1. Section 9
Cutting Tool Geometry
The Physics of Chip Making
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2. Overview
Critical Information!
Beyond Section 9, we begin exploring how
to cut and grind metal.
The knowledge you’ll gain in Section 9 is
of the highest priority toward real control
of the processes both on manual and
especially CNC equipment.
The ultimate knowledge will be how to
adjust the control factors for better
accuracy, faster cutting and longer tool
life – all with safety in mind.
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3. There is a lot of science and a few
surprises in this subject.
Though we’ve machined metal with
cutters for over 200 years, more technical
advancements are happening today than
at any time in the past, other than
tungsten carbide’s introduction.
As we press on into high speed
machining, a deep understanding of how a
cutting tool removes chips will be
increasingly important.
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4. Solve the Mystery?
When a cutter removes metal, there’s
heat, lots of it in some cases.
The heat comes from friction – no
mystery there.
But a steel chip has been cut away
using a carbide cutter, it lies on the
floor and turns brown/blue – reacting
to room oxygen, it seems to be getting
hotter after the friction is over! Why?
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5. Section 9 Goals
Unit 9-1 4 Universal Cutting Tool Features
> Identify Rake, Clearance and Cutting Angles.
> Explain the Benefits of Corner Radius on Tools
Unit 9-2 Chip Physics and Forces
> Define the shear line in a chip
> Identify Two Sources of Chip Heat
Unit 9-3 Taking Control
> Control Variables for Learners
> Improving Results
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6. Unit 9-1
Universal Cutting Tool Features
This bit is generic –
Rake
it could be a drill or
Angle
mill cutter spinning
to the left, or a
lathe bit moving
through the metal.
It demonstrates
two tool angles Clearance
Angle
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7. The Function of Clearance
Clearance is a
simple issue
It relieves behind
the cutting edge
such that only
the edge touches
without rubbing
the heel of the bit
on the newly cut Heel does not rub
surface.
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8. Just Enough Clearance
Ifthe heel did rub, there would be
excess heat and the bit would not cut.
Clearance is a Trade Tip of just enough
matter
and no more.is not one of the
Clearance
control factors. If there’s enough
From 3 to 7 degrees clearance is usual
clearance, then changing it has
Other than the minor advantage of a
bigger coolant cutter performance or
no effect on space, extra clearance
finish.
only weakens the bit
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9. Rake Angle
A Major Control Factor
On the other hand, rake is a big issue.
When a chip is made, it undergoes two
actions
The Cut and the Rake
The cut is exactly that, the sharp cutting
edge bites into the workpiece – starting
the chip formation.
But immediately afterward, the chip is
raked away – a ploughing action.
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10. Unit 9-2 Deforming Metal
When the chip is
raked, it is pushed or
wedged away from
the parent metal.
Rake changed direction -
The bit’s rake face
bends the chip as it
Trade Tip
flows up away from
the parent metal.
Changing rake angle on the bit
hasIt redirects the on results
a dramatic effect
direction in which it
– it is the prime control factor
is flowing.
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11. Steeper rake angle while pushing
Tilting the knife up – note that the
Zero Rake Angle – Not the way
A Claymetal is cut – clay, the chipoutside
Demo a up on action
clay is bunchingslicing the is now
it through the
curve because it’s being bent quite a
Carving school clay it’s being raked. can
being redirected, with a knife,
lot – metal chips do that too.
demo the concept.
Metal is not sliced away, as one would
carve wood, with the knife edge facing
the cut.
Rather as I progressively tilt the knife
up, it begins to plough the metal away –
that’s the rake.
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12. Positive Neutral and Negative
As the knife was tilted up, in the clay,
the rake angle went from a positive
angle to progressively more negative.
The more the bit interferes or wedges
the chip, the more negative the rake
angle becomes.
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13. Notice the length of these red lines
as the rake becomes more
negative
Positive Neutral Negative
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15. After watching the video, you probably
have a far different view of how metal
cuts.
You can now see why
I chose to use clay as
the demo material –
metal cuts almost the
same way by
deformation.
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16. 2 Important Facts Revealed
Look at the micro-photo again.
Note two revealing features of the chip being
cut.
The line along which the chip is deforming –
that’s the shear line
The dark spot near the cutting edge, called
the boundary zone.
Note that the total field of view is only a few
thousandths.
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17. Shear Line
Given what you know about rake and what
you saw in the movie, explain this dark area,
just beyond the cutting edge, the Boundary
Zone. Watch again.
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19. What Happens When the Shear Line Angle
Changes?
Inthe next frame, coolant will be added
causing the pink hue.
Watch what happens to the chip as the
angle changes.
What are the results of adding the
coolant – there are several.
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21. Changing the Shear Angle
Coolant isn’t the only thing that can
change the shear angle and effect the
boundary zone.
Discuss with your class: what other tool
or setup factors might change the shear
angle, boundary zone and overall heat
generation in making a chip?
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22. Conclusion
My objective wasControl Requires More
Gaining Full to convince you that there
really is a lot of science in this subject.
Study other fascinating
And that there’s a bunch of
facts yet to be revealed the textbook, like the
role lead angle and corner radius play in tool
geometry.
Only after studying them can we discuss
how to change and adjust the control factors
for better machining.
Dig deeply into Section 9, it’s all vital info for
truly controlling machining.
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