Why not just mount a microswitch on the extruder and write a program that positions it over each adjusting screw.  Then drive the print table up slowly until the switch trips.  Save the Z count for each of the three positions.  Then it would be a simple matter for the program to position the extruder/switch over each position and tell the user which way to turn the adustment screw until all three positions trip the switch at the same Z count.

The switch could be connected in parallel to the existing Z switch.

Run the program on the controlling computer with it sending G codes to the printer.

Nick

Back when the first version of these calipers was made, the external interface was much simpler.  The data port was intended to drive an external up/down counter.  One pin was a clock and another a direction signal. That's how they worked internally too.  (no uC in them back then)

How do I know?  Back in '78 - '80 I was the project engineer responsible for designing the electronic guts in the very first digital caliper with a liquid crystal display.  All of these new ones use the same sensor and operational concept.

The sensor concept, and the basic functionality, was invented and patented by a professor in Stockholm, Sweden.  I was the project engineer turning his patent into a product.

The attached photo is my personal one of the original model.  Over 30 years old and still working.

(Back to original programming)

Nick

That might be something we can implement depending on how much room is under the bed. Might need to sacrifice a little z height.

For those with the aluminum bed - we could attach another 8x8 aluminum plate to the bottom of the bed carriage, drill out and countersink holes in the corners of the bed, use longer versions of the leveling screws we already have through to the bottom plate (through oversized drilled out holes), with a compression spring holding up each screw which would each be "endstopped" by a nut and washer superglued/locktited onto it's end - then it's just a matter of printing a setscrew thing for each corner.

... I just don't feel so good about this acting on 4 corners and without close tolerance ball joints and blah, blah, blah...

Somebody else should try this and prove me wrong! 

I'm already playing with gear motors and dial indicators and an arduino for this task (no drilling required) - ok, well, at least I have a couple of things ordered....

Just gonna leave this here... http://www.youtube.com/watch?feature=pl … hDSFuC7HkQ

KIS method in all its splendor .. *bow*

...still slightly manual having to move the touch off plate around the bed, but otherwise pretty automated. if you wanted to avoid doing that, then just use a PCB blank the same size as the entire bed.

Yep - I really wish we had a level of scripting available in the gcode that could perform some basic actions/math and make use of auxilliary inputs and outputs on the control board.  Only way I could think to do that currently would be to run the printer on Mach 3 and a diffferent board.  Basically process the slicing and gcode in RH and Slic3r, save gcode to file, load in Mach 3, run leveling and z-stop scripts (with inputs and outputs), load gcode and print.  I'm not experienced with Mach 3 but I'm sure it could be configured to interpret the extruder and bed temperatures.

using one side of the loop connected to a PCB plate of a known thickness (or thin copper/brass plate) and the other side connected to the extruder bolt..

You could home the Z-Axis, drop it 10mm, move the head to a known place, then creep back till the circuit goes high. Then knowing the deltas on the different z-axis apogee's, you could calculate the level bed and set your z-height by removing the thickness of the copper plate.

Obviously backlash would need to be minimal to make this work, but i can't see too many other shortcomings. still slightly manual having to move the touch off plate around the bed, but otherwise pretty automated. if you wanted to avoid doing that, then just use a PCB blank the same size as the entire bed.

Consider this:
...put thick gears in place of the leveling thumbscrews, put a worm gear on little gear motors (3 - one for each screw)  to turn them [thick screw gears so that the worm gear can slide up and down along the teeth as they move up and down], drive the gear motors from an arduino (with simple on/off transistor switches to get enough current) connected to the dial indicator.  As long as the extruder is repeatably near the leveling screws, an algorithm to level a 3-point plane should pretty much solve it - just cycle around the 3 screws.  All the resolution you need will come from the dial indicator - don't worry about backlash in the leveling gears.

Really, if you manually put the back single thumb screw at a "middle" position, you would only have to adjust the other two screws to level the bed. ...but still use a measurement at the other screw.

Assuming you can repeatably position the extruder (either by direct g-code or by g-code scripts with pauses), theres no need to mess with anything else on the computer side of things.

THANKS....

There's another thread around here somewhere that mentions interfacing arduino avr or another microcontroller with digital calipers - I don't see why the same couldn't be done with a cheap dial indicator.  The cheap indicator that I bought from harbor freight this week has a similar output port.

Mine is labeled "Pittsburgh" but I'm sure they're the same thing:
http://www.harborfreight.com/1-inch-sae … 93295.html

Example connections and interfaces with the calipers:
http://www.robotroom.com/Caliper-Digital-Data-Port.html
http://nut-bolt.nl/2012/reading-digital … n-arduino/
http://robocombo.blogspot.com/2010/12/u … rface.html

I was contemplating something similar but was thinking more on the lines of an ultrasonic range finder or laser for distance reading.  Not sure if it's even feasible but one of the things on my "TO RESEARCH" list..... which seems to be growing more and more every day

I'm afraid laser will suck, due to being reflected from the glass surface.
ultrasonic and PIR sensors are not likely to work as well.
ultrasonic has minimal distance of about 30 to 60 cm.
PIR - I'm afraid glassy surface will screw up the readings and I don't think it's that precise. (but it IS on my checklist to try)