Take Precise Measurements with Comparatron

Reverse engineering physical objects is difficult when they're complex or very small. If a part has organic curves, it is difficult to find reference points to for calipers. If a part is very small, you can't even fit calipers on the features. To make it easy to take very precise measurements, I designed and built a digital optical comparator called Comparatron.

An optical comparator is an instrument that lets users measure distances and angles through a microscope. When I was a mechanical designer, I used an optical comparator to reverse engineer medical devices — including bone screws with threads too small to make out with the naked eye. But commercial optical comparators are very expensive (easily tens of thousands of dollars). While working on a recent project, I found myself pining for an optical comparator and so I decided to build Comparatron.

The base of this project is the iDraw Pen Plotter 2.0 that I reviewed a couple of months ago. That's meant for drawing on paper, but it has a CNC control board that accepts Grbl 1.1 commands for control. That allows for very precise movement (down to around +/-0.02mm in my tests).

First, I removed the pen lift mechanism and replace it with a 3D-printed microscope mount with a rack-and-pinion gear mechanism to adjust the distance between the microscope and the surface for. I also placed an LED-backlit tracing board on the pen plotter bed, which eliminates shadows and provides a clear view of the part.

After deciphering the g-code commands accepted by the iDraw, I programmed a software interface in Python. This interface lets the user connect to the control board and move it. A video feed from the microscope contains a crosshair for lining up features. Clicking a button draws a point at the current coordinates and those points appear in the plot window.

Once the user marks every important point, they can export a DXF file. Many programs, including Autodesk Fusion 360, Eagle, AutoCAD, KiCAD, and many others will let you import that DXF file and use it in a design.

This makes it very easy to recreate parts or PCBs, as you can see in the demonstration video above. You can measure parts as large as around 310 x 220 x 26mm (taller is possible with some workarounds) and as small as a grain of rice.