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True Position in 3 axis???

1CMM4ME
Ok, I have read the post regarding this so called argument that my boss and I got into today. He told me that you cannot measure true position in 3 axis, as I told him he was wrong. I know it depends on the way the FCF calls it out, but I tried to explain to him that it is possible to measure TP in 3 axis.
I have seen the formulas, so I know it's possible. I just wish I could get one of you gurus to reply back explaining that it is possible and why. I see the picture but he does not. I do not like to get into pissing matches with upper management, but I have learned a h**ll of alot from you guys, and I know this is possible. So if one of you fine gentleman can back me up on this, I would very much appreciate it. Thank you very much.
Now I am going to get a cold beer.Smiley
  • Whether there is a diameter symbol or whether there is a spherical symbol does not determine what axis are applicable (2 or 3) but rather the shape of the target zone. You can have a 3 axis basic dimension scenario for a feature with a diameteric (or cylindrical) tolerance zone. Like I said earlier it makes an odd drawing but I have dealt with it. I believe that is what Matt was getting at in regards to dimensioning a point that is the intersection of a diameter and a face. 3 axis define it but it is still a diameteric tolerance zone not a spherical one as the feature itself is a diameter.

    I disagree to a point. You MUST have three to have a spherical zone. When it is diametric tolerance zone you MUST have at least two BASIC Dims to locate it. You can have only one BASIC if it is a single axis tolerance zone.

    That said, I have seen points with spherical zones. Same with circles in sheet metal. Oh yeah, we have a lot of prints with BASIC Dims all over them. I have holes with 4 BASIC Dims (3 axis locators and a BASIC Angle).
  • I disagree to a point. You MUST have three to have a spherical zone. When it is diametric tolerance zone you MUST have at least two BASIC Dims to locate it. You can have only one BASIC if it is a single axis tolerance zone.

    That said, I have seen points with spherical zones. Same with circles in sheet metal.

    Actually you can constrain a feature to a spherical tolerance zone with one basic dimension provided it is a spherical feature (see P 155 fig 5-62). What I am saying is that just because you have 3 basic dimensions locating a feature that does not necessarily mean it is a spherical tolerance zone. I am saying you can have 3 basic dimensions locating a feature with an axis which means that you have a diametric (or cylindrical) tolerance zone (see attached picture). And yes you can locate a diametric (or cylindrical) feature with only one basic dimension for true position purposes. Actually, when specifying coaxiality you have zero basic dimensions.



  • Maybe it's too early in the morning but I still see NO way to locate a spherical tolerance zone with one BASIC unless as you illustrated there are ASSUMED dimensions as in coaxiality. The point I am trying to make is that you have to LOCK three Axis in place somehow to be able to use a spherical tolerance zone. You have to LOCK two axis in place for a diametric tolerance zone etc. No way around it. I don't see the shape of the feature as having any special significance. Example: A tooling Ball. Put it in space in any reference frame. Give it a locator in one axis ONLY and there is no way you can tolerance it with a spherical tolerance zone. The center point of a spherical tolerance zone MUST be pinpointed in all three axis. Unless all three axis converge on a theoretical center point there is no way to determine deviation from that point. Same goes with a diametric. It has to be locked in two axis in a given workplane to be able to determine deviation from the theoretical center.

    I do agree that there are times that there are three BASIC Dims locating a feature and there is only a diametric zone. However, you cannot USE all three to axis deviations calculate the vector deviation. With a diametric zone you can ONLY use two axis to calculate deviation OR you are constraining the feature to a spherical zone.

    That said, what is your criteria for determining workplane when performing diametric TP tolerancing when there are three or even four BASIC Dims?
  • Like I said look at P 155 fig 5-62. Also picture two spheres or two tooling balls. A basic dimension between the two will constrain to a spherical tolerance zone as well so that makes two examples of constraining to a spherical tolerance zone using one basic dimension.

    For your other example coaxiality is specified as true position, there are zero basic dimensions for coaxiality. Just the same you can control a cylindrical feature with one basic dimension for true position purposes. one example would be a distance between two bores where one is the datum and the other needs to be at a set distance from it while also being parallel to it. One basic dimension with a TP FCF will get that done for you.

