Machinery's Handbook, 31st Edition
2174 Diameter of Tap Drill medium fit series. Dimensional details for the radius under the head of fasteners made according to BS 3692 are given on page 1763 ; those for fasteners to BS 4168 are given on page 1827; those to BS 4183 are given on pages 1801 through 1805. Cold Form Tapping.— Cold form taps do not have cutting edges or conventional flutes; the threads on the tap form the threads in the hole by displacing the metal in an extrusion or swaging process. The threads thus produced are stronger than conventionally cut threads because the grains in the metal are unbroken and the displaced metal is work hardened. The surface of the thread is burnished and has an excellent finish. Although chip prob - lems are eliminated, cold form tapping does displace the metal surrounding the hole and countersinking or chamfering before tapping is recommended. Cold form tapping is not recommended if the wall thickness of the hole is less than two-thirds of the nominal diam - eter of the thread. If possible, blind holes should be drilled deep enough to permit a cold form tap having a four thread lead to be used as this will require less torque, produce less burr surrounding the hole, and give a greater tool life. The operation requires 0 to 50 percent more torque than conventional tapping, and the cold form tap will pick up its own lead when entering the hole; thus, conventional tapping machines and tapping heads can be used. Another advantage is the better tool life obtained. The best results are obtained by using a good lubricating oil instead of a conventional cutting oil. The method can be applied only to relatively ductile metals, such as low-carbon steel, leaded steels, austenitic stainless steels, wrought aluminum, low-silicon aluminum die- casting alloys, zinc die-casting alloys, magnesium, copper, and ductile copper alloys. A higher than normal tapping speed can be used, sometimes by as much as 100 percent. Conventional tap drill sizes should not be used for cold form tapping because the metal is displaced to form the thread. The cold formed thread is stronger than the conventionally tapped thread, so the thread height can be reduced to 60 percent without much loss of strength; however, the use of a 65 percent thread is strongly recommended. The following formula is used to calculate the theoretical hole size for cold form tapping: US: 0.0068 Theoretical hole size basic tap O.D. threads per inch percent of full thread = − × Theoretical holesize basic tap O.D. = Metric: The theoretical hole size and the tap drill sizes for American Unified threads are given in Table 9, and Table 10 lists drills for ISO metric threads. Sharp drills should be used to prevent cold-working the walls of the hole, especially on metals that are prone to work hardening. Such damage may cause the torque to increase, possibly stopping the machine or breaking the tap. On materials that can be die cast, cold form tapping can be done in cored holes provided the correct core pin size is used. The core pins are slightly tapered, so the theoretical hole size should be at the position on the pin that corresponds to one-half of the required engagement length of the thread in the hole. The core pins should be designed to form a chamfer on the hole to accept the vertical extrusion. 147.06 − × Threads pitch percent of full thread
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online