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  1. #1
    GlennM is offline Aluminum
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    How does one measure the diameter of narrow ID grooves, for example an o-ring or snap ring groove inside a bore?

    In addition to measuring the depth of the groove, how does one gauge it's width?

    - Glenn Minch

  2. #2
    JedClampit is offline Member
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    the tip of a pair of calipers? I've measured .5mm that way ok. Maybe spark plug gap indicators for thinner grooves? Metal shim stock would work too. Or measure the width of the cutting tool that made the groove - assuming no sideways movement.

  3. #3
    bluchip's Avatar
    bluchip is offline Stainless
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    There are a number of ways to do this and it depends upon your accuracy requirements and comfort/confidence level. I am perfectly content to measure these diameters to commercial tolerances using the little clamp on adaptors that are available for dial calipers. These are available with holes for pins that you can adapt to your groove width. Even if you don't trust the reading of the caliper you can take a reading note the caliper's reading ,retract reset to the noted reading and measure with a mic similar to T-gages.
    There are internal dial calipers available but due to the varied sizes we use the aforementioned method.
    You could also if workpiece allows, mic across the OD to the groove using a pointed anvil mic and calculate (OD-ID/2-mic reading=groove depth x 2 = groove od) Don't have a pointed mic?...drop a ball bearing in the groove and mic over that.

  4. #4
    jim rozen is offline Diamond
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    One shop I used to work at had a nifty little
    item that one mounted a last-word type indicator
    on top of, and it had two pins on levers that
    could be put inside bores to check snap ring
    gooves. One zeroed the instrument on a gage,
    and then read deviation from nominal dimension
    on the dial.

    Jim

  5. #5
    bluchip's Avatar
    bluchip is offline Stainless
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    Jim,
    It's called Indi-cal. The work as well. just didn't think any one would know of them!


    As far as the widths, JedClampit is just about spot on. Use gage blocks, feelers, the groove mic on bores that allow, and calipers on large bores.

    Another method is a pair of washer like tools you can make. Turn an OD and ream and ID then part and deburr a pair off to a width that slips in the groove width. Insert these diametrically opposite (as best you can) and measure across the ID's of disks using the knife or nibs of your caliper rocking to reach max reading(easy now don't bend the calipers with those silly thumb rolls) and calculate. The dims for disks depend on whats convenient.

  6. #6
    Toolbert is offline Stainless
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    There's a good explanation here:

    http://www.longislandindicator.com/intertest.html

    These are obscure enough, I found a set at a local auction for $30 ea. a ways back, maybe you'll luck out on ebay.

  7. #7
    tattoomike68 is offline Stainless
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    On small cheap screw machine parts I mill a sample in half and use an optical comparitor.

  8. #8
    GlennM is offline Aluminum
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    I should have quantified my question with an actual example.

    Recently I needed to cut an o-ring groove for a 1mm o-ring, 8mm down a 12mm bore. The work was a casting that didn't lend itself to fixturing on a faceplate (at least, not on the size lathe I have) so I did this using the mill.

    I made a small cutter like a woodruff keyseat cutter, then revolved the casting on a rotary table. I was able to use a new o-ring to check the result.

    Now that I've read the responses I see that checking the width of the groove is pretty easy. All that is needed is a disc whose thickness is the minimum width of the groove. Duh!

    I still don't see how to measure the dia of the groove in a small (<.5") bore, though. Calipers won't fit, so any plan that calls for inserting the blades of a caliper seems doomed. Modified spring calipers might work.

    I hoped there might be some simple tool similar to telescoping bore gages.

    Glenn Minch

  9. #9
    JST's Avatar
    JST
    JST is online now Diamond
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    Well, I have a couple of gizmos that are dial bore gages. Not the type with three plungers, these have measuring arms. There is a lever to compress them to go into the bore.

    On the arms are small points, and the dial reads the distance across the points, complete with cosine error, I think. (They are best used as comparators.)

    One of mine goes down to a fairly small size.

    Read the inside of the groove diameter with one, and note the reading.

    Get your calipers or mic, get the arms between the anvils, and adjust to read same on the dial of the bore gage. The mic is now set to the groove diameter.

    The points do have to fit in the groove, and not let the arms bottom out on the main ID. But it works.

    Eh..... late edit....
    Somehow I missed Toolbert's link.... that is what mine are, less the carbide tips, and with somewhat different design details. Yes, they are in fact "Intertest".

    The smaller one goes from 0.20 to 0.60, so the range under 0.5 is available.

    [ 12-05-2005, 11:57 PM: Message edited by: J Tiers ]

  10. #10
    olddude is offline Stainless
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    Glenn,
    You should of said something I could have lent you a groove micrometer.

    http://catalog.starrett.com/catalog/...sp?groupid=574

    Got the pictures yet?
    Jim

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