chapter24 - CHAPTER 24 MACHINING AND TURNING CENTERS,...

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Unformatted text preview: CHAPTER 24 MACHINING AND TURNING CENTERS, MACHINE-TOOL STRUCTURES, AND MACHINING ECONOMICS 24-1 Description Machining and turning centers have the capability of performing a variety of machining operations on various surfaces of a given workpiece Flexibility and versatility An automated transfer line Cell layout with material handling from station to station FIGURE 24.2 A horizontal-spindle machining center, equipped With an automatic tool changer. Tool magazines can store 200 cutting tools. Source: Courtesy of Cincinnati Milacron, lnc. Tool storage Tool interchange arm Tools (cutters) Traveling column Spindle MAX!!! Spindle carrier Computer numerical _ control I panel 14715 FlGU RE 24.8 Schematic illustration of a three-turret, two-spindle computer numerical controlled turning center. Source: Hitachi Seiki Co., Ltd. 2nd Turret head I st Turret head I #1 Spindle head / l ; : / i ‘ l l d l #3 Turret hes! #2 spindle head, 24-4 Major Characteristics of Machining Centers Capable of machining a variety of components with close tolerances (i0.0025mm or 0.0001in.) Multi axes of linear and anglular movement Rapid changeover Minimum load/unload time Can use fewer high skilled labor High level of automation, one operator can control more than one machine Have tool—condition sensors In-process and post-process gauging and inspection Machining Cells 24-6 Vibration and Chatter Caused by low stiffness in the machine tool frame/assembly If uncontrolled: -Poor surface finish -Loss of dimensional accuracy -More rapid tool wear -Damage to machine tool components -Objectionable noise 24—7 Forced Vibrations From periodic applied force Self-excited Vibrations Caused by interaction of the chip removal process and the machine tool structure Damping The rate at which vibrations decay 1.2 1.2 0.8 0-8 0.4 0-4 > 0‘0 l > 0.0 T -o.4 7 ‘0'4 a -0‘8 2 4.8 -1.2 -1.2 . d. 1.6 _ 1.6 Epoxy/granite -2.0 -20 0 1000 2000 3000 4000 ' 0 1000 2000 3000 4000 l 4 10-9 S S FEGURE 24.3 _————————-——— Relative damping capacity of gray cast iron and epoxy-granite composite material.lThe vertical scale is the amplitude of vibration and the horizontal scale is time. Source: Cincmnati Milacron. lnc. __«______________.__________,____d__ Figure 24.14 24-8 - Damping and the effect of joints in the machine-tool structure (internal damping) _._.. Increasing damping Bed Bed + Bed + Bed + Complete only carriage headstock carriage + machine I; headstock FIGURE>24 Damping of vibrations as a function of the number of components on a lathe. Joints dissipate energy; thus the greater the number of joints, the higher the damping will be. Source: J. Peters. l:qu 1‘}. :5 - External damping Vibration absorbers/special facilities 24-” . Machining Economics Cost of Machining (C) C=C1+C2+C3+C4 Where: C1 = nonproductive cost: labor, overhead, and machine-tool costs involved in setting up for machining, mounting the cutting tool, preparing the fixtures and the machine, advancing and retracting the tool, and so on. C2 = machining cost: labor, overhead and machine-tool costs while the cutting operation is taking place C3 = tool-change cost: labor, overhead, and machine-tool costs during tool change C4 = cost of cutting tool Tool cost Machining cost I 1 | Non productive [cost Cost per piece —-—> Cutting speed —-l' I l ——-}| }<—-— High-efficiency 1 l machining range (b) Machining time Time per piece e—u Nonproductive time [:1 q WUL 2L}. 1 go Tool-changing time Cutting speed —" FIGURE 24,16 # Graphs showing (a) cost per piece 'and (b) time per piece in machining. Note the optimum speeds for both cost and time. The range between the two is known as the high-efficiency machining range. 24-1 1 “— COST OF SELECTED CUTTING TOOLS TOOL SIZE (in.) COST (3) High-speed steel tool bits iSCI- X 2% long 1‘2 "3‘, sq. x 4 ' 3—7 . Carbide-tipped (brazed) tools for turning ' ‘zsq. 2 g sq. 5 Carbide inserts, square 96" thick Plain ,1, inscribed circle 2—5 Coated 3—6 Ceramic inserts, square § inscribed circle 5—8 Cubic boron-nitride inserts, square i inscribed circle 60—80 Diamond-tipped inserts (polycrystalline) g: inscribed circle 75—4 00 Tool holders 1 x 1 x 6 50 ‘71-, 24-155 . 7 ; COST OF SELECTED TOOLS FOR MACHINING* TOOL SIZE (in.) COST (S) Drills, Hss, straight shank k % 1.00—1.70 i 3.00-6.00 ; 2.60—3.00 1 I 7-1 2 Coated (TiN) Tapered shank 3; 2.50—7.00 1 1 6-42 2 80—85 3 250 4 950 8—1 0 1 2-14 7—9 20—25 1% 40-54 .1, 6—1 5 1 18-27 Carbide tipped § 30—33 1 1 I 1 Reamers. HSS, hand Chucking lEnd mills, HSS 45—60 30—70 1 80 1 2—1 8 50—60 1 I 1 Milling cutters, HSS, staggered tooth, g” wide 4 35-75 8 1 Solid carbide Burs, carbide 1 30—260 Collets (5 core) 7-18 " Cost depends on the particular type of material and shape of tool. 13 24M Example of Machining Vs. Cold Forming (a) H O O D H O O Machining Cost per bolt ($) H o H 9'11 10 102 103 104 105 106 Number of bolts (b) H—fi 0.980 0.700 ' :3 T— 0:546 43 21/2 dlam 11/16 T [ 23/16 diam 1 1/4 [:3 LE L Slug First upset Second upset Trimmed Chamfered All dimensions are in inches. 24-14 s Threaded ...
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chapter24 - CHAPTER 24 MACHINING AND TURNING CENTERS,...

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