Many of us have seen the screw locks before. Typically very cheap quality. But how do they work, and how do you pick one?
First, for those who are not familiar with the locks we are speaking of, they look like this.
How do they work? Well, here is some of my famous artwork depicting the inside of the lock and key. No, am not an artist by any stretch of the imagination, but I can get the idea across.
There are not many parts. The lock body, the shackle, the plunger/screw assembly and a spring. The key is a stepped tube with a threaded interior. No pics of the keys, because I got them without keys.
To open the lock, you put the key into the keyway until it stops, then start turning it. The screw inside the lock then gets picked up by the threads of the key. The key will wind down until the stepped part of the key contacts the face of the keyway. At that time, when you keep turning the key, the screw is drawn up into the key, pulling the plunger up, compressing the spring. Once the plunger is pulled back enough to clear the notch in the shackle, the shackle can be opened.
This is the edge of the plunger that functions as a locking dog that holds the shackle locked until the screw is pulled towards the keyway by the key.
OK, how to pick the lock. After all, that is what we do here, right?
First, estimate the diameter and thread pitch of the screw inside the lock. A flashlight can help.
This one should be close enough. Note: You do not need to use a screw to pick this lock. The screw is so I can demonstrate partly what is happening inside the lock.
Next, get some tubing that is about the same diameter in the inside as the outside of the screw.
Crimp the tube about 1/4" to 3/8" (about 6-8 mm) up from the end. This is so the end is wide enough for the screw of the lock to fit, but that it narrows enough for the threads of the screw to catch and hold.
Now you can screw the tube onto the threads of the screw inside the lock. The screw used in the picture can help a little if you are able to correctly estimate the thread pitch, since it will start threads in the tube. Imagine this is inside the lock, the tube threads onto the screw of the lock perhaps half way the length of the screw. DO NOT thread the tube down so far that it bottoms out on the screw.
Put the end of the tube in a vise, and thread the lock onto the tube.
When the tube is tight, then use a tool to pry the lock up away from the vise.
As you lift the lock away from the vise, the plunger will pull away from the shackle. (The lock is unlocked for these pics so you can see).
Once the plunger is pulled away from the shackle a short distance, the shackle toe can be removed from the lock body.
Hope this helps explain how these locks function, and how to pick them. Making a key would be as simple as cutting threads of the correct pitch in a tube, and add a washer partway up the tube that would allow the key to fit maybe 2/3 of the length of the lock body. Solder or epoxy the washer in place, and add a key bow so you have a little leverage to turn the tube. (It does not take much effort if there are decent threads in the tube). Will make a key someday, perhaps soon, and post the pics as an addition to this thread.