In this article we will discuss about:- 1. Definition of Machinability 2. Factors Affecting Machinability 3. Variables Affecting 4. Assessment 5. Evaluation of Metallic Materials 6. Relative Machinability 7. Economics.
Definition of Machinability:
The machinability of a metal is defined as “The most machinable metal is one which permits the removal of material with satisfactory finish at lowest cost.”
In other words, the most machinable metal is one which will permit the fastest removal of the largest amount of material per grind of tool with satisfactory finish. The operational characteristics of a cutting tool are generally described by its machinability—which has three main aspects, viz., tool life, surface finish and power required to cut.
Factors Affecting Machinability:
Factors affecting the machinability of metals are as follows:
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i. The type of work-piece, i.e. material of work-piece;
ii. Type of tool material;
iii. Size and shape of tool;
iv. Type of machining operation;
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v. Size, shape and velocity of cut;
vi. Type and quality of machine used;
vii. Quality of lubricant used during machining operation;
viii. Coefficient of friction between chip and tool;
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ix. Shearing strength of work-piece material.
To remove metal by machining, the tool must first penetrate the surface. Once the tool has penetrated the surface, the chip formed should readily break. The ideal method of breaking the chip is to provide a built-in breaker. It is wrong to assume that because a metal is soft it is easy to machine.
Toughness is usually a corollary of softness and as a consequent the chips formed separate only with difficulty. In steel, a completely spheroidized structure is considered to be easiest to machine.
Machinability could also be considered as the ease with which a given material can be machined and it is affected by machine variables like cutting speed, feed and depth of cut, tool form, tool material, cutting fluid, rigidity of machine tools, shape and size of work, nature of engagement of tool with work.
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For a given set of machine conditions, ease of machining is affected by the properties of the work material like hardness, tensile properties, chemical composition, microstructure, degree of cold work and strain hardenability.
Ease of machining can also be judged by the common criteria like life of tool between resharpenings (expressed in various ways), cutting forces encountered and power consumption, surface finish, and ease of chip disposal etc.
Variables Affecting Machinability:
Machinability is influenced by the following variables:
i. Machine Variables:
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Machine variables (like power, torque, accuracy and rigidity) indirectly affect the machinability. The machine should be rigid and have sufficient power to withstand the induced cutting forces and to minimise deflections. If not so then, both tool life and finish are affected, and to limit the cutting forces, the speed, feed and depth of cut have to be limited.
ii. Tool Variables:
These include tool material, tool geometry, and the nature of engagement of tool with the work. The cutting tool has to be optimised to obtain a reasonable value of tool life and remove maximum material. Proper tool geometry is essential for efficient machining and it is chosen depending on the work material and machining conditions.
Surface finish is greatly influenced by the tool geometry. Rake angle and nose radius result in large improvement of surface finish and other parameters have little influence. Tool rigidity affects tool life, surface finish and dimensional accuracy.
iii. Cutting Condition:
Cutting speed has the greatest influence on tool life. The surface finish, normally, is improved by increase in the cutting speed, due to continuous reduction of the built-up edge. Dimensions of cut and cutting fluids also influence tool life.
iv. Work Material Variables:
Hardness, tensile strength, chemical composition, microstructure and method of production of work material have influence on machinability.
Assessment of Machinability:
The machinability of a material may be assessed by one or more of the following criteria:
a. Tool Life:
Amount of material removed under standardised condition, before the tool performance becomes unacceptable or tool is worn by a standard amount.
b. Limiting Rate:
Limiting rate of metal removal-maximum rate at which the material can be machined for standard short tool life.
c. Cutting Force:
Forces acting on a tool or the power consumption.
d. Surface Finish:
They are achieved under specified cutting conditions.
e. Chip Shape:
Chip shape is important as it influences the clearance of the chips from around the tool.
It must be understood that machining behaviour is complex and it can’t be evaluated by a single measurement. Useful ad hoc tests can be specified for prediction of tool life, metal removal rate or power consumption under particular sets of operating condition, but these can’t be regarded as evaluations of machinability, valid for the whole range of operations encountered. Improved appreciation of machinability also calls for understanding of the interactions of tool and work material at the interface (bonding, diffusion and interaction).
Evaluation of Metallic Materials for Machinability:
Experience has shown that a correlation exists between the basic physical properties of the metals and their machinability. Fig 24.1 shows that machinability decreases with increase in tensile strength. This property together with hardness is probably a good criterion of machinability.
It is also true that under controlled conditions there is very little difference in the machinability of cast and hot rolled steels, of carbon and low-alloy steels, and of acid and basic steels provided tensile strength and hardness are the same. Cast steels and nonferrous alloys may be more difficult to machine than wrought material.
Factors which come into play while evaluating the machinability of any metal are:
i. Tool life;
ii. Form and size of chip and shear angle;
iii. Cutting forces and power consumption;
iv. Surface finish;
v. Cutting temperature;
vi. Rate of metal removal per tool grind;
vii. Rate of cutting under the standard force;
viii. Uniformity in dimensional accuracy of successive parts.
