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    Bygrave and MHR-1 locking mechanism construction
    From: Gary LaPook
    Date: 2009 Sep 29, 18:55 -0700

    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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