Threading Technologies - Chapter 5 5.1 Threads 5.3 Fluteless Taps 5.4 Threading Dies 5.5 Thread Turning – Introduction 5.5.1 Radial Infeed Techniques 5.5.2 Thread Helix Angles, for Single-/Multi-Start Threads 5.5.3 Threading Insert Inclination 5.5.4 Thread Profile Generation 5.5.5 Threading Turning – Cutting Data and Other Important Factors 5.6 Thread Milling 5.7 Thread Rolling – Introduction 5.7.1 Thread Rolling Techniques References 5.1 Threads An Introduction The originator of the first thread was Archimedes (287–212 BC), although the first modern-day thread can be credited to the Engineer and inventor Joseph Whitworth in 1841, where he developed the Standards for today’s screw thread systems. Whitworth’s 55° included angled V-form thread, became widely established enabling thread-locking and unlocking precision parts and of sub-assemblies – paving the way to the build-up of precise and accurate modernday equipment and instruments. Standardisation of Imperial thread forms in the USA, Canada, UK, and elsewhere, allowed for the interchangeablity of parts to become a reality. Around this time, both in France and Germany metric threads were in use, but it took until 1957 before both the common 60° included angled ISO M-thread and Unified thread profiles to become widely accepted and established (Fig. 95). Along with these and other various V-form threads that have been developed (Fig. 95i), they include quick-release threads such as the Buttress thread: this being a modified form of square thread, along with the 29° included angled truncated Acme form which is a hybrid of a Vform and Square thread. Tapered: gas, pipe and petroleum- type threads, were developed to give a mechanical sealing of the fluid, or gas medium, with many other types, including multi-start threads that are now in use throughout the world. V-form screw threads are based upon a triangle (Fig. 95 – top diagram), which has a truncated crest and root, with the root either having a flat (as depicted), or a more likely, a radius – depending upon the specification. If screw threads have an identical pitch , but different diameters, it follows that they would have dissimilar lead angles. Usually, threads have just one start, where the pitch and the lead are identical – more will be mentioned on multi-start threads later in this chapter. Referring to Fig. 95, the angle enclosed by the thread flanks is termed the included thread angle (β – as illustrated in Fig. 95 – middle right). This thread form is uniformly spaced along an ‘imaginary cylinder’ , its nominal size being referred to as the major diameter (d). The effective pitch diameter (d2) is the diameter of a theoretical co-axial cylinder whose outer surface would pass through a plane where the width of the groove, is half the pitch. Therefore, the pitch (p) is normally associated with this ‘effective’ diameter (i.e. see Fig. 95 – middle right). The minor diameter (d1), is the diameter of another co-axial cylinder the outer surface of which would touch the smallest diameter. Thread clearance is normally achieved via truncating the thread at its crest, or root – depending upon where the truncation is applied. These are the main screw thread factors that contribute to a V-form thread, which has similar geometry and terminology for its mating nut – for a thread having single-start.