Atom Minor Mk III
by Jan Huning
11. The Conrod
I usually mill con-rods to shape, rather than turning them, which suits the Atom Minor design. I use either HE15 (2014) round bar or more recently I have used 7075 flat plate. Whichever is used, the first operation is to mill the bar to a rectangular section encompassing the big end and small end. The centre section is then milled to final thickness before the bores for the big and little ends are machined (so that any distortion due to residual stresses in the bar does not result in misaligned bores). The rod is loosened and then retightened in the vice before machining the bores. The big and little end bores were simply drilled and reamed.
A simple jig plate is used for machining the external profile. A piece of aluminium plate is machined on one face, and two holes machined at the correct centre distance. Both holes are the same diameter, in this case the same size as the little end. One hole is machined with the centre of the rotary table in line with the axis of the spindle. Two stepped shouldered tubular bushes are required, the larger diameter of which fit in the big and little end bores, and the reduced outer ends are a close fit in the holes drilled in the jig plate. These bushes locate the part machined rod accurately on the jig plate and allow it to be turned end for end for machining both big and little ends.
Bolts fitted through the bushes, screwing into tubular nuts below the jig plate, clamp the rod securely for milling to shape. This is shown in the second photo. The outer profile of the ends are milled to shape by rotating the rotary table. Some simple trigonometry is needed to determine the angle of rotation for each end. Next the sides are milled to the correct profile. Determining the start and end points requires a little more head scratching when the rod is tapered, as on the Atom Minor.
For most con rods, that would now be the end of the milling, but I decided to follow the drawing and include the fluting. This was milled using a 3mm slot drill with the corners of the cutting edges radiused by hand. (A good use for a chipped cutter. It doesn’t matter if the radii on each edge are not exactly the same. The resulting machined radius on the rod will be a combination of the two radii on the cutter.)
The last operation was to machine the taper on each side of the big and little ends. This was completed by pushing the rod onto a brass mandrel.
This is probably not the quickest method of making a single conrod, but all steps are under complete control, and it is good if a number of conrods are to be made.