Hitting the spec .0015
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  1. #1
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    Default Hitting the spec .0015

    Hello all,
    I hope this is the right place for my question.
    I am turning a new 9" chuck mount for my DSG lathe.
    I have a copy of the OEM drawing for the mounting plate.
    They made their own version called a "Fastlock" for a few years. I was lucky enought to get one of these lathes....
    Anyhow, the id is 6.250 (-.00000) and (+ .0015) I have got the piece close to final size and I am concerned about hitting my mark, I have a very nice near new Moore and Wright inside mic, I also tried to double check with my 24" Starrett vernier caliper. I got two different readings.
    Which one do I trust?
    I don't want to dismount the chuck right now and measure my one and only reference. I know I should have measured it before I started to double check my inside mic, but I didn't (hindsight is 20/20).
    I am at about 6.230 right now.

    Maybe I am over concerned about this but I would really like to get it right, It has been a lot of work to get the large chunk of steel to this point.....
    Some help from those who have done this a thousand times would be great.
    Thanks
    Michael Moore

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    Presuming you have no setting rings for your inside mic, use the inside mic as a gauge, and measure that with an outside mic, similar to how you use a telescope gauge.

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    Quote Originally Posted by M. Moore View Post
    I have a very nice near new Moore and Wright inside mic...
    Moore & Wright make different styles of inside mic, including caliper and tubular. Which do you have?

    With any two-point measuring instrument the challenges are:
    1) To ensure that the plane of measurement is perpendicular to the feature axis; and
    2) To ensure that the measurement is taken on the diametral axis, not off to one side.

    Depending on your instrument and your familiarity with it, these may be minor points or major concerns.

    As Tony said, your best accuracy may be obtained by using the inside mic as a setting gauge, locked in position, then measuring it with an outside mic.

    In all cases, the accuracy of the measurement should be verified with a 6.2500" stack of gage blocks, with any error used to correct the reading. A ring gage would be nice, but one of this size would be VERY expensive if you could find it at all. (Meyer only makes ring gages up to 4.010".)

    - Leigh

    (p.s. - My 1" outside mic was made by Moore & Wright. I bought it used 45 years ago, and it still works great.)

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    Tony,
    Thanks for that, I will try it. No I don't have any setting rings.
    I am having a little trouble with the inside mic, it seems difficult to get a good reading, do you tighten it inside the bore and read the scale? I want to be able to just move the mic a little....but then I have a 1/2 thou or so of movement on the scale, anyway I am finding it difficult to get an accurate reading. How do you know if you are exaclty on centre when taking your reading with and inside mic?
    That is what I need some tips on. I will try your tip and go from there.
    Michael

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    Leigh,
    Thanks for that, you nailed it. I am guessing my inside mic is tubular?
    And thanks for the extra stuff regarding using gage blocks etc, I do have a set of gage blocks.
    You guys are great, this is what I need to know to get to where I want to go!
    Sorry for the super dumb beginner question.
    Michael

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    Hi Michael,

    One source of error with tubular mics occurs when attaching the extension tubes to the micrometer head. Any error in the mating torque translates to a dimensional error.

    One way to check the inside mic is to make a snap gauge using your 24" vernier and gage blocks. Make a block stack 6.250". Close the vernier jaws over the stack, with the blocks up against the long vernier back, and lock the movable jaw in place. This leaves the jaws protruding.

    Measure the distance between the vernier jaws with the inside mic. It should read exactly 6.250". If not, determine the source of the error and correct it.

    Beginner questions are always welcome. We were all beginners at one time or another (or still).

    - Leigh

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    Use the inside mike (inside caliper whatever) as a transfer gage and measure the bore to get a setting only. Then outside mike over the ID mike and use it to follow size on the back plate to finish dia. This is called "comparative measurement" and it eliminates instrument to instrument claibration error as a factor in obtaining mating fits.

    Also very shallow bores like rabbet fits in chuck backs can be difficult to check. Many mike thimbles are too fat to get the measuring contact inside the actual bore.

    Make a "pin gage" a one-time gage made from keystock or what ever is convenient. Cut it a bit over-size and bevel the ends sorta like pencil points. Then work down the length with a file or a stone til you get a definite drag fit in the rabbet bore. Rub it in the bore a few times to "wear it in". Remember the heat of your hand will affect the length of the gage. Always make the final fitting and checks with the gage and part at equal temperatures and use rags and gloves as insulators when you handle it. Make it too short? No problem. Bang it firmly in the center with a hammer against an anvil to "expand" its length a little and fit it again.

