I have a simple enough question but I cannot find the answer on Google, hope someone can help:
What is the purpose of a self tapper screw, and when would one use such a screw? Can they be used in place of wood screws? What about with wall plugs?
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).
Feeling very confused !
Steve
"Self tappers" do what the name suggests really. They actually cut their own thread.
They are usually used in sheet metal, but at a 'pinch' can be used in other materials ;-)
Self tappers have threads which are designed to cut their own thread in suitable material, usually sheet metal. Normally they have a thread along the full length of the shaft rather than having an unthreaded portion to act as a dowel as with wood screws, however other than this I can think of no reason not to use them with plastic wall plugs.
Yes, would do the job just fine. Some might consider stainless fasteners overkill but they will probably still be there holding strong when the plastic drainpipe has disintegrated in sunlight.
Self tappers can be used in most situations where you might otherwise use a wood screw. As the name suggests, they will cut a thread much better than an ordinary screw even without a pilot hole. As a result thay can be screwed directly into materials that would be difficult for a conventional screw to get a purchase in (chipboard etc).
The other notable difference is that most self tappers tend to be glavanised and hence rust resistent which is more the exception that the rule for ordinary wood screws.
Armed with a decent posidrive head they are ideal for power screwdriving, since they will locate accurately on the screwdriver bit and stay put as you drive them home.
I would say that 90% of the screws I buy are technically a self tapper type design.
yes and yes.
yes. There is one other difference not yet mentioned, and that is that the self tap has a much steeper taper on its pointy end. This is a small advantage when screwing into very shallow materials like chip, since a little more thread will grip the chip. The downside is its harder to drive in, which is an issue with wood at times.
Also theyre normally single thread rather than twin, so longer time to drive it in too.
Regards, NT
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.
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.
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.
So long as it has a reasonably coarse pitch (Screwfix's do), it's perfect.
The exception is where you want to screw planks or other bits of wood securely to something. With the plain shank, it will pull through lots easier, to pull the two bits flush. With a screw threaded all the way to the top, you have to apply enough torque to break the thread formed in the wood, before the screw will continue to turn and bring the two bits of wood flush.
..........or by pre-drilling a clearance hole in the top plank. Much easier and more precise, imho.
I read in teh book /understanding wood' of some tests done with wiodscrews.
They actually held better when nailed in and tightened the last few turns.
Good analysis of nail to wood shear failure in there IIRC.
Yes, I needed to get my "Birmingham" screwdriver out a few times when assembling my conservatory.
Christian.
That too. But if you happen to have screws with the right size of plain shank, then it means one less tool needed, which can be handy.
Hi,
This might not be good idea for outdoor stuff like window frames though.
cheers, Pete.
tsk tsk, thats an Irish screwdriver you're talking about
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