JAMES COOMBES ENGINE REVISITED

Part three by Jason Ballamy

I did most of the crank on the CNC - drilling, reaming and shaping. In this photo I have just completed the milling of a web between the two bosses which was done with a 4MM dia cutter with 1mm corner radius to leave a fillet for that cast look.


After machining it was Loctited onto a 10mm spigot that had been turned on the end of the 12mm PGMS crankshaft and once set was faced off ensuring the face was at right angles to the axis of rotation.



Stuarts show a soldered construction for the tuning fork shaped conrod but I decided on a bolt together one similar to that used on the Waller table engine. I roughed out the stock on the manual mill and then thinned the ends and reamed the holes on that machine, too.

Then over to the CNC where a scrap of aluminium was drilled and tapped so the job could be screwed to that for machining, I added a couple of spacers under the ends so that the work was meld down firmly.
This is it straight off the CNC, a little draw filing with emery wrapped around the file should clean it up nicely. Machining time was 24mins with most of that being the fine stepover used to do the sloping top surfaces and fillets where they meet the round bosses.

The short leg of the conrod that has the big end was turned from some 16mm dia stock, first doing the 4mm dia section before pulling out some more material for the half fish bellied rod.

This was then transferred to the spin indexer on the mill and the rectangular end formed.
And while still held in the indexer the main hole was reamed for the bearing and two holes drilled for the dummy strap retaining bolts
Progress so far including the big end bearing and a couple of square headed bolts, again from round stock and the milling done with the spin indexer.

.Part one here  Part two  Part three  Part four