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  • Parallelism on datum callouts

    Discussion in 'Engineering Drawings' started by mechie_01, Feb 28, 2012.

    1. mechie_01

      mechie_01 New Member

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      Hi All,
      I am new to this forum and this is my first post. So kindly bear with me if this question has been previously answered. I would greatly appreciate it if you could just point me to the correct answer or thread. :)

      My dilemma is this. I have two surfaces on my part that are very critical to function and need to be parallel to a high level of accuracy. There are other features on my part that are dimensioned with respect to either of these surfaces. So they are both datum surfaces in a sense. I need to call out parallelism of one of this datum surface with respect to the other.

      I have tried searching online and in my ANSI standards and not found anything relevant. May be this is not acceptable?

      So bottom line my question: "How do you call out parallelism between two datum surfaces?"

      Thank you !

      mechie_01
       
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    3. Ramana Rao

      Ramana Rao Well-Known Member

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      You can do this by using a surface plate and height gauge with an indicating dial. The part to be placed on the surface plate with one of the datum surface in contact, and in this position the second datum surface can be checked with the indicator dial. I hope this will be of help
       
    4. vic.blackall

      vic.blackall Well-Known Member

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      If I understand this correctly the question is how do you indicate this Datum tie up on the detail drawing. You explain that these two surfaces are closely linked with a parallelism tolerance. I would make one of these faces a master datum and then tie the other surface to this. The second face would be a secondary datum. This would not affect the other features which relate to each of these two datum's.
       
    5. mechie_01

      mechie_01 New Member

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      Hi Ramana,

      Thanks for the reply. My question was more towards using the right call out on the drawing.

      Hi Vic,
      Thanks for the reply. How do you call out one surface to be a master datum? What is the GD&T nomenclature to do such a call out?
       
    6. vic.blackall

      vic.blackall Well-Known Member

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      Decide which datum you are going to call the master and denote it as "A" , the secondary datum will ,for example be "B", with the relevant geometrical tolerances relative to "A". Features relative to the master datum will refer to "A" and features relative to the secondary datum will refer to "B" with there own geometrical tolerances. All other less important features will, if a geometrical tolerance is required should be referred back to "A" For clarification do not be frightened of putting clear notes on the drawing. For example "A" is the master Datum.
       

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