This article throws light upon the types of machining process. The types are:- 1. Shaping and Planning 2. Turning and Boring 3. Drilling and 4. Milling. It is necessary to have some exposure to the actual machining operations and their analyses. In this article, we shall not go into the extreme technological details and all possible types of operations, but we shall deal with only the very basic and common machining operations.
The basic nature of material removal process is the same in both the cases. The major difference between the two is that, in shaping, the primary (cutting) motion is provided to the tool and the feed is given to the work piece, whereas, in planning, it is just the opposite.
The cutting operation is intermittent in nature and takes place during the forward stroke. During the return of the tool (or the job, as the case may be), the feed motion is provided when there is no cutting action. Figure 4.34 shows some details of the cutting zone.
In an actual cutting operation, the major parameters are the strokes per unit time (N), stroke length (S), quick return ratio (R) (displacement / stroke), depth of cut (d), and the tool angles. To convert these parameters into the basic machining parameters, it would be enough to examine Fig. 4.34 showing a sectional view.
It should be remembered that, in general, the condition of orthogonal machining is not satisfied but we shall treat the process by assuming that the mechanics of orthogonal machining is applicable. As far as the power consumption is concerned, the results are not very inaccurate. The uncut thickness and the width of cut are given by the relations –
Where Ψ is the primary principal cutting edge angle. The rake angle is found to be α (also called normal rake) from the sectional view (Fig. 4.34). Figure 4.35 shows the cutting and thrust components of the force.
The cutting component FC acts against v and FT acts perpendicular to the transient surface. FT can be again resolved into two components, namely, Ff (feed component) and Fn (component normal to the machined surface), as –
The metal removal rate is given by LdƒN, where L is the length of the job and N is the number of cutting strokes per unit time. The cutting time can also be found out if the breadth (B) of the job, the total depth by which the work surface has to be lowered (H), the depth of cut (d), the feed (ƒ), and the cutting stroke per unit time (N) are given. The total time –
Turning is one of the most common operations. Surfaces of revolution are generally produced by this operation though the flat surfaces are produced by face turning. All turning operations are done in lathes. The major types of turning operations are – (i) turning of cylindrical and stepped cylindrical surfaces, (ii) turning of tapered and curved surfaces of revolution, (iii) turning of screw threads, and (iv) face turning and parting. When an internal surface is machined, the operation is commonly known as boring.
The boring operations can also be performed for producing different types of internal surfaces of revolution. We shall discuss here the mechanics of a simple turning operation. This then can be extended to the various other special operations whenever required. Figure 4.37a shows a simple turning operation. The tool used for such an operation is commonly termed as a single point tool.
The detailed geometry of this operation is illustrated in Fig. 4.37b. Figure 4.38 shows the different views and angles of a single point turning tool. The parameters in the corresponding basic machining operation can be found out as –
Where Ψ is the side cutting edge angle. The normal rake angle α can be found out when the tool angles are specified. In general, the condition of orthogonality is not satisfied, but to keep the discussion within the scope of this text, we shall assume orthogonal machining. The cutting speed is given as –
Where N is the number of job revolutions per unit time and D is the job diameter. Since the depth of cut d is very small as compared with D, the cutting speed may be assumed to be constant throughout the width of cut and equal to the value given by equation (4.41). To fulfill the condition of orthogonal machining, the cutting edge should be perpendicular to the velocity vector, and it can be easily shown that the condition to be satisfied by the tool angles is –
Type # 3. Drilling:
The most common hole making operation is drilling and it is usually performed with the help of a twist drill. Unlike shaping and turning, this involves two principal cutting edges. Figure 4.41 shows a drilling operation.
If the total advancement of the drill per revolution (the feed rate) is ƒ then the share of each cutting edge is ƒ/2 because each lip is getting the uncut layer the top surface of which has been finished by the other lip 180° ahead (during 180° rotation, the vertical displacement of the drill is ƒ/2). The uncut thickness t1 and the width of cut w are given as –
r being the radius of the point on the cutting edge where the normal rake is being evaluated, D the nominal diameter of the drill, β the half point angle (Fig. 4.41b), and Ψ the helix angle (Fig. 4.42).
