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- The pressure exerted by the rolls to
convert the square section to half-round
is transmitted throughout the section of
metal being squeezed. The incompressible
metal behaves rather like a very viscous
liquid, and instead of remaining
4mm wide, it oozes out of the
unsupported sides resulting in the
cross section shown in photo 10.
If I’d been working with copper,
or some other inexpensive metal,
this would have been easily
remedied by simply fi ling away the
unwanted extrusions. But precious
metal is far too expensive to waste
in this way. - As a result of there being
no lateral support in the gap
between the rolls, the pressure
on the twisted part causes it to
partially untwist, resulting in rather
distorted hoops in the fi nal ring.
This is totally unacceptable.
The only solution I could think
of was to make a new pair of rolls
with supporting sides, as shown in
photo 11.
Since I had two rings to make;
one 4mm wide and the other 3.5mm wide,
I needed two pairs of special rolls. The
prospect of machining 7mm deep, half-
round grooves into steel bars was not that
appealing, so I decided to make them in the
form of separate narrow inner rolls that use
the same pair of steel cheeks. Photograph
12 shows the two pairs of inner rolls and
the steel cheeks. One pair of inner rolls is
sandwiched between the outer cheeks by
means of high-tensile screws that screw
into threaded holes in the left -hand
cheeks.
Since the main rolls are 65mm in
diameter, both pairs of inner rolls had
to be 65mm too. That meant that one
pair of cheeks had to be 75mm and the
other only 55mm. The next job was to
make them.
The diameter of the roller axles is
28mm, and the width of the original
roller extensions is 20mm, which means
that the width of each cheek must be
8mm. I had two bars of mild steel (MS),
one 75mm and one 65mm in diameter. I
fi rst bored a 22mm deep, 28mm hole in
the 75mm bar, and another one, 50mm
deep, in the 65mm bar. I then faced both
bars at both ends and mounted the
75mm one vertically on the mill table,
and drilled three equally spaced, 22mm
deep 5.3mm holes, on a 39mm PCD
around the centre.
The bar was then returned to the
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