On 23.01.16 13:28, Peter Blodow wrote:
> Old fairy tale, fact ist that brass (depending on the zinc content)
> needs a lower cutting angle then steel, both tools ought to be as sharp
> edged as possible. For holes in brass larger than 10 mm, I use drill
> bits with a 90° edge (broken edge, as we say) since a 12 mm bit cougth
> once and almost smashed my ellbow by flinging the machine vise a me...
To avoid having to grind that zero rake onto a drill, I recently used a
set of German "All purpose" drills with a finer version of tungsten
carbide brazed-on wings. (Like a masonry drill.) They chewed through
hard brass at a very good rate, making a pleasant sound. The swarf was
more granular than flaky, so it just poured out of the flutes. (On the
lathe.)
So I haven't actually tried these notes:
Eliminating drill snatching:
----------------------------
Cartridge Brass: 70% Cu, 30% Zn. Does not need drill rake reduction.
In brass: Grind the drill's cutting edges to 5° rake on the outer 70%
of the edge. At the corner, do not reduce the land width by
more than half.
(MEW 21:53 sketch, 26:65, 42:31, 43:63 photo)
[That's Model Engineer's Workshop, issues 21,26,42,43
making mention of it.]
Erik
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