Part seven - complex milling - by Ramon Wilson

Before making the front housings it was necessary to make a simple sub-plate to fit on the rotary table, the housings locating on the boss and registering the centre line by that dowel.

The profile was 'proved' first ...
...before removing the waste on the bandsaw and the profile milled to 0.2mm up on major diameter.
After milling the profile the cavity was taken out. As drawn, and as machined on the first one there is no lower web. As it progressed I felt that a lower web would add a lot more strength so milled the second as such. The tapped hole for the jet was put in at this stage, too.
A mandrel was turned with the same diameter boss and set up on the dividing head at the requisite angle to mill the taper - this is on the second pass - much smaller radial movements to improve the finish.
The boss end was parted off the mandrel and used to mill the top of the intake and drill and ream the intake hole through to the jet position - this hole, usually tapered for venturi effect will have a close fitting moveable barrel by which to regulate the air to match the fixed jet size.

And with a bit of 'fettlin' they are beginning to look part of the case.
Next up was to mill the inlet pocket. Done by holding on an expanding mandrel it would have been much better to have done this early on when that front portion was fully cylindrical.
The front of the intake was rough milled at 45 degrees (the spigot is a filing aid, used here to level the part by eye)...
...before shaping the leading edge by filing - note the spigot was used as both support and guide.
With that filed to shape, the trailing edge was rough milled to 30degrees -
and that too was shaped by filing to a tear drop section. A series of 'flats' first, followed by gradual shaping. The ink really keeps tabs on what's done and what's to come off.
The front tank ends were internally screw cut so they could be used as thread gauges to cut the thread on the shaft housings.
The tank bodies were a simple taper turning job with a register to fit the shaft housings and once done the front ends were assembled, mounted on the expanding mandrel previously used and the secondary taper turned on as well as turning the front tank end angles.  This was all done in reverse due to the need to prevent those left hand threads unscrewing.
That little boss previously used to shape the intake tube was pressed into service again, this time mounted to a small block to allow setting it square in one direction to mill the relief for the air control valve...
...and then in the other to allow the milling of the air intake. (The brass tube was there to help eliminate burs on the inside of the intake tube).
Finally the block was set at the requisite angle to drill and tap the fuel filler hole square to the tank face.
And that was it, all done except milling between those three holes on the backplates. As that is such a useful means of holding the cases that will be left until assembly just in case anything needs a slight skim.
And here they are ready for the next phase - finish the heads and begin the internals.

part nine  part ten   part eleven   part twelve