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Thread: GD&T Position Tolerance and continuous Feature symbol application

  1. #1
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    GD&T Position Tolerance and continuous Feature symbol application

    Is it correct to use the continuous feature symbol in this case? and how to you interpret the true position tolerance? Is it applied to both axis because of the 'square symbol' or does it keep its original meaning of axial position tolerance only applied to one direction? Please refer to the attached image.
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    My own interpretation is that the <CF> symbol is wrongly used in this case. How can you consider the square hole features as a single feature? With regards to the true position without the diameter symbol, I take that as applied only to one axis even if the feature has a square symbol. Some people says the position should be applied to both axis because of the square symbol.

  3. #3
    Administrator Kelly Bramble's Avatar
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    First, the engineering drawing is hard to read and incomplete... The continuous feature specification is meant to define that a series of FOS or FWS are to be treated as though they are a single continuous feature. If a part has several coaxial holes that are non-continuous or have gaps of air between them, the CF designation is used to specify that these hole features are to be treated as though they are one continuous feature instead of several separate features. This has significant implications for Rule #1 and measurement requirements.
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    Sorry if the engineering drawing is hard to read and incomplete, I cannot upload the entire drawing for a reason and I have to remove the dimensions. The 4 square hole features as shown on the green 3D model are not coaxial. I have shared the same image attached earlier as well as a snap shot of the 3D model in google drive. The view in the drawing is basically showing the size of the square hole as well as the position control. That is why I could not accept the use of CF in the drawing because the 4 square holes are not coaxial and cannot be treated as a single feature. The drawing even used CF at the corner radius just to remove the 4X.

    https://drive.google.com/open?id=0B7...1Q2UjRVZGE5SkU

  5. #5
    Administrator Kelly Bramble's Avatar
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    The drawing you provided with CF specification is legitimate.. The four sets of the plane FOS are treated as though they are one continuous feature instead of four separate. See ASME Y14.5-2009, Figure 7-28 or 7-34. The CF specification works with the horizontal position specification not the vertical.

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    The drawing in your reply is the original drawing, and there is no confusion in that. When a new part number was created due to material change with all geometrical features remain the same, the drafter changed the drawing and used CF and removed the true position control of the vertical position. I would agree to think that the CF works on the horizontal walls/surfaces but not on the vertical walls/surfaces of the square holes. Therefore, the true position will only be applied on the vertical direction/horizontal walls? Figure 7-28 or 7-34 all shows individual position controls for both axis, but my drawing only shows one position control yet other people would still think that the position will be applied to both horizontal and vertical axis. I do not think that it is correct to use CF on the corner radius.

    The argument is this : I am only applying the true position control on the horizontal walls/vertical direction because there is no diameter sign in the true position, and obviously there is no position control on the vertical walls/horizontal direction. The drafter is saying that the true position is applicable to both horizontal and vertical direction because of the square and CF symbol added into the drawing.

    https://drive.google.com/open?id=0B7...3R2WlV4QjhIdFk


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    People are thinking that the true position will be applied to both vertical and horizontal direction because the feature being controlled by the true position has the square symbol. I am trying to explain that it does not work that way. When a true position has no diameter symbol, it is basically controlling only in one direction. What I find even more strange is how other people would explain that the diameter symbol is not used in the true position because the feature being controlled is square and not circle... I am not sure if they really understand what is the difference between true position with or without diameter symbol.

  8. #8
    Administrator Kelly Bramble's Avatar
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    Yes, in your example reposted below the application of CF is not clear...

    I suspect that the application intent is that the square features be controlled simultaneously relative to the DRF, which can be accomplished by simply adding the number of places since MMC is included in the FCF and removing the CF. If the drawing originator wants this to be double clear they could add an explicit note describing the requirements as well.

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  9. #9
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    The design intent is to control the location of the square feature with respect to the horizontal datum (common to all 4 square features) and the vertical datum (individual datum for each square feature). How would you apply the true position control if I ask you to manufacture this part?

  10. #10
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    This could also be resolved by simply having the same position feature control frame tagged to the horizontal span, similar to Figure 7-34 of the standard. (I would also add the word BOUNDARY, but the Y14.5 standard says that's optional.)
    Another option is to ditch the size tolerance on the .3873 and convert it to a profile tolerance (all around) and then it's easy to add the position tolerance. An example of this would be Figure 8-24 of the standard.

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