I was getting pretty sick and tired of using the paper method to set the height of my Replicator's build plate so I looked around for another solution. Using a height gauge looked like a good idea, so here is my version.
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Step 1: Parts List
1) Height gauge:
I used a digital one since I already had one lying around. AccuRemote Digital Electronic Indicator. It also has a data port which may come handy in other projects.
2) Neodymium magnets:
4 x 0.125' OD, 0.0625" thick disc magnets from MAGCRAFT.
3) 4-40 screw of your liking as set screw.
4) An already well calibrated 3D printer of course :)
Step 2: Printing and Assembly
The part will be mounted on the top rails and since the gauge is spring loaded, the mount has to be magnetic. Trying to hold it down with your hand to make a measurement won't work as you bend the rails ever so slightly, screwing up your measurements.
I was initially going to embed the magnets but decided not to for two reasons.
1) Even the slightest amount of plastic between the magnet and the metal reduces the holding force by quite a lot and the gauge mount won't stick properly to the rails.
2) The printed surface is too rough, hence additional leveling error.
So instead I filled up the magnet holes with thick ABS slurry (ABS melted in acetone) and pressed in the magnets using a 12 mm diameter rod that is quite close to the diameter of the rails, and then wiped off the excess slurry.
Step 3: Leveling/tramming
Leveling the build platform doesn't mean you've got the nozzle height right. The trick I used was to level first, by moving to the four corners, starting in the back. Then jog the plate up till the micro switch is triggered, zero the dial and then readjust the leveling screws to have the plate leveled at -0.1", cycling through the corners again.
You may need to run a test print to find the optimal nozzle-plate separation for your setup and resolution.
Hope you enjoyed this instructable and best of luck printing :)