BR 08 Shunter scaled and motorised

For the sake of posterity if anyone is looking to build this, please note my findings on the axle/wheel/hub/crank alignment.

Chassis Sides + Axleboxes

If using separate chassis sides and axlebox detail, one only needs to open out the chassis holes wide enough for the axles to pass through, and the axleboxes wide enough for the bearings to press into, that way they are held captive between the chassis side and the outside crank. In order that I could test the mechanism I opened out the chassis holes to the full diameter of the bearings - they are held captive by the cranks on one side and the wheels on the other and this permitted me to do some testing before adding the axlebox detail, but is not neccesary and may cause problems for me in future.

Bearings

On that topic, the wheel bearings are specified as MR63ZZ roller bearings, but if I were to build this again I would go with plain bronze or brass turned bearings, or nylon bushings - mostly because the bearings have a very tight tolerance and it was very difficult to insert and remove the axles even with any burrs filed down and the axle burnished - but it obviously can be built as specified.

Axle Assembly

There is a jig provided to file flats on the axle for grub screws to fix the helical gears, wheels and cranks in place. This puts the cranks at 90 degrees from each other, which is as desired. However, I found it difficult to juggle the positions of the wheel hub inserts and the wheels, this was my method:

1. Mark 3 wheels with a dot or cross on the inside face.
2. Place both wheel hubs onto an axle, then both wheels (no glue), then the cranks.
3. Use M2 grub screws or bolts to fix the wheel hub and cranks into their correct positions. Ensure that the axis of the cranks are perpendicular to the D-shape filed onto the end of the axle - I had to wiggle mine, gently tighten and repeat - as the friction of the fit may be enough to prevent them from rotating as the grub screw is inserted.
4. Hold an axle end-on and rotate it so all the flats are facing upwards except the one facing away - the crank facing you should be pointing downwards, and the wheel should be fixed so that the balance weight is directly opposite, which places the fixing screw between two spokes. Apply glue (I used EvoStik to give me some working time) and fix into place, repeating this process for all three axles.
5. When set, flip the axle over so you're looking at the other side, which should look like a correctly oriented D and the crank pointing to the right - and fix the wheel again with the balance weight opposite the crank, this time the fixing screw for the hub should be behind a single spoke.

It is not imperative that the wheels are set exactly 90 degrees apart as long as they are all identical, but that's the method I used. They can now be disassembled and inserted into the chassis.

Drive Shaft to Axle Interface

The last difficult bit is the interface between the helical gears and the worm gear. The helical gears must be inserted into their recesses loose and then the axles threaded through them.

One driveshaft sleeve has two worms on it, so the helical gears must be in an identical position. I used the grubs crew on the helical gear as an indicator. 

One of the driveshaft sleeves has a single worm on it, and so the helical gear can be place in any position during assembly. After the axle is properly in place, the grub screw on the worm can be slackened and the worm assembly rotated around the driveshaft so the position of the grub screw on the helical gear matches those on the double-worm, and then tightened up. 

Difficulty

I have rated this as a difficult model, not because of any problems but simply that it is a complex model with lots of parts and interfaces.