The Dale Maley Family Web Site

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Combination Lock Demo

Back several years ago, I built most the locks designed by Tim Detweiler.  A year or two ago, I saw this combination lock demo, and bought the plans from Matthias Wandel.  You can buy his lock plan from his web site  using this link.   He also made a YouTube video of his lock which you can see using this link.    I bought his plans, which include a Sketchup file, way back in 2009.  I decided to build it in November of 2020 because it would be a nice addition to my hand-cranked toy display.



Design Changes

The plans call for using several different thickness of plywood.  I don't keep any Baltic Birch plywood in my inventory.  I do have a ton of 3/16" Luan plywood in stock.


I decided to modify the plans and use 2 layers of my Luan glued together, or 3/8" thick plywood, for all the parts in the lock.  I copied the design assembly from the Sketchup file I bought, then modified it to use the 3/8" thick glued up Luan.


Getting Started on Assembly

Issue with 2 bottom curved pieces

I had to sand the curves deeper than the paper pattern I printed out from Sketchup.  By design, there is only 1/8" clearance, so stackup of tolerances can wipe out this clearance. No big deal to sand it deeper.  The design should have given maybe 1/4" clearance since it is not important.


Problem with Dial

I didn't like the design of using printed dial on photo paper.  Over the years, it will degrade.........and I design my toys to last for generations.  I think a paper dial covered by 1/8" thick plexiglass would last much longer.


The dial has to turn the 1st rotor, but not turn the fixed 3/4" shaft.


The plans use an 1/8" brass pin to link the thin disk to the hollow first rotor, and the knob is glued to the thin disc.  


I can't glue the knob to the thin disc because I have the plexiglass in between.


If I run the brass pin from the thin disc to the knob, then the knob has to have a big enough diameter for the pin to fit inside the knob.


If I use 2 screws from the back of the thin disc into the knob, the knob diameter also has to be be big enough.



Gave up on alternate dial design

There was not enough room to get 2 screws in from the back side of the thin dial, and hit the knob.  I gave up and went with original design, but with a longer knob. I glued the knob to the thin disk like the plans recommend.


Achieving a Targeted Combination of 3 numbers

The instructions just say to put in the slots for the tabs wherever  you want on the 3 wheels, then whatever combination you get, you get.


I wanted to target a specific combination like 36-24-36.


There are several ways you can design this lock.  The first method I did was:


-spin left several times, enough to get all 3 rotors linked together

-spin right until first number achieved

-spin left until 2nd number achieved

-spin right to final number

-unlock


My memory finally got jogged from 46 years ago in high school.........from my locker combination lock.  Another way to do it, which I switched to on this lock project:


-spin left several times until you achieve the first number

-spin left, probably past zero to 2nd numer

-spin right to 3rd number

-unlock


To actually do the 2nd method:


-spin left until front rotor slot is correct, this sets the 3rd number. Mark this on dial in pencil.

-spin all tumbers left several times, then turn right until 2nd rotor slot is correct. Mark where tabs go on front and middle wheel (front side)

-work on back rotor and its 2 tabs.


Rounded corner of plunger like a door lock

Preventing middle and back rotor from spinning after in proper location

I have done similar designs where the disc spins when you don't want it to......


1. Turn left several times until 1st number is achieved

2. Turn right until 2nd number achieved [while you are doing this, the back disc moves out of proper location]

3. Turn left until 3rd number achieved [while you are doing this, either or both the back and middle discs move out of proper location]


I reduced the thickness of the 2 spacer washers by sanding until I had no extra movement.



Final Assembly

As I do with all the moving parts in toys I build, I coat all rotating parts with Johnson paste wax.


Finished Project

YouTube Video of Lock in Action

You can use this link to see this video in action.


Closing Thoughts on this Project

This project was easy except trying to get the rotor tabs set correctly to achieve a specific 3 digit combination........in my case.............36-24-36.  I also had the opening sequence wrong initially in that I went left, right, left, right...........and most of these style locks work left several times to 1st  number, right twice to 2nd number, and left one time to final number.


I tried to model this in Excel, but it turns out to be much more difficult than I thought.


The sequence I used was:


-1st digit is set when you locate the brass pin which connects the front rotor to the knob.

-Now you can set tab on front of middle rotor to achieve 2nd number when turning right.

-Then set tabs to get the back number when turning left

This demo lock will be a nice addition to my display of wood locks and hand-cranked toys :)