In this article we will discuss about:- 1. Definition of Tool Life 2. Ways of judging Tool Life 3. Measurement.
Definition of Tool Life:
Under the modern methods of machining, it would be desirable to have as long tool life as possible. Tool life has many interpretations, but in general, it is defined “as the actual cutting time between two successive grinds”. Any lengthening of the grinding time results not only in increase in labour-cost but also indirect and overhead expenses.
The production machines are more expensive as compared to basic machine tools. As the production output increases, the more important it becomes to reduce down the sharpening time of their tools.
Tool life excludes the following points:
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(a) Removing,
(b) Regrinding,
(c) Resetting.
If somehow, we can manage to reduce the time for the above three operations, we can increase the productive time. Tool life is valuable measure of machinability in preference to other criterion, because it is a factor of direct importance to production department.
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Expected tool life:
(a) Cast tool steel = 22 minutes.
(b) High speed steel = 60 to 120 minutes.
(c) Cemented carbides = 240 to 480 minutes.
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Expected tool life for high speed steel (Different machining operations):
(i) Turning operation (rough cuts) < 30—40 minutes, (finish cuts) > 90 minutes.
(ii) Screw cutting and finishing cuts = 120 minutes.
(iii) Capstan and turret lathe = 150 minutes,
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(iv) Automatics. = 6 to 8 hours
During machining operation, the cutting edge of the tool gradually wears out and at certain stage it stops cutting metal. After a certain degree of wear, tool has to be resharpened to make it cut again. Tool life is the useful cutting life of a tool expressed in time or some other unit. This period measured from the start of a cut until such time when the tool no longer performs the designed function defined by the failure criteria.
The period during which tool cuts satisfactorily is called its life. Though usually wear (which makes cutting edge so worn that catastrophic failure occurs), other considerations like dimensional accuracy or poor surface finish, may demand earlier replacement. Tool life is the period between two consecutive tool resharpenings or replacements.
Ways of judging Tool Life:
Several ways of judging when the effective cutting life of a tool is over are:
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i. Fixed time of failure,
ii. Removal of a set volume of metal,
iii. Production of prescribed number of components,
iv. Machining until destruction of tool,
v. Use of limiting value of surface finish,
vi. Limiting change in component size,
vii. Fixed increase in cutting forces or power and
viii. Observation of the wear land pattern.
In any machining operation, the most important characteristics requiring due consideration are:
a. The life of a tool-work piece combination,
b. The surface finish produced,
c. The type of chip produced,
d. The power consumed in the formation of chips.
The relative importance of each characteristic depends upon the application, e.g., in removal of heavy stock the tool life is important; for finishing operation, the ability to produce a smooth surface of precise geometry is the criterion. The power consumed is generally not attached such importance till the metal removal is limited by the power available in the machine.
A tool may fall (i.e. cease to function satisfactory) due to any of the following reasons:
I. Temperature Failure:
a. Plastic deformation of cutting edge due to high temperature.
b. Cracking at the cutting edge due to thermal stresses.
II. Rupture of Tool Point:
a. Chipping of tool edge due to mechanical impact.
b. Crumbling of cutting edge due to built-up edge.
III. Gradual Wear at Tool Point:
a. Flank wear.
b. Crater wear on rake face of tool.
When a cutting tool surface slides over the surface of the work piece, a friction force is experienced due to:
i. Instantaneous welds formed at points of contact and the stress required to rupture these welds,
ii. Plowing of the high points of tool surface through those of work piece.
In addition to this friction force, the sliding of metal surfaces is also accompanied by wear. The plowing action of the hard particles of tool embedded in the relatively soft work piece matrix is similar to lapping process, and the rate at which material will be removed by this means will depend upon the size, hardness and number of hard particles per unit area and the relative hardness of the tool and work matrices.
Flank wear refers to abrasion or wear on the flank below the cutting edge. Crater wear refers to wear of a cup in the tool face back of the cutting edge by the flowing chip. The cup gradually grows larger and finally cutting edge crumbles.
It is obvious from above that the temperature present in the zone of cutting during machining has a direct effect on tool life. It has been found that a small change in cutting temperature has a large effect on tool life.
Temperature can be reduced either by using coolant or reducing the energy expended in cutting (comprising of energy for shearing of metal and that used in overcoming friction between chip and tool) which is controlled by the basic mechanical variables, viz. shear strength of the metal Ss, the coefficient of friction between chip and tool μ, and the machining constant C. Reduction of Ss, μ or increase in C results in improvement of tool life.
Next source of tool failure is abrasive action of the work on the tool which is due to the:
i. Inherently hard constituents present in the microstructure of the metal being cut (which produce rapid wear of the tool) and
ii. hardening induced in the chip and work surface by the cutting process, which depends on brinell hardness of metal, its strain hardenability and the amount of shearing strain that the metal undergoes during machining (function of μ and G).
