How to align this part on a CMM?
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    Default How to align this part on a CMM?

    Hi,

    We are having some debate about how to align this part for inspection, could you please tell me the correct way to align this part on a faro arm/CMM to measure the the hole true positions.

    At the moment I am:

    1. setting part down on granite table on red face.
    2.Measuring datum plane A
    3.Measuring datum dia B
    4. measuring all holes
    5. set up coordinate system with plane A to set Z direction and a line between datum datum dia B and a the csk hole that is at 12 oclock, then datum B dia as the origin.

    Then once the co-ord system is setup I just measure true positions to A and B.

    Is this correct way to measure true position of these holes? If not, what is the correct way?

    Thanks
    Attached Thumbnails Attached Thumbnails view-1.jpg   view-2.jpg  

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    It might help if we knew the dimensions and tolerances. Many or few made?

    Off hand what you're trying to do looks like overkill.

    How was it machined? Or on what if you prefer.

    BTW a one triangle finish doesn't look like high precision work.

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    Quote Originally Posted by Gordon B. Clarke View Post
    It might help if we knew the dimensions and tolerances. Many or few made?

    Off hand what you're trying to do looks like overkill.

    How was it machined? Or on what if you prefer.

    BTW a one triangle finish doesn't look like high precision work.

    Hi, unfortunately I can include all the dimensions and tolerances for obvious reasons, I just need to know what the correct way to measure true position of the holes as there is a little bit of a dispute at work because the first 3 were 100% inspected and found to be correct, then the rest of the batch of 8 were run off and were all later found to have holes out of position. But some people are getting the holes out and others are getting them in, hence the dispute.

    For reference the OD it 250mm and overall length is 55mm.

    Can you please explain why this is overkill? Im a novice when it comes to inspection, if its overkill what should I have done?

    The part is machined in 2 ops on two haas ST30's with live tools:

    OP1: grip on half of the largest OD, machine the large flat face highlighted in red, drill all holes and csk, machine the tapered bore.

    OP2: locate the sharp end of the tapered bore on a fixture and use a tapered washer that attaches to the drawbar to pull the part back to the fixture, a drive pin locates in one of the holes. Then on OP2 machine datum B and the counter-bore.

    What do you mean by a "one triangle finish"? Ive never heard this terminology before. I never said this was high precision work, but with any work the parts still have to be to drawing...

    Let me know if that doesnt make sense or you need more info, but basically I need adive on how to properly measure the true positions

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    Quote Originally Posted by KristianSilva View Post
    Hi,

    We are having some debate about how to align this part for inspection, could you please tell me the correct way to align this part on a faro arm/CMM to measure the the hole true positions.

    At the moment I am:

    1. setting part down on granite table on red face.
    2.Measuring datum plane A
    3.Measuring datum dia B
    4. measuring all holes
    5. set up coordinate system with plane A to set Z direction and a line between datum datum dia B and a the csk hole that is at 12 oclock, then datum B dia as the origin.

    Then once the co-ord system is setup I just measure true positions to A and B.

    Is this correct way to measure true position of these holes? If not, what is the correct way?

    Thanks
    Sounds good to me, only question is are you measuring the satellite holes as cylinders or 2d circles? The difference being cylinders measured to datum A will be evaluated like perpendicularity in addition to position, Where as 2d circles will only give you a coordinate value of where the holes are.

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    Quote Originally Posted by Jake E CEI View Post
    Sounds good to me, only question is are you measuring the satellite holes as cylinders or 2d circles? The difference being cylinders measured to datum A will be evaluated like perpendicularity in addition to position, Where as 2d circles will only give you a coordinate value of where the holes are.
    We are just measuring 2D circled projected onto datum plane A. Should I be measuring cylinders?

    A colleague is saying that to align the part in X a line should be taken between two holes that lie in the same position in X, is this right or wrong? or would it not make a difference? Just trying to get to the bottom of which way is right

    Thanks for you help

    Could you explain a little more about how measuring cyclinders would be evaluated as I don't quite follow. How would the position of the cylinder be determined as would that not effectivley be asking the software to evaluate the position of a line that isnt perfectly square to the datum plane? So which point on that line would the position be taken from?

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    Quote Originally Posted by KristianSilva View Post
    For reference the OD it 250mm and overall length is 55mm.

    Can you please explain why this is overkill? Im a novice when it comes to inspection, if its overkill what should I have done?
    My reference to "overkill" very much depended on the size and tolerance and of course quantity. What I'd do, rather than measure "horizontally and vertically" is to measure diagonally. Basically the same principle as when tightening on a flange or changing a tyre.

    Your diagonal dimensions should be easy to calculate (basic trig) and I'd use a "cylinder" with a "point" to find the center of the holes.

