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

After milling away the feet and bringing the front and rear faces to within .5mm of finishing, the side 4 deg. draft angles were milled.

The bolt holes were drilled using the jig made for the previous engines
Then the 'fun' bit - of milling that relief between top and lower sections.  2mm FC3 cutters were used which were not long enough to clear - first, the corner of the case then second, the protruding boss. Ground the shanks of two cutters back to clear - short one for the main milling, the second longer one, for milling underneath the boss. Cutters were ground off-hand on the bench grinder.
What makes this op tedious is the small rib on each side between the top and lower sections otherwise it would be a simple matter of sailing right round to depth.

Result with the cutters.

The engine mounting lugs were next shaped and drilled, and those angles on the front top edge of the lower case were milled on.
This would give a reference for milling the angle at the sides but first the radii on the buttresses were tackled. 

The cutter had been made for the Mk1 &2.

Once those rads were done the angles on the sides could be tackled but because of the compound angle there’s no way of milling to the depth of the relief around that radius other than in steps. Due to the small cutter used even that's limited by the tool holder potentially hitting the mounting lugs so theres only one way and thats out with the needle files. It is easier to do them one at a time - mill the angles then patiently file away the waste.
It is a mite tedious but with care it come out okay. Though the lower two look rather 'bitty' pre milling they are cleaning up well on the initial filing (top two)

Next are a couple of pics of said filing,  a tedious part of the process but by relaxing into it and not trying to 'force' it to see the end result it's surprising how soon the metal disappears......


Milling as much away as those mounting lugs would allow.

Initial attack made with a dental burr to remove a bit more. This is a 'dangerous' op as it is easy to scar the surrounding area where it would take a lot of filing to remove the witness.
Most of this was filed away using just two needle files - a square one with one face ground off as a safe edge and the other a barrette reduced at the end for 25mm or so to 3mm wide. Still quite a bit here to go though.
Nearly there but still a small witness to get rid of
And finally as finished.
They weigh 56/57 grams each without the top plate so that’s 2192 gms of swarf made in a fortnight.

They'll get another final going over with Scotchbrite and Garryflex block before being bead blasted but that won't be done until the backplates and front housings are finished.



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