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  1. #1
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default Building a "Mongrel" Repeat O Meter.

    This may end up being a long term project for me as I don't really need it but I would like to check the general condition of a couple of surface plates that I have. I have studied a lot of home built versions on the internet but I keep coming back to the original Rahn as my preffered design.
    I have most of the materials and have made a start by producing probably the easiest part - the differential screw assembly. The threads used are M8 x 1.25 and M12 x 1.5 which give a differential of .25mm which should provide a fairly fine adjustment for setting the indicator to zero, this is equivalent to a thread of approximately 102tpi.

    repeat o meter differential screw.jpg

  2. #2
    Join Date
    Sep 2008
    Location
    Narrabeen, Sydney NSW Australia
    Posts
    156

    Default

    I had to research differential screw assembly, much cleverness, colour me impressed!

  3. #3
    Join Date
    Apr 2019
    Location
    Adelaide
    Posts
    598

    Default

    Is that knob you showed above a single rod with 2 threads or is the courser of the threads a separate piece?

  4. #4
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    Yes, it is a separate piece, the 8mm stud is reduce to 1/4" diameter on that end and the 12mm piece is drilled and reamed to provide a nice transition fit on it, the two pieces are retained together by the 1/16" spring pin.

  5. #5
    Join Date
    Apr 2019
    Location
    Adelaide
    Posts
    598

    Default

    I understand differential screws when there are 2 different thread sizes on one shaft but I am not sure I understand what you have there. I THINK you have something like is animated in this video?

    I am looking to make a repeat-o-meter in the future so need a fine adjustment. Oxtool did a video on the NYCNC RoM design and stated that sufficient adjustment can be obtained from a relatively course thread. He is a smart man and knows his stuff but that doesn't feel right.

  6. #6
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    For this application it works more like this: https://en.wikipedia.org/wiki/Differ...ntialScrew.gif

  7. #7
    Join Date
    Apr 2019
    Location
    Adelaide
    Posts
    598

    Default

    hmmmm........hurry up and build it then I might understand!!

  8. #8
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    Don't worry too much about the differential adjustment screw, I am just trying to make it as close as possible to the genuine article, any fine thread will do the job, an ME thread would be my choice if I wasn't using the differential thread,

  9. #9
    Join Date
    Apr 2019
    Location
    Adelaide
    Posts
    598

    Default

    https://www.youtube.com/watch?v=7RC8WL2ngfA

    Oxtool explains it from 19 minutes onwards...........

  10. #10
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    I finished the top section today and gave it a quick test which did confirm that it works well. In the attached video the screw can be seen moving down quite a distance while the gap between the bars closes by a very small amount. The total travel on the 1 micron indicator after about 1.25 turns of the knob is approximately .16mm or around .006"


    https://www.youtube.com/watch?v=6geZ3JQ2S9k

  11. #11
    Join Date
    Apr 2019
    Location
    Adelaide
    Posts
    598

    Default

    Brilliant!

  12. #12
    Join Date
    Sep 2008
    Location
    Narrabeen, Sydney NSW Australia
    Posts
    156

    Default

    That travel movement is so smooth, especially for such tiny increments!

  13. #13
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    From what I have seen online the genuine Rahn repeat o meter uses a Mahr dial indicator with 20 millionths of an inch divisions (.00002"), I don't have one of those but I do have one with .0005" divisions which I fitted to see how it looks and I must admit that I like it better than the round face indicator for this application. I haven't see any .00002" Mahr indicators for sale at a reasonable price so I will get a.001mm version which will be plenty good enough for what I will be doing.

    repeat o meter 14.jpg

  14. #14
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    A bit more progress over the last couple of days, I also found a nice Pelican type case at Total Tools to store it in.

    repeat o meter 10.jpg

  15. #15
    Join Date
    Sep 2008
    Location
    Riddells Creek, Vic.
    Posts
    850

    Default

    Almost done now, just the handles and the 4 feet to go.

    IMG_6392.jpg

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