The Lulzbot printer comes with a variety of tools in a nice little tool bag, and one of the items is a 150mm ruler. It’s sufficient for short measurements where accuracy isn’t a major factor, but for the rather precise parts and fittings I’m building, I needed something a little more accurate.
While wandering around the Phoenix Fry’s Electronics store, I found (after asking the very helpful staff there) two electronic models from General Tools & Instruments. I ended up buying the larger Model 147 for just under $40. I read some reviews of this and similar models on the Amazon site, and a number of people berated the accuracy and battery life. But in my opinion, it works extremely well for my purposes.
It will read in millimeters, which is very useful when designing in FreeCAD. And if you want to go to inches, it’ll do both decimal and fractional inch measurements.
To alleviate the battery life problems, my model automatically shuts off after a few minutes. All you have to remember is to close the caliper all the way before turning back on, or hit the “zero” button after closing it.
This is the six inch version; they have a three inch version for about half the price.