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Turning Process II Dr. Chana Raksiri Introduction Tool geometry the designations for a right-hand cutting tool. Right-hand means that the tool travels from right to left. Tool geometry 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. 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 Material-removal rate Since there are N revolutions per minute, the removal rate is MRR Davgd 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 Material-removal rate 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. Forces in turning 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. Forces in turning 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. Forces in turning 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 workpiece. Roughing and finishing cuts In machining, the usual procedure is to first perform one or more roughing cuts at high feed rates and large depths- of-cut (and therefore high material-removal rates) but with little consideration of dimensional tolerance and surface roughness. These cuts then are followed by a finishing cut, at a lower feed and depth-of-cut in order to produce a good surface finish. Tools materials, feeds and cutting speeds Specific recommendations regarding turning process parameters for various workpiece materials and cutting tools are given. Example 1 Material removal rate and cutting fluid force in turning A 150-mm-long, 12.5-mm-diameter 304 stainless-steel rod is being reduced in diameter to 12.00 mm by turning on a lathe. The spindle rotates at N = 400 rpm, and the tool is traveling at an axial speed of 200 mm/min. Calculate the cutting speed, material- removal rate, cutting time, power dissipated, and cutting force. Solution The cutting speed is the tangential speed of the workpiece. The maximum cutting speed is at the outer diameter, and is obtained from the expression V Do N V 12.5400 15.7 m/min 1000 The cutting speed at the machined diameter is V 12.00400 15.1 m/ min 1000 From the information given, note that the depth-of-cut is 200 f 0.5 mm/rev 400 and the feed is 12.5 12.0 d 0.25 mm 2 According to Eq. (23.1a), the material-removal rate is then MRR 22.214.171.12400 1924 mm 3/min 2 106 m3/min Equation (23.1b) also can be used, where we find MRR=(0.25)(0.5)(15.7)(1000)=2×10–6 m3/min. The actual time to cut, according to Eq. (23.2), is 150 t 0.75 min 0.5400 The power required can be calculated by referring to Table 21.2 and taking an average value for stainless steel as 4 W–s/mm3. Therefore, the power dissipated is Power 41924 128 W 60 Since 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 of torque, T, and the rotational speed in radians per unit time; hence, 7680 T 3 .1 N - m 2 400 Fc 3.11000 506 N 12.25 / 2 Drilling, Drills and Drilling Machines When inspecting various large or small products, note that the vast majority have several holes in them. Hole making is among the most important operations in manufacturing, and drilling is a major and common hole- making process. Drills shows the two common types of drills: (a) Chisel-point drill. The function of the pair of margins is to provide a bearing surface for the drill against walls of the hole as it penetrates into the workpiece. Drills with four margins (double-margin) are available for improved drill guidance and accuracy. Drills with chip-breaker features also are available. (b) Crankshaft drill. These drills have good centering ability, and because chips tend to break up easily, these drills are suitable for producing deep holes. Drills Drills The capabilities of drilling and boring operations are shown in Table 23.10. Drills Twist drill Two spiral grooves (flutes) run the length of the drill, and the chips produced are guided upward through these grooves. The grooves also serve as passageways to enable the cutting fluid to reach the cutting edges. Drills are available with a chip-breaker feature ground along the cutting edges. This feature is important in drilling with automated machinery where a continuous removal of long chips without operator assistance is essential. Drills Other types of drills shows the Various types of drills and drilling and reaming operations. Drills Other types of drills A step drill produces holes with two or more different diameters. A core drill is used to make an existing hole larger. Counterboring and countersinking drills produce depressions on the surface to accommodate the heads of screws and bolts below the workpiece surface. Drills Other types of drills Spade drills have removable tips or bits and are used to produce large-diameter and deep holes. Fig 23.1 shows various types of drills. Drills Gun drilling Developed originally for drilling gun barrels, gun drilling is used for drilling deep holes and requires a special drill. Fig 23.22(a) shows the gun drill showing various features. (b) Schematic illustration of the gun-drilling operation. Drills Material-removal rate in drilling The material-removal rate (MRR) in drilling is the volume of material removed by per unit time. For a drill with a diameter D, the cross-sectional area of the drilled hole is πD2/4. The velocity of the drill perpendicular to the workpiece is the product of the feed, f (the distance the drill penetrates per unit revolution), and the rotational speed, N, where N=V/πD. Thus, Thrust force and torque The thrust force in drilling acts perpendicular to the hole axis; if this force is excessive, it can cause the drill to bend or break. An excessive thrust force also can distort the workpiece, particularly if it does not have sufficient stiffness (for example, thin sheet-metal structures), or it can cause the workpiece to slip into the workholding fixture. The thrust force depends on factors such as (a) the strength of the workpiece material, (b) feed, (c) rotational speed, (d) drill diameter, (e) drill geometry, and (f) cutting fluids. Example Material-removal rate and torque in drilling A hole is being drilled in a block of magnesium alloy with a 10-mm drill bit, at feed of 0.2 mm/rev, and with the spindle running at Calculate the material-removal rate and the torque on the drill. Solution The material-removal rate first is calculated from Eq. (23.3): 102 MRR 0.2800 12,570 mm 3 / min 210 mm 3 / s 4 Referring to Table 21.2, let’s take an average unit power of 0.5 Ws/mm3 for magnesium alloys. The power required is then Power 210 0.5 105 W Solution Power is the product of the torque on the drill and the rotational speed, which in this case is (800)(2π)/60 = 83.3 radians per second. Noting that W=J/s and J=Nm, we find that 105 T 1.25 N m 83.8 Drilling machines Drilling machines Reaming and Reamers Reaming is an operation used to (a) make an existing hole dimensionally more accurate than can be obtained by drilling alone, and (b) improve its surface finish. The most accurate holes in workpieces generally are produced by the following sequence of operations: 1. Centering 2. Drilling 3. Boring 4. Reaming Reaming and Reamers For even better accuracy and surface finish, holes may be burnished or internally ground and honed. A reamer is a multiple-cutting-edge tool with straight or helically fluted edges that remove very little material. Fig 23.26(a) shows the Terminology for a helical reamer and (b) Inserted-blade adjustable reamer. Hand reamers are straight or have a tapered end in the first third of their length. Reaming and Reamers Reaming and Reamers Various machine reamers (also called chucking reamers, because they are mounted in a chuck and operated by a machine) are available in two types: (1) Rose reamers have cutting edges with wide margins and no relief. (2) Fluted reamers have small margins and relief with a rake angle of about 5°. Reamers may be held rigidly (as in a chuck), or they may float in their holding fixtures to ensure alignment or be piloted in guide bushings placed above and below the workpiece. Tapping and Taps Internal threads in workpieces can be produced by tapping. A tap is a chip-producing threading tool with multiple cutting teeth. Fig 23.27(a) shows the terminology for a tap. (b) Tapping of steel nuts in production. Tapping and Taps Tapered taps are designed to reduce the torque required for the tapping of through holes. Bottoming taps are for tapping blind holes to their full depth. Collapsible taps are used in large-diameter holes; after tapping has been completed, the tap is collapsed mechanically and is removed from the hole without rotation. Chip removal can be a significant problem during tapping because of the small clearances involved. Tapping and Taps Tapping may be done by hand or with machines such as (a) drilling machines, (b) lathes, (c) automatic screw machines, and (d) vertical CNC milling machines combining the correct relative rotation and the longitudinal feed. Tap life can be determined with the same technique used to measure drill life. Self-reversing tapping systems also have been improved significantly and now are in use with modern computer- controlled machine tools.