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Someone on the site suggested that I use "self-tapping screws", and gave me a link to the Wikipedia page about them.

So, I went there, saw the definition:

A self-tapping screw is a screw that can tap its own hole as it is driven into the material.

but I know this to be true of a screw in general... and the photos on that page were simply of what I would call "a screw", period:

enter image description here

So what is a NON-self-tapping screw then? How do I tell apart self-tapping from non-self-tapping screws?

Note: I'm not asking about "self-drilling" screws - the ones whose tips look like the end of a metal drill bit. Those are markedly different than what I know as "just" screws.

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    The images on that Wiki page are pretty bad, and not really representative. – JPhi1618 Feb 21 at 15:27
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    Try screwing an ordinary steel machined thread screw into hardened steel - it won’t cut its own thread... – Solar Mike Feb 21 at 16:17
  • self tapping screws drill at the front end rather than pushing it sideways. It makes them slightly easier to screw into very hard wood which is not elastic and which doesn't compress sideways that easily, so the drill jumps off the screw-head less. – aliential Feb 22 at 7:33
  • So, you edited your question such that the title asks one thing and the body asks the opposite. Are you asking what "non" means?? – Hot Licks Feb 22 at 13:19
  • @HotLicks: In this context - sort of, yes. But I re-edited the title. – einpoklum Feb 22 at 16:34
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To "tap" in this context means to cut threads into a hole. For this topic, we can think of three basic things a screw can do - drill its own hole, tap its own threads, or just force its way into the material tearing out a hole or threads in the process.

Using this image from that wikipedia article:

Screw types

The top screw is self-drilling. The sharp, split point acts as a drill bit to create a properly sized hole. At the top of that drill point, the threads taper up, and this is the self-tapping portion. All self-drilling screws are also considered to be self-tapping, and the common vernacular is to just call them "self tappers".

The next three screws in the picture have a blunt tip, so they are not capable of drilling. They require a pre-drilled hole, but they have a notch or flute at the tip, and this is what the screw can use to cut its own threads or "self-tap". These are pure "self tapping" screws, but you don't see them for sale at the big home stores. They are used more in industry to assemble goods on a factory level. You can buy them - they're just not as common.


The Wikipedia entry has it wrong

This picture from the Wiki page is just regular sheet metal screws and shouldn't be on a page describing self tapping screws:

Regular screws

These do not have a drill tip, and they do not have a flute to cut threads. Sure they are a little sharp, but not sharp enough to make a hole in sheet metal. They need a properly sized hole. If driven into a hole that is too small, they will tear the hole wider (if they are stronger than the sheet metal) and create a weaker connection than a self-tapper that properly cuts the right threads.

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    @jphi1618; I agree in principle with what you are trying to convey; however, for the record, "regular sheet metal screws" are often described as self-tapping by their manufacturers. Yes, they don't have a thread-cutting flute, but when used in sheet-metal with a proper pilot hole, they do cut a thread (not a machine thread or standard thread of any sort, but a thread)... so... they "self tap". – Jimmy Fix-it Feb 21 at 17:30
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    @JPhi1618 I'm pretty sure that those screws will self-tap wood, though, even if they won't self-tap metal. – nick012000 Feb 22 at 12:47
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    How exactly do self-drilling ones drill any holes if there is no way to evacuate chips? – n0rd Feb 22 at 19:30
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    @n0rd I have extensive experience with the bolt at the top of the first picture (and screws with similar end) and chips have never been a problem. The sheets used usually aren't that thick anyway, so the chips fall away at either end. There is no evacuation problem with those at all. – Mast Feb 24 at 8:58
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    @n0rd self-drilling screws tend to be used with metal thinner than the fluted section, so there is a way to get chips out. They're sometimes used to screw metal sheet to wood in one shot, so the wood has to compress a little – Chris H Feb 24 at 9:48
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Partly you're being confused between "wood screw", and "machine screw" aka bolt. Wood screws self-thread by nature. Bolts are not meant to self-thread at all (unless they are).

