Fully 3D Printed Combination Lock With Changeable Combination




  • All 3D printed, fully functional, changeable combination, Pad Lock.
  • Combination can be changed to nearly any 3 digit 0 thru 9 combination.
  • Fun and educational for kids (and adults), but I wouldn't trust my life savings with it.

In this Instructable you will find printing notes, assembly steps, instruction on how to pick and set a combination number, how it works and some of the challenges I encountered in designing this.

The Combination Lock zip file has all the STL files and also HTML folder that contains all the assembly steps, operating instructions, printing notes and a handy tool for setting the combination numbers. In the HTML folder just click on "Combination_Lock.html", this should open in any browser that supports HTML5. I have tested on Windows with Chrome, FireFox and IE.

Step 1: How to Operate

This operates like most combination style Pad Locks.

  1. Turn knob at least 2 full turns clockwise, continue turning clockwise until you reach the first number of the combination.
  2. Turn knob counter clockwise one full turn passing your first number again, continue turning counter clockwise until you reach the second number of your combination.
  3. Turn knob clockwise until you reach your third number of your combination. Do not go past it. The shackle should now be able to be opened.
  4. To lock it simply insert shackle back into the lock. Then turn the knob (either direction) at least 2 full turns. You can also turn the knob before reinserting shackle .

Step 2: Printing

  • Printer: Rostock Max v2
  • Nozzle: 0.5mm
  • Filament: PLA (avg diameter 1.73mm)
  • Slicer Software: Slic3r 1.2.9
  • Bed Temp: 40° c
  • Nozzle Temp: 200° c
  • First Layer Height: 0.2mm
  • Layer Height: 0.15mm
  • Perimeter Width: 0.6mm
  • Ext Perimeter Width: 0.6mm
  • Solid Fill Width: 0.5mm
  • No Supports required
  • Notes
    • Print both Tumbler Pegs at the same time, this prevents the pegs from gobbing.
    • Make sure tumblers spin freely on spacers.
    • Make sure Latch moves freely on base.

Step 3: Assembly Steps 1 and 2

Step 1: Insert Shackle into Base as shown. Then install Clip onto Shackle, this keeps the shackle from coming out all the way when opening the lock.

Step 2: Insert Tumbler 3 into Base tumbler shaft with groove facing up. Then insert one of the Spacers with flange up and aligning flat of spacers inside diameter with base tumbler shaft flat side. Slide spacer all the onto tumbler 3. The purpose of the spacer is to allow tumblers to turn without causing the other tumblers to turn with it.

Step 4: Assembly Steps 3 and 4

Step 3: Insert one of the Tumbler Pegs into Tumbler 2. There are 10 different positions, position 0 is considered to be the peg directly under the notch of the tumbler and is counted up in a clockwise direction. In this step the position effects only the 1st number of the combination. Later in this Instructable will show how to know which position to insert the tumbler peg depending on the combination numbers desired. Please note that position 0 can not be used, the lock will not function properly. The picture 'Assembly Step 3' shows the peg inserted in position 4.

Step 4: Insert Tumbler 2 onto the Bases tumbler shaft with the groove up, then insert the other Spacer onto the bases tumbler shaft just like Assembly Step 2. Make sure Tumbler 2 spins freely on the Spacer.

At this point rotate both Tumbler 3 and Tumbler 2 so that the notches are pointed directly to the left side of the base. This makes it easier to perform the next step of installing the Latch. Though not shown in the pictures, it is also better to pull the short end of the Shackle out of the Base.

Step 5: Assembly Step 5

Step 5: Insert Latch onto Bases latch shaft as shown. The Latch right arm should fit into the notches of the tumblers and make sure the Latch moves freely on the latch shaft. The shackle should slide in and out freely. Now turn tumbler 2 so that the latch right arm is not in the tumblers notch. Now the shackle should be locked in position.

Warning:Do Notpull or push the Latch or the shackle hard, without the top installed the latch shaft is weak and may snap off.

