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Thread: Finding the center of a tapped hole

  1. #1
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    Default Finding the center of a tapped hole

    Is there a way to do it without using one of those screw in gauges?

  2. #2
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    No different than a plain hole, really. Just use the thread ID as your reference. When I have been tired and not thinking straight, I've worked a tap down into the hole sometimes and centered on the end of the tap. Its pretty fast and somewhat accurate if all you need is to rebore or something.
    Sometimes the professional is hidebound by tradition while the skilled amateur, not knowing it can't be done blazes a new trail. -JCHannum

  3. #3
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    Quote Originally Posted by Kibby
    No different than a plain hole, really. Just use the thread ID as your reference. When I have been tired and not thinking straight, I've worked a tap down into the hole sometimes and centered on the end of the tap. Its pretty fast and somewhat accurate if all you need is to rebore or something.
    This is my prefered method as it also tells you if the hole is taped sq

  4. #4
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    Allen grub screw fitted upside down, eyeball the point or centre of the dimple.

    .
    .

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.




  5. #5
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    What is this for? How critical is this measurement?

    Do you have a screw with a shoulder on it that you can thread into the hole to measure from?

    Depending on the hole and the drill bits you have available, you may find that a drill bit will sit fairly snug in the hole. Or maybe a dowel pin.

    If you have a mill, or at least a drill press, you may find that lightly chasing out the mouth of the hole with a countersink can be done by eye with a fair amount of accuracy. This requires a bit of patience and maybe some side lighting with a lamp or a flashlight. There is no need to drill very much here. Just take off enough to make a bright evenly sized shoulder around the rim. The size of the edge will tell you if you are centered or not. When you are finished, your quill will now be lined up on the exact center of the hole. (This procedure assumes that the material isn't hardened and that this slight modification will not affect the performance of the item.)

    If this is a production item, parts are frequently sized to have similar characteristics throughout and other holes of the same size will often be the same distance from the edge or spaced the same. If this hole appears to follow a pattern shared by other holes, you may be able to use a little common sense to figure out where the center of the hole would be. Take note of the origin and the measurement standard used. If it's metric it will likely be a nice round number like 100mm or 150mm. If the standard is SAE you can usually round to the nearest 1/8" (0.125) and still be safe. (your mileage may vary on this and there are no hard and fast rules here. just keep in mind that engineers are just as lazy as the rest of us and will repeat steps as much as possible to save time and effort.)

  6. #6
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    Take a SHCS with the same thread as the hole you are checking. Turn the head down in a lathe so the head is concentric with the threads. Screw in the cap screw and indicate the the turned head. That should tell you exactly where the tapped hole is.

    Below are some transfere buttons I made over the years to locate blind holes.
    Often used in Die work for mounting punches. These can be used for threaded holes if the shank is the same as the tap drill size.



    Outback
    So much to learn, so little time

  7. #7

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    Screw in a shoulder bolt and indicate the shoulder near the surface of the part.

  8. #8
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    ok, so it looks like most of the replies are based on some variant of "screw something into the hole". But what if it's some weird thread that you don't have a screw for? I assume that if I jam a center finder into the hole, I'll be off by no more than the pitch depth, but that may or may not be good enough.
    It seems that the shallower the angle of the center finder point, the more accurate this will be.

    The other methods of "drop a smooth-walled object into the hole" require me to have or make something with the same inner diameter. My goal, in this specific case, is to make a countersink for an existing tapped hole, of which I don't have a screw for. I may have to rely on the eyeball method after all.

  9. #9
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    Quote Originally Posted by beanbag
    The other methods of "drop a smooth-walled object into the hole" require me to have or make something with the same inner diameter.
    Drop a transfer punch into the hole and indicate off it.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  10. #10
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    I usually screw a bolt into the hole and use a co-ax to indicate it in. If you have to be super accurate I guess you would have to machine a shouldered plug to fit in the hole and indicate off the shouldered area.
    It's only ink and paper

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