The method relies on a close fit and it is essential that the hole is parallel. Also,
if several parts are to be machined, the hole in each one must be made with minimum
variation, say within + 0.01 -0.0mm. Even a small difference may, prevent the component
fitting onto the mandrel if small, and be loose if large. For multiple parts therefore,
a reamed, or accurately bored, hole will be necessary. Ideally, if bored, the parts
should first be made with the hole slightly small and each one then returned for
final boring. In this case, bore the first one returned to the diameter require and
then lock both the cross and top slide. Subsequent one can then be bored at that
setting ensuring that they are all the same diameter.
I fully appreciate that the initial shape machined may not permit the part being
returned a second time. In this case, particular care will be necessary when machining
the bore to size, each one being done individually.
It is though preferable, even for a one off that the hole is parallel even though
the bore diameter, in some cases, is not critical as the mandrel can be made to suit.
The viewer will most likely appreciate that even a new drill will produced a hole
marginally on the large size. What though may not be realised is that the hole size
reduces to drill diameter as the drill breaks through. The larger the drill is the
larger the distance over which this occurs. This can cause confusion when attempting
to make a mandrel to fit the hole especially if it is made to the size at the smaller
end. Because of this, a drilled hole will only be suitable if the reduced diameter
is removed by some means, a triangular scrapper being a good method.
Another important factor is the hole diameter to length ratio. It should be obvious
that, with a part having a large hole but thin, typically of washer proportions,
that fitting the part to a solid tapered mandrel would be
difficult. Unless some form of guide were employed, say the tailstock barrel, getting
the part to run true would be difficult. Even then, machining would have to be carried
out with extreme care. Hole diameter no greater than twice its length would be ideal
but greater will be OK providing the limitations are taken note of, Photograph 2
being a typical example.
Unfortunately, there can though be no strict rules, as it depends on so many factors.
These include, outer diameter large, intermittent cut and difficult material such