When you fit a screw, there are two functions going on.

Drilling the hole in the virgin material

This is not what you are asking about. Wood screws can self-drill, as can certain sheet-metal screws when they are going into thin material of known density. They will not self-drill into an engine block.

Carving the spiral grooves into the existing hole

This is what self-tapping means. All wood screws are this, of course. For metal bolts aka "machine screws", it means the very front threads are tapered, fluted to give a cutting edge, and hardened. The hardening happens at the expense of other desirable attributes like tensile strength.

enter image description here enter image description here

This is a proper tap, dedicated to the tapping task. Next to it is a machine screw that self-taps (but not self-drills).

Needless to say, the self-tapping screw is good for somewhere between 0.3 and 2.0 taps, before it loses its edge. It's not hardened nearly as much as the tap; because it is a compromise between tap cutting and bolt strength.

A non-tapping bolt (machine screw) is simply one that does not have this feature.

It has no chance of self-tapping into a hard surface like metal. But its tensile strength (performance as a bolt) is much better.

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    The "of course" part was not clear to me. Actually, I would consider making that part boldface. – einpoklum Feb 21 at 17:17
  • @einpoklum I changed it all... – Harper - Reinstate Monica Feb 21 at 17:28
  • Note that machine screws/bolts do self-thread in plastic very nicely with an appropriate size drilled hole. – R.. GitHub STOP HELPING ICE Feb 22 at 3:19
  • @R..GitHubSTOPHELPINGICE Well, they self-tap so nicely you can easily create an extra thread into the existing one, completely destroying the hole. – Nelson Feb 22 at 16:47
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Pretty much any machine screw --

machine screw

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    You're the only one who is actually answering the question, although you could have explained why your statement is correct. – KlaymenDK Feb 22 at 19:11
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All screws "self tap" into its material it is intended for. It is its nature. When a screw is specifically stated to be self tapping, it is usually used for metal since the material is to difficult to allow the screw to simply turn in. Of course, there are some screws that do not use the term self tapping that are for metal that will turn right into the metal.

enter image description here (image courtesy Wikipedia}

These screws, pictured above, are what I have always known to be called self tapping. They are not drill point screws, that are "self drilling tapping screws". At least what I have known them to be In the picture, it is the one at the top. The other three are variations of self tapping. They do not have any kind of point for starting a hole, all are blunt for starting in a predrilled hole and will cut/tap threads while being driven in. I see a lot of theses screws being used on commercial door hinges where the finish paint gets into the screw holes, and if it were a regular threaded screw would ind up while going in. I have done that in the past, before theses screws were packaged with the hinges.

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    Good point - those self tapping screws without a drill point to a great job cleaning out a tapped hole that has paint or corrosion in it. It's like chasing the threads and installing the screw in one step. – JPhi1618 Feb 21 at 15:59
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I believe you'll find that probably all self-drilling screws are also self-tapping, but not all self-tapping screws are self-drilling.

The 1st pic on the left of the wiki page is a good example of a self-tapping, but not self-drilling, screw.
enter image description here
What make a self-tapping screw recognizable over a non-self-sapping screw is that the self-tapper will have at least a slightly tapered end, and it will have a 'flute' at least at the end, but possibly some or all the way up the shaft across the threads. Note the blunt end on this screw which clearly makes it not self-drilling.

What differentiates a self-drilling screw from a self-tapping screw is that the self-drilling screw will have a sharp drill-bit-like end with no threads at all, much like the top screw in this pic (also from the wiki page):
enter image description here

A non-self-tapping screw would be something like a machine screw:
enter image description here
or a wood screw:
enter image description here
In either case, no matter whether the tip is sharp and pointy or flat, there is no flute up the side to enable the threads to cut through the material they're being screwed into.