    Of course this all depends on your dimensions and tolerances but I can't imagine it is precision work. Something like this might be enough depending on the tolerances and can be done by whoever is machining.

    http://www.f-m-s.dk/Pitch%20circle.pdf

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    Quote Originally Posted by KristianSilva View Post
    We are just measuring 2D circled projected onto datum plane A. Should I be measuring cylinders?

    A colleague is saying that to align the part in X a line should be taken between two holes that lie in the same position in X, is this right or wrong? or would it not make a difference? Just trying to get to the bottom of which way is right

    Thanks for you help

    Could you explain a little more about how measuring cyclinders would be evaluated as I don't quite follow. How would the position of the cylinder be determined as would that not effectivley be asking the software to evaluate the position of a line that isnt perfectly square to the datum plane? So which point on that line would the position be taken from?
    Both ways you explained are right (your coworkers method is also valid - if only more technically correct, but it really shouldn't affect the outcome in the method you originally described), if you generally don't have perpendicularity issues with the holes then measuring the satellite holes as a 2d circle is fine, just keep in mind that they will only have the position evaluated (X and Y value). With how the datum scheme is called out, position to -A- doesn't really mean anything unless evaluated as a cylinder - then perpendicularity comes into play. When position of a cylinder is called out to a perpendicular surface than it will factor into the evaluation.

    It all boils down to a cylinder being a line reduceable feature and a circle being a point reduceable feature. So when the position is called out to datums -A-, and -B-. With -A- being a plane, then the cylinder needs to be perpendicular (or parallel depending on the situation) to -A- or your position will be off. With a 2d circle you won't have that problem.

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    Quote Originally Posted by KristianSilva View Post
    We are just measuring 2D circled projected onto datum plane A. Should I be measuring cylinders?

    A colleague is saying that to align the part in X a line should be taken between two holes that lie in the same position in X, is this right or wrong? or would it not make a difference? Just trying to get to the bottom of which way is right

    Thanks for you help

    Could you explain a little more about how measuring cyclinders would be evaluated as I don't quite follow. How would the position of the cylinder be determined as would that not effectivley be asking the software to evaluate the position of a line that isnt perfectly square to the datum plane? So which point on that line would the position be taken from?
    I don't see a perpendicularity call out for the "satellite" holes, so that doesn't need to be measured. If the designer wanted that measured he should have added it to the control frame.

    To answer your question about cylinders though, PC-DMIS would evaluate a best fit cylinder inside the hole. Then it would determine the position of the cylinder by extending its axis to the point where it intersected the datum plane. In this case the holes themselves intersect the datum plane, so position would be evaluated at the center of the "bottom" of the cylinder.

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    Quote Originally Posted by Rewt View Post
    I don't see a perpendicularity call out for the "satellite" holes, so that doesn't need to be measured. If the designer wanted that measured he should have added it to the control frame.

    To answer your question about cylinders though, PC-DMIS would evaluate a best fit cylinder inside the hole. Then it would determine the position of the cylinder by extending its axis to the point where it intersected the datum plane. In this case the holes themselves intersect the datum plane, so position would be evaluated at the center of the "bottom" of the cylinder.
    Thank you for more clearly elaborating on my point about cylinders and positions. I only bring it up because if evaluated as a cylinder and there is a perpendicularity issue, it will incredibly skew your results for position. If the best fit cylinder is not perpendicular the intersection point will be moved in whatever direction your best cylinder is orientated.


    Again I brought up the cylinders and perpendicularity idea because while the blue print does not specify it exactly, a engineer may ask for that data if there is a issue with the satellite holes at the customer. If a part is rejected by the customer, my first question would be how they evaluated it, if the customer evaluates the holes as cylinders and the manufacturer evaluates them as 2d circles, there will be a drastic difference in measurements of true position which may result in the customer rejecting a shipment. While not specifically stated on the print, it is my experience that it is a good idea to try and see any potential issues before they arise.

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    Quote Originally Posted by Jake E CEI View Post
    Both ways you explained are right (your coworkers method is also valid - if only more technically correct, but it really shouldn't affect the outcome in the method you originally described)
    So are you saying my co-workers method is more valid? If so, why?

    Quote Originally Posted by Rewt View Post
    I don't see a perpendicularity call out for the "satellite" holes, so that doesn't need to be measured. If the designer wanted that measured he should have added it to the control frame.
    I agree, although there is a 0.8 perpendicularity tolerance "built in" to the 0.8 position tolerance, but I am certain we are achieving that.

    Quote Originally Posted by Jake E CEI View Post
    Thank you for more clearly elaborating on my point about cylinders and positions. I only bring it up because if evaluated as a cylinder and there is a perpendicularity issue, it will incredibly skew your results for position. If the best fit cylinder is not perpendicular the intersection point will be moved in whatever direction your best cylinder is orientated.
    So just to clarify, when measuring the position of a cylinder the point that is evaluated is the point at which the best fit cylinder intersects the datum plane, hence why if the cylinder isnt perpendicular, there will likely be big issues with hole position?

