Building the
Atom Minor Mk III

by Jan Huning

4. Crankshaft

Making the crankshaft from solid creates a lot more swarf than crankshaft. It was machined from high strength low alloy steel, EN16M (old outdated spec) or 606M36 (current spec), in the 'T' condition. A piece long enough to produce both the crankshaft and prop driver was cut, and held in the 4-jaw with enough sticking out to machine the main journal. The main journal was turned and polished, using tailstock support as the diameter reduced. 600 grit "wet and dry" paper, held against a fine file, followed by 1200 grit paper, was used for polishing. The thrust face and outer diameter of the crankweb were turned at the same setting. The bar was then turned around, leaving enough protruding to machine the propdriver. ETW recommends case hardening the propdriver, I assume to provide a good wearing surface for the contact breaker plunger. I decided not to do this, but instead use the same high strength material as for the crankshaft. The propdriver was machined complete at one setting, including the tapered bore, then parted off. (Machining propdriver and crankshaft from one piece was done to eliminate chucking pieces.)

The crankpin was machined by holding the finish machined journal in an ER collet fixture set on the faceplate, offset from centre to give the correct crank throw. This method was described to me by John Maddaford. The throw is easily checked by using a clock held in the toolpost, set at centre height, measuring against the crankweb. With the crank in the 9 o'clock position (looking from the tailstock) the cross-slide is wound in till the clock touches the web. The face plate is rocked back and forth to obtain the highest reading, and the cross-slide dial zeroed. The faceplate is then rotated 180 degrees to the 3 o'clock position, and the cross-slide wound in until the clock touches the crankweb. Again, the faceplate is rocked a little, this time to find the lowest reading, winding in until the clock reads the same value as at the 9 o'clock position. The distance the cross-slide has been wound in is then twice the crank throw, ie the stroke. The fixture will need adjusting to obtain the desired stroke, but this is quite a quick process. The offset was checked after tightening the hold down bolts, and then the pin roughed out. The section of the web between the counterbalance and the pin was machined with a short trepanning type tool, fed in axially. The pin was finally polished in the same way as for the crank, but a little more care is needed to a avoid the sharp corners of the counterbalance.

The collet fixture was transfered from the faceplate to the rotary table for machining the sides of the crankweb. A milling cutter was used to remove the bulk of the material, and the curves added using a flycutter set to the correct radius. Finally the collet chuck was mounted directly on the lathe spindle for centre drilling the web. This centre was provided simply to allow the main journal to be polished later, if required, by holding between centres. It is not used for any of the machining processes.

 


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