controlling tool wear

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a white paper

Controlling Tool WearReplacing worn cutting tools is a fact of machining life that we all accept,

but that doesn’t mean we have to like it. Sure, in the grand scheme of

manufacturing costs, the amount of money we spend on tooling is relatively

small Of course this doesn’t include the costs of labor and production time

lost when replacing worn tools. That’s why it’s important to consider

all of the factors that affect tool wear and have a plan to address them.


Face the ConsequencesThere are many factors that contribute to tool wear including high surface loads, high spindle speeds, surface temperature, and material composition.

Therefore each application creates different tool wear

problems. This means you must take all of these factors into

consideration when determining when to replace a tool.

Just how important is it to replace an insert before it

becomes badly worn? Naturally you want to get the most

mileage out of your cutting tools, but excessive tool wear

can have grave consequences. Severely damaged inserts

will damage shims that, in turn, can damage the tool holder.

If the tool holder and cutting insert are no longer properly

positioned or able to maintain rigidity, they affect the

accuracy of the cut and the quality of the finished part you’re

cutting. The result could be costly scrap and/or rework.

Even more expensive is the potential for damage to the

machine tool itself. That’s why monitoring tool wear and

determining the optimum life of tooling is so important.

Types of Cutting Tool Wear

Flank Wear

• The insert breaks down quickly when flank wear achieves a critical width.

• Lowering cutting speed helps reduce flank wear, but you lose productivity. The solution is to switch to

an insert that has greater wear resistance.


Crater Wear

• Found on the rake side of the insert. Excessive crater wear weakens the cutting edge and can cause fractures.

• Caused by a chemical reaction between the material and the cutting tool.

• A change in chip composition may indicate crater wear because it changes the geometry of the insert.

• Reducing cutting speed can help, but choosing a more

compatible insert coating is the longterm solution.

Built-up Edge

• Caused by pressure welding of chips (adhesion) to the insert.

• Most common when machining sticky materials such as low carbon steel, stainless steel and aluminum.

• Lowering cutting speeds generally makes the problem worse. Increasing cutting speed and adjusting geometry can help.

Notch Wear

• Another effect of adhesion that forms oxidation and excessively damages both the rake face and flank at the depth of the cut line.

• More common when machining stainless steel and heat resistant super alloys (HRSA).

• Notch wear on the trailing edge occurs where the cutting edge and material part, while notch wear on the leading edge indicates a harder material that requires a more wear-resistant insert.

• A larger lead angle may be a short-term solution.

Plastic Deformation

• Excessive heat and/or pressure cause the tool material to soften.

• Calls for a more wear-resistant, harder grade and possibly reducing cutting feed or speed.


Chipping

• Caused by an overload of mechanical tensile stresses.

• Usually means the insert is not appropriate for the application.

• The remedy is to use an insert with a stronger cutting edge.

Thermal Cracking

• Multiple cracks appear perpendicular to the cutting edge.

• Often the result of rapid temperature changes from hot to cold.

• Consider how you are using coolant to regulate cutting temperature, and move to a tougher grade of insert.

Edge Fracture

• Usually caused by other wear factors.

• Reducing speed and feed can help, or select a different insert.

Tool Holder Maintenance

An often-overlooked way to maximize tool performance

is tool holder maintenance. The proper care of these

devices should be part of a regularly applied preventative

maintenance program. The reason is quite simple:

Tool holders that don’t hold the insert securely or

in the right position will cause premature tool wear,

and likely create problems in the machining process.

The net result of not adequately maintaining tool holders

is lost production time, premature insert wear, reduced

part quality and additional labor costs as machine operators

spend time resolving the problems caused by the

tool holders.


Fundamental tool holder maintenance includes:

• Checking for shim damage.

• Keeping the insert seat clean and free of debris.

• Indexing or replacing the shim.

• Keeping the spindle, taper, flange, collet and

collet pocket free of dirt and other debris.

• If the insert doesn’t seat properly in the pocket sides,

the pocket may have become oversized due to wear.

• Using a piece of .001 shim stock, look for small

gaps in the corners between the sides and bottom

of the pocket.

• Use the proper size

and type of wrenches

when installing inserts

to make sure you

don’t strip the screw

and that the screws

and clamps are

tightened with the right amount of torque.

• Replace worn screws as soon as you see any signs

of wear. It’s much less expensive to replace a screw

than a damaged insert or tool.

• Apply screw lubricant to the threads to prevent

screws from locking up.

• Always check supporting and contact faces of tool

holders, milling cutters and drills, to make sure there

is no damage or debris.

Boring Bar Issues

• In boring operations, it is especially important to have

the most secure clamping possible. If the bar is not

supported to the end of the holder, increased overhang

will create vibration.

Extending Tool Life

The most significant influence on tool wear is cutting speed.

Therefore adjusting speed will affect the tool’s useful life.

