Crater Wear

7/29/2019 Crater Wear

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TOOL LIFE

Optimizing tool life and performance

Tool wear is damage on the cutting edge caused by the different types

of insert wear.

Productivity and tool life can be optimized by observing the wear

development during machining and adjusting accordingly.

This ensures that the best insert geometry, grade and cutting data are

applied for the operation.

To optimize turning, milling or drilling operations, inspect the cutting

edge as it wears, or after it has worn, and determine what type of

wear is present. Once the wear type is identified, the machining canbe modified to prevent excessive wear from occurring.

However, reducing any type of tool wear to achieve hours of tool life is

useless if it causes a drastic reduction in productivity.

Keep in mind that the main objective of a modern machining

operation is to find the balance between tool life and productivity. For

every metal cutting operation, there is always an 'ideal' wear

development. The essence of machining optimization is to get as close

as possible to that ideal wear pattern to maintain good tool life andhigh productivity. Selecting the right modern cutting tool; using the

optimum starting values for cutting data; stable machining

conditions; high quality workpiece materials; and metal cutting

experience, are all factors that contribute to achieving the best tool

life.



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FLANK WEAR

The most commonly occurring type of insert wear.

As the name indicates, this wear takes place on the clearance sides and/or

flanks of the cutting edge.

The main cause is abrasive wear, the result of hard particles in the

workpiece material rubbing along the insert as it passes.

Controlled flank wear is considered an ideal way for an insert to

deteriorate.

However, excessive flank wear leads to increasing friction and poor results

in finish and

If flank wear development is occurring too rapidly, check the cutting

speed to make sure that it is not too high for the insert grade and operation in

question.

Also, a more wear resistant grade, higher up in the ISO application chart,

is often the best remedy.



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CRATER WEAR

Crater wear: takes place on the top face of the insert, where the chippasses over the cutting edge.

The main wear mechanisms that create crater wear are abrasive and

diffusion wear.

In this case, the tool material is continuously removed by workpiece

material chips as they pass over the tool at high temperature and

pressure.

Crater wear is also considered normal wear in many operations if kept

within acceptable limits.

Excessive crater wear changes the cutting geometry, and in time, leads to

a dangerous weakening of the edge.

Rapid crater wear is often the result of cutting speeds that are too high for

the operation in question. A more wear resistant grade should be

considered.

The feed rate should also be checked, as the pressure of the cutting forces

may be too high when combined with the excessive heat of a high cutting

speed.

A more positive geometry can usually help to reduce crater wear.

PLASTIC DEFORMATION



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Plastic deformation: occurs when high pressure

and temperatures weaken and depress the tool

material in the insert.

This can happen either when the edge is pressed downward in the

direction of machining or when the clearance side and/or flank is

pressed inward.

The tool material must retain sufficient hot hardness in order to resist

plastic deformation.

Unfortunately, the bulging of the edge leads to even more friction heat,

geometry deformation and deteriorating chip control, escalating to a

critical state.

In finishing, the deformed edge leads to inferior part quality.

A typical cause of plastic deformation is using an excessively high cutting

speed and feed rate for the operation.

Combat plastic deformation with a more wear resistant grade.



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BUILT UP EDGE

Built-up edge: the formation of layers of workpiece material that become

smeared and/or welded onto the cutting

edge.

Built-up edge (BUE) is usually a result of low cutting zone temperatures

due to low cutting speed, often combined with insufficient or complete

lack of coolant.

BUE alters the cutting geometry, often making a positive insert more

negative or reducing the clearance by smearing material onto the flank of the insert.

Low temperatures combined with the pressure of metal cutting make

certain materials more prone to this type of wear. Some of the very sticky

materials require that certain measures be taken to prevent excessive

BUE.

Fortunately, the temperature and cutting speed areas of BUE formation

are well defined and can be avoided. Most modern machining should be

done at cutting data ranges that lie above the range at which BUE forms.

Adjusting the cutting speed is always the first corrective action to take if

BUE occurs.

Make sure there is sufficient coolant or cutting fluid used.

A more positive insert geometry should also be considered, since negative

cutting angles are more likely to cause BUE.

It may also be helpful, especially in finishing operations, to switch to a tool

material that is less prone to reacting with the workpiece material andthat creates less friction.

