On Wed, 25 Aug 2004 14:34:02 -0700, Karen Story
wrote:

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)


I regularly use a backmounted HSS cut off tool and
the comparative freedom from tool jams and chatter is well
worth the bother of providing the backmount facility.

Geo Thomas was a well respected model engineer and
I've no doubt that a parting tool lovingly crafted the way
he recommends would perform pretty well. However since my
inclinations are nearer to constructive laziness than
patient perfection I use a simpler grind.

The key feature is a longitudinal groove along the
flat top face of the tool. Because the plunge cut chip of a
parting tool is in an extreme state of compression its
natural width is WIDER than the width of the cut and it will
try real hard to jam against the sides of the cut. If the
top of the tool is flat, the flat chip will try to form a
close coiled swiss roll which is probably the worst possible
shape for generating excessive side pressure.

The effect of the groove is to produce a chip which
is curved across its width instead of flat and this
curvature is more easily compressed by the sides of the cut.
The shape of the groove doesn't seem to be very critical - I
aim for a centred roughly semi circular groove a bit less
than half the width of the top face. A tool and cutter
grinder would make short work of this but I content myself
with a cutoff disc in a dremel grinder and a fairly steady
hand. I run the groove back for about 1/4" so that it
survives several resharpenings.

Resharpen on the front face only - about 10 deg
front clearance. Unless you're limiting cutting to small
diameters with relatively wide blades keep the front face
grind square on to the tool axis. Any angle here to move the
final pip to waste or the workpiece generates undesirable
sideways deflection

Because the side clearance on parting tool blanks
is so small and there is no side rake it's important to be
careful that the cutter is mounted truly vertical and
accurately square on to the spindle axis. Even with the
curved chip face there's still pretty severe pressure on the
sides of the cut so continuous brush or drip oil feed
lubrication is essential on difficult work material.

Jim