Measurement of Tool Life:
The various criterions to specify tool life are:
(i) Failure of tool,
(ii) Presence of chatter,
(iii) Poor surface finish,
(iv) Sudden increase in power and cutting force,
(v) Overheating and fuming due to heat of friction,
(vi) Dimensional instability.
The criterion for tool life is generally taken as the time for total destruction in the case of a high speed steel tool or the time to produce a 0.75 mm wear and for carbides. It is the common practice to present tool life in the form of a curve in which the log of the cutting speed (surface metre per minute) is plotted against the log of the tool life measured in minutes.
Experimentally it is found by the accelerated tests that the tool life curves plotted in this way are linear and, therefore, the tool life can be expressed by the relation:
VTb = C (Taylor Equation)
where V = cutting speed in cm/minute,
T = tool life in minutes, b and C are constants.
For high speed steel cutting steel, b @ 0.1, and C @ 50.
For cemented carbide tool, b @ 0.125, and C @ 100.
Expressing the tool life in minutes has the advantage in computing machining costs; it is not a good way of measuring a tool life in tool wear studies. Since tool wear is related to the total chip area that passes across the tool face, it is more advantageous to express tool life in terms of the volume of metal removed. The plots of log of tool life in terms of material removed for different tool materials versus log of cutting speed are shown in Fig. 24.2. Tool life is expressed in volume of metal removed as (L) = TV fd
where d = depth of cut, f= feed rate.
Expression of tool life VTb = C can be rewritten as:
where b’ = (1 – b)/b = constant and C’ = fd(C)1/b = constant.
It has been observed that the above equations of tool life obtained on the basis of accelerated tests do not hold in certain cases, e.g. according to this equation it would appear that the tool life would become infinite as the cutting speed approaches zero. Further it is found that the tool life rises very rapidly below a speed of 30 s.m.p.m. for high speed steel tool and 120 s.m.p.m. for carbide tool which is not in accordance with the above equations.
Thus, obviously the definition of tool life is quite arbitrary. In any tool the primary cause of failure is due to wear. The poor cutting conditions obtained with a worn tool usually necessitate regrinding long before failure would occur.
It is thus evident that the criterion of tool life should be measure of a specified amount of wear rather than complete failure. The wear can be easily judged by measuring the length of the wear land, which it is found, varies linearly with time. The mean tool life for various operations according to this criterion can be specified as 0.75 mm of wear land.
From above it is seen that tool life varies with cutting speed; in the same way it also varies with feed rate. At a low feed rate, the area of chip that passes across the tool surfaces will be relatively large for a given volume cut, and relatively small for a high feed rate.
From this, it seems that tool life should increase with increase in feed rate, but as the cutting forces on tool also increase with increase in feed rate, it leads to decreased tool life. Thus these two opposing influences of the feed rate upon tool give rise to an optimum rate of feed which is about 0.25 to 0.50 mm/rev.
It will be appreciated that measurement of wear land is not a simple task, even when it be measured under microscope.
The reasons of this are:
(i) It is difficult to determine the exact extent of the wear land due to the variation of the wear land across the tool, and
(ii) the difficulty also arises due to a more subtle cause that is inherent in the wear land technique.
When an amount of metal MTL is worn from the clearance face of a tool, the extent of the wear land is TL [Refer Fig. 24.3].
Diamond Indentor Technique:
It is found to be a very useful method for determining the tool wear. In this method, on a freshly sharpened test tool, 0.25 mm wear land is first carefully ground on the clearance face; and then on an indenting fixture, an impression of the wear land is made as shown in Fig. 24.4.
A small amount of cut is then taken to condition the wear land and cause it to wear-in and readings are taken of the major diagonal of the impression. In this way the subsequent wear can be conveniently and precisely determined.
It is important to note in this method that the tool should not be removed from the machine after an impression is made on it and, therefore, it is customary to fit a microscope bracket directly on the tool holder.
Radioactive Techniques:
Radioactive technique has also been employed for the rapid measurement of tool life. Utilising the fact that about 90% of the radioactive tool material worn away remains attached to the chips, this method measures the tool life by measuring the radioactivity of the chips by using the radioactive tool material.
But this method measures tool wear both on the cutting face as well as the clearance face. However, it has been established that the wear as determined by radioactivity is easily correlated to wear measured in the conventional manner.
Short Time Tests:
These tests, though not as reliable as long time tests (tool life-cutting speed relationship), yet are good enough for comparing different materials with regard to their machinability.
These include:
(i) Tests at Elevated Cutting Speeds:
High cutting speeds are used to effect rapid tool wear. The results are likely to be distorted and may thus not give a true picture of material behaviour. These are used for rapid comparison of different materials.
(ii) Facing Tests:
In it, facing is carried out from the centre towards periphery and the diameter at which the tool fails is the measure of the machinability. Results may not be quite reliable but can be adopted for comparison purposes.
(iii) Test with Low Wear Criterion:
In order to reduce the test time, low values of wear can be taken as the criterion.