Instructional objectives

At the end of this lesson, the students will be able to

(i) State how the cutting tools fail.

(ii) Illustrate the mechanisms and pattern of tool wear.

(iii) Ascertain the essential properties of cutting tool materials.

(iv) Define and assess tool life.

(v) Develop and use tool life equation.

Failure of cutting tools

Smooth, safe and economic machining necessitates:

• Prevention of premature and catastrophic failure of the cutting tools.

• Reduction of rate of wear of tool to prolong its life.

To accomplish the aforesaid objectives one should first know why and how the cutting tools fail.

Cutting tools generally fail by:

i) Mechanical breakage due to excessive forces and shocks. Such kind of tool failure is random and catastrophic in nature and hence is extremely detrimental.

ii)Quick dulling by plastic deformation due to intensive stresses and temperature. This type of failure also occurs rapidly and is quite detrimental and unwanted.

iii) Gradual wears of the cutting tool at its flanks and rake surface.

The first two modes of tool failure are very harmful not only for the tool but also for the job and the machine tool. Hence these kinds of tool failure need to be prevented by using suitable tool materials and geometry depending upon the work material and cutting condition. But failure by gradual wear, which is inevitable, cannot be prevented but can be slowed down only to enhance the service life of the tool. The cutting tool is withdrawn immediately after it fails or, if possible, just before it totally fails. For that, one must understand that the tool has failed or is going to fail shortly. It is understood or considered that the tool has failed or about to fail by one or more of the following conditions:

(a) In R&D laboratories:

• Total breakage of the tool or tooltip(s)

• Massive fracture at the cutting edge(s)

• Excessive increase in cutting forces and/or vibration

• Average wear (flank or crater) reaches its specified limit(s)

(b) In machining industries:

• Excessive (beyond limit) current or power consumption

• Excessive vibration and/or abnormal sound (chatter)

• Total breakage of the tool

• Dimensional deviation beyond tolerance

• Rapid worsening of surface finish

• Adverse chip formation.