small length on cylinder

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  • small length on cylinder

    Ok I need to know what others have done in the past.

    I have a part that has a counterbore from a face. The counterbore diameter is 114mm and the depth is only 5mm. Print callout has cylindricity on it of 0.25. The interesting part is the also happens to be the A datum. B datum is the face the counterbore goes into and that has a flatness of 0.25 as well as a perpendicularity of 0.25. The opposite side has a similar face and counterbore, but this bore has 32mm of length and the callout is 0.25 runout to datum A. On each face there are bolt hole patterns and one of them has a TP callout as well of 0.25 to A and B.

    Question is has anyone had this before?

    How sound will the cylinder be with so little length and it being a datum.?

    I do not have parts yet so I am just prepping in advance to make sure I can get the best results.

    Any suggestions are welcome as to what tip is best to use etc.

    Thanks in advance for all the help.

  • #2
    I would use the plane to align the part and measure it, to help keep my hits normal to the surfaces.

    For the output, -A- is -A-. You're stuck with it.

    I would measure both bores with the same number of hits per circle (doesn't matter how many levels for what I'm about to say), and start them so that they hit the same side of the probe to start. Put a dot on the shank of the probe, start at I=1.0 measure. Move the head to the other side, where now I=-1.0 to contact the same side of the ball to start. I would do this to factor out the tri-lobing from my probe by making it hit at the same approach vectors in the mechanism of the probe.

    The short cylinder sucks, but I've been stuck with it too. Keep the number of hits per level prime, adjust the quantity, and pray. It is runout, so you need to hit more than three times per level, but I would test from 13 to 23 to 31 to see what gave me a reading I could come close to verifying on a surface plate (since you presumably won't have a fixture to spin the part on -A- with while you indicate the other bore to measure it accurately and correctly).

    Good luck.

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    • #3
      Thanks for the feedback. I know it will be rough but I figured to check here if anyone else had ideas to make sure I was not missing anything. Is there any value in measuring the A datum with more than 3 levels with how short it is? I am not sure I will gain much more accuracy. I will make sure to hit both cylinders with same start point. I know the lobing is a factor. Has anyone purposfully rotated a part to elminate the worse part of the lobing if they know where it is? I am not sure for what I have to tolerancing that it is the largest factor, but I know it still can change results.

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      • #4
        I've never noticed a probe qualify and have a sizeable difference in the way it lobes, though I'm sure it is possible.

        On parts I do with a deep bore (20X the length of the diameter) I generally never worry about more than 3 levels on the datum, as I'm simulating a perfect cylinder for measurement purposes. What I do, however, is measure a cylinder, and align it. I then measure the same cylinder again. If the X, Y or Z and I, J or K have movement in the first four decimal places, I align that. I then measure it again, and so on and so on. When I notice it doesn't shift anymore, that is the point I stop aligning.

        On your short cylinder, I would suggest the same type of thing. Keep measuring it and aligning until you notice no changes that you consider large enough to have concern over. If you are curious about the accuracy based on levels, get it plumb and then add a level. See if it stays true. For time's sake, you want the fewest number of hits and levels that will give you accurate and repeatable results.

        Please note, I only do 3 levels on a 20 inch long bore for the datum. When measuring the bore, I need form, so I'll pop a circle in it every .5" or so to measure it and look at my axis for possible curvature. You won't have this problem though

        Also, watch the way the probe qualifies. If the probe is at A90 B90 and hits the sphere moving straight on Y, hit the part straight on Y. The CMM learned the probe's form with the tri-lobing, copy that over to the part with the same number or hits around the diameter of your part that went around your sphere (this is only if you really have a tight tolerance and/or repetition problem). I do this when measuring coaxiality .001" to factor out all the error I can, as I have noting to play with. You may not have to worry about it so much with .25mm. But if you are seeing problems, it might be worth a try to factor something out.

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