Centre drill the outer boss and support using the tailstock centre ensuring that
it cannot fall from the chuck. Machine both sides and the outer diameter of the rim,
leaving the dimensions a little over size, say + 0.010” on diameter and 0.020”on
width, for final machining later, Photograph 36. Give the chuck another tighten,
just in case the copper ring has relaxed and then remove the tailstock centre, drill
a through hole 6.8mm diameter and, with a small boring tool, accurately open up the
hole to 9/32” diameter, Photograph 37. Check the result with the hole gauge made
Taper Stub Mandrels
At this stage a taper stub mandrel is required to support the flywheel and for those
readers who are not conversant with these the following explanation should be of
This type of mandrel is used for supporting parts for machining, either where they
cannot easily be held in any other way or where an outer diameter being made needs
to be concentric with an internal diameter already made, the engine's flywheel for
example. Where concentricity is vital then machining the workpiece should be done
immediately after making the mandrel as the mandrel is unlikely to run true if removed
from the chuck and replaced.
To make these, set your lathes top slide to an angle of no more than 1/2° and in
a direction that increases the diameter towards the chuck. Machine a parallel portion
a close fit in the bore of the part to be held and of a length about equal to its
length. When nearing the required diameter winding the top slide back will enable
very shallow depths of cut to be easily set. When making the final pass for the parallel
portion, then complete the machining by traversing the top slide to produce a gradually
increasing diameter that will permit the eventual workpiece to be held securely using
With a piece of 5/16” diameter steel in the chuck, support the end with the tailstock
centre and produce a 9/32” taper stub mandrel as detailed above. With that done,
push the flywheel onto the mandrel, using a turning action will improve the grip.
Adding a very slight chamfer on the end of the bore will also help with the process.
Turn the outer diameter and both sides of the rim ensuring, as a result, that the
wheel runs true, Photograph 38. Whilst still on the mandrel also machine the outer
diameter and face of the boss. Remove the flywheel, reverse, refit and machine the
second boss. Do note that one boss projects more from the rim than the other.
Also regarding the flywheel I noticed that where I had machined the sides of the
outer rim that there was a slightly jagged edge between it and the unmachined casting.
I therefore decided to machine this so that there was a clear demarcation between
the two, in addition to appearance, an advantage of this being that painting will
be much easier. Photograph 39 shows that it was done whilst still mounted on the
taper stub mandrel.
Mount the flywheel onto fixture 2, using one of the M6 fixing points, and set at
a suitable angle in a vice, drill a 2.7mm diameter hole, Photograph 40, and tap 5BA.