Tail wheel and yoke
Wednesday, 12th March, 2014
With the strut complete I began on the U-shaped tail wheel yoke: Starting with some 1/8-in. thick sheet copper, I cut a blank roughly 5 in. long by 1.25-in. wide and heated it to a cherry red. Fortunately, I had some mild steel bar exactly the internal diameter of the yolk, so I used this as a former around which to bend the softened copper. The bending resulted in a good, although not perfectly parallel, U-shape, so I milled a rectangular off-cut of brass to match the diameter of the steel bar exactly, and corrected things by inserting this into the U and squeezing tight in the vice.
Machining began by milling the rough-cut upper and lower edges of the copper blank parallel and to the overall depth of the yoke. Next I turned the work-piece through 90 degrees, curved face uppermost, and milled a vertical slot, making this as deep as possible, without actually cutting the piece in two! As stated in my previous blog, I had already turned the brass boss; now I milled a flat along its length and cut this down on either side so as to create a projecting rectilinear key to fit the groove in the yolk.
With the boss in situ and checked for square, I silver soldered the two parts together. Silver solder provides a very strong bond, but it requires considerable heat, and even with two propane-butane torches, I needed a simple hearth contrived from an old tin box and barbecue coals to do the job satisfactorily. As the photographs show, the soldered fillets turned out quite nicely.
Now the remaining machining – and here my little brass block proved invaluable when clamping the work-piece tightly in the milling vice without fear of distortion or damage. I marked out the yoke’s arms in side-view and cut them in a series of milling passes, prior to drilling pilot holes at the axle positions. In plan-view the arms taper gracefully towards the axle, which required further milling, and since I would be working flank-side uppermost, I secured the piece temporarily to its brass block with soft solder. I turned the axle in brass, and threaded one end for the tow lug and the other for a ‘round’ castellated retaining nut. I also made two internal bushes that retain the wheel. The complete assembly is shown in the attached photos.
The Spitfire’s tail-wheel has a sheet metal cover or shield around the heel of the yoke, presumably to protect the caster mechanism. This final task was demanding, not only because of the shape, but because the upper surface of the part has to be made to surround and slot neatly into the grooved bush mentioned previously. I began by turning a hard wood dowel with two filed flats to represent as closely as possible the plan-view curvature of the shield, and I milled the end to an angle corresponding to that between the trailing edge and upper surface. This provided an albeit crude pattern around which I could bend a scrap of litho plate and, holding it there, turn over the top along the plane of the mitre by gently ‘panel beating’ with a tiny hammer. After a couple of tries I got something that had the makings of the correct shape, and once I had established that, I could drill and tap four 8 BA fixing holes in the yoke, mount the shield into position then mark out and cut the rest of it to shape. Finally, with the job re-assembled, I marked the outline of the circular bush and filed the opening in the top surface. It took most of a day and two broken 1mm drill bits, but I got there in the end.
By comparison the tail wheel was easy. I chose to fit the early version, as depicted in Paul Montforton’s drawings, and made it in two parts, both turned from stubs of aluminium bar left over from my Mustang project. They are held together by four 10 BA steel screw sets, much as the original. The lathe work was simplicity itself, as can be seen, and even the detail was done in a couple of hours in a few simple milling and drilling operations. I’ve included the air valve, but the tyre? Well, I’m sure about the methodology yet, so that’s a job for another day.