Welcome to
Steve's Britannia
5" gauge Adams O2 - Calbourne
Drawing E1(a). Drawing E1(b). Drawing E2(a). Drawing E2(b).
Drawing E3. Drawing E4. Drawing B1. Drawing B2.
Drawings S1 and S2 Boiler Assembly Assembly and Alignment Errors and Ommissions
Assembly and Alignment
  1. Wheel Assembly  
Before I could assemble the wheels to the axles, I had to fix the coupling rods pins into place. These were a light press-fit done on the drilling machine but, belt and braces, I used a locking screw to keep them in place. The wheel was held on the pin using a collet block. The block was then loaded to the milling vice and an M3 tapped hole made on the perimeter of the pin. There is an M3 grub screw down the hole.
Following on from that, I needed to fabricate the balance weight plates and these were cut from 20swg mild steel for the backs and 16swg brass for the fronts. I've also engraved the wheels as left driver, right driver, left coupled and right coupled so that they go back in the same place after any future dismantling. The front axle was made at the same time as the bogie axles, so no need to cover that again. I did need to put the keyways in the end, though, and converted the fixture I made for the Britannia axles. It just fitted end-to-end which saved having to make a whole new block.
The wheels are kept in radial alignment with the keys and I have set a locking screw down through the hubs at 20 degrees to keep them pressed back against the shoulder. The wheels are a stiffish fit to the axles but don't need hammering on. No doubt I will get told off for doing this as well but I want my wheels removable. The balance plates have been drilled and fixed with 8BA screws and the gullies between the spokes can be filled with some laed or printers metal, whatever low-melt material I can get hold of. Once cast and set, the fronts of the screws will be sanded off flush. The bolt-heads on the back will just be painted over.
The axles have been asssembled, set into the frames and the coupling rods fitted on. By some stroke of good fortune, the wheels turn with just the tiniest high-spot at one point in their revolution. This should work it's way clear after some running time.
  2. Building the Cylinder Block  
The steam chest sits between the two cylinders so there is no access to the valves once the cylinders are assembled. Because of this, I needed to make sure that I knew exactly where the valves were when it came to timing everything and I have already made a pair of access ports at the front of the steam chest for measuring things. I did each side separately by putting the studs in one of the cylinders and fitting the steam chest over them. Because they are a fairly tight fit, I'm confident that the steam chest will return to the same position on re-assembly. With a piece of 1/8" gauge plate fitted to the back port, the valve was pressed against it and the distance measured through the first access hole. The packing was then placed in the front port, the valve pressed against it and a second measurement taken. With the average of these two figures written down, and with all the other dimensions already measured, I now know when the valve is exactly in the middle of the two ports. By repeating the process with the other cylinder and it's valve, I have accurate measurements for both of the valves once they are buried within the steam chest. One of my older calipers was modified to enable these measurements.
  3. Adjusting the Pistons  
The next job undertaken was to adjust the positions of the pistons and lock them to their piston rod. The first thing to do was find the exact positions of front dead centre and back dead centre on each side and I used the same method that I used on the Britannia. I did a fairly comprehensive write-up on the method HERE for anyone who may be interested. With the loco resting on it's wheels and with the axleboxes packed and clamped at the calculated ride-height, the approximate BDC postion was found by eye. The loco was then rolled forward by about 30 degrees and a DTI zeroed against the front of the crosshead, at which point the wheel was marked with the special scriber which, surprisingly, had not got lost! The loco was then carefully rolled bacwards until the crosshead went round the corner and came back to give the same reading on the DTI. A second line was scribed on the wheel and the centre-point between these two lines found using dividers. This was then centre-popped. The process was repeated on the other side to get that BDC, then the loco was rolled forward and the FDC's found for both sides (DTI measuring the other side of the crosshead, of course) and these also centre-popped.
I had put an M3 grub screw in the sides of the crossheads to retain the piston rod and these came into play now. With the front covers off the cylinders, each wheel was set at FDC and the piston set at 1/8" from the front face. The shaft was locked with the screw, the wheel set at BDC and the piston checked for the correct 2.1/8" from the front face. With both sides correctly placed, the complete drive assembly was removed from the frames and set up on the angle-table. Holes were then drilled and reamed to take a pair of 1/16" taper pins for permanent fixing and the grub screws removed. I didn't take pictures at the time but this later photo shows the crossheads pinned.
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