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Datums in a drawing with thread 3

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ivanovi

Mechanical
Nov 20, 2015
7
Hello everybody,

I am new in mechanical design and I would like an advice from professionals. I have a drawing of a part with thread. I wand the part to be very precise and adding a datums is a good option, but this sounds me very lightly, because threads are very loose and even if the part is precisely machined it does not matter. Am I right or I can add datums in this drawing, maybe there is a rule that I don't know?

Thank you!
Ivaylo
 
 http://files.engineering.com/getfile.aspx?folder=a1e52e98-97d8-41cd-a00b-3f259ea2a84e&file=1.jpg
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ivanovi,

If you want to specify centreing, and other relations between your features, you need to call up datums. It does not matter how accurately you call them up.

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JHG
 
Yes, do not assume that datums, or more generally use of 'GD&T' as opposed to just +- tolerances is only appropriate for tight tolerances.

If you're European I assume you're working to ISO standards? They have some options for default tolerances that off the top of my head I can't recall how they may play into tolerance of features shown alligned.

Posting guidelines faq731-376 (probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484
 
Yes, I understand, I will need time to read the standards. I meant actually that if I call up very tight concentric datum with 0.02 mm tolerance for example, that can be even funny for somebody, because the concentric features are on the same axis with the thread. Then it is better not calling up datums at all. I hope you understand me.

Thanks for the fast answers!
 
ivanovi,

I am not sure about ISO, but the ASME Y14.5 standard requires datums to be applied to features, not centre-lines. If you apply datums to the bottom of your part and to the minor diameter of the thread, a positional tolerance on the hole is completely logical.

--
JHG
 
For female threads both the minor diameter and the pitch diameter can be used as datums. Properly designing gauging to accurately pick them up can be rather difficult.


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