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Thread: Trick to measuring a hole diameter

  1. #1

    Default Trick to measuring a hole diameter

    I am boring out some holes for tool holders and I am having a problem measuring the diameter of the holes.

    Typically I use my calipers, put the inner jaws in the hole and pull them tight against the walls in a couple of places to make sure I am at the max diameter of the hole. Then I adjust my lathe to take off some more working towards the final diameter of 1.125.

    The last couple of holes I have been measuring and based on my measurement and lathe adjustment, should be .005 undersized (ready for a final finishing cut), yet when I put in my actual toolpost into the bored hole, the hole is too big already. Measuring again with the calipers I get a diameter bigger than it should be.

    I suspect my problem is my hole diameter measurement technique. Any suggestions for a technique to get a repeatable accurate diameter measurement?

    Hobby machinist having fun,
    - T

  2. #2
    Join Date
    Aug 2005
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    Atlanta Ga USA
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    Default

    Calibration of measuring instruments has been the forever problem.
    Many, MANY reasons.

    The best low cost standards system for many decades has been stacking/ wringing Jo-Blocks to use to set an outside micrometer or test gage. This allows setting inside micrometers or test gage.

    In your case, calipers (dial or vernier) are usually considered inferior to micrometers.

    Suggest you purchase a set of ball plug gages and telescope gages and use them with a micrometer, which (the micrometer) usually come with a secondary standard.

    To be sure "diameter" micrometers and gauges also have problems spotting lobed out of round situations, but they usually beat calipers all hollow.

    More can be said, but depends on how you reply.

    Hth Ag

  3. #3
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    You asked about a round hole for your boring bars....
    As you know the target dimension... make a stepped plug gage with steps in .001+/-.0002 increments x .2 long. The step diameters are easily held over the short distance by stoning them to size. Start .005 under your final size so you know when you're getting close.

    Use a split dowel and emery clothe to take the last .001-.0015 out.

    Now here's a question for you... why does the boring bar have to go into a round hole? a "V" with a bolt on top clamp is much easier to make.

    http://www.kdktools.com/catalog.PDF Refer to page 6, 5-B boring bar holder, for the general idea. Can be made to fit any size.
    Ignorance is curable through education.

  4. #4
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    calipers only do a very rough job of measuring ID's, well rough in general but particularly bad at ID's......use mics when you want to do accurate work.

    The first step up is a set of telescoping gauges and small hole gauges. you measure them with a mic. there are expensive items that will do a better job, but they're not common in the home shop

    There is a very accurate home shop solution. An close fit can be had using plug gauges, and a trick is to turn some steps on them. Turn an go/no go to keep you work between to limits....all based on the notion that is easier to measure OD than ID

    the good news is you can buy all this stuff Starrett or Mit for not money on ebay these days

  5. #5
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    Rusty
    About bar sockets.
    Rigidity, Ruggedness, and equal deflection in all directions.
    Ag

  6. #6

    Default

    T,
    Your measuring techniques for the hole may be just fine. Every time I've experienced what you describe when boring on my lathe, the discrepancy always comes from my having the cutting tool not properly on center. This renders measurement in the movement of the compound un-reliable in determining the amount of increase the movement causes in the diameter of the hole.

  7. #7
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    Mar 2001
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    East Iowa
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    Have you been getting the cuts you expected each time you made a lathe adjustment, ie: if you turned the dial .020" were you getting a corresponding cut, or something slightly different??
    You could be running into a tool deflection/machine rigidity problem were your last cut is cutting more than you thought it would.
    Did you happen to make 2 passes on the last setting when you had only been making 1 pass before??
    One of the tricks to running any machine is knowing what to expect, and adjusting accordingly.

  8. #8
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    May 2002
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    NL
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    Not to go to far off post, but to answer the question:

    "why does the boring bar have to go into a round hole? a "V" with a bolt on top clamp is much easier to make"

    The round clamping type is much better then the Vee type in preventing vibration. You do not have to take my word, just try it sometime
    e2die
    please visit my webpage:
    http://motorworks88.webs.com/

  9. #9
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    Using calipers to measure a bore is not a good idea to get accuracy. you use the calipers to get within say, .030" and then measure with telescopic gage and a micrometer.

    You are probably trying for a slip fit and you won't get it using calipers.
    It's only ink and paper

  10. #10
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    Every caliper I have, or have used to measure a hole diameter, shows the hole being about 2 to 4 thou smaller than it actually is. This is because the ID jaws overlap by a small amount so they don't jam when the caliper is closed all the way. Drill and ream a hole in some scrap, and call that a reference hole. You should be able to confirm it being on size within very close tolerances by using a test rod of known diameter. In my case, I use a piece of hardened .250 diameter rod. I'll ream a hole then check it with the rod, then measure both the rod and the hole with my caliper. The reading for the hole will be about 2 1/2 thou less than the actual hole size. When boring a hole to size then, I stop when the reading is short by that amount.

    I agree that it isn't easy or particularly accurate to use a caliper to measure a hole diameter, but you can be a lot closer than what you're seeing by taking into account the error which your caliper introduces in the reading, and by taking much care in using the caliper.

    Two more things- one is that it's so easy for an off-height cutting edge to oversize a hole, especially for small diameters, and secondly the likelyhood is that the tool holder, compound, etc will want to 'relax' towards you without being dialled in. On my lathe, that can result in about 20 thou or more difference in the positioning of the cutting edge, without dialling in or out.

    When turning an OD, the cutting edge is relaxed towards me, so I can take multiple passes without the tool changing relative position. The position of the tool is fully controlled by the leadscrew.

    When turning an ID, I will often put some forward pressure on the tool holder to keep the 'float' in the slides towards the rear, or away from me. That lets the leadscrew define where the cutting edge is going to be. Multiple passes without this method of play reduction will have the cutter drifting towards me, and subsequently a bored hole will become progressively large with multiple passes, even though the feed dial hasn't been moved.

    It doesn't end there. You have to characterize the cutting action when you're close to final size. If you dial in, say 5 thou, does it really increase the hole diameter by 10 thou, which is what you would expect? If you dial in 2 thou- do you get a 4 thou size increase? You need to find out how much material is actually coming out for a given depth of cut via the dials. You might find that dialling in for 5 thou is actually removing 12 thou total, and that dialling in for 2 thou might actually not be taking off much at all.

    Sometime you find that even if you started with a sharp cutter, by the time you get near your desired size the cutter won't be able to take off very small amounts, and would either just burnish the surface, or hog out a few thou instead.

    Then there's chip weld. Sometimes the effective position of the cutting edge will change because of this.
    Last edited by darryl; 08-03-2009 at 06:50 PM.

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