Machinery's Handbook, 31st Edition
Diameter of Tap Drill 2177 power varies, of course, with the conditions. More power than that indicated in the table will be required if the cast iron is of a harder quality or if the taps are not properly relieved. The taps used in these experiments were of the inserted-blade type, the blades being made of high-speed steel. Power Required for Pipe Taps Nominal Tap Size Rev. per Min. Net Power Thickness of Metal Nominal Tap Size Rev. per Min. Net Power Thickness of Metal inch mm H.P. kW inch mm inch mm H.P. kW inch mm 2 50.80 40 4.24 3.16 1 1 ∕ 8 28.58 3 1 ∕ 2 88.90 25.6 7.20 5.37 1 3 ∕ 4 44.45 2 1 ∕ 2 63.50 40 5.15 3.84 1 1 ∕ 8 28.58 4 101.60 18 6.60 4.92 2 50.80 a 2 1 ∕ 2 63.50 38.5 9.14 6.81 1 1 ∕ 8 28.58 5 127.00 18 7.70 5.74 2 50.80 3 76.20 40 5.75 4.29 1 1 ∕ 8 28.58 6 152.40 17.8 8.80 6.56 2 50.80 a 3 76.20 38.5 9.70 7.23 1 1 ∕ 8 28.58 8 203.20 14 7.96 5.93 2 1 ∕ 2 63.50 a Tapping cast steel; other tests in cast iron. High-Speed CNC Tapping.— Tapping speed depends on the type of material being cut, the type of cutting tool, the speed and rigidity of the machine, the rigidity of the part-holding fixture, and the proper use of coolants and cutting fluids. When tapping, each revolution of the tool feeds the tap a distance equal to the thread pitch. Both spindle speed and feed per revolution must be accurately controlled so that changes in spindle speed result in a cor- responding change in feed rate. If the feed/rev is not right, a stripped thread or broken tap will result. NC/CNC machines equipped with the synchronous tapping feature are able to control the tap feed as a function of spindle speed. These machines can use rigid-type tap holders or automatic tapping attachments and are able to control depth very accurately. Older NC machines that are unable to reliably coordinate spindle speed and feed must use a tension-compression type tapping head that permits some variation of the spindle speed while still letting the tap feed at the required rate. CNC machines capable of synchronous tapping accurately coordinate feed rate and rotational speed so that the tap advances at the correct rate regardless of the spindle speed. A canned tapping cycle (see Right Hand Thread (G84) and Left Hand Thread (G74) on page 1369 in the CNC NUMERICAL CONTROL PROGRAMMING section) usually controls the operation, and speed and feed are set by the machine operator or part programmer. Synchronized tapping requires reversing the tapping spindle twice for each hole tapped, once after finishing the cut and again at the end of the cycle. Because the rotating mass is fairly large (motor, spindle, chuck or tap holder, and tap), the acceleration and deceleration of the tap are rather slow and a lot of time is lost by this process. The frequent changes in cutting speed during the cut also accelerate tap wear and reduce tap life. A self-reversing tapping attachment has a forward drive that rotates in the same direction as the machine spindle, a reverse drive that rotates in the opposite direction, and a neutral position in between the two. When a hole is tapped, the spindle feeds at a slightly slower rate than the tap to keep the forward drive engaged until the tap reaches the bottom of the hole. Through holes are tapped by feeding to the desired depth and then retracting the spindle, which engages the tapping-head reverse drive and backs the tap out of the hole—the spindle does not need to be reversed. For tapping blind holes, the spindle is fed to a depth equal to the thread depth minus the self-feed of the tapping attachment. When the spindle is retracted (without reversing), the tap continues to feed forward a short distance (the tapping head self-feed distance) before the reverse drive engages and reverse drives the tap out of the hole. The depth can be controlled to within about 1 ∕ 4 revolution of the tap. The tapping cycle normally used for the self-reversing tap attachment is a standard boring cycle with feed return and no dwell. A typical programming cycle is illustrated with a G85 block on page 1369 . The inward feed is set to about 95 percent of the normal
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