The chassis selector control
Thursday, 20th August, 2015
The undercarriage or chassis selector quadrant is neither an easy item to interpret nor to model. Even with GA drawings and good photographs, the shapes and how they interact and lock together are hard to appreciate. And because the bulky unit is constrained by extant fixing points both horizontally beneath the waist longeron and vertically (via the chain guard assembly) to the front of Frame 8 there is little room for error in the overall dimensions.
Because of this I began with the mounting bracket that is installed beneath the longeron. This is shown in Photo 1 bolted to a knee-shaped back plate, the latter being a contrivance of my own onto which to build the assembly – it does not exist in the real thing. It is worth mentioning before moving on that there are actually two brackets, but to simplify things and aid installation I made them as one then cut away part of the middle at a later stage to give the appearance of a pair (the result is seen in Photo 4).
Photo 1 shows the five principal components of the assembly as I chose to build it: an alloy cube from which to machine the valve assembly, the aforementioned mounting bracket and back plate, the part-machined quadrant barrel, a disc-like faceplate and a brass spindle shouldered and threaded at one end for a 10 BA nut. This serves to lock the latter three items together and as the pivot for the big actuating lever.
The valve block, with its angular geometry, is made for the milling machine (Photo 2); indeed it would require an expert to do a crisp, accurate job with hand tools. The part-finished component is clear in Photo 3.
Photo 3 also shows the chain guard, the sixth major item in the mix. In the real thing this is made of sheet metal, folded twice through 90 degrees to achieve the step-like geometry. Again I simplified things and the picture reveals the ruse: my chain guard is made in two pieces with a chunky block of milled resin glued in between. The rest is down to the fine detail, which can clearly be seen in Photos 5-7. Note also the additional slight of hand by which a delicate strip of styrene attached with thin superglue stands in for the chain guard flange, thus circumventing an awkward metal forming operation.
The various hydraulic pipe unions and other brass fittings that adorn the valve block need no commentary from me except to say that they illustrate the wisdom of keeping a stock of hexagon brass in various sizes. The whole lot was produced in an hour or so on the lathe.
I left the hand lever to last, mainly because I was hesitant about milling the tapered channel along its length. It seemed sensible to make the channel first and then cut the leaver out around it; even so, it took three goes to get it even approximately right! Turning the elegantly shaped handle from a hard black plastic was a lot more enjoyable.
I have covered the main components of the quadrant, omitting the finer detail. However, before concluding I want to mention the circular faceplate. This is clearly a casting as evidenced by the heavy relief in the form of the ovals and arcs that adorn it. These look hard to do, but they are not, being simply cut out from styrene sheet of the appropriate thickness and glued on. The secret is in the finishing: Once the superglue is dry I gave the whole piece a vigorous working over with steel wool, which serves to round off the plastic corners. Beneath several coats of paint the effect is convincing.