I haven`t seen anybody else do it this way (maybe there`s a reason for that ), so thought I`d share this with you in case somebody else can make use of idea. This is I think the third stem clamp I`ve made this way, and I`ve done a number of other misc clamps by the same general method. The positives for it are that it gets the bolts in very close to the tube being clamped and that it`s almost impossible to have anything crooked. Ends up looking pretty cool too, IMO. On the negative side, it`s time consuming and it pretty much requires a lathe and mill. Pardon the crappy pics- I tried.
1. Turn and bore the shell at the end of a piece of stock with enough left over to chuck or clamp. Cut the final OD, but leave the ID a little under. In this case, it was for a 26mm stem and I wanted .050 wall, so I cut the outside to 1.125 and the temporary ID at about 1.010
2. Work out where the center of the binders need to go and vise or clamp to a milling table, locate center. I leave .010 to .015 meat between the major dia of the binder screw threads and whatever it`s clamping around, but in theory it could be as little as zero if you`re brave.
3. Mill pockets for all the binders with an endmill the same size as the stock the binders will be made from- for me, that`s usually either 5/16 for 4mm screws or 3/8 for 5mm screws. If I`m using 3/8 for the binders, I`ll cut the pockets with a 10mm endmill to give a little extra flow room for brazing- 5/16 and m8 are so close together that it doesn`t help much, so I just use whichever. If you stop cutting down before the bottom of the pocket exits the shell, it leaves a little lip that works just dandy to ensure that all the binders are at equal heights when you go to braze them on. I hope that makes sense- my pictures don`t show it very well either. Make a purdy hole in the center of the cap if you`re so inclined- it looks extra cool if countersunk.
4. Measure the height of the pockets, cut and face rods for binders, drill through to tap drill size for the screws you want to use. Sometimes I leave the tops of the binders sticking up a little bit, sometimes just go to the top of the pocket.
5. Dry clamp and check, final clean up, flux, clamp for real, braze. For a single bolt (like a seat post clamp), it`s easiest to just set the binder in the pocket and put a weight on top for tacking. For pairs of binders opposite each other, I made clamps to hold them in place.
6. Soak and clean, go back to the lathe and bore the final ID. Part off.
If there`s no more brazing or welding to do, I`ll slit the shell at that point, then tap the threads and drill out the through holes in the cap to accomodate the screws. Since I still have to braze this one to the rest of my stem, I`ll take an adjustable reamer to it after brazing, and then slit and finish the holes. Yeah, a lot of steps, but for the small number of clamps I have to make I can live with that. What do you guys think?
EDIT: a few tips-
To figure side to side locations for the pockets (center of shell to center of pocket), you can use (D+d)/2 + m where D + tube (bar) dia, d = screw dia, and m = "meat" between the two.
For end to end distance, it helps to leave a little extra space between the binder and the end of the shell (1/32 is nice).
The thicker your shell wall, the deeper you can make the pockets and still have a lip to push the rods up against.
I just use solid roundstock (cold roll, usually) for these because it`s available and easy enough to drill and turn to any size under the sun. Mine are all thick walled, so I think pretty much any steel is strong enough.
Don`t try to make biners from drillrod! After you braze it, you`ll never get even the first hole tapped without breaking off the tap! (trust me on that)
If you sart with stock that`s way bigger than the shell you`re making, be sure to cut the OD long enough that the shoulder doesn`t get in your way when you braze. Also nice to cut the ID a little longer so that you aren`t brazing right next to a heat sink. BUT if you have a lot of thin walled shell sticking out, the vibration makes it tough to machine smoothly.
For the binders, you can use rod smaller than the head of your screws, but the rod sections need to stick up higher then (so the screw heads clear the shell), and that ends up looking kind of unballanced.
I tried a few of these with aluminum and they tore apart- problem is that welds don`t sweat in to cover all the inside surface like braze filler does.