Syd: I will take a shot at answering your questions, & I betcha you get
plenty more.

No, rake angles and front relief are NOT the same. Front relief is the
angle ground into
the front of your tool which allows ONLY the front cutting edge to contact
the workpiece.
If the tool contacts the workpiece below the cutting edge, you will get a
"rubbing" action,
and the tool cannot bite into the workpiece. Side relief (or clearance)
is the angle ground
into the side of the tool which allows only the side cutting edge of the
tool to contact the
workpiece.

There are two rake angles, both on the top of the tool. Back Rake is the
angle from the tip
of the cutting tool toward the back of the tool. It may be either
positive, neutral, or negative.
If it slopes down from the tip of the tool toward the back of tool, it is
positive rake, an upward
slope would be negative rake, neutral is self explanatory.

Rake angles, particularly back rake, may be built into the tool
holders. The old lantern type
holders I grew up on usually had a positive back rake angle built in. This
was a help when grinding
your tools, as you did not have to grind any back rake into the tool
itself. When grinding threads
tho, a neutral back rake angle is desirable, thus one had to grind a
negative rake angle (point of
tool pointed downward) to compensate for the positive back rake angle built
into the tool holders.

Side rake is the angle from the side cutting edge of the tool toward the
opposite side of tool
(across the top of the tool). It can also be ground for negative, neutral,
or positive side rake.

A negative rake angle is usually used for brittle materials, such as brass,
which are notorious
for "hogging in" as you cut. A positive rake angle would increase this
"hogging in" action, while
a negative rake angle will push the tool away from the work, eliminating
the tendency to "hog in".
"Hogging in" is an old term for the material grabbing the tool, pulling it
into the material for a
deeper cut than you are set up for. Backlash in your machine increases the
possibility for such
"hogging in". Many projects are spoiled, as well as tools broken, etc, by
this action.

A very general rule of thumb. For heavy, roughing cuts, use less clearance
and rake angles.
This leaves more material in the cutting tool to withstand the pressures of
heavy cutting, plus
more "beef" in the tool, means more ability to carry heat away from the
cutting edges.
For finish cuts, and turning such "soft" materials as aluminum, use more
clearance and more
rake angles for a better finish.

I have seen may references on this reflector to Top Rake, which is
confusing to me, as it does
not define whether side rake, or back rake is being referred to.

Dub
At 06:03 PM 9/12/2002 -0400, you wrote:
>Well, I am all confuzzled yet again! Is rake and front cutting edge
>relief the same thing? The TCxx carbide inserts with 7 degree relief are
>said to have positive rake in the literature. Yet the TNxx carbide inserts
>with 0 degree relief are said to have "negative relief". Is there really
>"negative rake" or does that simply mean no relief? If there is true
>negative rake, what would it be used for?
>
>I can get my hand ground bits to cut thanks to Mert and Dave M. and I have
>quit using the Plastitool indexable holders in favor of positive rake
>tools, so this is an academic concern. But inquiring obsessive minds
>wanna know...
>
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-
J. W. (Dub) Thornton WA5YFY
Minco, OK.