University of Nevada Las Vegas

MEG426/626 Manufacturing Processes

Department of Mechanical Engineering

Fall Semester 2000

 

 

Tool Life

 

 

Three common modes of tool failures:

1). Fracture failure: this mode of failure occurs when the cutting force at the tool point becomes excessive, causing it to fail suddenly by brittle fracture.

2). Temperature failure: this failure occurs when the cutting temperature is too high for the tool material, causing the material at the tool point to soften, which leads to plastic deformation and loss of the sharp edge.

3). Gradual wear: gradual wearing of the cutting edge causes loss of tool shape, reduction in cutting efficiency, accelerated wear, and final tool failure in a manner similar to a temperature failure.

 

Note: Modes 1) and 2) result in premature (or unexpected) loss of the cutting tool, therefore, it is undesirable tool failure.

 

Tool Wear

 

Abrasion: this is a mechanical wearing action due to hard particles in the work material gouging and removing small portions of the tool.

Location: both on rake and flank faces.

 

Adhesion: as the cutting chip flows across the tool under high temperature and high pressure, small particles of the tool are "welded" to the chip surface and taken away.

Location: mostly on the rake face.

Diffusion: is a process in which an exchange of atoms takes place across a close contact at the tool-chip boundary between two materials.

Location: on the rake face.

 

Tool Life

Tool life is defined as the length of cutting time that the tool can be used.

1) Rapid initial wear;

2) Steady-state wear;

      1. Failure-state wear.

 

Taylor Tool Life Equation

 

v Tn = C

where v = cutting speed, ft/min (m/min), T = tool life; and n and C are parameters whose values depend on feed, depth of cut, work materials, tool material, and tool life criterion used.

 

 

Tool life criteria in production

 
  1. Complete failure of the cutting edge;
  2. Chips becomes ribbony, stingy, and difficult to dispose of;
  3. Degradation of the surface finish on the work;
  4. Cumulative cutting time or workpiece count.

 

 

Tool Materials

Required Properties for better wear resistance

Toughness: the capacity of a material to absorb energy without failing.

Hot hardness: the ability to retain its hardness at high temperatures.

 

 

Plain carbon and low-alloy tool steels;

High-speed steel;

Cast cobalt alloys;

Cemented carbides, coated carbides;

Ceramics;

Cubic boron nitride;

Synthetic diamonds and nature diamonds.

 

Cutting Fluids

 

A cutting fluid is any liquid or gas that is applied directly to the machining operation to improve cutting performance.

Functions: 1). Reduce cutting temperature;

2). Reduce friction at tool-chip and tool-work;

3). Reduce cutting forces;

4). Improve dimensional stability of the part;

5). Improve surface finish.

 

Types of Cutting Fluids

  1. Coolant-type: mostly water-based solutions;

  2. Lubricants: usually oil-based fluids.

Created by Dr. Wang