Tool Wear and Tool Life Analysis for Machine Work Study

Tool Wear and Tool Life

Tool wear and failure mechanisms are of great practical interest because they affect machining costs and quality. If tool life is small, tool needs to be changed many time and time is lost because of it and also tool cost increases as multiples of tool  cost of edge or tool.

Tool failure may result from wear, plastic deformation, or fracture.

Tool wear may be classified by the region of the tool affected or by the physical mechanisms that produce it.  Wear occurs on both the rake and relief faces of the tool. Flank wear results from abrasion of the cutting edge. The extent of flank wear is characterized by the average or maximum land width. After an initial wearing in period corresponding to the initial rounding of the cutting edge, flank wear increases slowly at a steady rate until a critical land width is reached, after which wear accelerates and becomes severe. Flank wear progress can be monitored in production by examining the tool or (more commonly) by tracking the change in size of the tool or machined part. The flank wear land is generally of uniform width, with thicker sections occurring near the ends. Flank wear can be minimized by increasing the abrasion resistance of the tool material and by the use of hard coatings on the tool.

Flank and crater wear are the most important and thus the most widely measured forms of tool wear. Flank wear is most commonly used for tool wear monitoring since it occurs in virtually all machining operations. Tool wear is most commonly measured by examining the wear scar on the tool using a micro scope or (less commonly) a stylus tracing instrument.

A number of standardized tool life tests have been developed to help rank the performance of cutting tool materials or the machinability of workpiece materials. These include the ISO standard tests for single-point turning , face milling, and end milling, equivalent tests defined by national standard organizations, the ASTM bar turning test, and the Volvo end milling test.

The standard tests strictly define the tool and workpiece geometries, cutting conditions, machine tool characteristics, and tool life criteria needed to construct repeatable tool life curves. They typically use a maximum average flank wear criterion to define tool life; in the ISO turning test, for example, tool life is assumed to be over when the average flank wear reaches 0.5 mm under roughing conditions.

TOOL LIFE EQUATIONS

Tool life has a strong economic impact in production operations. The development of  quantitative methods for predicting tool life has a long history and F.W. Taylor's 1907 papers is the popular formula still used now.

Taylor tool life equation  relates the tool life T in minutes to the cutting speed V through an empirical tool life constant, Ct:
VT^n =  Ct
Where
T is specified in minutes
Ct  is the cutting speed which yields 1 min tool life

The exponent n determines the slope of the tool life curve  and depends primarily on the tool material; typical values are 0.1–0.17 for HSS tools, 0.2–0.25 for uncoated WC tools, 0.3 for TiC or TiN coated WC tools, 0.4 for Al2O3-coated WC tools, and 0.4–0.6 for solid ceramic tools. The constant Ct  varies widely with the tool material, work material, and tool geometry and is typically on the order of 100 m/min for rough machining of low carbon steels.

The basic Taylor equation reflects the influence of only the cutting speed on tool life. The feed rate and the depth of cut can also effect tool life. For this reason, a modified version of Taylor’s equation, called the extended Taylor equation, is often used:


A number of researchers have developed models for predicting the rate of tool wear due to specific
mechanisms.

Models have also been derived to calculate wear rates due to diffusion. The best known is Kramer and Suh model, which is applicable to WC tools used to cut steel.


TOOL FRACTURE AND EDGE CHIPPING

Tool fracture occurs when the tool is unable to support the cutting force over the tool–chip contact area. When fracture occurs near the cutting edge and results in the loss of only a small part of the tool, it is referred to as edge chipping or frittering. Chipped tools produce poor surface finishes but are often still usable. Fracture that occurs away from the cutting edge and results in the loss of a substantial portion of the tool is referred to as gross fracture or breakage; when this occurs, the tool is unusable and must be changed.

THERMAL CRACKING AND TOOL FRACTURE IN MILLING

In milling, tools are subjected to cyclic thermal and mechanical loads and may fail by mechanisms not observed in continuous cutting. Two common failure mechanisms unique to milling are thermal cracking and exit failure.

TOOL WEAR MONITORING

Tool wear is one important factor contributing to the  surface finish. In transfer line applications, worn tools are often changed on a statistical basis at a rate dictated by the shortest life expectancy for multi-tool operations. In such cases, significant useful life of the tools may be wasted and system productivity may be reduced. Alternatively, an in-process tool wear monitoring and control approach may be used to predict in-process tool wear throughout the life of specific tools. This is a common approach in aerospace machining and in CNC-based high volume production systems.