The chart and formulas below can help you make

adjustments to cutting speed that will influence tool life.

The industry benchmark for tool life is 15 minutes of

in-cut time. If you speed up, you’ll increase tool wear, while

slowing down makes the tool last longer. However, slowing

spindle speed also affects your productivity because the

metal removal rate is reduced. Therefore you must weigh

the advantages of reducing speed to extend tool life against

the effect on your production time.


Tool Life (Mins.)

Correction Factor

10 1.11

15 1.0

20 0.93

25 0.88

30 0.84

45 0.75

60 0.70

Cutting Speed and Feed Data Compensation for Turning

Increased feed fn inch/r

Decreased feed fn inch/r

Example: if the recommended cutting speed (v c) = 720 ft/min. A tool life of 10 minutes gives you: 720 x 1.11 = 800 ft/min

-20% -15% -10% -5% +5% +10% +15% +20%

+.010

+.008

+.006

+.004

+.002

-.002

-.004

-.006

-.008

-.010

0

Starting Value

Example 1

Example 2

How to use the diagram

This diagram shows a simple method of adjusting the starting value for cutting speed and feed recommendations. Cutting date on insert dispensers are based on a tool life of 15 minutes and will remain the same with the values taken from this diagram.

Example 1: Increase the feed by .006 inch/rev (+0.15)

Result: Decreas the cutting speed by 12%

Example 2: Increase the cutting speed by 15%

Result: Increase the feed by .007 inch/rev

Standard corner radius

Wiper radius (feed rate x2)

Another way to increase tool life and productivity is the use of wiper inserts.

Wiper inserts not only increase productivity, they also produce better surface finish.

For example: A turning operation requiring 125Ra is achievable with a conventional

nose radius of 0.031” feeding at 0.012”/rev. However a wiper insert using the same

0.031” nose radius feeds at 0.024”/rev and yields the same 125Ra. In this illustration,

if the cycle time with the conventional insert is one minute, the user will make 15 parts

using the industry average, while the wiper insert will make 30 parts using the same

average. Both inserts have 15 minutes of tool life, but the wiper insert removes more

metal to make twice as many parts with the same tool life.


Insert CompositionsEarlier in this paper we discussed various types of cutting

tool wear. In many cases the remedy for the wear problem

was to use a cutting tool better suited to the application.

The difference in cutting tools has much to do with the

material used. Here is an overview of the various types of

inserts and their primary characteristics.

• Coated cemented carbide currently represents 80-90%

of all cutting tool inserts. Its success as a tool material

is due to its unique combination of wear resistance and

toughness, and its ability to be formed into complex

shapes. It combines cemented carbide with a coating

that is customized for its application.

• Uncoated cemented carbide grades are either straight

WC/Co or have a high volume of cubic carbonitrides.

Typical applications are machining of HRSA (heat resistant

super alloys) or titanium alloys, and turning hardened

materials at low speed. The wear rate of uncoated

cemented carbide grades is rapid yet controlled.

• Cermet grades are used in smearing applications where

built-up edge is a problem. Its self-sharpening wear

pattern keeps cutting forces low even after long periods

in cut. This enables a long tool life in finishing operations

and close tolerances, and produces shiny surfaces.

Typical applications are finishing in stainless steels,

nodular cast irons, low carbon steels and ferritic steels.

Cermets can also be applied for trouble shooting in all

ferrous materials. Hints: use low feed rates and depth of

cut; change the insert edge when flank wear reaches 0.3

mm; avoid thermal cracks and fractures by machining

without coolant.

• Ceramic grades can be applied in a broad range of

applications and materials, most often in high speed

turning operations, but also in grooving and milling

operations. The specific properties of each ceramic

grade enable high productivity when applied correctly.

Knowledge of when and how to use ceramic grades

is important for success. General limitations of

ceramics include their thermal shock resistance

and fracture toughness.

• Cubic boron nitride (CBN) grades are mostly used for

finish turning of hardened steels, with a hardness over 45

HRc. Above 55 HRc, CBN is the only cutting tool which

can replace traditionally used grinding methods. Softer

steels, below 45 HRc, contain a higher amount of ferrite,

which has a negative effect on the wear resistance of

CBN. CBN can also be used for high speed roughing of

grey cast irons in both turning and milling operations.

• Polycrystalline diamond (PCD) tools are limited to

non-ferrous materials, such as high-silicon aluminum,

metal matrix composites (MMC) and carbon fiber

reinforced plastics (CFRP). PCD with flood coolant can

also be used in titanium super-finishing applications.

Gosiger thanks Sandvik Coromant for the information

and illustrations they contributed to this white

paper. Visit them at myyellowcoat.com for extensive

information about cutting tools.

http://www.myyellowcoat.com

www.gosiger.com | (937) 228-5174 www.gosiger.com | (937) 228-5174

http://www.gosiger.com

controlling tool wear

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