Thursday, 13 January 2011

Judith Edge 06 shunter

I have been working on and off for several months on a Barclay Class 06 shunter, based around an etch from Judith Edge (shot down from 4mm).

First stage was to build up the main bodyshell using the parts provided:

This prototype has a rather narrow bonnet (something like 7mm inside width) that won't fit most types of motor, so it presents something of a challenge to design a workable mechanism. Particularly as it is my first scratchbuilt chassis!

The next photo shows the body attached to the basic structure of the chassis. The frames are made from thin PCB, and spacers are solid brass (as per Nigel Cliffe's DY1 article). The Judith Edge etch does include frames but I wanted to use the PCB method with solid brass spacers to maximise weight.

The third photo shows the spacers more clearly. They are tapped for the 12BA screws that hold the frames on - the screw heads will be hidden behind steps eventually. They are drilled to a clearance fit for two further screws which attach to captive nuts on the top of the footplate (inside the cab and front bonnet), thus holding the body and chassis together. The third axle bearing in each frame will support the jackshaft.

I decided to try one of Nigel Lawton's "micro-motors" as these are narrow enough to fit in the bonnet, thus keeping the cab empty. I spent quite a considerable time pondering how to fit in a mechanism and also experimenting with how to mount the motor etc. I settled on the arrangement you see below, with a 3-stage reduction. The initial stage is via one of Nigel Lawton's rubber band drives, followed by a conventional Association 21:1 worm, and finally 14:20 spur gears. The latter are the new metric M0.3 gears, as these had the most suitable diameters. Due to the narrow width I was severely limited in the choice of gears to fit on the wormwheel cross-shaft. There was also a limitation due to the rather small 7mm driving wheels. The overall reduction works out at something like 108:1.

The intermediate gear shafts are held in a simple "gearbox" folded up from a scrap part of the kit etch. The "motor mount" is simply a piece of PCB hollowed out to an interference fit. It is bolted on to the gearbox so that the whole can be dismantled and the motor and/or rubber band drive can be replaced. Notice also the resistor between the motor and frame, recommended because these motors are rated only for 6V.

Amazingly, the mechanism now fits inside the body. However, it is rather temperamental, and I'm in the process of "tweaking" the setup to get the motor to turn happily in both directions. I got it to work initially but rather infuriatingly ruined the alignment somehow while modifying it to fit in the body! The problem is that the gearbox needs bending slightly to achieve minimum resistance on the worm and the correct meshing distance for the gears. In theory this will allow me to correct for my inaccuracy in marking, drilling and folding it up. In practice it is remarkably difficult to get right. Hopefully I will be able to get it turning OK, fit the wheels, and be able to move along the track! I'm not particularly confident in its running abilities, but it is after all my first chassis.