Several of the plastic feet on my patio chairs have gone missing over the years. I should be able to do a 3D part replacement by printing some new ones. That will be the easy part of this project. The difficult part is coming up with a 3D model of part. Since no one has published a 3D model for this part on Thingiverse or elsewhere, I am on my own.

There are two ways to create a model for a 3D part replacement. The first is to design it yourself in a CAD program. The second way is to use a 3D scanner. While I don’t have a 3D scanner, there is a potential solution called 123D Catch from Autodesk, available free for non-commercial use online.

123D Catch tries to build a 3D model from a collection of still pictures. This should be perfect for 3D part replacement. The idea is to take between 20-70 digital photographs of the part from all different angles. 123D Catch will then analyze these images and stitch together the same sort of 3D model you would get from a 3D scanner. The process is called photogrammetry, or “the use of photography in surveying and mapping to measure distances between objects.” It is also referred to as reconstructing 3D structure from motion. There is a wonderful open source tool for this process called VisualFSM, but it is very complex to use. 123D Catch uses the same ideas but automates the process for the beginner.

I thought that using 123D Catch would be faster than trying to build a 3D part replacement in CAD. After a week of trying, I suspect that I was wrong. The 3D models provided by 123D Catch are coming close, but still not good enough.

3D Part Replacement– Getting Ready to Create a Model with 123D Catch

Getting ready for my 3D part replacement project involved the following steps.

1. Download and install 123D Catch software. This is software to transform a series of still pictures into a 3D model. You need to take at least 20 pictures of the part from different angles.
2. Digital camera. At first, I used my Samsung Galaxy Smartphone with Camera. which automatically uploads its pictures into my Dropbox Image Capture folder on my computers. Later, I switched to my wife’s Panasonic digital camera. It was easier to control and hold steady, as well as had better features.
3. Download and install Meshmixer. This is companion software to 123D Catch. It is used to fix or repair the model created by 123D Catch and output an STL file for the 3D printer. That is, if I ever get that far.

3D Part Replacement– Using 123D Catch

The main requirement is to take at least 20 good pictures of the part. Ideally, this is a series of pictures taken from different angles around the part, as explained on the 123D Catch tutorials. Not being much of a photographer, this was difficult for me. The problems I encountered were keeping the part focused, the camera steady and the lighting consistent. When these problems happen, the software has trouble stitching in individual images into a 3D representation so you can do 3D part replacement.

On my first effort I got a good partial 3D model based on four images. But 123D Catch rejected the other sixteen images – see picture to the right. You will notice that I had the part mounted on top of a glue stick to keep it positioned vertically and off the counter surface. Nevertheless, I was encouraged.

Another thing I learned was that the 123D Catch software needs the images to have a diverse background. I tried taking pictures against a solid, constant background, rotating the part rather than myself. That did not work at all.

Eventually, I switched from my phone camera to my wife’s digital Panasonic. I also laid the camera on a small box of the right height, so that I did not have to worry about holding the camera steady. I closed the curtains to try to keep the lighting constant and reduce reflections.

None of the next five attempts worked as well as the first. Finally, I figured out that as the morning wore on, increasing sunlight was causing too much reflection and distraction in the pictures. The first attempt happened while it was still fairly dark. So I went in search of a room with less sunlight. Since you are not supposed to use a flash when taking pictures for 123D Catch, finding a location with “just right” lighting is hard.

Eventually, I learned that the best lighting for a 123D Catch capture is outdoor ambient lighting on a cloudy day. I took a fresh set of 30 pictures on the front driveway under a cloudy sky. Three of these pictures are shown at the top of this article. The results were again disappointing. For some reason, the software thinks all the pictures were taken from only one side of the object.

At this point, I should probably give up on 123D Catch and just design a simplified 3D part replacement in CAD. But I don’t like to fail! I will keep trying 123D Catch a bit longer and let you know what happens.

As an aside, you can also use a video to capture the part, and then extract still images from the video using software like Windows Movie Maker. Also, I should mention that 123D Catch is a “cloud app”. This means that none of the processing is done on your computer. You just upload your still images and then wait for Autodesk to create and return the 3D capture to you. Sometimes this takes a while.