Out of these eight mentioned factors, the first four factors are the basis of comparing the machinability of different materials. The relative importance of these four factors depends upon the kind of machining operation. This has been drawn in a tabular form for various operations.
The relative importance of various considerations in judging the machinability of metals depends upon the type of machining operation. Tool life is the most important parameter for assessing machinability. Since tool life is a direct function of cutting speed, a better machinable metal is one which permits higher cutting speed for a given tool life.
Cutting force becomes important criterion for machinability where it has to be limited considering the rigidity and vibrations in machine. The material which requires higher cutting forces for machining under given cutting conditions is less machinable.
Where surface finish is an important criterion for a part, then it is the consideration for machinability. For roughing operation, the primary consideration is the maximum material removal rate. In the case of finishing, the surface finish forms the criterion for machinability.
Machinability of a metal is influenced by the machine variables (rigidity, power and accuracy of machine), tool variables (tool material, geometry and type of cut), cutting conditions (speed, feed and depth of cut) and the work material properties. How each of these variables affect machinability is discussed below.
If a machine tool is not very right or it has less power, then tool life can’t be high and the accuracy and surface finish will also be poor. In order to reduce cutting forces on such a machine values of speed, feed and depth have to be reduced, thereby affecting the machinability.
Tool material is an important factor for machinability as cutting operation can be performed efficiently only when proper tool material and tool geometry have been selected. Tool life and specific cutting speed (speed for a predetermined tool life) are dependent upon tool material. Selection of a particular tool material is influenced by work material, machining conditions, wear resistance and cost of tool materials.
Clearance angle, nose radius and type of tool have a marked effect on the tool life. Tangential cutting force is directly related to rake angle. Radial and axial forces are altered by nose radius, side cutting edge angle. Surface finish can be greatly improved by proper rake angle, end cutting edge angle and nose radius.
Interrupted cuts (as with cutting slots or keyways in cylindrical work piece) subject tool to impact and shock loading and influence tool life. In the case of intermittent cuts (milling cutters) the sharp cutting edge should not make initial contact (possible by adopting negative rake angle).
Where tools are not rigid and supported firmly (end mills), it is necessary to use light cuts to reduce cutting forces and cutter deflection.
Relative Machinability of Some Ferrous and Non-Ferrous Alloys:
Soft grey cast-irons do not contain hard excess carbides or phosphide particles. Malleable iron is the most readily machinable of all ferrous materials, usually ranking higher than high sulphur steel. Free cutting materials contain about 1 to 4.0% lead.
Economics of Machinability:
In most of the cases, machining is the most economical method of production. Where large volumes of material have to be machined, it is essential that economic selection be done for components.
The various cost elements in machining cost are:
(i) Set up and idle time cost/piece (C1);
(ii) Machining cost/piece (C2);
(iii) Tool changing cost/piece (C3);
(iv) Tool re-grinding cost/piece (C4); and
(v) Too depreciation cost/piece (C5).
Let us consider a case of turning a cylindrical piece of length I mm, diameter D at velocity V m/sec and a feed rate of f mm/rev. Let us assume that cost/min of labour and overhead is K1. If ts be the time required for set-up and idle time/piece, then set-up and idle time cost per piece. C1 = K1. ts.
If tm = machine time per piece = π Dl.N/f.V (N = no. of cuts)then machining cost/piece C2 = K1 . tm
If tc is the time required to change a tool, then tool changing cost C3= K1 x tc x no. of tool changes/piece and no. of tool changes/piece = tm/T [T = (tool life)]
It would be appreciated that cost of re-grinding a tool is made up of two components, viz, K2—the setting cost and K3—cost/mm of tool ground. However, in the case of throw away tips, the cost of tool re-grinding can be ignored. Let w be the permitted size of flank wear land on the tool and δ = flank clearance angle. Then the metal to be reground from tool corresponding to this wear is shown by dotted line and is equal to w sin δ.
... tool re-grinding cost/piece
C4 = (K2 + K3w sin δ) x no. of tool changes/piece
If total amount of material which can be ground off the flank face of new tool before discarding it be M, then:
If R be the cost of a new tool, then depreciation cost per tool life:
and tool depreciation cost/piece C5
... Total cost C = C1 + C2 + C3 + C4 + C5
In the above equation, the variables are feed f and velocity V.
The constant C is dependent upon the tool parameters like nose radius and side-cutting angle and is limited by chatter considerations and the geometry of the work piece.
The value of feed can be increased to obtain minimum value of machining cost, but its value is limited by the maximum feed available on the machine, the surface finish obtainable, or the onset of chatter. Further, very high feed is associated with a very low cutting speed at which the tool is likely to fail by crumbling of the cutting edge rather than flank wear.
Further high feed and low speed may call for power in excess of motor capacity. Thus considering these factors suitable value of maximum feed needs to be selected.
The value of optimum cutting speed depends upon the tool material and work piece material. It has been found that optimum value of speed for solid brazed tools,V = (0.1n/1-n)n C.
where n and C are the constants dependent upon material, and their values can be determined by tool life tests.
It may be mentioned that selection of optimum speed reduces the tool life and thus may have to be reset frequently which may be costly. Hence overall compromise for the given requirements needs to be made.