    The best fit cylinder is best fit to the actual hole not a best fit cylinder that takes perfect form, i.e perfectly round and perpendicular to the datum plane?

    Think im starting to get my head round this

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    I'm going to write what I'm thinking. The drawing in the OP has got to have been drawn by someone that has never set foot in a shop.

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    Quote Originally Posted by KristianSilva View Post
    So are you saying my co-workers method is more valid? If so, why?



    I agree, although there is a 0.8 perpendicularity tolerance "built in" to the 0.8 position tolerance, but I am certain we are achieving that.



    So just to clarify, when measuring the position of a cylinder the point that is evaluated is the point at which the best fit cylinder intersects the datum plane, hence why if the cylinder isnt perpendicular, there will likely be big issues with hole position?

    The best fit cylinder is best fit to the actual hole not a best fit cylinder that takes perfect form, i.e perfectly round and perpendicular to the datum plane?

    Think im starting to get my head round this
    Both methods will work because there isn't a tertiary datum, so there isn't a specified clocking feature. You could align the part with either a CSK hole or through hole in any position, all you're doing is telling the software where the specified feature is so it knows where all the other satellite holes are.

    Putting whichever hole you're aligning to in specifically the X or Y axis, like your coworker suggested, makes more sense as far ease of use and understanding after measuring the part, but I wouldn't say your method is wrong or will make much of a difference to how the measurement will be evaluated.

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    Quote Originally Posted by Jake E CEI View Post
    Both methods will work because there isn't a tertiary datum, so there isn't a specified clocking feature. You could align the part with either a CSK hole or through hole in any position, all you're doing is telling the software where the specified feature is so it knows where all the other satellite holes are.

    Putting whichever hole you're aligning to in specifically the X or Y axis, like your coworker suggested, makes more sense as far ease of use and understanding after measuring the part, but I wouldn't say your method is wrong or will make much of a difference to how the measurement will be evaluated.
    When aligning one of the holes to the X axis, this means that the hole we align of has zero deviation in X, but the problem is we are getting two of the holes out of position when we align the part like this.

    Is it a "legal move" for us to put a rotation onto the established co-ordinate system so that all the holes that are close to nominal move a little further from nominal but so the holes that are outside the true position tolerance move inside the tolerance zone?

    I know this wouldnt be an option if we had a datum C but we are only referring to 2 datums here so is there anything stopping us rotating the co-ordinate system to "share" the inaccuracies across the holes instead of having one hole bang on nominal which we have aligned from?

    Hope this makes sense

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    Quote Originally Posted by KristianSilva View Post
    When aligning one of the holes to the X axis, this means that the hole we align of has zero deviation in X, but the problem is we are getting two of the holes out of position when we align the part like this.

    Is it a "legal move" for us to put a rotation onto the established co-ordinate system so that all the holes that are close to nominal move a little further from nominal but so the holes that are outside the true position tolerance move inside the tolerance zone?

    I know this wouldnt be an option if we had a datum C but we are only referring to 2 datums here so is there anything stopping us rotating the co-ordinate system to "share" the inaccuracies across the holes instead of having one hole bang on nominal which we have aligned from?

    Hope this makes sense
    The hole you are aligning the part with, is there any deviation in the Y axis? I'm curious if the satellite holes aren't concentric with Datum -B-. If you build a circle with the center point of all the satellite holes it should be concentric with the Datum -B-. Something sounds a little off, just not sure if it is the origin point not being established in the most robust manner or something else. I would believe that you are right and that all the holes are deviating from nominal and not one hole being perfect and the rest being out of tolerance, just not sure why you're seeing that issue, hard to diagnose without seeing the measurement being performed.


    The other thing you could do, if you are using a CAD model. Measure the satellite holes and report their profile deviation value, you should be able to make a graphical report and see which direction(s) the holes are off in.

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    Quote Originally Posted by Jake E CEI View Post
    The hole you are aligning the part with, is there any deviation in the Y axis? I'm curious if the satellite holes aren't concentric with Datum -B-. If you build a circle with the center point of all the satellite holes it should be concentric with the Datum -B-. Something sounds a little off, just not sure if it is the origin point not being established in the most robust manner or something else. I would believe that you are right and that all the holes are deviating from nominal and not one hole being perfect and the rest being out of tolerance, just not sure why you're seeing that issue, hard to diagnose without seeing the measurement being performed.


    The other thing you could do, if you are using a CAD model. Measure the satellite holes and report their profile deviation value, you should be able to make a graphical report and see which direction(s) the holes are off in.
    There is some deviation in Y, but not too miuch to be causing problems, maybe 0.2mm.

    I think the point of origin is robust, I just think something went wrong during machining, holes being maybe 0.2mm out in Y, then bolt hole circle not being perfectly centre to the datum. Think its a combination of a few things. Will investigate further nect time the job is on but for now I managed to get all holes in tolerance by rotating the co-ordinate system, JUST.

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