Here’s an illustration of how this jig will work: If you draw a line from the rear dropouts which meets the seat tube at 90 degrees and then extend this all the way to the steering axis, you get this:



The angle of the headtube to the horizontal axis (hereinafter called the jig axis) is 90 degrees plus the difference between the HTA and STA. This varies only a few degrees either side of 90 for all practical frame designs. This is the only angle that needs to be adjusted in this jig design, everything else is orthogonal. To do this accurately we need a couple of precision XY translation stages. Normally these are very expensive but a cheap substitute is easily found in cross slide vices like these:



In a later post I’ll show you how to do the calculations using RattleCAD But for now lets skip straight to construction.
I used two pieces of steel square tube (aka RHS) welded into an L shape to hold three mounting plates:



The mounting plates are rectangular steel plates with holes for the adjustment screws. I got two of the plates welded on but that isn’t strictly necessary, the third is bolted in place which works well. A second set of plates bolts to the first so that the plates can be adjusted to be coplanar, on which more later. Here’s the jig with the second plates mounted:



The second plates are mounted on studs which bolt through the plates mounted to the frame, allowing the orientation of each plate to be adjusted. There are four studs for the hevy plates to which the vices will be mounted:



But only three for the small plate for the tail end:



The extra holes in the second set of plates are to mount the Cross slide vices and a short piece of aluminium slotted extrusion similar to that sold by 80 / 20:



Hopefully you are now getting the picture: basically the vice on the left will hold the head tube mechanism, the one on the middle will hold the seat tube and bottom bracket and the ally extrusion will hold the dummy axle.


Costs to date: $70


Bill of materials to date:

2 x Cross slide vice $60
Assorted nuts and bolts $10
300mm 80 x 20 extrusion*
1 metre 40 x 40 RHS*
300mm 150 x 12 bar*
200mm 100 x 12 bar*
400mm 80 x 6 bar*

Items marked with an asterisk came from the offcuts pile. If you have to purchase these they will add to the cost. The aluminium extrusion costs around $20 per metre so the costs aren't huge.

Next post I’ll start detailing how these various bits get held in place.