Construction began by preparing two lengths of 15 thou nickel-silver for the coupling rods and two pieces of 0.64mm brass for the frames. In the photo below you see the latter stuck together with double-sided tape.

It is critical to get the holes in the rods the same distance apart as the holes in the frames, so I temporarily soldered the two pieces of nickel-silver together and to one of the frames. (I didn't want to rely on two layers of double-sided tape). In this photo I scribed a straight line and marked the position of one of the holes:

I don't have a vertical drill press but I do have a Proxxon MF70 mill, so I used this to drill the three holes (diameter 0.5mm), measuring the distances using the handwheel. These awkwardly turn out to be 15 2/3 and 12 2/3 mm, so it is an approximation!

The result after separating the coupling rods:

The coupling rods were now put (safely) aside for later, to concentrate on the frames. Before opening the axle holes out further, I decided to mark the other critical hole, which is for the stub axle holding the worm wheel and 14T gear. For this purpose, I used the 2mm Association gear meshing tool. Below you see the two M0.3 gears on the tool, 14T and 25T. I found that this meshing distance was too small for the tool as supplied, but I was able to attack the sliding portion with a file and bring it within the range. The gears are mounted on bits of plastic muff.

To use the tool, you set it up so that the gears turn nicely, then put one point in the axle hole and use the other to scribe a circle (sorry for the blurriness):

The other horizontal line was measured from the drawing to get the correct x-position of the hole. This was centre punched, then drilled in the Proxxon as before (0.5mm). I then stuck on the template and drilled the other holes (two body fixing screws and three for pins to locate the brakes). I then opened out the axle holes by hand up to 1.5mm, running up through 0.1mm increment drills and finally using a broach. I did this by hand to avoid the risk of drifting the holes by having to centre them again on the mill. Unfortunately I discovered that I had managed to drill the gear hole slightly outside the scribed circle, so I had to drift it sideways with a round file. This then made it larger than 1.5mm, so I opened it all the way out to 3mm and will fit a brass bush later at the correct centre. The photo below shows the frames with all of their holes: