94

I am now in Poland and see these everywhere:

enter image description here

The plate can rotate freely,when you insert the key, matching the groove, you rotate the key so it is aligned with the lock and then insert the key.

What is the purpose of this?

  • 2
    Probably the grandfather of a wafer-tumbler mechanism without a waffer – tungsten Sep 5 at 14:09
  • 4
    Lock-related questions are allowed. Locks and lock picking are explicitly on-topic according to Meta. – forest Sep 7 at 11:01
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    It's also on topic according to What topics can I ask about here? which lists as on topic "physically securing the office, datacentre, information assets etc.". Security properties of physical locks (which would include the purpose of a part of a lock) certainly seems to me to fit within that bullet point. – a CVn Sep 7 at 11:09
114

It's called anti-drill plate.

With the anti-drill rotating plate, the drill bit won't be able to go through the cylinder. It's supposed to be in hardened steel and act as an other layer of security.

When you drill a cylinder lock, you have to go with small drill bit first. You will then use bigger and bigger drill bit on the little hole you made. The plate will prevent you from doing so because the bigger drill bits will just make the plate rotate.

Here is how drilling attack works

Examples:

  • 10
    I don't understand why the drill bit won't be able to go through the cylinder. If they key can go through the slot, then why not a drill bit? Looking at your first link, it seems the OP's plate would not prevent following those instructions. – Jon Bentley Sep 5 at 19:26
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    @JonBentley I think the idea is that once the drill bit gains purchase in the plate, the plate just spins with the bit, preventing the bit from drilling any deeper. – Carl Kevinson Sep 5 at 19:46
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    @CarlKevinson I am sure I must be missing something obvious, but I still don't understand. Why do you need the drill bit to gain purchase in the plate at all? You just bypass the plate entirely through the slot (the same slot the key goes through). – Jon Bentley Sep 5 at 20:27
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    @JonBentley - What I'm seeing is that if the bit were large enough to drill out the whole cylinder in one go, it'll end up gaining purchase in the plate just due to size. Of course smaller bits will go through with no trouble at all, but will drilling out such a small diameter in the lock gain the attacker anything (and to that I genuinely don't know; I've never drilled out a lock cylinder before)? – Broots Waymb Sep 5 at 20:51
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    @Deunis but several videos on the process do not show using bigger bits. That's what is causing confusion for many (including myself). – schroeder Sep 6 at 8:24
38

What Deunis said, except it is not only an anti-drill plate. The fact that it rotates freely makes it anti-drill on top.

Originally, these were an upgrade for existing lock cylinders which didn't have built-in protection against being pulled out, which became a popular method (the most popular?) of breaking in the 80s. Locks at that time didn't have the rotating bolt sufficently reinforced, so you would basically just shear it off with brute force. Fast, easy, not much noise, and upon casual inspection people who are passing by do not even notice, once you are inside and have closed the lockless door again (really, who looks at the keyhole!).

Modern quality locks (albeing not the cheap ones!) have the bolt made of massive, high quality steel to prevent that, or have other measures such as a thick end on one side, so they can only be inserted and taken out from one side (well, without smashing the whole door to smithereens). For example, Keso locks with an internal knob have the knob explicitly manufactured in such a way. You would have to punch a fist-sized hole into the door to open it that way.

I don't know the English term, in German they're called "Ziehschutzrosette" (translates approx. to "pull-protection collar", though I'll admit that sounds quite clumsy).

Which makes drilling the lock the second most lucrative option.

It is true that you can still insert a small drill in the slit, but that is pretty useless. Though you'll likely want to start small because it's really no fun drilling in steel with a large caliber, you will need to use increasingly larger drills up to about 10mm to do the job. You can't do that, at least not easily, because the two plates which are made from hardened steel will prevent the drill from entering. And, as they are rotating, they'll turn away as the drill tries to drill through them. So that's drill protection as an extra feature.

Ironically, modern doors are (at least in Europe) optimized for energy-saving craze, not for keeping people out. The commonly available "burglar-safe" doors are built to withstand an "unexperienced occasional" burglar armed with a screwdriver for at least 3 minutes.

  • Attacking a lock with a slide-hammer (attached by a screw threaded into the keyway) seems to be the most common way to do this. – jberryman Sep 21 at 0:25
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    @jberryman: That'll work, but it will make an awful lot of noise. The classic gear-puller (or the lockbreaking-optimized form, which is basically the same) has the immense advantage of making no noise, except for one click/snap sound when the bolt breaks. Other than that it's just a screw being turned, which is very quiet. – Damon Sep 21 at 16:58
-6

You don't show whether this is inside or outside. In the UK, locks commonly had disks over the keyhole, suspended from a hole at the top. The disk could be lifted up to unlock the door, and would then drop back with gravity. This gave some protection against draughts through the keyhole for older locks, and also (for old and new locks) protected the lock from environmental damage.

If this is an exterior door in a snowy country, it could well be to protect the lock from getting iced up.

  • 2
    It’s inside locks as well; it’s freely rotating in front of the lock and almost every modern building has them (I’m in Wroclaw) – Thomas Sep 5 at 22:42
  • This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – Vipul Nair Sep 8 at 18:08
  • @VipulNair Regardless of whether it's right or not - and I'm happy to be told I'm wrong - it certainly does answer the question directly. Down votes are fine and valid, but please do not misuse the review feature for answers which do attempt to answer the question but are simply incorrect. – Graham Sep 8 at 23:04

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