NavList:

The alignment does not change when you twist the locking mechanism because the wedging effect of the two pieces being pulled together causes the locking rubber ring to expand radially, straight out, and it does not apply a torque on the cosine tube. (It works just like the DeBange obturation system used on separate loading artillery.) It is only necessary to turn it 60º to go from full freedom to solidly locked.


I also found that I do not need the

"piece of one inch PVC pipe a little bit
shorter than the cotan tube and put it inside the cotan tube. This is to
prevent the plastic piece from being drawn into the tube under tension"

as the plastic piece fits very firmly inside the cotan tube.

BTW, I have gotten real good with my Bygrave and I can do the whole computation in 45 seconds

gl



Wolfgang Hasper wrote:

Gary,
thank you for your reply to my earlier question and those most interesting
experiments.
I am courious to learn about your results:
Does the tightening torque affect the alignment, is a 90° turn sufficient...?

I myself will shortly start some experiments on tube-making to custom
diameters which -given the results were positve- might solve that cumbersome
problem. My idea is to use a bending apparatus to bend pieces of various
materials to form my own tubes. This should be possible with sheet metal the
seam then soldered/brazed/welded, plasic (in heated state) with the
advantages of low weight, no corrrosion, cheap price, easy to glue and
possibly even with cardboard.
I have a printer that is ready to print on cardboard up to 1 mm thickness.

Maybe it is even possible to first print/engrave scales onto flat (plastic)
material and then bend it...
Should that work it seems promising to use this
http://www.gravograph.com/english/engraving-products/gravoply_ultra.php
Since it can be laser-engraved any design could be realised easily.

THe experiments will take some time, just too busy right now.
I will report as soon as results are ready.

Wolfgang








Am Mittwoch, 30. September 2009 03:55 schrieb glapook@pacbell.net:
  

In the past we have discussed the superiority of the MHR-1 to the Bygrave
due to its mechanism for locking the two tubes together to prevent unwanted
movement while resetting the cursor. See prior posts:

http://www.fer3.com/arc/m2.aspx?i=109049&y=200907


http://www.fer3.com/arc/m2.aspx?i=109054&y=200907



I have made several copies of the Bygrave using as its base a 1.500 O.D.
standard sink "tail pipe" or drain pipe twelve inches long on which to
mount the cotan scale.  See my prior post on this:

http://www.fer3.com/arc/m2.aspx?i=108949&y=200907


I have also suggested in the past that a three and a half buck off the
shelf "test plug" might be the basis for a locking mechanism, see:


http://www.fer3.com/arc/m2.aspx?i=109056&y=200907

The problem with making a copy of the Bygrave or the MHR-1 is finding tubes
that are close enough in size to provide the proper sliding fit. My cotan
scale is printed on paper and wrapped around the 1.500 inch O.D tube and
then covered with a clear plastic sheet expanding the diameter to 1.520
inches. I measured the I.D of standard one and a half inch PVC pipe and
found it is 1.566 which is a good fit and within the range of adjustment of
the "test plug." So today I decided to give it a try. I purchased a piece
of such pipe, a piece of 5/16th inch threaded rod, a 5/16 inch nut, a
5/16th inch self locking nut, and another test plug, about ten bucks. The
original bolts in the test plugs are 5/16ths.


The attached pictures shows the process. Disassemble the two test plugs and
use the two smaller plastic pieces, one of the large pieces, one of the
rubber rings one of the washers, and one of the wing nuts. The other pieces
are not used. The small plastic pieces are designed to use carriage bolts
so have square sockets for the heads of the carriage bolts. Modify one of
the small plastic pieces by opening up the square socket to allow the use
of a normal hex nut (or modify a nut to fit in the existing square socket.)
 Tilt the unmodified small plastic piece and you can insert it into the end
of the cotan tube. Straighten it out and position it so it protrudes from
the end of the cotan tube. Cut a piece of one inch PVC pipe a little bit
shorter than the cotan tube and put it inside the cotan tube. This is to
prevent the plastic piece from being drawn into the tube under tension. Cut
the threaded rod so that it will go through the tube and protrude out the
end. Put the wing nut on the rod and place the self locking nut on the rod
and twist them together to lock them solidly to the rod, "double nutted."
Place the rod though the washer and then through the large plastic piece
and slide this whole assembly through the tubes so that the rod protrudes
from the other end. Place the rubber ring on the rod followed by the
modified small plastic piece and a regular nut. Turn the rod using the wing
nut to tighten up the whole thing. Now slide the one and a half inch PVC
pipe over the assembly, tighten the rod and it locks the two tubes
together.

I  have not yet made the cosine scale to go on the one and a half PVC pipe
but that should not be a problem. I plan on making the same type of cursor
using clear plastic sheet. The only thing I don't like about this is the
PVC pipe has a thick cross section and I will continue to look for a
thinner piece of tubing with the same I.D. to eliminate the parallax
problem with the cursor so far away from the cotan scale.


gl


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