Step 6: Assembly Steps 6 and 7

Step 6: Insert the other Tumbler Peg into Tumbler 1 following the same procedure as step 3. The position of this peg effects combination numbers 1 and 2. Again see the step on setting the combination later in this Instructable for the correct position depending on your selected combination number.

Step 7:Insert Tumbler into Knob, again there are 10 different positions the tumbler can be inserted into the knob. The position is determined by which number on the Knob the notch of the Tumbler is under. In the picture it is difficult to see but the Knob is in position 3. This position effects all three numbers of the combination. It is OK if the Tumbler fits tightly into the Knob.

Step 7: Assembly Step 8

Step 8: Insert Knob with tumbler 1 assembly onto the base tumbler shaft with the notch of the tumbler pointed toward the Latch right arm, as shown. Make sure the knob turns freely on the base tumbler shaft.

This is a good time to test you combination numbers to make sure it works. When the last number of the combination is reached the Latch left arm should move away from the shackle.

Trouble Shooting Operation

If your combination doesn't work go through these steps.

1) Turn Knob at least 2 full turns clockwise then continue turning clockwise until you reach your first number. Remove the knob and look at tumbler 3 (the bottom one). The notch on tumbler 3 should be lined up with the right arm of the latch. If not your position of the knob and tumbler 1 is incorrect or the the tumbler pegs in tumbler 1 and/or tumbler 2 are wrong.

2) Reinsert Knob assembly. Turn Knob at least 2 full turns clockwise then continue turning clockwise until you reach your first number. Turn knob 1 complete turn counter clockwise and continue turning until you reach the second number of you combination. Remove knob assembly, check if notches of tumbler 3 and 2 are lined up with the right arm of the latch. If tumbler 2 is lined up but tumbler 3 has moved, it is possible that when tumbler 2 turns it wants move tumbler 3 with it. This can be caused by the peg on the tumbler peg being malformed during printing causing it to rub on tumbler 3 groove. Sand or file the tumbler peg until it can turn without effecting the tumbler below it. This is why it is a good idea to print both tumbler pegs on the same print, while the printer jumps between pegs it give time for the print to cool down before printing the next layer.

3) Reinsert knob, do the the complete combination, remove the knob. Again check the positions of tumbler 2. If the notch is not lined up with the latch, check the tumbler peg on tumbler 1 as in 2 above.

If everything is moves freely and your combination still doesn't work. Please double check that your combination meets all the rules for setting the positions of the tumbler pegs.

Step 8: Assembly Step 9 Final Step

Step 9: Insert the Front onto the Base, align the shafts on the Front with the holes in the Base. Don't push or wiggle too hard, the shafts on the Front are weak when not fully assembled into the base. When fully in place flip lock over, you should see that the shafts have locked into the base.

Step 9: Setting Combination Numbers

If you assembled the pad lock exactly as shown in the pictures the combination will be 2 4 6. However if you wish to select your own combination numbers use the following guide.

Below is the math (OMG not Math) to find the positions of the tumbler pegs and knob positions used during assembly to meet your custom combination numbers.

If you would like, The Combination Lock zip file (in the intro page) has a HTML folder that contains a handy tool for setting the combination numbers. In the HTML folder just click on "Combination_Lock.html" then click the "Combination Tool" tab. This makes it easy to find the positions in a more graphical way.

Rules for selecting the combination numbers

  1. The tumbler peg can not be positioned directly under the tumbler notch (position 0) .
  2. Consecutive numbers in the combination can not be the same. For example the combination can not be 4 4 6, but 4 6 4 is ok.
  3. The second number in the combination can not be 1 less than the first number in the combination. For Example 4 3 8 will not work.
  4. The second number in the combination can not be 1 less than the Third number in the combination. This will put the tumbler 1 peg in the 0 position.

