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(Ben Nguyen) writes:
Im looking for ideas to do for a school project and came up with
a digital micrometer.

Does anyone know whats involved? I want something challenging, but not
something impossible!

You need a precision screw and a precision nut, probably adjustable to
reduce or eliminate play. You need some sort of rotary encoder to
measure rotation of the shaft to high angular precision, and a computer
of some sort to read the output of the encoder, keep track of position,
and convert that to a displayed measurement. For a practical measuring
device, you also need a ratchet or friction clutch device, like on a
real micrometer, to standardize the measuring force.

The easiest place to obtain the precision screw and nut and the
ratchet/friction device is an old mechanical micrometer, if that's not
considered "cheating". Then you need a rotary encoder, which you might
build yourself (a challenge), or you might find one in the carriage motor of
an old inkjet or dot-matrix printer, or an old tape drive, or even just
buying a surplus encoder.

Then you need to couple the encoder to the spindle (screw). This is
easiest if the encoder has a hollow shaft. Keep in mind that the screw
moves along its length as well as rotating, while the encoder only
wants to rotate, so you'll need a precision sliding coupling between
the two. Alternately, you could use gears to couple the spindle to the
encoder.

Then you need software to take the two sine or square waves coming out
of the encoder, decode transitions into distance and direction info,
keep a running count of where the spindle is, and display the position
in inches or mm (or both). You should also allow for setting zero at an
arbitrary position. The simplest way to write this software might be to
program a PC, using the parallel port for input and the frame buffer for
display. A more professional way is to use a microcontroller like a PIC
or AVR with a LCD or LED display.

What level of school is this for? How long do you have?

Dave