Introduction: How Do I Model and Build a 3D Model for Studying Compliant Mechanism

About: I do engineering design for machines and electronics but do love science, art and history.

This project is inspired by the projects at Brigham Young University's Compliant Mechanisms Research (BYU CMR, and Dr. Robert Lang ( Here are the original models provided by BYU CMR.


These models are very interesting and we would be able to make more fun with Lego bricks. Thus, I would like to build my model from scratch.

Step 1: MAKE a PLAN~

Honestly, this project is a reverse engineering project. I have to duplicate the critical features of the Lego brick in order to make the thing to be used with standard Lego pieces. The critical features of Lego brick are the holes and the pocket spans. The first priority is to identify the centre-to-centre distance and the depth of the pocket snap.

So, I will measure the geometry and then draw a 3D model with FreeCAD. Then, I will make several trials by 3D printing in order to verify the dimension, the precision and the quality of products.

Here is my strategy of reverse engineering or simply my personal tips:

  1. Engineer loves integer.
  2. Engineer loves to enlarge the cavity by 0.05 mm to 0.2 mm in diameter for fitting.
  3. The error between 0.01 mm to 0.1 mm has no big difference in 3D printing.
  4. Try to measure a larger feature in order to reduce the error from the measurement tools.
  5. Make trials and check the dimension unless you are happy with your products. If you found mistakes or errors, then you should amend the 3D model or the print setting.

Here are the materials.

  1. A Lego brick, Technic
  2. A calliper
  3. A pen or a pencil
  4. A note or a computer
  5. A 3D printer

Step 2: Measurement

In order to make a precise and accurate model, I would like to measure the geometry of the Lego brick and make my own specification by myself.

I always have a wrong result in the first measurement or the first build because of the tolerance and clearance in manufacturing. For the repeated features, I would like to measure the size of a larger piece and then do the calculation. This is because it can reduce the measurement's error.

The measured values cannot be simply round up or down for drawing a 3D model because of the concern of precision. Therefore, I usually have to revise the specification and the 3D model.

Step 3: Drawing the 3D Models

I used the FreeCAD to build the 3D model. This is because you can download the FreeCAD for free. If you do not have FreeCAD, you can download it here.

I have already determined the dimensions of the holes and the pocket snap by several trials. In addition, I have determined a suitable width of the bending beam for the compliant mechanical model. With the FreeCAD model, you can change the width of the beam and the number of holes.

If you would like to learn more about FreeCAD, you can find many tutorial here.

I have attached a 2D draft which has shown the dimensions of the key features. You will be able to find my finalized models in the last step.

Step 4: Print the Model and Try to Make Some Fun!

I have prepared two basic models. You can download the print files here or at this link (

The following is the setting of my print file. You may have to adjust the setting according to the characteristics of your 3D printer.

  1. Layer height: 0.08 mm
  2. No. of print shell: 2
  3. Raft: Yes
  4. Base print speed: 30 mm/s
  5. First Layer speed: 10 mm/s (You should set a lower speed if you do not use raft for your print.)
  6. First Layer travelling speed: 50 mm/s (You should set a lower speed if you do not use raft for your print.)
  7. Dimension adjustment, internal compensation: 0.2 mm (The setting is depended on the performance of your printer.)

Step 5: Explore the Application and Have Fun!

You can try different combinations to build your own structures and mechanisms. The video has shown a few characteristics of these model.

If you would like to edit the model, you can download the FreeCAD files at this link (Instructables does not allow me to upload FreeCAD files), .

You can download them and print it for studying the compliant mechanism. Enjoy!