I've been musing on rear-mounted parting tools again. The late renowned
British model engineer George Thomas wrote knowledgeably about them. I
have his book "The Model Engineer's Workshop Handbook". In that book he
recommends grinding some complicated angles on his HSS parting tools. I
find I can scarcely comprehend what he's talking about, which is normal
for me when reading British publications despite speaking English. Sigh.
It *appears* he suggests grinding the wider face of the business end
(what would be the top were it mounted normally) with an angled groove
with an included angle of 140° so in cross section its top looks like
'M' (a capital emm), back maybe 1". He also suggests grinding awat the
front of the tool, top to bottom, so that it has an angular tip with an
included angle of about 160°. He has line drawings in his book but I
can't figure them out. Realizing how confused I am, I think it is likely
that anyone reading this will be even more confused so I will describe
how these angles are ground, maybe that will help.
He has a 1/8" wide wheel which he dresses so it is wider in its center
than on either side. He mounts this on his T&C grinder spindle. He
mounts a parting tool on the table of the T&C grinder so its top
surface is the wider one, and is flat, then he grinds away an angular
groove in about 1" from the cutting point. Then he removes the parting
tool (HSS) from the T&C grinder to his tool grinder, which has a table
rest which can be set to an angle. He drops the table 10°, then, using
a cup wheel, he holds the parting tool so its top surface is parallel to
the axis of the cup wheel but lowered by 10°, and grinds away half of
the very end of the blade. Then he turns the blade over and grinds away
the other half. Now the parting tool is longer in its center than on
either side, because the front sticks out like the prow of a ship.
He claims the grind on the front helps with digging in on deep parting,
and he claims the grind on the top (where the chips slide along) tend
to "break the back" of the chips so they won't bind in the cut.
OK: thanks for reading all that. Can anyone confirm that I understand
his topology correctly? Has anyone tried this?
End of first question. On to the second one.
Mr. Thomas also asserts that a parting tool should have about a 7°
top rake, i.e. the part of the parting tool where the chips slide
should not be horizontal, rather canted by 7°. He used to grind this
into the front of his horizontally-mounted parting tools but then he
realized he couldn't correctly sharpen his parting tools without
completely removing the top rake portion and starting over, which is
tremendously wasteful, so his toolholder design holds the entire blade
at 7° so it can just be sharpened on its end. I have an MLA rear parting
toolholder which I'm about to machine, and I thought I might design it
to accommodate this feature. The MLA toolpost holds an Armstrong
toolholder. It doesn't directly hold the parting tool the way GT's does.
It wouldn't be hard to modify the MLA design to hold the toolholder at
a 7° angle. Does anyone think this is worth it?
Whew. Next I'm going to complain about the lack of information on
parting tool angles when you buy them. On many parting tool blades -
but not all - the top and bottom are not parallel, rather they are
at a slight angle. These would seem to be difficult to hold in many
toolholders like the Aloris AXA-7, but they seem to work in the
Armstrong type holders. Yet no tool catalog I am aware of does a good
job of describing all the blade topologies in enough detail so you can
buy these (expensive) little blades and know they will work.
Grant Erwin
(posting from Karen's account since my hard drive died)
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