On Sat, 04 Sep 2004 21:32:05 +0100, Steve Jones
wrote:

What is the purpose of a self tapper screw, and when would one use such
a screw?

Machine screws go into ready-threaded holes, cut with a separate tool.

Traditional woodscrews are quite crude things that cut their own way
in, usually with a thread that tapers in diameter considerably.
Because they mangle the wood so much, they usually have a plain
parallel section to the top of the shank. This locates in place like a
nail. If a long parallel woodscrew was threaded all the way long, it
would have drilled itself a clearance hole by the time you got to the
top of it.

Self tappers appeared in the '50s (they're tricky to make - took quite
a bit of steel metallurgy to make something hard enough, yet cheap).
First intended for thin, soft sheetmetal like aluminium, they're quick
and cheap to install. They also cut a reasonably neat thread, so you
can take them afterwards and replace them, just like a machine screw.
Cross-thread them in the hole though and you'll strip the threads out.

Some self-tappers are even self-drilling - they have a sharp point.
Others like the "Taptite" series are less aggressive and really are
thread-forming machine screws. They're less prone to wearing their
threaded holes out if re-used, but they also need accurately drilled
holes and soft metal.

"Modern" woodscrews, like the Screwfix Goldscrews, are somewhere
between the two. They look like coarse self-tappers. In use they cut a
thread into wood that's coarse pitch, yet regular enough to work all
along its length.

In the modern world of high speed bodging, just use Goldscrews for
everything.


Can they be used in place of wood screws?

More or less - depending on the size and thread pitch. For soft
materials (softwoods, plastic wallplugs), go with a coarse pitch and
think about using traditional woodscrews. For modern materials (MDF,
UPVC, sof aluminium) go with the modern parallel woodscrew.

Use a self tapper in wood and it will just drill a hole. Too fine a
pitch.

Use a soft screw in hard steel and the material will re-shape the
screw, not the other way round. This is sometimes a problem with
stainless self tappers, as they're a bit soft.


What about with wall plugs?

You can set wall plugs perfectly well into a wall just by using a
nail. All they need is a wedge in the middle. To hold into the plug
itself, your screw needs a moderately coarse pitch.

There are many sorts of wallplug. If your holes are drilled parallel,
use a parallel plug and a parallel screw (modern woodscrew). The
older plugs sometimes had thin walls and solid noses, so as to be more
compatible with the old tapered screws. Avoid these today, as they're
harder to drive in and fussier about getting the depth right.


For example, is an A2 stainless steel self tapper suitable for attaching
downpipe clips to an external brick wall? (With suitable wall plug of
course).

So long as it has a reasonably coarse pitch (Screwfix's do), it's
perfect.
--
Smert' spamionam