. But the seller knew how much those are in demand from hobbyists, due to the small size, and was asking about twice what I could afford. That would have been near perfect for me but a little weird, like with morse-taper spindle vs R8 on the Enco (R8 is the Bridgeport size, super common tooling), and the Enco has twice the horsepower. And one third the price. There's a huge community of RF-30 sufferers out there, so plenty of Youtube how-tos, add-ons, fixes for their weaknesses etc.

But I digress! Anyway, do as I say, not as I do.

Mark B

unterhausen

If through some miracle I get my mill running, it's possible I might do rear triangle miters on it. I wouldn't mind being able to miter fork blades too. Path of least resistance is hand tools though. I do have a set of paragon hole saw mandrels and some nice hole saws. With the current state of my equipment, I would mess up a t47 bb, I'm pretty sure of that.

bulgie 

Sorry to totally hijack the thread, but I mentioned having to winch the mill down, and today I disassembled it enough that each piece can be hand-carried. No winching, maybe some wincing. But I still have all my fingers and other body parts, so far. I have to weld a base for it before I can reassemble it though.

By the time I get this mill working, I could have built multiple frames in the time I spent shopping, shipping, dis-and re-assembling. Just watching all those youtube videos alone consumed several hours. Good thing I'm not trying to make money at this.

unterhausen

I always wanted a mill, nothing to do with bikes really. I rarely use my lathe for anything bike related. But my kids are going to curse me over the mill when I die