The cutting forces are normally increased by wear of the tool. Crater wear, flank wear (or wear land formation) and chipping of cutting edge affect performance of the cutting tool in various ways. 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.

Mostly, such cracks are perpendicular to the cutting edge and begin formation 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. An insufficient coolant can promote crack formation.

Tool wearandtoollife PDF

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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 chipping.

When the wear size increases to a critical value, the surface roughness of the machined surface decreases, cutting force and temperature increases rapidly, and the wear rate increases. Then the tool loses its cutting ability. In practice, this zone of wear should be avoided.

Tool wearmeasurement

C = Machining constant, found by experimentation or published data book. Depends on properties of tool materials, work piece and feed rate.

Abrasive wear is basically caused by the impurities within the work piece material, such as carbon nitride and oxide compounds, as well as the built-up edge fragments. It is a mechanical type of wear. It is the main cause of the tool wear at low cutting speeds.

First, calculate the cutting length per min. from the feed and spindle speed. ... Substitute the answer above into the formulae. ... 0.5×60=30(sec) The answer is 30 ...

L Dan · 1990 · 458 — This paper provides a review of the numerous techniques and methods of monitoring tool wear particularly in turning operations. By and large, these techniques ...

iv. The crater wear can increase the working rake angle and reduce the cutting force, but it will also weaken the strength of the cutting edge.

Flankwear

v. It is more common in ductile materials like steel which produce long continuous chips. It is also more common in H.S.S. (High Speed Steel) tools than the ceramic or carbide tools which have much higher hot hardness.

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It is common in case of milling operation. In milling, tools are subjected to cyclic thermal and mechanical loads. Teeth may fail by a mechanism not observed in continuous cutting. Thermal cracking can be reduced by reducing the cutting speed or by using a tool material grade with a higher thermal shock resistance.

Cutting tools are subjected to an extremely severe rubbing process. They are in metal-to-metal contact between the chip and work piece, under high stress and temperature. The situation becomes severe due to the existence of extreme stress and temperature gradients near the surface of the tool.

iv. Increase in cutting speed results in increased chip velocity at rake face, this leads to rise in temperature at chip-tool interface and so increase in crater wear.

Due to high pressure and temperature at tool-chip interface, there is a tendency of hot chips to weld on to the tool rake face. This concept leads to subsequently formation and destruction of welded junctions. When the weld intermittently breaks away picking particles of cutting tool. This leads to a crater wear. Fig. 9.19 shows adhesive wear.

The cyclic variation in temperature in milling process induce cyclic thermal stress at the surface layer of the tool expands and contracts. It may leads to the formation of thermal fatigue cracks near the cutting edge.

i. It is the most important wear that appears on the flank surface parallel to the cutting edge. It is most commonly results from abrasive/adhesive wear of the cutting edge against the machined surface.

Diffusion of carbon in a relatively deep surface layer of the tool may cause softening and subsequent plastic flow of the tool. It may produce major changes in the tool geometry.

Tool wearin machining

The gradual wear is unavoidable but controllable. It is the wear which cannot be prevented. It has to occur after certain machining time.

vi. The parameters used to measure the crater wear can be seen in the Fig. 9.18. The crater depth KT is the most commonly used parameter in evaluating the rake face wear.

Material removal rate (MRR) is the volume of material removed per time unit during machining operations such as milling, turning, drilling, and grooving.

Wear on the flank face (relief or clearance face) of the tool is called flank wear. The flank wear is shown in Fig. 9.17 (a, b, c).

Edge chipping is commonly observed in milling operation. It may occur when the tool first contacts the part (Entry Failure) or, more commonly, when it exits the part (Exit Failure).

Diffusion wear is usually caused by atomic transfer between contacting materials under high pressure and temperature conditions. This phenomena starts at chip-tool interface. At such elevated temperatures, some particles of tool materials diffuse into the chip material. It can also happen that some particles of work material also diffuse into the tool materials.

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The average width of allowable flank wear varies from 0.2 mm (for a precision turning operation) to 1 mm (for a rough turning operation).

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After reading this article you will learn about:- 1. Meaning of Tool Wear 2. Types of Tool Wear 3. Causes 4. Growth 5. Forms 6. Consequences.

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It can be observed in high temperature region. The high temperature results in the formation of thermal-couple between the work piece and the tool.

Tool wearmechanism

iii. Increase in feed results in increased force acting on tool interface, this leads to rise in temperature of tool-chip interface.

ii. The chips flows across the rake face develop severe friction between the chip and rake face. This produces a scar on the rake face which is usually parallel to the major cutting edge.

Due to this effect voltage established between the work piece and tool. It may cause an electric current flow between the two. However, this type of wear has not been clearly developed.

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The following Table 9.11 gives some recommended values of allowable average wear land (VB) for various operations and cutting tools:

These results in a variety of wear patterns observed at the rake face and the flank face. We call this gradual wear of the tool.

As the tool wear increases, the surface roughness of machined component also increases. This is particularly true for a tool worn by chipping .Although, there are circumstances, in which a wear land may burnish (polish) the work piece and produce a good finish.

As we decide to sharpen a knife edge when the quality of the cut begins to deteriorate and the cutting forces required increase too much, similarly re-sharpen or replace cutting tools when.

Tool wear is generally a gradual process due to regular operation. Tool wear can be compare with the wear of the tip of an ordinary pencil. According to Australian standard, the tool wear can be defined as “The change of shape of the tool from its original shape, during cutting, resulting from the gradual loss of tool material”.

iii. It is somewhat normal for tool wear and does not seriously degrade the use of a tool until it becomes serious enough to cause a cutting edge failure.

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Tool wearandtoollife

The gradual wear can be controlled by remedial action. The gradual wear can be divided into two basic types of wear, corresponding to two regions in the cutting tool as shown in Fig. 9.16.

The facture wear usually caused by breaking of edge at end or length. The bulk breakage is the most harmful and undesirable type of wear, and it should be avoided as far as possible.

Causes oftool wear

Initially, for the new cutting edge, the growth of wear is faster. The initial wear size is VB = 0.05 to 0.1 mm normally.

A wear land increases the tendency of a tool to dynamic instability or vibrations. When the tool is sharp, the cutting operation is quite free of vibrations. On the other hand, when the tool wears, the cutting operation is subjected to an unacceptable vibration and chatter mode.

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 angle cutters.

After the initial wear we found that the wear rate is relatively steady or constant. In this zone, the wear size is proportional to the cutting time.

Due to flank wear, the plan geometry of a tool may disturb. This may affect the dimensions of the component produced. It may influence the shape of the component.

Tool wearformula

Wear on the rake face of the tool is called crater wear. As the name suggests, the shape of wear is that of a crater or a bowl. The crater wear is shown in Fig.9.18 (a, b, c).

The tool matrix or a major strengthening constituent may be dissolved into the work and chip surfaces as they pass the tool. For example: Demand tool, cutting iron and steel is the typical examples of carbon diffusion.

vii. It occurs approximately at a height equal to the cutting depth of the material, i.e., Crater wear depth ⋍ cutting depth.

iii. It results in the formation of wear land. Wear land formation is not always uniform along the major and minor cutting edge of the tool.

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