Chapter 25 Machining Centers, Machine-Tool Structures, and Machining Economics

SOME PRODUCTS NEED VARIOUS PROCESSES (TURNING, FACING, BORING, THREADING

FIGURE 25.1 Examples of parts that can be machined on machining centers using various processes such as turning, facing, milling, drilling, boring, reaming, and threading; such parts ordinarily would require the use of a variety of machine tools to complete. (a) Forged motorcycle wheel, finish machined to tolerance and subsequently polished and coated. (b) Detailed view of an engine block, showing complex cavities, threaded holes, and planar surfaces. Source: (a) Courtesy of R.C. Components; (b) courtesy of Donovan Engineering, programming by N. Woodruff, and Photography by E. Dellis, Powersports Photography.

CONCEPT OF MACHINING CENTER traditional method is to produce parts on several machines Transfer (dedicated) lines are used in mass production If production change for any reason, rearranging the machines is costly Machine center: is an advanced computer controlled machine tool that is capable of performing a variety of machining operation on different surfaces and at different orientations of a work piece without need to remove it from the work holding device

CONCEPT OF MACHINING CENTER work piece generally stationary cutting tools rotate in transfer lines or traditional factories: the work piece is brought to the machine in machining centers: the machinig operation is brought to the work piece

FIGURE 25.3 Schematic illustration of the principle of a five-axis machining center. Note that, in addition to possessing three linear movements (three axes), the pallet, which supports the workpiece, can be swiveled around two axes (hence a total of five axes), allowing the machining of complex shapes, such as those shown in Fig. 25.1. Source: Courtesy of Toyoda Machinery.

FIGURE 25.5 Swing-around tool changer on a horizontal-spindle machining center. (a) The tool-exchange arm is placing a toolholder with a cutting tool into the machine spindle; note the axial and rotational movement of the arm. (b) The arm is returning to its home position; note its rotation along a vertical axis after placing the tool and the two degrees of freedom in its home position.

FIGURE 25.7 A vertical-spindle machining center; the tool changer is on the left of the machine, and has a 40 tool magazine. Source: Courtesy of Haas Automation, Inc.

FIGURE 25. 8 A computer numerical-controlled turning center FIGURE 25.8 A computer numerical-controlled turning center. The two spindle heads and two turret heads make the machine very flexible in its machining capabilities; up to three turret heads are commercially available. Source: Courtesy of Mori Seiki Co., Ltd.

MACHINE-TOOL MATERIALS Gray cast iron Welded steel structures Ceramic components Composites

VIBRATION AND CHATTER IN MACHINING OPERATIONS UNCONTROLLED VIBRATION AND CHATTER CAN RESULT IN: 1. Poor surface finish 2. Loss of dimensional accuracy of work piece 3. Premature failure of cutting tool 4. Possible damage of machine-tool components 5. Noise

FACTORS INFLUENCING CHATTER 1. Cutting forces 2. increase in hardness 3. Type of chip produced (continuous chips involve steady cutting forces)

GUIDELINES FOR REDUCING VIBRATION 1. Minimize tool overhang 2 GUIDELINES FOR REDUCING VIBRATION 1. Minimize tool overhang 2. improve stiffness of work-holding devices 3. Modify cutter geometry to reduce forces 4. Change cutting speeds, feed, depth of cut and cutting fluids 5. Improve the damping capacity of the machine tool