As some of the holes need making to quite precise diameters, because it is a requirement
of the engine in some cases, or they are to fit onto a taper stub mandrel in others,
hole gauges are required. These are made to a very close diameter, and with a short
length to a slightly smaller diameter. The purpose of this is as a warning that you
are now very close to the required diameter and to start taking a much shallower
depth of cut. One, the 3/4” diameter gauge, is dual purpose and has a 1” diameter
head and is drilled 5.2mm. The drawing shows the sizes required. These are best made
at the commencement as all being similar it will make sense to make them when set
up for machining the first.
One important point that needs deciding on at this early stage is whether the crankshaft
will be fabricated or made from one piece as I detail later. If fabricated using
the supplied bar then its diameter will normally be a little on the small side, and
therefore, wherever I make mention of 9/32” diameter throughout the series this should
be treated as equal to the diameter of the material supplied. If though the one piece
approach is taken then the shafts can be turned accurately to 9/32” (0.281”) and
should be the dimension used.
The casings supplied machined easily, except for the corners of one of the two Valve
Chest covers, and similarly one of the Box Beds I was machining. That is of course
no guarantee the you will experience any or the same problems. Preparatory work will
though be needed before machining proper takes place. Some, have quite sizeable
extensions, no doubt where the iron was poured, that need to be removed.
The castings also have a thin projection of iron, “flash” I think the technical term
for this is, where the two halves of the mould met. This needs removing though mostly
it is not important at this early stage.
Rather than keep repeating myself through the series, the following applies when
securing any of the castings to the faceplate. Before doing this, check that the
surface being placed on the faceplate, or fixture, is sufficiently flat to avoid
it rocking or being distorted, dressing (filing/grinding) the part appropriately
to minimise the effect if it does. A piece of thin hard card between the workpiece
and the mounting surface will help compensate for any minor errors.
Make fixture 1 and clamp 1 and using these, fit the assembly to the faceplate and
and machine the underside as seen in Photograph 6. Leave the fixture in place but
remove the soleplate, turn over, and secure using clamp 2, face the six available
surfaces, Photograph 7 3. Next, remove the fixture and use this as a drilling jig
to position the four 7BA holes on the larger faces. Centralise the fixture and secure
it with a single screw and Clamp 1, then drill the four 2.1mm holes, Photograph 8.
Actually, this is a good example of where organising your machining sequences will
pay off, as the fixture could remain on the faceplate for use with other parts, the
Box Bed typically. Also, with there being so many 7BA tapped holes tapping then
all at the same time, as far as is practicable, would be a time saver. Having now
stressed the need of planning, I will not repeat the advice except where it may be
All pictures can be clicked on to provide a larger view