36 http://www.model-engineer.co.uk Model Engineers’ Workshop
to show the pitfalls and point the way
forward.
My intention was to make the broach
out of the appropriate size of HSS lathe
tool steel. The tool would be backed by a
bush to match the bore. see photo 35. The
fl at on the bush was suffi cient position the
tool for its fi rst cut of about 0.3mm deep.
Each pass would be a small cut of about 0.3
mm deep. Subsequent passes, of similar
depth, would be achieved by placing shims
behind the tool to progressively move the
tool forward and deepen the slot, see. On
the fi rst tool I tried to have a cutting edge
on all 3 sides, see photo 36. This proved
too fragile and shattered. Photograph
37 shows the simplifi ed profi le I ended up
with. Although this appeared to work well,
it showed further problems. Even with very
shallow cuts, swarf built up on the cutting
edge and at the end of the cut burst out of
the slot destroying the shape, photo 38.
28 29
30 31
32