Wear and peace: Turning Performance

Understanding tool wear is the first step to extending tool life—which can be further enhanced with new coating technology.

Tool wear is one of the most basic propositions of machining. Defining and understanding it can help toolmakers and users extend tool life. Also, today’s tool coating technology, including new alloying elements, provides a means to extend tool life further while improving productivity.

Wear Elements

Energy is an expression of the heat and friction that develop during metalcutting. Heat and friction—produced by high surface loads and from chips sliding at high speed along the tool rake face—subject cutting tools to extremely challenging conditions.

Cutting forces tend to fluctuate depending on conditions such as the presence of hard inclusions in the workpiece or during interrupted cutting. Therefore, cutting tools require several characteristics to maintain strength at high temperatures, including extreme toughness, wear resistance and high hardness.

Courtesy of All Images: Iscar Metals

Inserts showing extensive wear; they are no longer usable for cutting metal.

While temperature at the tool/workpiece interface is a key factor in the wear rate of virtually all tool materials, it is difficult to establish values for the parameters required for calculating the temperature. However, experimental measurements can provide the basis for empirical approaches.

It is commonly assumed that the energy generated when cutting is converted to heat and 80 percent of that heat is typically carried away in the chip (this varies based on several factors—particularly the cutting speed). This leaves about 20 percent of the heat going into the cutting tool. Even when cutting mild steel, tool temperatures can exceed 550° C, the maximum temperature HSS can withstand without losing hardness. Cutting hard steels with PCBN tools will typically result in tool and chip temperatures exceeding 1,000° C.

Tool Wear and Tool Life

Several types of tool wear exist, including:

• flank,

• notch,

• crater,

• edge rounding,

• edge chipping,

• edge cracking, and

• catastrophic failure.

There is no universally accepted definition of tool life, which is typically based on the workpiece and tool materials and cutting processes. One way of quantifying an end point for tool life is to put a limit on the maximum acceptable flank wear, known as VB or VBmax. Tool life can be expressed in the Taylor equation for tool life expectancy:

VcTn = C

A more commonly used form of the equation is:

VcTn × Dx fy = C

Where:

Vc = cutting speed

T = tool life

D = DOC

f = feed rate

x and y are determined experimentally

n and C are constants found by experimentation or published data; they are properties of the tool material, workpiece and feed rate.

Developing optimal tool substrates, coatings and edge preparations are crucial to limiting tool wear and combating high cutting temperatures. These elements, together with the use of embedded chipbreakers and corner radii on indexable inserts, determine the suitability of each cutting tool to various workpieces and applications. An optimal combination of all of these elements can extend tool life and make machining more economical and reliable.