NavList:

   

Reply

On Fri, 25 Mar 2011 15:10:57 -0700, Patrick Goold  wrote:

> It took me 15 minutes to level the mirror assembly.

As you discovered, a lot of time would be saved by using three screws
instead of four.  Perhaps the third screw (or just a point) could be
placed in an outrider beam for better stability of the board.  Also, it is
possible to get quite fine elevation adjustments with coarse threads by
using a differential motion effect.

For example, I used an electrical light fitting consisting of a nut-plate
and a hollow tube of 0.385 (roughly 3/8) inch diameter with 27tpi; and a
bolt and two nuts of 0.181 inch diameter by 24 tpi.  The bolt was a 10-24
 from my scrap drawer, but I went out to buy the electrical fitting, used
for chandeliers, etc.

I tightened the two nuts on the number 10 machine screw, chucked one end
in a electric drill, and held the turning assembly against an electric
grinder until the nuts were a slight interference fit into the hollow
shaft; and the head of the screw was ground to a nice point; rounded off a
bit on some abrasive paste.  I spaced the nuts to almost the full length
of the hollow shaft, then heated the shaft to stuff in the nuts and
machine screw.  It turned out the machine screw was a bit tight on the nut
threads, even with a little tapping on the blunt end, so I added a bit of
abrasive to the screw threads and ran it forward and reverse a few times;
 from both ends of the hollow shaft.  I cleaned it up, and drilled a hole
through the machine screw near the point, to allow a length of coat hanger
wire to slip through and be peened/riveted in place.

Assemble the whole works so the electrical nut-plate is centered on a wood
lath over a large hole, the machine screw is threaded point down through
the electrical tube in the nut-plate, and a good length of coat hanger
wire is inserted through the little hole in the machine screw near the
tip.  The other end of the coat hanger wire is bent into a short elbow and
inserted in a slot in the lath from the underside.  I think a longer
length of music wire would have been better.

As you rotate the *electrical* shaft clockwise 27 turns, it travels down
one inch.  However, the machine screw, prevented from rotating by the coat
hanger wire, will climb one inch upward in 24 turns; or 27/24=1.125
inches.  The point of the machine screw will end upward .125 inches in 27
turns, with respect to its initial position, the nut plate, or the wood
lath.  That is roughly 0.00463 inches per turn, and tenths of a turn are
easy to mark on a bit of masking tape wrapped around the hollow shaft.

Note that there is an huge source of error if the final turn (or portion)
is not made in the same direction to the pause position; the slop is in
the threads of the machine screw, the threads of the electrical fitting,
and the clearance of the slot for the coat hanger wire.

For a crude construction there is good repeatability, according to a dial
indicator sitting on a glass plate.  The construction is not stable enough
through changing moisture, temperature, and sun conditions to make an
accurate range-finder of the tilting mirror design, especially now that
longer range laser devices have become available.   Besides, I still have
a heavier Barr and Stroud rangefinder of better design and much better
construction.

--
Richard . . .

Using Opera since the"Dog" died

   

Reply