BB Mount

 

pdf plans here

Before you start debur and wire brush all the metal pieces.

PLEASE NOTE: This is not the way I recommend constructing the BB Mount, its just the way I did it the first time. There may still be some useful info here, but have a look at the way I constructed the BB Mount for the Jetrike MkII for a better method.

13 Jan 2007

First I began by cutting out the pieces of metal for the BB Mount from the cut list.

The bottom bracket shell is a standard British 1.37" x 24 TPI x 68mm mild steel shell purchased from Nova Cycles <www.novacycles.com> frame building supplies in the US (I bought a hole bunch of these and other braze on bits at the same time, to save a little on postage).

To make clamping and welding the shell onto the sleeve easier, I ground a flat edge along the bottom. I hand finished this with a flat file to ensure that it was completely flat.

Here you can see the flat edge of the shell sitting on the sleeve (Item 14).

Using the drill press, I drilled two holes in to each side of the sleeve. Then using the fine cut-off saw on the rotary tool at about 5000rpm, I cut out the left over metal in the middle. To do this job you need to brace your hand against the vice, and while holding the rotary tool firmly with both hands, you have to apply only the lightest amount of pressure. It requires the deftest touch, if the blade catches, is shatters into pieces. Once you get the hang of it, its pretty easy -- like all things delicate, you just have to be patient.

20 Jan 2007

The next job is to machine the BB Mount binder (Item 25). This should be machined as a single piece and welded on. Then it and the bottom sleeve face should be cut in down the center using a hack saw.

To machine the binder you will need an M6 tap, a 13/64" bit and a " bit.

Start by center punching one end of a 30mm length of 14mm square bar.

Now using the 13/16" bit, drill a hole through the center of the binder. Next put the " bit into the drill chuck, and drill the wider aperture on one side 15mm to the center of the binder. You should have a depth gauge on your drill press to help you do this.

Now tap the other side with an M6 thread. Squirt cutting compound onto the tap and work piece to lubricate it. To cut a thread, you do a half turn at a time, then back off a quarter turn (you will feel it click as it cuts back) then proceed with the next half turn and back off again, repeating until the thread is cut. I use compressed air to blow out the metal shavings from time to time to keep the work piece clean.

Now the binder is ready to clamp and weld. Once its welded, place it in the vice, and cut it down the center with a hack saw. You should end up with a thread one side and the wider aperture on the other.

With the binder done, its time to tac weld the shell to the other side of the sleeve. To center the sleeve against the shell, place a piece of 3mm sheet metal under it. Then using some short ends of box, clamp it and tac weld each corner on each side. Make sure that they are substantial tacs, but don't weld it yet.

At this point you should slide your BB Mount onto a length of 35mm x 35mm box (or the front of the drive train if you have already started it) and mount your cranks to make sure everything is square. If its out a bit, put it in a vice and see if you can bend it (screw in the old BB races shown below before you do this to protect the shell). If its out a lot, you may have to grind off the offending tac, bend then re-tac.

Now using a 40mm hole saw on the drill press, cut a fish mouth into the front derailleur post (Item 5). For the endcap, I used the 30mm hole saw on my 1.5mm plate to cut out a disk, then welded that on the other end of the front derailleur post. You can just make it out in the picture below.

Now position the front derailleur post against the BB shell. This was a hard one to clamp, so I just braced it and held it with one hand and tac welded it in four places. Then, as described above, slide the BB Mount onto the length of 35mm x 35mm box and mount your cranks and front derailleur onto it and make sure everything is square.

Once you are happy with it, screw some old BB races into your shell before you start welding to minimize the warping. Here I am using a tool I made out of a bit of 5mm x 30mm flat to screw the left race in.

When you weld a job like this, you have to be mindful that the weld will warp the metal and pull it out of being square. Start welding at the front and do a short weld in the center, then do the same at the back. Now weld a little bit more on each side of that weld. Finally weld the sides, a little at a time and alternating sides. If you have enough metal on the front it should stop the side weld from pulling too much.

When its all done, clean it up with the heavy wire brush on the angle grinder and spray it with some Proetch undercoat to keep it from rusting. I left the BB races in to protect the threads, then sprayed the inside of the shell and spun it around a bit to make sure the inside it was fully undercoated.

Thats the BB Mount done!