Quickly calibrate the X, Y and Z motion of your RepRap or other 3D printer to 0.01mm accuracy with an inexpensive dial indicator and this simple mount. No fuss, no muss.

Buy one of these inexpensive dial indicators from Harbor Freight. (They're useful for other things, too!). http://www.harborfreight.com/hand-tools/measuring-tools/1-inch-sae-metric-digital-indicator-93295.html

Print the mount part.

Clean up the part and drill as shown in the PDF. (Note the "R4.00" hole is an 8mm hole to fit over a RepRap carriage guide rod.)

Attach the mount to the dial indicator. You'll need a 1/4" bolt and nut to mount the indicator, and a 4mm bolt and nut for the clamp end. There's a nut capture in the part so the 4mm nut will stay attached.

Now you're ready to calibrate as follows (seems complicated at first but it goes really fast once you get it):

Mount the assembly on you printer's 8mm X, Y, or Z guide rod in such a way that the printer carriage can compress the dial indicator when it moves. May take a few minutes of thinking. See my photo of how I did it for the Z axis, for example.

Turn on the indicator. Set to mm units (normally).

Use the printer control software to move the carriage little by little until it just starts to compress the dial indicator pin. Press the "zero" button on the dial indicator. Indicator reads 00.00.

Use the printer control software to move the carriage a nominal 20mm (indicator has a max 1" travel, i.e., 25mm, so we use 20mm). Read the actual distance traveled on the dial indicator. If it reads 20.00mm, you're all set!

If it reads some other value, you have a calibration error you may want to correct. Let's say it reads 19.85mm.

Use this formula to compute the new steps/mm value:

steps-mm-new = (20.00/distance-read) * steps-mm-current

e.g. on my printer:

steps-mm-new = (20.00/19.85) * 52.504 = 53.141

Plug this new steps/mm value into your printer firmware, recompile and reload, and repeat the above procedure to check. May take a couple of iterations to get right on.

Once you get the first axis calibrated, move the dial indicator to the next axis and repeat.

Sit back and marvel at your perfectly-calibrated printer!