The part now needs to be slit so as to provide adjustment for the fit between it
and the Eccentric Sheave (43). With a slitting saw fitted to a mandrel, place this
in the three jaw chuck and with the eccentric strap mounted on the lathe's top slide
make the slit as seen in Photograph 53. Whilst in this position the saw can also
be used to make the flats on which the screw and nut eventually sit, finishing the
recesses using a suitable file, then drill the 7BA clearance hole. File a small flat
where the tapped hole is required and drill and tap 7BA, the flat ensuring that the
screw head will seat tidily, also drill the lubrication hole.
To machine the outer faces at the end use the same method as that for the con rod,
photograph 49. However, with the arm being longer, and more slender, it will tend
to flex under the pressure exerted by the cutting action so it will be necessary
to limit the depth of cut and feed rate. I used a depth of 5 thou and with a steady
feed managing it without there being a problem but after each cut I took a second
cut without placing on any more feed. Taking a spring cut being the term for this.
It is essential though that you use a sharp cutter.
If you are making the horizontal engine the part is longer and therefore even less
ridged but I still machined the part using the same method but did reduce the depth
of cut to around 3 thou.
Eccentric Sheave 43
Whilst the eccentric strap above was initially bored 5/8” diameter this may have
closed a little when the part was slotted. In this case the eccentric sheave should
be turned to a diameter that is a close running fit rather than 5/8”. However, the
material supplied is from 5/8” diameter bar and will be, as is normal, a thou or
two under size so may not need turning, requiring just the 1/16” wide groove to be
made and the end faced. If the bore has opened up, or at least not closed down, you
should turn the part from a piece of 3/4” (or larger) rather than using the material
supplied. This is probably preferable in any case.
The bore and boss are off centre by 3/32” needing the part to be fitted into the
four jaw chuck and set off centre such that one gets a total indicator reading of
0.1875” (3/16”). If you do not possess a dial indicator (a dial test indicator will
not do) use a piece of 3/16” material such that a tool in the tool post just touches
the workpiece at one point and just allows the test piece to pass between the tool
and the workpiece when rotated 180°.
Now produce the boss, drill 6.8mm and bore accurately to 9/32”, Photograph 54. The
part now needs parting off but the cut will be intermittent. If though the part was
set to run true for the operation it would eventually break into the bore (now running
off centre) and an intermittent cut would again result and could result in problems
as the cut progressed. I chose therefore an initially intermittent cut that would
nicely finish by breaking into the bore that was running true. Photograph 55 shows
the task in progress, and being done with a parting tool having a 2mm wide tip being
ideal for use on the smaller lathe,
If you do not have a rear toolpost this is not a task that should be attempted with
a top slide mounted parting tool so it will be a case of manually sawing the part.
Next, set up the 9/32” mandrel once more and face the end to length. You could alternatively,
hold the part with the boss in the three jaw but this may mark the machined surface.
Finish by drilling and taping the 7BA hole.