[A version of this article appeared in Make: magazine volume 3, August 2005]
When a friend asked for my help removing some non-standard screws from his apartment doorframe without damaging them, I expected little resistance. Many manufacturers use so-called tamperproof or security screw heads to prevent casual would-be hardware hackers; tamperproof torx, spanner bits and tri-wing being some of the more popular types. This security-by-obscurity approach can usually be foiled with a security bit set available in most hardware and electronics stores though, and I assured him that we’d have that panel off in no time.
The screws in question, however, were not of the standard varieties. Rather than having a bit pattern cut into the centre of the screw head, this was basically a round head with three notches removed from the edges to form an equilateral triangle. Some research online revealed that the screw was a Tri-groove design (for an excellent reference on standard security screw types, see: http://resources.tannerbolt.com/articles/what-type-of-screw-is-this/). What’s more, individual driver bits for this head type can cost upwards of $10 a piece.
When our attempts to use pliers and brute force failed, I decided to manufacture a bit myself. I would need to get a cast of the screw head for reference, find a suitable source material for the driver, and then use a Dremel to handle the metalwork.
There are better moulding compounds than Silly Putty but few are as cheap and its weaknesses, particularly its tendency to “flow” and lose definition, shouldn’t matter for the short span required. I worked the putty to soften it, then pressed it onto the panel. One advantage of such a soft casting material is that it easily fills countersunk holes. Once removed, I covered the reverse-mould of the screw head with spackle and let it dry. It gets fragile once dried, so handle it carefully; it should be mostly an eyeball reference anyhow.
I took the cast back to my shop. The decision to use Allen keys was automatic – they are an excellent self-contained tool, they are cheap, and made of solid, uniform metal. I selected an appropriate gauge and went to work. Since the Tri-groove screw head is basically just three notches, I would have to remove metal from the head of the hex key until only three “posts” remained, to match the notches of the head. The fact that the keys are hexagonal in shape helped here with the equilateral spacing of the posts.
Using the Dremel with a cut-off wheel, I began the process of removing material from the hex head to create my bit. It is essential whenever doing metal work like this to have a glass of water handy, and to quench (immerse) the key every few seconds. If the metal starts heating it gets harder to hold, of course, but the greater problem is that you can overheat a section and temper the metal. This will make it very hard but also brittle which is not a desirable feature in a driver bit. Make a cut, squelch, make another cut, squelch. Of course, eye protection is essential.
Throughout the process I referred to the cast I had made to ensure that the posts were positioned and shaped correctly. Several times I thought I was finished only to find that when I tried to match the key to the spackle cast, the posts would align correctly, but were too fat: I was leaving too much material from the centre of the key on the posts, so they would have impacted the screw head instead of sliding into the notches. Eventually though, the driver matched the cast and after some sanding with medium grit sanding cloth to remove any burrs (a Dremel grinding wheel works here too, or skip this step altogether) I had a functional, if unbeautiful, Tri-groove driver. Because of the extensive comparisons to the poly version, I was only mildly surprised that the driver worked with the real screws on the first attempt. With an appropriate selection of keys and a bit of practice with the Dremel, most security bits should be equally hackable.