Machinery's Handbook, 31st Edition
2200 THREAD MILLING Development of Helix: If one turn of a helical curve were unrolled onto a plane surface (as shown by diagram), the helix would become a straight line forming the hypotenuse of a right-angle triangle. The length of one side of this triangle would equal the circumference of the cylinder with which the helix coincides, and the length of the other side of the trian gle would equal the lead of the helix.
Development of Helix
Helix
Lead of Helix
A
B
Circumference of Cylinder
Helix Angles.— The triangular development of a helix has one angle A subtended by the circumference of the cylinder, and another angle B subtended by the lead of the helix. The term “helix angle” applies to angle A . For example, the helix angle of a helical gear, ac - cording to the general usage of the term, is always angle A , because this is the angle used in helical gear-designing formulas. Helix angle A would also be applied in milling the helical teeth of cutters, reamers, etc. Angle A of a gear or cutter tooth is a measure of its inclination relative to the axis of the gear or cutter. Lead Angle: Angle B is applied to screw threads and worm threads and is referred to as the lead angle of the screw thread or worm. This angle B is a measure of the inclination of a screw thread from a plane that is perpendicular to the screw thread axis. Angle B is called the “lead angle” because it is subtended by the lead of the thread, and to distinguish it from the term “helix angle” as applied to helical gears. Finding Helix Angle of Helical Gear: A helical gear tooth has an infinite number of helix angles, but the angle at the pitch diameter or mid-working depth is the one required in gear designing and gear cutting. This angle A , relative to the axis of the gear, is found as follows: tan helix angle Lead of gear tooth 3.1416 pitch diameter of gear # = Finding Lead Angle of Screw Thread: The lead or helix angle at the pitch diameter of a screw thread usually is required when, for example, a thread milling cutter must be aligned with the thread. This angle measured from a plane perpendicular to the screw thread axis is found as follows: tan lead angle 3.1416 pitch diameter of screw thread Lead of screw thread # =
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online