38 http://www.model-engineer.co.uk Model Engineers’ WorkshopAft er each pass the tool tended to jam in
the slot and it was necessary to dismantle
the setup to prise it out. This meant
the tedium of resetting the tool again
accurately each time until the full depth was
achieved.
Since that wasted eff ort, I’ve noticed
that “push” broaches are available on the
market, including even the appropriate
backing bushes and shims (I thought these
were my original idea – it seems that there
is nothing new under the sun!).
This is defi nitely unfi nished business.Broaching Square holes
Before the press, cutting square holes was
tedious. It was done the hard way, either
fi ling or using a vice to press HSS cutting
tools through a root pilot hole to equip
boring bars. The press gives a much more
positive and better control. To start it off
and keep it parallel with the required axis, I
found it useful to drill oversize (the diagonal
dimension of the square required) for a
small distance. It did mean I was limited in
sizes to whatever HSS steel was available.
A lead angle was ground on the business
end of the HSS to encourage it to bite. This
particular piece was one used the hard way
(before the press was built) but the principle
remains the same. On thin plate it works
well. On boring bars I was equipping, the
HSS size was 1/8 inch. To avoid the swarf
buildup the initial drilled hole was 11/64
inch (my best guess). Since the load is on
the corners, loss of contact in the middle of
the fl ats was not important (that was my
excuse and I was stuck with it – it worked).
Cutting square holes in plate needs
a die of larger dimensions beneath the
work piece. In industry the die would be
hardened and tempered as well. Initially
I talked to a company to see whether it
would be worthwhile buying tooling. It’salmost a Black Art, dependant upon the
work piece material and its thickness. They
are designed for each job. Clearly too rich
for me. For the few times I was going to do
it, I had to accept a mild steel die that was
ruined each time. My results were less than
perfect but suited me at the time. I’ll do
better next time.Injection Moulding
This, I’ve no experience of, but the article
in MEW 213 on this topic intrigued me. It’s
worth a try. Whether the spring return will
drag the piston out of the molten plastic is a
possible impediment. It might be necessary
to fi t all four return springs rather than the
two shown. One of these days perhaps!Aft erthought
My guesstimate of 10 tons for the jack
size was I think a little light. When folding
thicker material up to 300mm wide it
tended to struggle a little. The 15 ton jack
available would fit the frame I made.
A hydraulic press is a “must” for
Broaching keyways. Although I
knowingly sized my press to suit the
material available, if I’d seen the “Push
Broaches” available I almost certainly
would have increased the press height to
accommodate them.
The press has now been in use for a
year or so and has proved its worth many
times. ■36 37
38
.
.
.