All work which is executed on a mandril partakes in some
measure of the irregularities in the form of that mandril; and
the perfect circularity of section which ought to exist in every
part of the work, can only be ensured by an equal accuracy in the
mandril and its collar.
149. Rose engine turning. This elegant art depends in a great
measure on copying. Circular plates of metal called rosettes,
having various indentations on the surfaces and edges, are fixed
on the mandril, which admits of a movement either end-wise or
laterally: a fixed obstacle called the 'touch', against which the
rosettes are pressed by a spring, obliges the mandril to follow
their indentations, and thus causes the cutting tool to trace out
the same pattern on the work. The distance of the cutting tool
from the centre being usually less than the radius of the
rosette, causes the copy to be much diminished.
150. Copying dies. A lathe has been long known in France, and
recently been used at the English mint for copying dies. A blunt
point is carried by a very slow spiral movement successively over
every part of the die to be copied, and is pressed by a weight
into all the cavities; while a cutting point connected with it by
the machine traverses the face of a piece of soft steel, in which
it cuts the device of the original die on the same or on a
diminished scale.
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