Well a couple of weeks stretched into five due to illness but we’re back.
One of the details omitted previously was the flat bar which will hold the bottom bracket post. This was mostly because I’d lost it – the jig has been moved around a few times and somehow it got left behind. Having run out of the scrap I’d rescued, I needed a piece of steel with which to make a new one. I went down to my local metals supplier to buy some 50 x 12 bar but they didn’t have anything appropriate in less than 6 metre lengths so I bought a short piece of heavy channel and cut one side of it off.
I then filed this part flat and drilled and tapped four M12 holes. The two lower holes are alternate positions for the bottom bracket post, this extends the adjustability of the centre line to BB dimension – it can now be anything from 100mm to 250mm. To allow extra working room I decided to mount this plate 16mm behind the top plate. To get this space accurately I used two short pieces of 16mm key steel which can also be seen in the phot.
Because the two bolt holes are fairly close together I used M12 socket headed cap screws to hold the two plates together. I lined the two plates up edge on to a flat steel plate whilst bolting them together – I think I’ll loctite this part of the assembly together to make sure it keeps in alignment.
The other part that was omitted was the piece of aluminium T-Slot extrusion to which the dummy axles will be attached. This was cut to 250mm and one end notched to clear the fixings on the mounting plate. This allows adjustment for any chainstay length from 400mm to 500mm without moving the dummy axles on the plate, and 100mm beyond that if the axles are moved.
Before putting everything together, here’s how I lined up the mounting plates to make them co-planar. I have two laser line generators with magnetic bases bought for another project. I attached one to each mounting plate and aimed them at the far wall of the shed.
The plates were then adjusted using the four mounting bolts mentioned in the post above. You need to get three things right at once: the lateral displacement, the horizontal angle and the vertical angle. The last is the easiest as the two generated lines cross if the angles are wrong. Horizontal angle and lateral displacement are adjusted together and then double checked by turning the line generators around and aiming them at the other wall. One phot was taken with flash, the other without. You can see a slight degree of divergence in the line without flash – I hadn’t finished yet.
The last phot is the jig as it stands – sorry about the quality of the shot, but hopefully you can see where the remaining parts will go.
Cost to Date: $133
Bill of materials:
300mm of 16mm Key Steel $11.
200mm of 12 x 50 flat $5 (plus almost half an angle grinder cutoff wheel).
Bolts $6.
Next post we get down to the business end: the parts we need to get made to finish this thing.