Re: making the case for self-tapping screws


Earl T. Hackett <hacketet@...>
 

There is a wide variety of "self tapping" screws available for plastic
assembly. They fall into two classes, self tapping that cuts threads,
and thread forming that displace material as they enter the predrilled
hole. The common sheet metal screws are of the thread forming type.
The material displaced by the thread forming screws creates internal
stress in the plastic. Some plastics tolerate this well, others
develop stress cracks. It can be quite important to provide some
place for this displaced material to go.

Rather than me yammering about this, go to the McMaster Carr web site

http://www.mcmaster.com/

search on "screws," and check on the various "About ..." topics. Not
all of the designs are available in the small sizes we use, but many
are.

Be careful when replacing a self tapping screw in a hole that you
engage the original threads. Turn the screw backwards (counter
clockwise) until you feel the threads click into place then as you
turn it clockwise you should note that it enters the hole with almost
no resistance. If you don't follow this procedure, after a few
changes you may find that the hole has grown to be over sized and the
screw will not hold at all.

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