GEOMETRY OF SINGLE POINT TURNING TOOLS

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1 GEOMETRY OF SINGLE POINT TURNING TOOLS LEARNING OBJECTIVES Introduction to Features of single point cutting tool. Concept of rake and clearance angle and its importance System of description of Tool geometry ASA, ORS and NSA systems Interrelation between different Systems of Nomenclature Tool geometry is basically referred to some specific angles or slope of the salient faces and edges of the tools at their cutting point. Both material and geometry of the cutting tools play very important roles on their performances in achieving effectiveness, efficiency and overall economy of machining. Cutting tools may be classified according to the number of major cutting edges (points) involved as follows: Single point cutting tools : Examples: turning tools, shaping, planning and slotting tools and boring tools Double or Two point cutting tools : Example : drills Multipoint (more than two) cutting tool: Example: Milling cutters, Broaching tools, Hobs, Gear shaping cutters etc. The geometry of a single point cutting tool is determined by the following features: Face or rake surface, which is the surface of cutting tool along which the chips move Flank surface that face the workpiece. There are two flank surfaces, namely principal and auxiliary flank surfaces. Cutting edge that remove material from the workpiece. There are two cutting edges. The principal cutting edge performs the function of major material removal and is formed by the intersection line of rake face with the principal flank surface. The auxiliary cutting edge, often called as end cutting edge, is formed by intersection of rake face with the auxiliary flank surface. Corner or cutting point, which is meeting point of the principal and auxiliary cutting edges. Often a nose radius is provided to avoid a sharp corner The above mentioned features are shown in the figure 2. 1

2 Figure 2: Features of a single point tool. CONCEPT OF RAKE AND CLEARANCE ANGLES OF CUTTING TOOLS. Tool geometry is basically referred to some specific angles or slope of the salient faces and edges of the tools at their cutting point. Rake angle and clearance angle are the most significant for all the cutting tools. The rake angle and clearance angles are illustrated with figures below Rake angle (γ): Angle of inclination of rake surface from reference plane Clearance angle (α): Angle of inclination of clearance or flank surface from the finished surface The rake angle has the following function: It allows the chip to flow in convenient direction It reduces the cutting force required to shear the metal and consequently helps to increase the tool life and reduce the power consumption. It provides keenness to the cutting edge. It improves surface finish Rake angle may be positive, or negative or even zero as shown in Figure 3. Figure 2: Different types of rake angle on tool 2

3 RAKE ANGLE- TYPES AND THEIR SIGNIFICANCE. Positive rake : Positive rake or increased rake angle reduces compression, the forces, and the friction, yielding a thinner,less deformed and cooler chip But increased rake angle reduces the strength of the tool section and heat conduction capacity Some areas of cutting where positive rake may prove more effective are, when cutting tough, alloyed materials that tend to work harden, such as certain stainless steel, when cutting soft or gummy material or when low rigidity of workpiece, tooling, machine tool, or fixture allows chatter to occur. Negative rake: To provide greater strength at the cutting edge and better heat conductivity, zero or negative rake angles employed on carbide, ceramic, polycrystalline diamond, and poly crystalline cubic boron nitride cutting tools. These materials tend to be brittle but their ability to hold their superior hardness at high temperature results in their selection for high speed and continuous machining operation. Negative rakes increases tool forces but it is necessary to provide added support to the cutting edge. This is particularly important in making intermittent cuts and in absorbing the impact during the initial engagement of the tool and work. Negative rakes are recommended on tool which does not possess good toughness ( low transverse rupture strength) Negative rake causes high compression, tool forces and friction, resulting in highly deformed, hot chip. The rake angle for a tool depends on the following factors 1. Type of the material being cut: A harder material like cast iron may be machined by smaller rake angle than that required by soft material like mild steel or aluminum. 2. Type of tool material. Tool material like cemented carbide permits turning at high speed. At high speeds rake angle has little influence on cutting pressure. Under such condition the rake angle can minimum or even negative rake angle is provided to increase the tool strength. 3. Depth of cut: In rough turning, high depth of cut is given to remove the maximum amount of the material. This means that the tool has to withstand severe cutting pressure. So the rake angle should be decreased to increase the lip angle that provides the strength of the cutting edge. 4. Rigidity of tool holder and machine: an importantly supported tool on old or worn out machine cannot take up high cutting pressure. So while machining under above condition, the tool used should have larger rake angle. 3

4 CUTTING TOOL ANGLE AND THEIR SIGNIFICANCE. Side cutting edge angle The following are the advantages of increasing this angle, It increases tool life as, for the same depth of cut; the cutting force is distributed on a wider surface It diminishes the chip thickness for the same amount of feed and permits greater cutting speed. It dissipates heat quickly for having wider cutting edge. The side cutting edge angle of the tool has practically no effect on the value of cutting force or power consumed for a given depth of cut and feed. Large side cutting edge angles are lightly to cause the tool to chatter. End cutting edge angle The function of end cutting edge angle is to prevent the trailing front cutting edge of the tool from rubbing against the work. A large end cutting edge angle unnecessarily weakens the tool. It varies from 8 to 15 degrees. IMPORTNANCE OF NOSE RADIUS. The nose of a tool is slightly rounded in all turning tools The function of nose radius is as follows: Greater nose radius clears up the feed marks caused by the previous shearing action and provides better surface finish. All finish turning tool have greater nose radius than rough turning tools It increases the strength of the cutting edge, tends to minimize the wear taking place in a sharp pointed tool with consequent increase in tool life Accumulation heat is less than that in a pointed tool which permits higher cutting speeds. TOOL POINT REFERENCE SYSTEM To determine the orientation and inclination of the rake face and flank surfaces, a coordinate system is essential, resulting in a set of planes with reference to which the orientation or inclination can be determined. Two planes are of significance Tool reference plane or principal plane, which is perpendicular to the cutting velocity vector Cutting plane, which is tangential to the cutting edge and contains the velocity vector. The cutting plane is also perpendicular to the principal plane or the tool reference plane. There are three different types of coordinate system that are popular, when it comes to tool nomenclature, they are: 4