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    Quote Originally Posted by The real Leigh View Post
    \

    With any two-point measuring instrument the challenges are:
    1) To ensure that the plane of measurement is perpendicular to the feature axis; and
    2) To ensure that the measurement is taken on the diametral axis, not off to one side.
    Let me add a very important third challenge... the lobing condition of the bore. With an odd number of lobes in the bore, a two-point system could easily see it as round.

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    Quote Originally Posted by Peddler View Post
    Let me add a very important third challenge... the lobing condition of the bore. With an odd number of lobes in the bore, a two-point system could easily see it as round.
    Very true.

    I was assuming that the feature has a uniform profile.

    If that's not true, you'll never get a meaningful measurement with anything other than an air gauge.

    You can make a good argument that an air gauge measurement is inaccurate because it's only showing you an average, not max and min.

    - Leigh

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    Thanks Forest for your input.
    If I had been smart enough to make a gage before I mounted the stock I would be fine. I am hesitant to remove the chuck at this point and then reinstall, in theory it should go right back to where it is? But I can't be sure, never having done that with a critical part in the jaws. It is a buck adjust true chuck but I can't get at the mounting screws to loosen them with such a large diameter in the chuck. OD is 9" necked down to 7.75" and then bored to 6.250.
    There is no problem with the depth of bore, at 1.675" deep there is lots of room to work. My current measurements using the inside mic as a bore gage gives me 6.220 on the starrett calipers, and the same on the inside mic, so that is checking out nicely.
    My question is on using the inside mic properly, if I just barely tighten inside the bore then I can't get it out of the bore, If I loosen it just enough to remove it, then it seems like it is measuring too small?
    Anyway I am feeling more confident about hitting my mark, now that I have had your help.
    One more question, do I creep up on the size on thou at a time or?
    Thanks again all,
    Michael

    ps, Peddler and Leigh, you have left me behind....

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    Quote Originally Posted by M. Moore View Post
    My question is on using the inside mic properly, if I just barely tighten inside the bore then I can't get it out of the bore, If I loosen it just enough to remove it, then it seems like it is measuring too small?
    Insert the mic so the fiducial (the indicator line) is visible.

    Tighten the gauge so it's a snug fit (not a force fit) and note the thimble reading at the fiducial.

    Loosen the thimble, remove the mic, then return the thimble to the noted position. Lock the thimble position if there's a provision to do so, or support the mic so that the setting won't change.

    Then read the length.

    - Leigh

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    Quote Originally Posted by M. Moore View Post
    I am hesitant to remove the chuck at this point and then reinstall, in theory it should go right back to where it is?
    In the general case.... no, it won't go back to the exact same position.

    There are ways to accomplish this, and some people do it routinely.

    But it requires careful planning and preparation, and extensive knowledge of the tools and components.

    This is where the concept of "run-out" becomes important.

    Any chuck will hold a part such that you can bore a hole concentric with the spindle axis of rotation. This axis is defined by the machine geometry and can't be changed.

    Run-out is a measure of how closely the chuck holds the workpiece axis to the machine axis. In an ideal world, the run-out would be zero and the piece would rotate on the spindle axis exactly.

    That never happens in practice. Where it becomes important is when a part is removed and then re-installed in the chuck, with the intent of doing additional work which should be placed accurately WRT the previous cuts.

    In all cases you should make a mark on the work corresponding to some reference point, like jaw #1, so you can put it back in as close to its original orientation as possible.

    - Leigh

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    I got brave tonight, I removed the chuck with the work in the jaws, checked the bore on the mounting plate and checked the spindle size. Then I reinstalled the chuck, no problem right back where it was, so far so good!
    Then I got really brave and bored it out to final spec, 6.250 exactly.
    Well as far as I could measure, it was right on. I checked at several points on the bore and all was fine.
    Finished the outside and removed from chuck, tested onto spindle and it slipped right into place like magic!
    How can you tell I'm a woodworker??
    Now all I have to do is bore 4 one inch holes around the perimeter for the eccentric locking pins. Should be no problem on the K&T 2H with dividing head.
    Thanks all for your help.
    Michael

    Thanks everyone.

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    I maybe a bit late here but maybe you could source some old ball bearing assemblies and look up their manufacturing tolerances from the design sheets and the use them as gauges?
    As I said maybe too late for now but maybe useful in the future.
    Cheers
    Mike
    Last edited by stroker; 09-22-2009 at 11:02 PM. Reason: Missed last sentence


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