Saturday 17 October 2020

Machining the Crankcases – Part 2

As there was now a flat surface on the inner surface of both crankcase halves the timing-side could be machined to provide a flat surface for further machining operations and this was done using a fly-cutter in the milling machine.

Unfortunately, this was the last operation that I could undertake on the milling machine without providing more “headroom”. I have a Tom Senior Major milling machine with the vertical quill head and this is an excellent machine tool in every respect except in its capacity, or lack of it, for milling large items. It is all too easy to fill the space between the table and the tool  and whilst I have often overcome this deficiency in the past, for example, by shortening drills or turning taps with a spanner rather than a tap wrench, it was impossible to even fit the crankcases under the vertical head without some drastic modifications.

I decided that I would raise the head by approx. 7.5” by machining a very substantial (300mm x 200mm x 50mm) piece of aluminium which would then allow the crankcases and tooling to fit. As this needed to be very rigid I used 7075 T6 aluminium alloy which is an extremely high strength aluminium, often used in aerospace applications, and is nearly as strong as EN8 steel. A large piece like this is also not cheap! …but it has increased the utility of my milling machine beyond belief.

The first piece of machining on this was to remove the corners at both ends to enable it to fit in the lathe without hitting the lead screw.

And to then machine the various holes that were required to take a new short length of stainless steel bar to support the head and to mount the head on the milling machine.

 And with the extension fitted it was now possible to machine the crankcases.

It seems that I didn’t take a picture of the crankcases and the jig at the time of machining so the picture below shows the simple jig to support the crankcases to the left of the spare set of crankcases. Very simply, the 1 1/8" diameter bar of the jig is centred and then passed through the main bearings and offset from the drill to allow each of the main clamping holes to be bored through both halves simultaneously. The crankcases are clamped together  with studs through the already-bored holes until all 8 holes have been bored


Having completed all the holes for the main clamping the next step was to machine the top faces and bore the holes for the cylinders.

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