5 a) Machine Reference System ( MRS) b) Orthogonal tool Reference System (ORS) c) Oblique or Normal tool Reference System (NRS) Machine Reference System MRS This system is also called ASA system; ASA stands for American Standards Association. Geometry of a cutting tool refers mainly to its several angles or slope of its salient working surfaces and cutting edges. Those angles are expressed w.r.t. some planes of reference. In Machine Reference System (ASA), the three planes of reference and the coordinates are chosen based on the configuration and axes of the machine tool concerned. For example in a Lathe, XX is the spindle axis or workpiece axis, YY is the tool axis and ZZ is the vertical axis. Figure 4 depicts the xyz coordinates of machine Figure 4 : Planes and axes of reference in ASA system 5

6 The planes of reference and the coordinates used in ASA system for tool geometry are : π R - π X - π Y and Xm Ym Zm π R = Reference plane; plane perpendicular to the velocity vector π X = Machine longitudinal plane; plane perpendicular to π R and taken in the direction of assumed longitudinal feed π Y = Machine Transverse plane; plane perpendicular to both π R and π X [This plane is taken in the direction of assumed cross feed] The axes Xm, Ym and Zm are in the direction of longitudinal feed, cross feed and cutting velocity (vector) respectively. Figure 5. Tool angles in ASA system 6

7 Rake angles: [Refer to Figure 5] in ASA system The shape of the tool is normally specified in a special sequence, often referred to as tool signature, as shown below: Back rake angle Side rake angle Front or end clearance or relief angle Side clearance angle or relief angle End cutting edge angle Side cutting edge angle Nose radius 7

8 Orthogonal Tool reference system ORS Generally referred to as ORS (orthogonal rake system). This system is also known as ISO (International standard organization) old. The principal or tool reference plane π R, which is the same as for the machine reference plane The cutting plane π C, which contains the cutting edge, normal to π R plane The cutting plane π o, which contains the cutting edge, normal to π R plane Figure 6 shows the orientation of the rake face and flank surfaces with respect to the orthogonal too reference system and tool angles in symbolic form. Figure 6: Planes and axes of reference in ORS The main geometrical angles used to express tool geometry in Orthogonal Rake System (ORS) and their definitions will be clear from Figure 7.0 8

9 Figure 7.0 : Tool angles in ORS system. 9

10 Normal Rake System NRS Figure 8.0 Auxiliary orthogonal clearance angle. This system is also known as oblique tool reference system or ISO new. ASA system has limited advantage and use like convenience of inspection. But ORS is advantageously used for analysis and research in machining and tool performance. But ORS does not reveal the true picture of the tool geometry when the cutting edges are inclined from the reference plane, i.e., λ 0. Besides, sharpening or resharpening, if necessary, of the tool by grinding in ORS requires some additional calculations for correction of angles. These two limitations of ORS are overcome by using NRS for description and use of tool geometry. The basic difference between ORS and NRS is the fact that in ORS, rake and clearance angles are visualized in the orthogonal plane, π o, whereas in NRS those angles are visualized in another plane called Normal plane, π N. The orthogonal plane, π o is simply normal to π R and π C irrespective of the inclination of the cutting edges, i.e., λ, but π N (and π N for auxiliary cutting edge) is always normal to the cutting edge. The differences between ORS and NRS have been depicted in Figure 9.0 The planes of reference and the coordinates used in NRS are: π - π - π and X Y Z RN C N n n n Where, π = normal reference plane RN π N = Normal plane: plane normal to the cutting edge 10

11 X n = X o Y n = cutting edge Z n = normal to X n and Y n It is to be noted that when λ = 0, NRS and ORS become same, i.e. π o π N, Y N Y o and Z n Z o. 11

12 Interrelation ship between different systems of nomenclature Interrelations can be established between MRS or ASA and ORS or between ORS and NRS systems and vice versa. Rake face angles Case 1 Given bake rake angle and Side rake angle in ASA Find inclination angle and Orthogonal rake in ORS system Assume either φ or φ s is known Note Case 2 Given inclination angle and orthogonal rake in ORS Find back rake and side rake angle of ASA system Assume φ s is known Case 3 12

13 Given inclination angle and orthogonal rake in ORS Find normal rake angle in NRS system Case 4 Given, inclination angle and orthogonal clearance in ORS Find normal clearance angle. Clearance angles Given orthogonal clearance and inclination angle of ORS Find front clearance and side clearance angle of ARS system Assume φ is known. Alternatively we can write 13