Barber and Colman Ltd are the manufacturers of 'Shakeproof' thread-cutting screws and washers.
'Shakeproof' thread-cutting screws made from carbon steel are subjected to a special heat-treatment which provides a highly carburized surface with a toughened resilient core. The additional strength provided enables higher tightening torques to be used, and will often permit the use of a smaller-size thread-cutting screw than would normally be specified for a machine screw. Thread-cutting screws actually cut their own mating thread; in any thickness of material a perfect thread-fit results in greatly increased holding power, extra vibration-resistance, and a faster assembly. The hard, keen cutting edge produces a clean-cut thread, from which the screw can be removed, if desired, without damage to screw or the cut thread. The most suitable drill sizes for use with these screws are generally larger than standard tapping-drill sizes, but this apparent loss of thread engagement is more than offset by the perfect thread-fit obtained.
Both the screws shown in Fig. 16.33 are interchangeable with standard machine screws. Type 1 is recommended for use in steel and non-ferrous sheet and plate, and they are manufactured with a wide shank slot and are eminently suitable for paint-clearing
Fig. 16.34 Type 25 thread-cutting screw applications, as they completely eliminate the need for expensive pre-production tapping of painted assemblies. Type 23 screws incorporate a special wide cutting slot with an acute cutting angle for fast, easy thread-cutting action and ample swarf clearance. These screws are specially designed for application into soft metals or plastics where a standard thread form is required.
The Type 25 thread-cutting screw has a specially spaced thread form which is designed for fast efficient fastening into plastics and sheet-metal applications.
Figure 16.35 illustrates a 'Teks' self-drilling screw which, with a true drilling action, embodies three basic operations in one device. It (1) prepares its own hole, (2) either cuts or forms a mating thread, and (3) makes a complete fastening in a single operation. These screws consist of an actual drill point to which a threaded screw-fastener has been added. Several different head styles are available. During the drilling stage, Teks must be supported rigidly from the head. Some bench-mounted, automatically fed screwdrivers provide a holding means which retracts as the screw is finally driven home. Other drivers connect with the fastener only through the bit or socket. A good-fitting Phillips or Pozidriv bit will normally drive several thousand of these screws, and a hex socket, for hex-head designs, will drive even more.
For long screws or applications requiring absolutely guaranteed driving stability, a special chuck is available which holds the screw with three fingers and retracts upon contacting the work surface. These screws are suitable for fastening sheet steel of 16 gauge, or thicker, within 5 seconds maximum while using a power tool.
Figure 16.36 shows alternative head styles available for thread-cutting screws.
The examples and dimensions of nuts, bolts, screws
(a) Slotted round t|T
(b) Slotted pan
(e) Pozidriv flange
(f) Posidriv pan
(c) Slotted cheese
(d) Slotted fillister
(e) Pozidriv flange
(f) Posidriv pan
and washers given here are intended especially to be of use to students engaged on design projects. There are however literally hundreds of industrial fastening systems available, associated with automobile, construction, electronics and aerospace developments. Manufacturers' catalogues are freely available to provide technical specifications and necessary details for designers. One further advantage of CAD systems is that such information can be used to build a library of useful data and drawings, which are invaluable, where contract drawings use a repetition of similar parts.
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