Section09 tool geometry

Machining and CNC Technology

Section 9Cutting Tool Geometry

The Physics of Chip Making


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.


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.


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?


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


Unit 9-1 Universal Cutting Tool Features

This bit is generic – it could be a drill or mill cutter spinning to the left, or a lathe bit moving through the metal.

It demonstrates two tool angles

Rake Angle

Clearance Angle


The Function of ClearanceClearance 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 surface.

Heel does not rub


Just Enough Clearance If the heel did rub, there would be

excess heat and the bit would not cut.Clearance is a matter of just enough

and no more.From 3 to 7 degrees clearance is usualOther than the minor advantage of a

bigger coolant space, extra clearance only weakens the bit

Trade Tip

Clearance is not one of the control factors. If there’s enough clearance, then changing it has no effect on cutter performance or finish.


Rake Angle A Major Control Factor

On the other hand, rake is a big issue.When a chip is made, it undergoes two

actionsThe Cut and the RakeThe 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.


Unit 9-2 Deforming Metal When the chip is

raked, it is pushed or wedged away from the parent metal.

The bit’s rake face bends the chip as it flows up away from the parent metal.

It redirects the direction in which it is flowing.

Rake changed direction -

Trade Tip

Changing rake angle on the bit has a dramatic effect on results – it is the prime control factor


A Clay DemoCarving school clay with a knife, can

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.

Zero Rake Angle – Not the way metal is cut – a slicing actionTilting the knife up while pushing it through the clay, the chip is now being redirected, it’s being raked.

Steeper rake angle – note that the clay is bunching up on the outside curve because it’s being bent quite a lot – metal chips do that too.


Positive Neutral and NegativeAs 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.


Notice the length of these red lines as the rake becomes more negative

Positive Neutral Negative


Micro PhotographyHere’s what it looks like in reality


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.


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.


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.

Shear Line


Watch it Again.


What Happens When the Shear Line Angle Changes?

In the 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.


Adding Coolant


Changing the Shear AngleCoolant 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?


Conclusion Gaining Full Control Requires More

Study

My objective was to convince you that there really is a lot of science in this subject.

And that there’s a bunch of other fascinating 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.

Documents

Section09 tool geometry