Inch and metric versions of a two piece printable micrometer. The Inch micrometer has a travel length of 2.5 inches and the metric has a travel length of 30 mm. I have included an additional base piece for each one with a slightly longer anvil. This should allow you too file down one end to make your micrometer as accurate as possible. I have printed both versions and the inch version is generally accurate with in +-3 thousandths. The metric version can be really accurate (+-40 microns).
I strongly recommend printing the calibratable versions of the base piece. This version is designed so that you can sand down the clamping point until you get the accurate measurements. If you use the standard base piece you may end up with measurements that are offset by 15 thousandths or so.
EDIT (1/12/16): I re-uploaded the rod piece because I thought the knurling was too deep and was painful to turn. It's only a slight change though and you probably don't need to re-download it.
EDIT (1/15/16): I uploaded the part Base_Inch_Calibrate in which I added 20 thousandths to the anvil (the static end). I did this to compensate for the fact that mine measures 16 thousandths too short and I suggest sanding or filing it down until the micrometer reads the correct value.
EDIT (1/16/16): I have uploaded stls for a 30mm metric version. To print this one you need 4.3 inches vertical. I strongly suggest printing the Base_Metric_Calibrate part so you can adjust it to be as accurate as possible.
Also check out Ultimaker's cool time lapse print of it here!
The base part will need support but the rod part should not have support generated for it. I believe if you're using 'normal' support settings in your slicer you should be able to print both pieces at the same time.
The base part is is 6.7 inches tall so make sure your printer has enough room.
With 10% infill and .15mm layer height, I printed both parts at the same time and it took 18 hours and used about 200 grams of filament.
It may take some breaking in once you make it but it should be sturdy enough that you can twist pretty hard on it. Once you twist through it a few times it should turn nicely.