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    A wood screw is definitely self-tapping... – Harper - Reinstate Monica Feb 21 at 16:26
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    @Harper-ReinstateMonica - well - in the same sense that a nail is 'self-drilling' maybe. It forces its way into the material and happens to leave a hole which conforms to is shape. It doesn't 'cut a thread' in the same sense that a proper thread tap does. – brhans Feb 21 at 17:25
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    @brhans The type of screw that most DIY retailers sell as a "self tapping screw" doesn't actually cut a thread either, that's rather the point of most of the argument on this page - they are typically used in sheet metal, which gets forced into the shape of a thread, but no material is actualy cut / no swarf is produced. – Mike Brockington Feb 24 at 12:49
  • @MikeBrockington I'd argue that this kind of labeling by retailers only contributes to the problem (as evidenced by the question asked by the OP and the multiple conflicting answers ...). IMO those sheet metal screws are in the same class as wood screws and are not really 'self-tapping'. If the only definition for self-tapping is that a screw forces its way into the material and leaves a threaded hole behind when removed - then under the right circumstances (pre-drilled hole in compatible material) any screw is 'self-tapping' ... which doesn't really help as a definition. – brhans Feb 24 at 12:58
  • I half agree with you, however the class of item commonly referred to as a "machine screw" WON'T cut a thread in any reasonable hard material, not least because they typically have a perfectly square end, so will not be stable enough if used that way. – Mike Brockington Feb 24 at 13:03
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What you are finding is the result of no authority regulates screw names, so what you get depends on supplier.

The only things I found consistent between the two major manufacturers Dottie and Cully that I find at wholesale supply houses is if you ask for sheet metal screws you get the pointy ones like the ones you said you used in your previous question. If you ask for "tek" screws you get the drill bit tip.

If you ask for self drilling or self tapping all bets are off.

Self tapping machine screws also come in a variety of styles.

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Self tapping screws which are usually of the "machine screw" (sheet metal) type, are called "tek" or "tek head" screws.

The easiest way to picture what they do is like this: If you're trying to screw something into metal, you would normally drill a pilot hole with a drill bit, then insert your screw. Tek heads have a built in drill bit at the tip of the screw thereby eliminating (in most cases) the need to drill a pilot hole.

Note: I do not recommend using tek heads when drilling into thick steel or very thin material. Use a drill bit instead.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Feb 24 at 2:17
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    I presume that 'tek' is a famous brand in the USA, but not worldwide, as I've never heard them called that. In the UK, a "self-tapper" is essentially the opposite of a "machine screw". The former is used in sheet metal, (or certain specific fixings) while the latter only with a nut or a pre-formed thread. – Mike Brockington Feb 24 at 12:53
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Self tapping screws have a small drillbit-like flute that cut into the threads at the tip like this: enter image description here

Regular screws do not have that notch at the tip

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    Technically a "self drilling" screw, which is kind of a subtype of a self tapping screw. – JPhi1618 Feb 21 at 15:28
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    No. You can see the notch in the first couple threads. This is intended to cut thread grooves into the material. Thus, self-tapping. – isherwood Feb 21 at 15:31
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    This is described on the Wikipedia page as being "self-drilling", hence being drillbit-like. Are you saying that self-tapping = self-drilling? Or that all screws are self-tapping, but not all are self-drilling? – einpoklum Feb 21 at 15:33
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    @einpoklum, it's also possible to have a self tapping screw that does not have a drill tip. They are made to go into a pre-drilled hole that doesn't have threads. The screw will then cut its own threads (tap). All self-drilling screws are also self tapping. – JPhi1618 Feb 21 at 15:41
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    Self-drilling is only one kind of self-tapper. Only half an answer. – Tim Feb 24 at 13:04
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Wow people are making this too hard. It simply means the screw forms the threaded hole. It does not matter if it done by self drilling and then cutting, cutting, or deformation. All are self tapping. Wood and plastic screws are self tapping by default. Most Sheet metal screws are as well. The term self tapping however is usually used on thread forms that are not inherently self tapping by default. Machine screws designed to fit a nut or threaded hole. Some of these screws use a cutting method, others deform the material to create the threaded hole (often these are triangular in cross section to reduce friction during forming.

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    Machine screws are most definitely not self tapping. – JRE Feb 24 at 17:11

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