The last two weeks were spent on making the cylinder blocks for the engine. As the cylinders are between the frames, they have a square shape and are relatively easy to manufacture. That said, it takes a fair amount of machining to make them, and the sawing machine, shaper, lathe and milling machine came to use to complete them.
It started with sawing bronze blocks of about 67 mm long, from stock bar 52 x 52 mm. Especially bought at a model engineering exhibition a few years ago. Expensive, but much cheaper than a casting.
Machining is only one part of the job; the design and drawings come first. Many holes have to be placed in exact locations to avoid unwanted intersections and to ensure that everything fits and can be assembled properly. Working in 3D CAD is a real advantage here. From the 3D model, the 2D working drawings are produced.
Shaping the blocks to the outside dimensions. My new (secondhand) vice is a delight to use: everything is dead square and parallel.
Shaping has the advantage that the surface finish is perfect, and sometimes the crystals of the bronze become visible. Very fine grooves stay on the surface, which will be perfect for holding an oil film on the port face. The slide valve will also be shaped, but with the grooves rotated 90 degrees. I've done this on previous locomotives, and this kind of surface finish will give long‑life valves.
The center of the cylinder bore is located, drilled, and countersunk on the milling machine
In the 4-jaw chuck the hole is set to center with the aid of a center and dial indicator.
Pilot drilling
and drilling 22 mm (biggest drill I have)
With the boring tool, the cylinder is turned to the final dimension of 30 mm.
It takes several light cuts to get to the final dimension.
After boring, in the same setup in the 4‑jaw chuck, the end is faced so that it is dead square to the cylinder bore.
The steam chest, which sits between the cylinders, was also shaped from a brass block of stock material.
Milling the chambers for the bolts that will connect the cylinders to the steam chest.
The exhaust port drilled.
I had to re‑machine the top side of the cylinder slightly. This was because the cylinder bore face was no longer perfectly square to the block. Most likely, when clamping the block in the four‑jaw chuck, it had shifted slightly out of alignment. I had used three aluminium shims and one set of copper shims on the exhaust‑port side. A distance of 2.5 mm had to be bridged there: the aluminium bears directly on the block, but the copper has to clear the exhaust port. After taking off 0.25 mm, everything was square again.
Milling out the steam chest.
Drilling and tapping M3
The first test assembly of the cylinders and steam chest
The ports are milled 4,5 mm deep.
Putting the block at an angle of 30 degrees, necessary for milling the hole that connects the steam port with the exhaust port on top of the cylinder block.
But even though the drawing states this angle, I double‑checked that everything was OK before plunging the end mill in.
Drill the steam ports. The design has been done so that manufacturing is easy: three holes parallel to each other and straight in, until you reach the ports.
A recess is milled to connect the ports to the bore itself.
Next, the holes for holding the cylinder blocks onto the frames are drilled and tapped.
A test fit between the frames.
Drilling and tapping 44 M3 holes for the cylinder covers. Careful working is needed to avoid breaking the small M3 tap. As a precaution I used cutting oil, normally not needed for bronze or brass—just to be on the safe side.
Two M5 holes for the cylinder drain cocks.
Cylinders completed; next step finishing the steam chest.
By the way, the picture above shows the cylinders I made for my 5" gauge GWR 0‑4‑2 Didcot—but that was back in 1997, almost 30 years ago…
Still a lot to do.