Manufacturing Processes - 2.6 Cutting Tools

2.6.1 Introduction

Metal cutting tools are subjected to extremely arduous conditions, high surface loads, and high surface temperatures arise because the chip slides at high speed along the tool rake face while exerting very high normal pressures (and friction force) on this face.

The forces may be fluctuating - due to the presence of hard particles in the component micro-structure, or more extremely, when interrupted cutting is being carried out.

Hence cutting tools need:

  • strength at elevated temperatures
  • high toughness
  • high wear resistance
  • high hardness

During the past 100 years there has been extensive research and development which has provided continuous improvement in the capability of cutting tool.

A key factor in the wear rate of virtually all tool materials is the temperature reached during operation, unfortunately it is difficult to establish the values of the parameters needed for such calculations, however experimental measurements have provided the basis for empirical approaches.

It is common to assume that all the energy used in cutting is converted to heat (a reasonable assumption) and that 80% of this is carried away in the chip (this will vary and depend upon several factors - particularly the cutting speed). This leaves about 20% of the heat generated going into the cutting tool. Even when cutting mild steel tool temperatures can exceed 550oC, the maximum temperature high speed steel (HSS) can withstand without losing some hardness. Cutting hard steels with cubic boron nitride tools will result in tool and chip temperatures in excess of 1000oC.

2.6.2 Tool Wear and Tool Life

One or more of the following wear modes may occur:

  • i) flank
  • ii) notch
  • iii) crater
  • iv) edge rounding
  • v) edge chipping
  • vi) edge cracking
  • vii) catastrophic failure

There is no single universally accepted definition of tool life. The life needs to be specified with regard to the process aims. A common way of quantifying the end of a tool life is to put a limit on the maximum acceptable flank wear, VB or VBmax. Typical figures are:
HSS tools, roughing 1.5 mm
HSS tools, finishing 0.75 mm
Carbide tools 0.7 mm
Ceramic tools 0.6 mm

Mathematically the tool life can be expressed in the following equation (the Taylor equation):

vc Tn = C

where

vc cutting speed
T tool life
n and C constants

The constants n and C may be found for specific workpiece and tool material and feed, f, either by experiment or from published data.

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David J Grieve, 17th March 2009, (previous 22nd October 2002).