Wednesday, December 2, 2009

Causes of tool wear

Hard particle wear (abrasive wear)
Adhesive wear
Diffusion wear
Chemical wear
Fracture wear

a) Hard particle wear (abrasive wear)
Abrasive wear is mainly caused by the impurities within the workpiece material, such as carbon, nitride and oxide compounds, as well as the built-up fragments. This is a mechanical wear, and it is the main cause of the tool wear at low cutting speeds.

b) Adhesive wear mechanism
The simple mechanism of friction and wear proposed by Bowden and Tabor is based on the concept of the formation of welded junctions and subsequent destruction of these.
Due to the high pressure and temperature, welding occurs between the fresh surface of the chip and rake face (chip rubbing on the rake face results in a chemically clean surface). [Process is used to advantage when Friction Welding to produce twist drills, and broaches, and in tool manufacturing]
Severe wear is characterised by considerable welding and tearing of the softer rubbing surface at high wear rate, and the formation of relatively large wear particles.

Under mild wear conditions, the surface finish of the sliding surfaces improves

c) Diffusion wear

Holm thought of wear as a process of atomic transfer at contacting asperities (Armarego and Brown).

A number of workers have considered that the mechanism of tool wear must involve chemical action and diffusion. They have demonstrated welding and preferred chemical attack of tungsten carbide in tungsten-titanium carbides. They have shown the photo-micrograph evidence of the diffusion of tool constituents into the workpiece and chip.

This diffusion results in changes of the tool and workpiece chemical composition.

There are several ways in which the wear may be dependent on the diffusion mechanism.

1. Gross softening of the tool
Diffusion of carbon in a relatively deep surface layer of the tool may cause softening and subsequent plastic flow of the tool. This flow may produce major changes in the tool geometry, which result in high forces and a sudden complete failure of the tool.

2. Diffusion of major tool constituents into the work. (chemical element loss)
The tool matrix or a major strengthening constituent may be dissolved into the work and chip surfaces as they pass the tool.
In cast alloy, carbide or ceramic tools, this may be the prime wear phenomenon.
With HSS tools, iron diffusion is possible, but it seems unlikely to be the predominant wear process.
Diamond tool - cutting iron and steel is the typical example of carbon diffusion.

3. Diffusion of a work-material component into the tool
A constituent of the work material diffusing into the tool may alter the physical properties of a surface layer of the tool. For example, the diffusion of lead into the tool may produce a thin brittle surface layer, this thin layer can be removed by fracture or chipping.

d) Chemical wear
Corrosive wear (due to chemical attack of a surface)

e) Facture wear

Fracture can be the catatrophic end of the cutting edge. The bulk breakage is the most harmful type of wear and should be avoided as far as possible.

Chipping of brittle surfaces

Other forms of tool wear

Thermo-electric wear can be observed in high temperature region, and it reduces the tool wear.
The high temperature results in the formation of thermalcouple between the workpiece and the tool. Due to the heat related voltage established between the workpiece and tool, it may cause an electric current between the two. However, the mechanism of thermo-electric wear has not been clearly developed. Major improvement (decrease) of tool wear has been seen through experimental tests with an isolated tool and component.

Thermal Cracking and Tool Fracture

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

The cyclic variations in temperature in milling induce cyclic thermal stress as the surface layer of the tool expands and contracts. This can lead to the formation of thermal fatigue cracks near the cutting edge. In most cases such cracks are perpendicular to the cutting edge and begin forming at the outer corner of the tool, spreading inward as cutting progresses. The growth of these cracks eventually leads to edge chipping or tool breakage.

Thermal cracking can be reduced by reducing the cutting speed or by using a tool material grade with a higher thermal shock resistance. In applications when coolant is supplied, adjusting the coolant volume can also reduce crack formation. An intermittent coolant supply or insufficient coolant can promote crack formation; if a steady, copious volume of coolant cannot be supplied, tool-life can often be increased by switching to dry cutting.

Edge chipping is common in milling. Chipping may occur when the tool first contacts the part (entry failure) or, more commonly, when it exits the part (exit failure). WC tool materials are especially prone to this.

Entry failure most commonly occurs when the outer corner of the insert strikes the part first. This is more likely to occur when the cutter rake angles are positive. Entry failure is therefore most easily prevented by switching from positive to negative rake cutters.


Consequences of tool wear

1. Increase the cutting force;
2. Increase the surface roughness;
3. Decrease the dimensional accuracy;
4. Increase the temperature;
5. Vibration;
6. Lower the production efficiency, component quality;
7. Increase the cost.

Influence on cutting forces
Crater wear, flank wear (or wear-land formation) and chipping of the cutting edge affect the performance of the cutting tool in various ways. The cutting forces are normally increased by wear of the tool. Crater wear may, however, under certain circumstances, reduce forces by effectively increasing the rake angle of the tool. Clearance-face (flank or wear-land) wear and chipping almost invariably increase the cutting forces due to increased rubbing forces.

Surface finish (roughness)
The surface finish produced in a machining operation usually deteriorates as the tool wears. This is particularly true of a tool worn by chipping and generally the case for a tool with flank-land wear - although there are circumstances in which a wear land may burnish (polish) the workpiece and produce a good finish.

Dimensional accuracy:
Flank wear influences the plan geometry of a tool; this may affect the dimensions of the component produced in a machine with set cutting tool position or it may influence the shape of the components produced in an operation utilizing a form tool.
(If tool wear is rapid, cylindrical turning could result in a tapered workpiece)

Vibration or chatter

Vibration or chatter is another aspect of the cutting process which may be influenced by tool wear. A wear land increases the tendency of a tool to dynamic instability. A cutting operation which is quite free of vibration when the tool is sharp, may be subjected to an unacceptable chatter mode when the tool wears.

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