    There are no rules as to how many basic dimensions must be used to constrain a feature with a FCF although I would say your example of four basics would be over constrained.
  • I have had numerous prints where features have a TP callout to all three
    axis and I am given Basics to all three axis. When that is the case I call
    the TP to all three axis. This calculation makes a spherical Tolerance zone.
    Even if the S is not in the Feature Control Frame using the third axis makes
    it spherical.

    My biggest pet peeve is a TP callout to a surface. I say that a TP to a surface is a 'Profile'.
  • I have had numerous prints where features have a TP callout to all three
    axis and I am given Basics to all three axis. When that is the case I call
    the TP to all three axis. This calculation makes a spherical Tolerance zone.
    Even if the S is not in the Feature Control Frame using the third axis makes
    it spherical.

    My biggest pet peeve is a TP callout to a surface. I say that a TP to a surface is a 'Profile'.

    Actually I posted an example where the tolerance zone is not spherical even though three axis are used. Guys, tolerance zone shape is not dictated by the number of axis constraining the feature's location/orientation. Tolerance zone shape is dictated by the shape of the feature. Axial features have an axial tolerance zone not a spherical one.

    BTW you can not TP a surface a surface is not a FOS so that is actually an illegal callout.

    Bill, here is another example where a cylindrical feature is adequately constrained using one basic dimension and a FCF.




  • I agree that it is an illegal callout but I see it on a lot of my prints.
    I report these as Profiles.
  • Just my little input (DMIS help file)...

    By default, the FCF True Position dialog box has the Deviation Perpendicular to Centerline functionality enabled in the background, as this is consistent with the ASME Y14.5 standard.

    While a Deviation Perpendicular to Centerline check box was not added to the FCF True Position dialog box (as it is the legacy dimension's True Position dialog box), you can still turn off this option by having your FCF True Position specify a spherical zone (rather than a cylindrical or planar zone). In this case the deviation is calculated from the X, Y, Z axes.

    Note: The only time you can select a spherical zone is when applying FCF True Position to a Point or a Sphere.
  • OK, you CAN have LESS as long as the proper number of Axis are controlled in some manner. You CANNOT report 3 axis with a diameteric and you CANNOT report only two axis with a sperical. In your tooling ball scenario TWO of the Axis must be implied to be the SAME for each ball (you still have 3 Axis Nominals that you can get from the print and the locations have no tolerance. I said I will GIVE you that. They ARE controlled in some manner but notnecessarily with BASIC Dims.

    What I want to know is what is the criteria for diametric when there are 3 axis AND orientation controlled?

    Please see attached sketch. 4 BASIC Dims. Assume a diametric TP to ABC (A = Z Axis, B = X Axis, and C = Y axis). In this particular case I call ALL the Dims as SET and translate / rotate to the NOM of the feature and report it. I do NEED 3 of them (the BASIC Angle controls orientation) but the 4th is ignored for Diametric TP. To include it would change the shape of the tolerance zone to spherical. Agree?

    It isn't the number of actual BASIC Dims on the print. It is how MANY AXIS are controlled and reported.
    Attached Files
  • Actually I posted an example where the tolerance zone is not spherical even though three axis are used. Guys, tolerance zone shape is not dictated by the number of axis constraining the feature's location/orientation. Tolerance zone shape is dictated by the shape of the feature. Axial features have an axial tolerance zone not a spherical one.

    BTW you can not TP a surface a surface is not a FOS so that is actually an illegal callout.

    Bill, here is another example where a cylindrical feature is adequately constrained using one basic dimension and a FCF.







    Yes, BUT, there are TWO Axis thare are reportable. The IMPLIED ZERO is the second Axis. Also, the SHAPE of the feature does NOT necessarily dictate the shape of the tolerance zone. You do NOT have to use a Diametric tolerance zone for a hole if you do not want to. You CAN use spherical for a point if you want to.

    BTW, that is NOT a diametric True Position. That is an AXIAL true position called out Craig. If it were Diametric then the Diametric Symbol would be in front of the TP tolerance and it is not.

    Sorry about the pic that I did not include. I will try to add it.
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