DIY CNC Scanner Application NotesJuly 30, 2010 by: Andrew Grevstad
The shareware version of CNC ScanCAD has created a lot of interest in playing around with our new CNC Scanner software (as well as the article in Digital Machinist). This is very cool, and its been a fun week talking with people who’ve been trying it out. The availability of low cost cameras makes scanning pretty easy to do, but we do want people to be aware of the relationship between camera quality and scan quality.
If you buy CNC Scanner from us, you’ll be getting our custom designed mount, one of the better cameras from Dino-Lite, fully functional software, and support direct from Tormach. As for the Dino-Lite cameras, these are Taiwanese made cameras, but there are numerous knock-offs made in China. Unfortunately, none of the knockoff designs are made to the same quality. They do, however, generally all share the same style and size housing, which means that the majority will work very well with the holder that we have.
We tried a lot of different lower cost cameras from a variety of vendors and found some pretty significant limitations with many of the models, which is why we settled on the camera models we offer. If you do want to play with CNC Scanner using a cheap camera, you should recognize these limitations. There are a lot of parameters in optics and USB microscopes. I won’t get into them all, but rather the most important parameters we found. These are lens quality, lens alignment, retention of focus, and illumination.
Lens Quality and Lens Alignment
Really cheap cameras, such as those you see for $25 on Ebay (search USB Microscope) do work, but we found they are not without siginificant performance compromises. Basic lens quality and lens alignment can be tested by focusing on a bit of graph paper. If you just want to get an idea of what this is about, you can get some very fine graph paper and afix it to a piece of plate glass or something else that is suitably flat. To properly evaluate magnification we use a square contact reticule like this one: Edmund NT39-463.
The image should be clear with a consistent focus across the full field of view. The lines on the resulting digital image should be absolutely straight, particularly on the edges. Most of the cheap cameras we tried had lens alignment issues that would allow us to focus in one point over the field of view, but a perfect focus over the full field of view was not dependable. You might get one that works, but we would purchase bunches of cameras and test several. Sometimes there was a good one, but in the cheap cameras we found it was a crap shoot and lots of them were bad.
Retention of Focus
The other issue is the ability to retain focus. The cheap cameras hold focus using heavy grease. As the LED illuminators warmed up the camera body, the grease would get soft and the lens would move. You can see this with a large area scan, where the beginning of the scan is clear but the camera drifts out of focus as the scan continues. Better quality scopes do not have this problem to the same severity.
Some very basic scopes have illumination issues, especially if the scope needs to be positioned close to the surface of the work to achieve the feature resolution you want. Generally speaking, the more illumination control settings, the better off you will be as these can be adjusted to provide the best possbile image. Otherwise, the image can be either so poorly illuminated that you cannot distinguish feautures or so brightly illuminated that the features are washed out.
We encourage you to continue experimenting with the software. We just wanted to make you aware of the relationship between hardware and software. And please do send me a note if you have feedback or other questions about CNC Scanner.