Parametric Printable Combination Lock

by profezzorn, published

Parametric Printable Combination Lock by profezzorn Dec 23, 2012

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A simple combination lock. The scad file is parametric and you can select the combination and how many numbers are required. Some ideas borrowed from http://www.thingiverse.com/thing:2047, but designed from scratch to make it easy to print.


Print all the pieces. (NOT exploded_view.stl) Print 3 of "disc_holder.stl" (one less than numbers in the combination).
You will also need 1 M4x50 (or longer), 2 x M4 nut and one M3-15 (or longer).
The tolerances in the printed pices are low, so unless your printer is really good, you'll probably need to spend a few minutes to clean the pieces up and make sure they move smoothly. Also, make sure to drill out the hole in the last disc.

First put the bolt in place and screw the M3 screw into the bolt through the slot in the door. Put each disc on it's disc holder. Put the M4 screw through the knob, then the door, and then the disc holders. The peg on each disc holder should fit snugly into the slots in the previous one. Finally put the last disc on (the one with the nut trap), put the M4 nuts on and tighten them in such a way that the knob spins easily, but the whole disc pack is kept together.

If you want to change the combination, start with the scad, and change the combination at the top. Then set part_number=1 and export the stl, then set_part_number=2, export the stl, etc. etc. until there are no more parts. You will also need the Write scad module (http://www.thingiverse.com/thing:16193)

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I don't think determining the combination is as straight-forward as it seems. The actual combination depends on the alignment of the dial with the final (ie rearmost disc) and this can be in any of six positions depending on the hexagonal head of the bolt, so don't trust the combination in the scad file to be correct - work it out from the dial as you align the disks.

To start with push the bolt into the locked positiobn (ie as if the door were locked, not locked by the plates). Then turn the dial several times to scramble the disks.

To unlock it, turn the dial clockwise at least five times to be sure you've picked up all the disks- you will probably feel them make the turning heavier one by one as you collect them. Stop on your first number (ie when the notch on the disk closest to the door lines up with the bolt). If you are doing this for the first time, make a note of the number on the dial next to the little cutout triangle.

Then turn the dial anticlockwise until you feel the third disk pick up. Keep turning it anticlockwise until the notch in the third disk lines up with the bolt. Note the number on the dial.

Then turn it clockwise until you feel the second disk pick up - keep turning it clockwise until the notch on the second disk lines up with the bolt - note the number on the dial.

Then turn it anticlockwise again until the notch on the final disk lines up with the bolt. Note the number.

Rather than feel the disks get picked up, you can just count the revolutions. So clockwise until your first number passes the mark, stopping on the fourth time . Then anticlockwise until your second number goes past the mark stopping on the third time, then clockwise again until the third number passes the mark stopping on the second time, and finally anticlockwise stopping on the first time your final number lines up.

It all sounds a bit complicated, but then it is supposed to be a lock. It wouldn't be much use if it was too easy.

It also demonstrates why when you see someone in the movies with a stethoscope listening to the plates lining up, it is a bit of a fib as you have no way of knowing if they are lining-up until the last one does, because until then nothing can move.

I have printed this all out and assembled it correctly. All the discs turn freely on their holders and I have put the bolt through with a locknut on the end so that it does not undo.
I cannot get the combination to work unless i watch it from the other side and turn the knob forwards and sometimes backwards.
is there something I am missing or is there a particular sequence required?

As with all combinations locks, you have to alternate left / right turns. Each turn should be one less revolution than the previous one. Make sure that you align the knob so that the last number of the combination is aligned while the lock is unlock-able.

I must admit that I forgot to check that the combination comes out right, so if there is a mistake in my math, I apologize for that.