ETA 15D MK 3 AND ELITE - 5cc ENGINE BUILDS
Part 12 - by Ramon Wilson
Part four - by Ramon Wilson

The crankshafts are made from En24t (4340) a decision was made to do one conventionally and make it from one piece. Began by facing the blanks and centring one end before roughing the front ends down to a consistent 14mm diameter. This would be thick enough to withstand milling around the crank pin without leaving too much to come off and, as a V-block was to be used  for centre drilling the crankpin location would give the consistency of offset required through all four.....

One of the rare times the lathe sees carbide tooling - it does help deal with roughing such tough material but the power drive isn't really up to the capability of the tool - I lost a couple of tips when the spindle slowed rapidly to a stop.

The shorter, composite, shafts were also drilled and reamed 7mm at this stage as well.

Using the crank pin turning fixture to centre drill the crank pin location
Held in a V-grooved plate to mill away the waste around the crankpin.  Major area first then rotated 90 degrees each side to mill the sides.
Last roughing op was to bring the crank pin to circular section using the fixture above
ready for making and fitting the secondary shafts before the finish turning between centres.
The reason for the composite shafts is none other than conserving the limited supply of En24t. The secondary shafts will be turned  from High Tensile cap head bolts and made a good press/loctite fit. So far this method of manufacture has proved more than capable to withstand the forces involved though of course they are only ever run on the bench - hence the decision to make the potential flying version from one piece - just in case!


The 'secondary' inner shafts were turned from the shanks of 8mm high tensile cap head bolts and with a smear of high strength Loctite pressed in using a small home made flypress.

Set back in the lathe in soft jaws the shafts were roughed down to plus .5mm on diameter and faces on the front ends.

I don't use the leadscrew for such turning - just the saddle hand wheel but it was always felt that the standard Myford version is a bit small as far as leverage is concerned so a while back I made a new saddle hand wheel which has improved matters considerably on this kind of op as well as general turning - certainly wouldn't go back to the old one now.
After the secondary roughing they were ready for finishing between centres
This little driver plate has seen considerable use now. The centre is removable as it is trued each time it's used to ensure concentricity - it's just about ready for that next time.
The shaft can now be finish turned using the crankpin as a driver - the red line denotes the limit of thread. These were turned to 0.03-04mm up on diameter and then polished down to size with fine emery and oil.
The shafts could be screw cut at the same set up but were done as a secondary op to the finishing.
With the front ends finished that fixture was pressed into service on the faceplate for turning the crankpins to finished size - the reduced portion is the drive for the rotor disc.
The shafts after finish turning ready for that last op - shaping the web and one which is always a bit fraught after all the work gone before.

See part one here  part two  part three  part four  part five  part six  part seven  part eight 

part nine  part 10  part 11  part 12  part 12+1