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  1. #1
    whiskeykid is offline Aluminum
    Join Date
    Feb 2008
    Western Canada

    Default Large ID Measuring Advice

    I'm building something larger than I have the means to measure. It's an 18" rotating head (oilfield thingy) and both the ID and OD of the two separate parts have to be turned to fairly tight tolerances due to a huge tapered roller bearing that gets slip fit in between the two.

    Anyhow, I'm honestly not sure how I'm going to accurately measure the two parts without purchasing a large set of internal mics, which I feel would be an excessive cost for a 'one-off' product. My original intention was to get the bearing first, rough out each piece, and then sneak up on a fit. It looks now like the bearing will be the last thing to be delivered.

    18" caliper maybe?

  2. #2
    Ferrous Antiquos is offline Titanium
    Join Date
    Mar 2004
    Lawn Guylin, Noo Yawk

    Default Mic heads to the rescue

    Why dont you just make your own temporary inside mic to suit?
    Get an inside micrometer head off fleabay.
    Get a hunk of bar stock, a few inches shorter than the ID of the workpiece.
    Press a rounded-end pin into one end.
    Fix the mic head onto the other.
    Voila ! A precision, adjustable inside mic.

    You could use the mic head to make an outside one also:
    Hunk of flat stock, one end bent over at 90deg.
    Fix the mic head into a block that can be clamped to the other end for rough adjustment.

    On second thought, forget part two - if you have the OD part turned, just clamp two blocks onto a piece of flat stock to make a "snap gage" which will establish a standard for the OD. Then use this to set the temporary ID mic to the required ID.
    Last edited by Ferrous Antiquos; 05-02-2008 at 01:16 PM. Reason: a better idea

  3. #3
    PeteM is offline Diamond
    Join Date
    Jan 2002
    West Coast, USA


    The entire ID mic from Ebay could be cheap -- $50 or so. Unless you can use the bearing as a reference, calibrating the mic or the gages noted above may be a slight headache depending on what equipment you have. Thermal effects will also be an issue in accuracy, if the part, mic, and bearing are at different temperatures.

  4. #4
    Forrest Addy's Avatar
    Forrest Addy is online now Diamond
    Join Date
    Dec 2000
    Bremerton WA USA


    Hell, make longer rods for your existing inside mike. Calibrate it anyway you can and note the error. Use it as a comparative device compensating for the knwn error. A hunk of rod with an indicator on one end makes a handy ID comparator. Set a zero and double check. OD mike to zero on the indicator dial and you are there.

    Make a pin gage: a rod whose ends are roughly spherically raduised and precision fitted to an ID. If you accidentally make it too short lengthen it by cold forging the middle.

    You don't need precision measuring gear to get good fits if the parts to be fitted can be referred to. All it takes is some rod and a dia indicator and a little imagination - and an understanding of what the heat of your hand can do to precision measurement.

    You only need measuring tools if you have to certify a measurement on paper.

  5. #5
    winger is offline Hot Rolled
    Join Date
    Dec 2005


    A PI-tape would be an inexpensive alternative to seldom used outside mikes over a large range.The last one I used read to .001 . I don't know if they read closer.

  6. #6
    FlatBeltBob is offline Stainless
    Join Date
    Jul 2007
    central WI


    A pi tape will get you close on the OD if it is a true circle , maybe within .001".
    If you can make an adjustable OD mike , and use the bearing as a reference to calibrate it , you are half way home .
    I have a few shop made "bow mics " . Just saw or torch a half circle , make a few collars on to hold the mic head , and an adjustable rod , and set it as close as you can .
    We used to take them to the gage room to calibrate , then stick a tag on that would say something like " reading is .0055" larger than actual ".
    I always view inside mics as measurement transfer tools , instead of true measuring tools .
    Especially in the case of the 2-12" and 8-32" Starret sets that you assemble from rod & spacers . I always use an outside mic to check an inside mic.
    Forget any calipers , they are just slightly more accurate than a 6" rule .

    I would not make a single chip untill you have that bearing sitting on your bench
    Last edited by FlatBeltBob; 05-03-2008 at 12:14 AM. Reason: added comment

  7. #7
    Troup is offline Titanium
    Join Date
    Jun 2007
    New Zealand


    I think the original poster stated his problem as being that he won't get the bearing in time to use it.

    Is there a local bearing house which could lend you a bearing cup of the same OD?

  8. #8
    gmatov is offline Banned
    Join Date
    Feb 2006
    SW PA


    I'd buy an inside set from 1 1/2 to 32, though I don't know what they cost today. Mine's way more than 40 years old, bought the toolbox from my lead man when he retired, 40 years ago.
    Probably prohibitive from a one off job aspect.

    This is not the For Sale Thread, but I have some mike heads with sleeve, shown here:

    They take a 3/8 rod tapered on the other end, radius the end a bit, set with the means of your choice, large outside mic, maybe, if you have one.

    We used these, and had them calibrated in the tool room, to about 20 feet. Tape on the sleeve is because that is where it was supported on the calibrator, you also hold it there, and over 20 foot, temp transfer was negligible.

    ALWAYS set at least 24 hours in advance so could acclimate to the temp in the machine area.

    Innyhoo, I have no idea of their value, I would entertain an offer for one.



  9. #9
    whiskeykid is offline Aluminum
    Join Date
    Feb 2008
    Western Canada


    Thank you all for the replies. I think that building an extension for an internal mic is the most practical solution. Incidentally, the part is due to go out to the field on June 1st, but the bearing isn't due to arrive until the last week of may, hence my choice to start building the part before the bearing arrived. Unfortunately though, the bearing was delayed even more so the design has changed to fit a more common and readily available double tapered roller bearing. I haven't heard when that one should arrive, but I'm assuming it'll be here within a week or two.

    Thanks for all the advice though, it's always good to learn from the vast experience here at PM!


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