A Work-saving Device with Countless Uses
Page 63
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TAPERED, slotted and hardened, I like a tap, Shakeproof screws cut their own threads and can be driven into metal after merely forming a plain hole. Moreover, when screwed home, they do not work loose.
A product of Barber and Colman, Ltd., Marsland Road, Brooklands, Manchester, Shakeproof screws are made in eight diameters, ranging from the equivalent of 6 B.A. to in., and are available with eight different types of head—round, hexagon, cheese, countersunk, etc. They are cadmiumplated, a finish which affords protection and serves as a lubricant.
The key to the successful functioning of the device lies in the formation in the side of the screw, extending from its end for a distance of about six threads, of a single narrow slot, which
is just offset from a radial plane. For f-in. screw this slot is about 1-32 in. wide and, in depth, ranges from nothing at the seventh thread to about half the diameter of the screw at the end.
The offset is on the leading side of the radial plane when the screw is turned in a clockwise direction. Thus the metal on the leading side of the slot is thinner than that on the other, and yields under the resistance offered to the rotation of the screw by The wall of the hole. As a result, the serrated edge on the stiffer side is exposed and is thus enabled to cut threads in the hole.
An additional function of this springiness is to secure the screw in its hole and to prevent it from unscrew Mg through vibration. At the same
time, it neither interferes with the extraction of the Screw by normal means nor leads to its suffering any damage or permanent deformation. The swan f formed during thread cutting accumulates in the slot.
We have tested the tapping abilities of Shakeproof screws and found that they can be inserted into a plain hole of the appointed diameter in steel without difficulty, cutting a clean thread and fitting snugly, provided they are not slacked back far enough to release the stressed portion. It is of note that the thread is really cut, not pressed or swaged, and is of standard pitch, depth and angle.
In our opinion the product is a practical and useful addition to the equipment available to the automobile engineer.