Atom Minor Mk III
by Jan Huning
The crankcase was held in the four jaw, as described in the ETW book, for machining the bore and front face. It was set so the outside of the case ran reasonably true, holding it quite lightly to minimise distortion.
A mandrel was machined to a light push fit on the crankcase bore for machining the rear face. This ensures the front and rear faces are parallel. The mandrel was then reduced in diameter a miniscule amount, so that the crankcase bore was a free fit, and the front face skimmed true. It was drilled and threaded, then parted off a little shorter than the crankcase thickness. This provided the locating fixture for machining the cylinder bore. This plug had a flat machined on it, so the cylinder bore could be machined full length. A small hole was machined on the flat, half the crankcase axial thickness from the end, and offset from the axis by 1/16", for setting purposes. The plug was bolted loosely to an angle plate which was in turn mounted on the faceplate. The flat was set square to the lathe axis, the plug tightened, and then the assembly adjusted so the setting hole on the flat ran true.
The crankcase was then fitted over the plug, rotated so the cylinder mounting face ran true, and clamped tight, all pretty much as described by ETW really. The distance from the flat face to the centre of the plug is known, so by measuring with a depth mic from the end of the cylinder to the flat face, we know the distance from the cylinder mounting face to the centre of the crankcase bore, so we know when to stop machining. I put a piece of paper between the angle plate and crankcase, to improve the grip and to protect the machined faces. The photos were taken the following day, when the paper had soaked up the cutting oil, hence the rather tatty state of it.
The crankcase was mounted on a machined block for machining the exhaust stub. This block had previously been machined square to use as the fixture for machining the cylinder bore on another engine. After marking out, the crankcase was bolted to the block at the correct angle, and then the whole lot put on the face plate with the marked centre of the exhaust stub running true. It looks a little precarious, but the block is relieved on the underside against the faceplate, so it contacts only around the outer edge, so is quite secure.
The underside of the lugs and the mounting holes were machined with the crankcase sitting on the cylinder location face, centred on a plug in the centre of the rotary table.
The tapped holes in the front and rear faces were also machined with the case centred on a plug in the centre of the rotary table, but no photos of this. The holes were drilled at the positions defined on the drawing, rather than to best advantage, which worked out ok.
The inside of the exhaust port was cleaned up with a small fly cutter when the crankcase was set for drilling the cylinder mounting holes. Some filing was still needed to blend the flycut part with the outer bored section.
Last operation was to spotface the top of the lugs to provide a flat seating for the mounting bolts. A cutter was made from silver steel, hardened and tempered, and attached to another piece of siver steel (same diameter as the mounting hole) with high strength loctite (grade 603). The crankcase was positioned by eye so the spotfacer went smoothly through the hole. Just enough clearance against the machined exhaust stub. This worked ok, and all the spotfaces are at the same height.
I used 4-40 UNC rather than 6BA for all the holes. The castings are quite soft, so I prefered to use the coarser thread. I machined all the screws anyway, so the heads are the same as for 6BA.