This section of our website is to allow users of our products and other wise engineers to specify and detail the external threads when they desire to use a Whittet-Higgins retaining device to secure a bearing, power transmission, machine or other equipment assembly. This is done because many of the threads are different from those detailed in Machinist's and other craft handbooks, and because here all necessary data is located for the person seeking to specify our products. By using this data and Whittet-Higgins products, every engineer can feel more secure they are making the best possible assembly.
Although screw threads date back to the third century B.C. and are best archeologically evidenced by such tools as the screw in wine presses or the clamps used to assemble constructs, abundant use and popularity did not emanate until Henry Maudslay invented the screw cutting lathe in 1797. This machine caused a plethora of threads to be manufactured for a plethora of needs. In the early nineteenth century chaos reigned. The need for thread standards became acute. Different countries established differing standards for the needs as their engineers perceived them. With the Great War in 1914 the United States became pressured to create a uniform standard in order that war ordnance could be consistently and efficiently manufactured. This brought forth the American National thread form.
With the onset of the second World War this American National form presented interchangeability problems with the Whitworth form of Great Britain, a thread form which had reached many parts of the world where English economic influence prevailed. After the war Canada, Great Britain and the United States agreed upon a Unified thread form that would allow interchangeability on a more universal basis. The Unified thread form is essentially the same as the American National except that it has a rounded root and either a rounded or flat crest. Countries using metric measurement also unified as to a standard under the International Standards Organization coming to a comprehensive system agreement in 1969.
Review of the nomenclature of threads helps make understanding their form and function. Major Diameter is the largest diameter of a thread whether it be external or internal; it is the diameter used to designate a thread in abbreviated form. Minor Diameter is the smallest diameter. Pitch Diameter is the imaginary diameter where the width of the grooves equals the width of the material mass. Pitch is the distance, measured parallel to the axis of the threads, between corresponding points on adjacent threads in the same axial plane; it is the theoretical measurement in contrast to the lead which is the axial distance moved in relation to the amount of angular rotation. It is important to distinguish between pitch and lead for uniformity of pitch does not assure uniformity of lead and misleads are a common assembly problem. A thread under the American, Unified, and ISO metric form have an included thread angle of 60 degrees from the flanks, or sides of the threads. The curvature of the forward motion of a thread known as the helix angle is a trigonometric resultant of the number of threads per unit and the diameter. A right-handed thread requires right-hand or clockwise rotation for tightening while a left-handed thread requires left hand or counterclockwise rotation.
Groupings of these particular characteristics result in particular performance characteristics. For instance, the more threads the finer the adjustability for the lead is smaller; the tighter the tolerances the greater the precision of alignment for the thread fit allows less movement between the external and internal threads. For bearing and similar precision applications finer threads have been chosen and a Class 3 inch and a 4H,5h6h metric tolerance level have been chosen. Whittet-Higgins Company uses five groupings of threads, one of long historic tradition and four it has chosen within the standards that have been established. They are:
Today more threads are turned with a single pointed tool as new cutting materials and advanced computer controlled equipment have made easier and less expensive the creating of threads through turning. Threading by turning is well coordinated turning with a particular preformed tool. The computer has made the coordination very easy allowing for the replacement of supplementary machine equipment such as lead screws and cams. The form tool with its' relatively small and fragile 60 degree point angle is very susceptible to the tremendous forces and stresses of metalcutting. To reduce these stresses a machinist wisely takes multiple passes, each time making the groove deeper until it reaches the designated depth. It is prudent to reduce the depth of cut with each succeeding pass for with each succeeding pass a greater amount of material is being removed from the developing thread flanks. Finally one or more dwell passes burnish the tears prevalent to turned metal. These steps to manufacture a very good thread are very important for applications in which Whittet-Higgins industrial retaining devices are use to increase the quality of performance both alignment and holding.
External threads are also manufactured by thread rolling, thread milling and thread grinding. There is no doubt a ground thread is the best possible thread, however, it is the most expensive. It allows the performance of prevailing torque retaining devices to function best. Thread milling significantly reduces time to manufacture long threads such as for leads screws, linear bearing screws and multiple start threads. Thread rolling is extraordinarily economic in very large volume production. But foremost is the importance of manufacturing very high quality external threads to assure the precision of the assembly is such to provide long and efficient use of the equipment.
The details of the external features of threads here provided are all that are needed to manufacture such threads. Please invest in ring gages if quantity is significant. Please invest in pins, micrometer or other measuring tool if quantity is small and measure each piece made. Do not sample small quantities. Please do not make an external thread specifically fitted to a component with an internal thread for if the internally threaded component needs to be replaced the replacement may not fit.
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