Table 4.12 gives the typical values of the drill angles and parameters.
It should be noted that in the drilling operation the variations of cutting speed and other parameters along the cutting edge are appreciable and the whole phenomenon is very complex. However, all our calculations are based on the middle point of each cutting edge. The effect of all the forces acting on the drill (Fig. 4.43) can be represented by a resisting torque M and a thrust force F. The action at the chisel edge is not truly a cutting action; rather it is one of pushing into the material like a wedge. But the effect of the chisel edge on the torque is negligible as it is on the axis of rotation.
The contribution of the chisel edge to the development of the thrust force is considerable. The total thrust force F can be expressed as –
Milling is perhaps the most versatile machining operation and most of the shapes can be generated by this operation. It is especially more indispensable for machining the parts without rotational symmetry. Unlike turning, shaping, and drilling tools, the milling tool possesses a large number of cutting edges. The shaft on which the cutter is mounted is commonly known as the arbor.
The milling operations can be classified into two major groups, namely – (i) horizontal milling, and (ii) vertical milling. In the horizontal milling operation, the cutter axis is horizontal. Figure 4.44 shows some common horizontal milling operations. Horizontal milling can, again, be divided into two groups depending on the relative directions of cutting and feed motion. When the arrangement is like what is shown in Fig. 4.45a, the operation is called up milling.
When the cutting and the feed motion are in the same direction (Fig. 4.45b), the operation is called down milling. Since in down milling there is a tendency of the job being dragged into the cutter, up milling is safer and is commonly done. However, down milling results in a better surface finish and longer tool life. When the cutting edges are helical, the cutting operation is smoother and a better finish is obtained. This is due to the gradual engagement of the cutting edge.
The cutter axis is vertical and perpendicular (generally) to the work surface in vertical milling. The scheme of chip formation during plain slab milling using a straight cutter is explained in Fig. 4.47a. The cutter has a diameter D and the depth of cut provided is d. When milling is done with a straight-edged cutter, the operation is orthogonal and the kinematics of chip formation is as shown in Fig. 4.47b.
Since all the cutting edges take part in machining, a study of the process is facilitated by considering the action of only a single tooth. If ƒ is the feed velocity of the table in mm/min, the effective feed per tooth in mm will be ƒ/(NZ), where N is the cutter rpm and Z is the number of teeth in the cutter.
The material removal rate per unit width of the job is given by ƒd. It is clearly seen from Fig. 4.47b that the thickness of the uncut material in front of the cutting edge increases gradually, reaching a maximum near the surface, and then again drops to zero quickly. If the feed velocity is small as compared with the circumferential velocity of the cutter, then –
So, the cutting force components FC and FT (shown in Fig. 4.48) not only change in direction but also in magnitude as the cutting edge moves along the cut surface.
It is obvious that when cutting with a straight cutter, there is no component of the cutting force along the cutter axis. The average uncut thickness can be taken as half of the maximum value. Thus –
The average values of FC and FT can be approximately found out using this value of uncut thickness. Since FT acts in the radial direction, it does not produce any torque and the arbor torque is due only to the component FC. So, the torque M due to one cutting tooth is FC(d/2) and varies approximately as Fc. Figure 4.49 shows the variation of arbor torque (M) with arbor rotation for the action of only a single tooth.
Now, to get the overall torque (M̅), the moments due to all the teeth should be properly superimposed. This leads to three different possibilities, namely- (i) β < 2π / Z, (ii) β = 2π / Z, and (iii) β > 2π / Z. Figure 4.50a shows the three different possibilities; the arbor torque corresponding to each of these is shown in Fig. 4.50b. It is apparent from Fig. 4.50 that with a straight-edged cutter the force and arbor torque have sharp variations which can cause vibration problems.
When a helical cutter is used, the contact between the cutting edge and the work piece starts and ends gradually. Here, the arbor torque due to a single tooth and the overall torque are of the type shown in Figs. 4.51a and 4.51b, respectively. The machining power can be calculated by taking the product of the arbor speed and the average overall arbor torque. The average thrust force can be considered to be acting along the mid radial line of the work-cutter contact arc.