For the math below:

C1 = first number in your combination;

C2 = second number in your combination;

C3 = Third number in your combination;

TK = The position of the knob to tumbler 1;

T1 = The position of the tumbler peg in tumbler 1;

T2 = The position of the tumbler peg in tumbler 2;

Step 1:

Select three numbers 0 thru 9 for your combination, make sure they meet the rules above.

Step 2: To find the position of the tumbler peg in tumbler 2. Use this in assembly step 3.

  1. T2 = (C2 - C1 + 2);
  2. If T2 > 10: T2 = T2 - 10;Tumbler pegs positions must be between 1-9;
  3. If T2 < 0: T2 = T2 + 10; Tumbler pegs positions must be between 1-9;
  4. If T2 = 0: Change your choice of either C1 or C2 and start over;
  5. Use T2 as your tumbler peg position in assembly step 3.

Step 3: To find the position of the tumbler peg in tumbler 1. Use this in assembly step 6.

  1. T1 = (C3 - C2 - 1);
  2. If T1 < 0: T1 = T1 + 10; Tumbler pegs positions must be between 1-9;
  3. If T1 = 0: Change your choice of either C3 or C2 and start over, if you change C2 repeat step 2 above;
  4. Use T1 as your tumbler peg position in assembly step 6.

Step 4:To find the position of the knob to tumbler 1. Use this in assembly step 7.

  1. if C3 > 2: TK = C3 - 3 ;Knob Position must be between 0 and 9;
  2. If C3 < 3: TK = C3 + 7;
  3. Use TK as your knob position in tumbler 1 in assembly step 7.

Step 10: Challenages of Design

I actually spent more time than I originally thought in designing this. I use Autodesk Inventor 2011 for creating the parts. Export the STL format and run it through Slic3r.

The main challenge was keep all parts 3D printer with no support. I hate supports.

The first challenge was keeping the tumblers from turning on top of each other, that causes the tumbler below it to turn when it wasn't supposed to. To do this I came up with the spacers, now I needed to prevent the spacers from turning with the tumbler. To do this, on the base, the tumbler shaft was designed with a flat side and the spacers accordingly. So now the tumbler where independent.

The next problem was how to design the latch. It has 2 criteria to meet. First it has to be able to be forced into the tumblers notches when the combination is completed. To do this I added a thin spring that would push against the wall of the base, applying just enough force to move the latch into the tumblers. My first attempt I had made this spring too thick and it actually held the latch open even when it was supposed to be locked. The second criteria was that the latch had to be able to accept the shackle even when the tumblers notches where not lined up. To do this I had designed the latch with 2 arms, 1 to lock the shackle and 1 for the tumblers. This allowed the the shackle arm to have a strong spring effect so that it could move back when the shackle was inserted and spring back to lock the shackle.

Because of the latch design being a bit bulky and in order to keep the tumblers in the center of the base, I had to reduce the diameter of my tumblers. This created 2 more problems, the notches on the tumblers where more shallow and would not allow the latch to move far enough to unlock the shackle. Back to redesigning the latch . I made the tumbler arm shorter and the shackle are longer on the latch, this gave me more movement on the longer arm. The second problem with smaller tumblers was the groove became smaller, this made the tumbler pegs design small. My first try in printing a single tumbler peg resulted in a malformed peg, due to very short layer print time did not allow the previous layer to cool and kind of gobbed the peg. That's when I discovered printing both tumbler pegs at the same time resolved this problem.

Overcoming problems is aggravating, disappointing, tedious but most of all rewarding with great sense of accomplishment. Even if it is just a toy.

Step 11: How It Works

3D Printing Contest 2016

Runner Up in the
3D Printing Contest 2016



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    Trash to Treasure
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    Tape Contest
  • Arduino Contest 2019

    Arduino Contest 2019

11 Discussions


2 years ago

Awesome print! I really like functional designs and this is a good one!


2 years ago

Works great until someone realizes it's just plastic *clip* !

1 reply

2 years ago

This is really great! Wonderful job designing a working lock out of plastic! Well done!

1 reply

Reply 2 years ago

Thanks, It was a bit of a challenge