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  1. #1
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    Default What's going on inside a 3-point bore mic

    Found some old posters lying around at trade school and was told I could have them. There's a few on metrology (all the various limits and methods of measurement for specific features/tolerances) and this one, which is a labelled cutaway of a TESA Tri-o-bor.

    IMAG3554.jpg

    Dozens of parts and yet they still have a 2 micron resolution and +/- 4 micron tolerance, I can't imagine what the manufacturing tolerances must be to achieve that. Makes you realise why they start around $1500 each

  2. #2
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    Default

    Thanks for that.
    While there are dozens of parts, the important bit is the cone against the sliding bits. Always wondered how they did that (balls was another possibility).

    Michael

  3. #3
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    Default

    That's just one way they do it, their Imicro goes one step further and actually has a threaded cone


  4. #4
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    Hi Guys,

    I've seen something similar that uses four steel balls and a micrometer type head.

    Also a DIY version that uses three steel balls with two at 180 degrees apart and one in between pushing them apart, again using a commercial micrometer head. I'm sure that this one is on the web somewhere.
    Best Regards:
    Baron J.

  5. #5
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    Default

    I have a set of the bowers three point mics, very similar in design to the first style, but a bit simpler (they dont have that whole geared section in the thimble which i assume is there to provide more reduction/sensitivity?). the real advantage of the first style is you can get right to the bottom of blind holes.

    Also setting up to grind that cone/helix must have been so much fun!

  6. #6
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    Quote Originally Posted by BaronJ View Post
    I've seen something similar that uses four steel balls and a micrometer type head.
    Also a DIY version that uses three steel balls with two at 180 degrees apart and one in between pushing them apart, again using a commercial micrometer head. I'm sure that this one is on the web somewhere.
    I've been trying to wrap my head around this concept and I keep stumbling on what seems to me to be an obvious problem; the rate of outwards motion of the pushed balls is not going to be consistent across the downward travel of the pushing ball, so how can you directly read an accurate measurement on a standard mic head? Am I missing something obvious here?

    Quote Originally Posted by caskwarrior View Post
    I have a set of the bowers three point mics, very similar in design to the first style, but a bit simpler (they dont have that whole geared section in the thimble which i assume is there to provide more reduction/sensitivity?). the real advantage of the first style is you can get right to the bottom of blind holes.

    Also setting up to grind that cone/helix must have been so much fun!
    I believe the geared section in the head is purely for the readout; it's "digital" but displayed on 2 revolving cylinders, much like a car odometer. They used the same mechanism on the Digitmaster mics; the left window is 1mm/div and geared to the spindle, the right window is 0.002mm/div and directly attached.


    As for setting up to grind the cone, I'm sure it's all just drawn in cad and done on a cnc grinder. It probably could be done manually in a universal indexing head, but you'd need a machine that could geartrain the X and Z axes plus the rotary head together.

  7. #7
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    Quote Originally Posted by elanjacobs View Post
    I've been trying to wrap my head around this concept and I keep stumbling on what seems to me to be an obvious problem; the rate of outwards motion of the pushed balls is not going to be consistent across the downward travel of the pushing ball, so how can you directly read an accurate measurement on a standard mic head? Am I missing something obvious here?
    Hi Elanjacobs,

    I've never actually given it any thought.

    They are both designs that I've come across on the net at some time. I particularly remembered the three ball one because it looked comparatively easy to build and it used a standard micrometer head, which are quite cheap and easy to find.
    Best Regards:
    Baron J.

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