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30079_33b - Template machining utilizes a simple...

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Template machining utilizes a simple, single-point cutting tool that is guided by a template. However, the equipment is specialized, and the method is seldom used except for making large-bevel gears. The generating process is used to produce most high-quality gears. This process is based on the principle that any two involute gears, or any gear and a rack, of the same diametral pitch will mesh together. Applying this principle, one of the gears (or the rack) is made into a cutter by proper sharpening and is used to cut into a mating gear blank and thus generate teeth on the blank. Gear shapers (pinion or rack), gear-hobbing machines, and bevel-gear generating machines are good ex- amples of the gear generating machines. 33.9.2 Gear Finishing To operate efficiently and have satisfactory life, gears must have accurate tooth profile and smooth and hard faces. Gears are usually produced from relatively soft blanks and are subsequently heat- treated to obtain greater hardness, if it is required. Such heat treatment usually results in some slight distortion and surface roughness. Grinding and lapping are used to obtain very accurate teeth on hardened gears. Gear-shaving and burnishing methods are used in gear finishing. Burnishing is limited to unhardened gears. 33.10 THREAD CUTTING AND FORMING Three basic methods are used for the manufacturing of threads; cutting, rolling, and casting. Die casting and molding of plastics are good examples of casting. The largest number of threads are made by rolling, even though it is restricted to standardized and simple parts, and ductile materials. Large numbers of threads are cut by the following methods: 1. Turning 2, Dies: manual or automatic (external) 3. Milling 4. Grinding (external) 5. Threading machines (external) 6. Taps (internal) 33.10.1 Internal Threads In most cases, the hole that must be made before an internal thread is tapped is produced by drilling. The hole size determines the depth of the thread, the forces required for tapping, and the tap life. In most applications, a drill size is selected that will result in a thread having about 75% of full thread depth. This practice makes tapping much easier, increases the tap's life, and only slightly reduces the resulting strength. Table 33.13 gives the drill sizes used to produce 75% thread depth for several sizes of UNC threads. The feed of a tap depends on the lead of the screw and is equal to I/lead ipr. Cutting speeds depend on many factors, such as 1. Material hardness 2. Depth of cut 3. Thread profile Table 33.13 Recommended Tap-Drill Sizes for Standard Screw- Thread Pitches (American National Coarse-Thread Series) Number or Diameter 6 8 10 12 J/4 3/8 V2 3/4 1 Threads per Inch 32 32 24 24 20 16 13 10 8 Outside Diameter of Screw 0.138 0.164 0.190 0.216 0.250 0.375 0.500 0.750 1.000 Tap Drill Sizes 36 29 25 16 7 5/16 27/64 21/32 7/8 Decimal Equivalent of Drill 0.1065 0.1360 0.1495 0.1770 0.2010 0.3125 0.4219 0.6562 0.875
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4. Tooth depth 5. Hole depth 6. Fineness of pitch 7. Cutting fluid Cutting speeds can range from lead 3 ft/min (1 m/min) for high-strength steels to 150 ft/min (45 m/min) for aluminum alloys. Long-lead screws
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