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Chapter7b machining turning

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Chapter7b machining turning

1. 1. Turning•• One of the most basic machining processes is turning, meaning that the part is rotated while it is being machined.• Fig 23.1 shows the miscellaneous cutting operations that can be performed on a lathe. Note that all parts are circular—a property known as axisymmetry.• Table 23.1 shows the characteristics of machining processes and typical dimensional tolerance.
2. 2. • These machines are very versatile and capable of producing a wide variety of shapes as outlined below:1. Turning: to produce straight, conical, curved, or grooved workpieces such as shafts, spindles, and pins.2. Facing: to produce a flat surface at the end of the part and perpendicular to its axis useful for parts that are assembled with other components.3. Cutting with form tools: to produce various axisymmetric shapes for functional or aesthetic purposes.4. Boring: to enlarge a hole or cylindrical cavity made by a previous process or to produce circular internal grooves.5. Drilling: to produce a hole which may be followed by boring to improve its dimensional accuracy and surface finish.6. Parting: also called cutting off, to cut a piece from the end of a part, as is done in the production of slugs or blanks for additional processing into discrete products7. Threading: to produce external or internal threads8. Knurling: to produce a regularly shaped roughness on cylindrical surfaces, as in making knobs
3. 3. Tool geometry • Fig 23.4 shows the designations for a right-hand cutting tool. Right-hand means that the tool travels from right to left. • Rake angle is important in controlling both the direction of chip flow and the strength of the tool tip. • Side rake angle is more important than the back rake angle, although the latter usually controls the direction of chip flow. • Cutting-edge angle affects chip formation, tool strength, and cutting forces to various degrees. • Nose radius affects surface finish and tool-tip strength. The smaller the nose • radius (sharp tool), the rougher the surface finish of the workpiece and the lower the strength of the tool.
4. 4. Material-removal rate • The material-removal rate (MRR) in turning is the volume of material removed per unit time with the units of mm3/min. • d. The volume of this ring is the product of the cross-sectional area (f)(d) and the average circumference of the ring, where Do + D f Davg = 2 • Since there are N revolutions per minute, the removal rate is MRR = π Davg d f N ( 23.1a ) • Note that Eq. (23.1a) also can be written as MRR = d f V ( 23.1b) where V is the cutting speed. • Since the distance traveled is l mm, the cutting time is l t= ( 23.2) fN • The cutting time does not include the time required for tool approach and retraction. • The foregoing equations and the terminology used are summarized in Table 23.3.
5. 5. Forces in turning • Fig 23.5 shows the forces acting on a cutting tool in turning. Fc is the cutting force, Ft is the thrust or feed force (in the direction of feed), and Fr is the radial force that tends to push the tool away from the workpiece being machined
6. 6. • The cutting force acts downward on the tool tip and, thus, tends to deflect the tool downward and the workpiece upward. • The product of the cutting force and its radius from the workpiece center determines the torque on the spindle. • The product of the torque and the spindle speed determines the power required in the turning operation. • The thrust force acts in the longitudinal direction. It also is called the feed force because it is in the feed direction of the tool. • The radial force, acts in the radial direction and tends to push the tool away from the workpieceTools materials, feeds and cutting speeds • Specific recommendations regarding turning process parameters for various workpiece materials and cutting tools are given in Table 23.4Cutting fluids
7. 7. Example: Material removal rate and cutting fluid force in turningA 150-mm-long, 12.5-mm-diameter 304 stainless-steel rod is being reduced in diameterto 12.00 mm by turning on a lathe. The spindle rotates at N = 400 rpm, and the tool istraveling at an axial speed of 200 mm/min. Calculate the cutting speed, material-removalrate, cutting time, power dissipated, and cutting force.SolutionThe cutting speed is the tangential speed of the workpiece. The maximum cutting speedis at the outer diameter, and is obtained from the expression V = πDo N V = (π )(12.5)( 400) = 15.7 m/min 1000The cutting speed at the machined diameter is V = (π )(12.00)( 400) = 15.1 m/ min 1000From the information given, note that the depth-of-cut is 200 f = = 0.5 mm/rev 400
8. 8. and the feed is 12.5 −12.0 d= = 0.25 mm 2According to Eq. (23.1a), the material-removal rate is thenMRR = (π )(12.25)( 0.25)( 0.5)( 400 ) =1924 mm 3 /min = 2 ×10 −6 m 3 /minEquation (23.1b) also can be used, where we find MRR=(0.25)(0.5)(15.7)(1000)=2×10–6m3/min. The actual time to cut, according to Eq. (23.2), is 150 t= = 0.75 min ( 0.5)( 400 )The power required can be calculated by referring to Table 21.2 and taking an averagevalue for stainless steel as 4 W–s/mm3. Therefore, the power dissipated is Power = ( 4 )(1924) = 128 W 60Since 1 W = 60 N-m/min, the power dissipated is 7680 N-m/min.The cutting force, is the tangential force exerted by the tool. Power is the product oftorque, T, and the rotational speed in radians per unit time; hence, 7680 T = = 3.1 N - m ( 2π )( 400) Fc = ( 3.1)(1000) = 506 N 12.25 / 2