The following points highlight the seven factors influencing tool life of metals in the industries. The factors are: 1. Cutting Speed 2. Physical Properties of Work Piece 3. Area of Cut 4. Ratio of Feed to Depth of Cut (f/d) 5. Shape and Angles of Tools 6. Effect of Lubricant 7. Nature of Cutting.
Factor # 1. Cutting Speed:
Cutting speed has the maximum influence on tool life. Tool life decreases as the cutting speed increases. The criterion of wear is dependent on cutting speed because the predominant wear may be flank wear or crater wear if speed is excessive. It has been found that for carbide turning of steel, the crater wear becomes more predominant above a cutting speed of 100 m.p.m.
Before F.W. Taylor came in the industrial field, the relation between cutting speed and tool life was given by the following equation:
where V = Cutting speed in ft/minute,
T = Tool life in minutes
C = Constant.
But F.W. Taylor, one of the efficiency experts, later on reached to a conclusion that equation (1) does not hold good under all conditions and hence the modified equation (1) to
where b = the index or exponent which depends upon machine tool and work piece characteristics.
Equation (2) is known as Taylor’s equation.
Generally the value of b varies from 1/5 to 1/10 for all tool materials, but the comparative figures for different tool materials are given below:
b = 0.1 to 0.15 for H.S.S. tools, = 0.2 to 0.25 for carbide tools, = 0.6 to 1.0 for ceramic tools.
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Equation (2) when plotted on a graph will give a curve, the shape of which will depend upon the index or exponent of tool life equation. From this graph, it is also concluded that as the cutting speed increases, tool life decreases.
Factor # 2. Physical Properties of Work Piece:
The cutting speed depends upon the work piece and tool material. Table below shows the comparative figures for different tool and work piece materials.
It is found that tool life can also be correlated with the microstructure of work. In general hard micro-constituents in the matrix result in poor tool life. Also tool life is better with larger grain size. It is also found that similar metallographic structures will exhibit similar machining characteristics, regardless of their relative properties.
The effect of properties of materials is given by the equation:
HB = Brinell hardness number.
Factor # 3. Area of Cut:
The cutting speed V is inversely proportional to the area of cut and is represented by the equation given below:
where V = Cutting speed,
A = Area of cut,
k, b and c = Constants,
since in equation (3), k, b and c are constants, hence V α 1/A.
If a graph is drawn between cutting speed and area of cut, it is found that as the area increases, the cutting speed decreases. In Fig. 24.6 is a graph drawn between cutting speed and area of cut for different depth of cuts—for 6 mm, 3 mm and 1.5 mm.
Factor # 4. Ratio of Feed to Depth of Cut (f/d):
When the feed is more, more localised action and heating of tool at chip tool interface takes place. For the same area of cut, if the depth is increased twice and feed is decreased to one-half, the cutting speed can be increased up to 40%.
It has been found that the desirable ratio of depth to the feed is 8 (d/f » 8,), but actually it varies from 5 to 10.
The effect of feed and depth of cut on tool life is given by following relationship:
where f = feed in mm/min.,
d = depth of cut in mm.,
T = tool life in mts,
V = cutting speed in m/min.
This equation is applicable for turning low carbon steel by a cemented carbide tool.
Another relation between cutting speed for a given tool life, depth of cut and feed is:
where V’ = cutting speed in m/min for a given tool life,
C’ = a coefficient depending upon machine and work piece variables.
Values of x and y depend on the mechanical properties of the material being machined. Average values of x and y are found to be of the order of 0.37 and 0.77 respectively. It is very obvious from these values that the combination of large depth of cut and a high rate of feed with a low cutting speed will allow a large amount of metal to be removed during a given life of tool.
Two important relations relating dimensions of cut are:
Cutting force FC = CF d0.9 f0.8 and metal removal factor
where, CF = constant whose value depends on material being cut and true rake angle of the tool.
W = specific power consumption and K = constant.
Factor # 5. Shape and Angles of Tools:
(i) Effect of Rake-Angle:
When the back rake angle increases, the cutting force decreases, because of small shear- strain. When the negative rake angle is used, the shear strain is more, but for practical range, the negative rake angle has higher cutting force than positive rake-angles.
Large side rake angle produces chipping and smaller rake angle generates greater heat or an excessive wear and deformation in tool.
In Fig. 24.9 is drawn a graph for both +ve and –ve rakes between cutting force and cutting speed for a carbide tool when 20% nickel steel is machined.
Change in end cutting angles has little effect on tool life. However, it is found that larger is this angle, longer is tool life. Similarly larger is the side cutting edge angle (fs), more is the tool life. But an angle larger than 15° produces chipping and the tool life decreases: V0.11 = 78 (fs + 15°)0.264
(ii) Cutting Angle:
The cutting speed is dependent upon true cutting angle. The relation between cutting-speed and true cutting angle is represented by a graph, drawn for different depths of cut (d) and feeds (f) for the same tool-life.
Also as the cutting angle increases the horse-power required to machine a metal increases. In Fig. 24.11 is drawn a graph between horse power and cutting angle for various depths of cut and feed.
(iii) Cutting Force and Cutting Speeds:
The increase in cutting speed tends to decrease the cutting force over the lower range of speed but over the practical range the effect of cutting speed on cutting force is negative. This is shown be a graph drawn in Fig. 24.12.
(iv) Nose Radius:
Increase in nose radius improves tool life. Small nose results in excessive stress concentration and greater heat generation.
The relationship is:
VT0.9927 = 331 r0.243, (r = nose radius).
Factor # 6. Effect of Lubricant:
Lubrication decreases the cutting forces. The effect of cutting fluid is more predominant over the lower range of cutting speed, rather than at higher cutting speeds. At low speeds the cutting fluid acts as lubricant and reduces friction at tool-chip interface.
Factor # 7. Nature of Cutting:
It has also great influence on tool life; e.g. in the case of continuous cutting the tool life is much better than in intermittent cutting. The intermittent cutting gives regular impacts on the tool leading to its failure much earlier.