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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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