THERMAL CRACKING



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Thermal cracking: heat-related wear, with fatigue occurring due toexcessive thermal variations during machining.

Usually, thermal cracks run perpendicular to the cutting edge, although

in some cases the cracks will run parallel to the edge.The edge is weakened as these cracks appear and tool material may be

broken away between the cracks, leading to rapid breakdown of the cutting

edge.

This type of wear is a typical problem in milling, although in turning,

large variations in chip thickness during cuts can also lead to the formation

of thermal cracks. In finish turning, poor surface finish occurs when the

cracks start to form.

Incorrect or insufficient application of coolant is often the cause of

thermal cracking.

Milling is best performed without coolant and many times, turning can

also be performed dry, since modern inserts do not rely on the cooling effect

of cutting fluid for good performance.

In general, coolant should either be used copiously to flood the workpiece,

or not used at all so the machining takes place in dry conditions.

Drilling and boring are exceptions to this rule, since these operations

typically depend on coolant to help evacuate chips from the holes being

machined.A tougher insert grade, lower down on the application chart, should also

be considered as an option to improve metal cutting.

CHIPPING

Chipping: when small particles of the

cutting edge break away and the

important edge line of the insert is

damaged.



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Instead of wearing, the cutting edge is broken away

prematurely.

Chipping is usually fatigue-related, an indication that the

cutting edge is not strong enough for the operation.

Intermittent cutting also causes chipping due to the varying

cycles of pressure being applied.

Insert grades and geometries can also make an insert too

brittle. An insert with more toughness may be required.

This can usually be reduced by combining a wear resistantfinishing grade with a roughing geometry with a reinforced

cutting edge line.

In some cases, changing to a tougher insert grade can also

reduce chipping.

EDGE FRACTURE

Edge fracture: a catastrophic situation

where the cutting edge breaks.



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If edge fracturing occurs, something has gone seriously

wrong or the tool and cutting data selection were totally

incorrect.

If cutting edge fracture occurs, every possible aspect of the

whole operation should be re-assessed.

Checking the cutting data and tool choice are especially

important. Was the right tool selected for the operation?

Was the cutting data too excessive for the tool used?

The stability of the operation must also always be checked.

For example, in roughing operations, a single-sided, rather

than a double-sided, insert is necessary to maintain sufficient

production security.



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TOOL WEAR

1. The development of flank wear

is measured in relation to the time

the cutting edge has actually beenmachining so maximum values can

be established.

2. A rise in the power needed to

take cuts in the operation is an

indication of wear. Carefully

monitor the scale for increases in

Hp power consumption.

This indicates that cutting forces

are growing due to the changes in

the cutting edge.

3,4. In finishing operations, where there are limits on accuracy and surface

finish, a worn cutting edge becomes apparent very quickly because the

component will be out of tolerance or the surface finish will deteriorate. Most

wear types lead to problems in this area.

5. Burr formation, especially in stainless steel machining, is also a sign that the

cutting edge is not sharp enough or that the geometry is not as positive as it wasin the beginning. Excessive flank wear, plastic deformation and BUE can cause

the cutting edge to become blunt, causing burr formation.

6. An excessive or increasing amount of heat is also an indication that the

cutting edge is no longer cutting as smoothly as it should due to wear. A blunt

edge cause more friction and therefore more heat, during the metal cutting

process.

TOOL WEAR



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7. Chipped or broken tools can often be spotted in the

machine even without magnified inspection. This indicates

more serious problems requiring careful assessment of the

operational set-up and the tool and cutting data

application. Vibrations will cause trouble, eventually

leading to poor part quality, insert damage etc.

8,9. Visible wear on the chip and poor chip

breaking occur when tool wear has been

allowed to develop too far.

10. Noise is a widely recognized sign that something is

going wrong in the metal cutting process. Excessive noise

can be the result of vibrations caused by changes in the

tool's cutting geometry due to wear.

11. Vibration tendency in the machining process may

indicate that the cutting edge is blunt. This is

especially true if there is chatter on the machined

workpiece or if the quality of the surface finish

deteriorates.

12. Cutting edge tool life is determined by its ability to

maintain the required production tolerances and

surface finish.

Crater Wear

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