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Related Topic Tags
bilateral dimensional tolerances, unilateral dimensional tolerances, tolerances (dimensional or other units)
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  #1  
Old 28th October 2002, 12:41 PM
David E. Wagher David E. Wagher is offline
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Please Help! Unilateral / Bilateral Tolerances for dimensions with Double Plus or Minus Tolerance

What if any rules apply to the Unilateral / Bilateral Tolerances for dimensions with
Double Plus or Minus Tolerance?
Example: 0.250 -.010 to -.020

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Old 28th October 2002, 02:15 PM
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I dont know about any specific rules, or what context you are thinking of other than in broad statisitcal terms. However I dont believe there to be anything special about them, like any other tolerance you try to set your process mean on the mean of the tolerance. So a double minus or double plus of a nominal value is nothing special, just perhaps makes you think when you look for the mean of the tolerance.
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Old 28th October 2002, 03:33 PM
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I would question the logic of using a tolerance that - XXX to -XXX. Appears to add nothing but confusion.

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Old 29th October 2002, 08:50 AM
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If by uniateral, you mean one sided; then yes there is a rule. If you have a one sided tolerance (..surface roughness, perpendicularity, roundness) then the Process Capability Cpk should be calculated from whichever tolerance that is available.

I assume you are refering to rules for process centering and Cpk calculatons. If you are talking of any other rules please clarify.
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Old 30th October 2002, 12:10 AM
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I would question the logic of using a tolerance that - XXX to -XXX. Appears to add nothing but confusion.
This convention evolves from the use of shaft basic and the hole basic systems. Even under the Geometric Dimensioning & Tolerancing, if a 20 mm diameter shaft has to assemble with a 20 mm diameter hole, tolerances on size, location and form will lead to worst case envelopes (Virtual Conditions... as defined by the GD&T standard) on the hole as well the shaft. These virtual condition envelopes will have to clear each other if assembly is desired. The 20 mm nominal diameter hole will then have both the tolerances on the positive side, and the 20 mm nominal diameter shaft will have both the tolerances on the negative side.

This could be one of the explainations. For process control however, as Martin suggests, there is nothing special about it. You have to put your process center on the mean tolerance.
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Old 22nd November 2002, 03:17 PM
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I thank you all for the responses.
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Old 3rd June 2006, 07:10 AM
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Query on that subject

Hi All,
It is just related to this topic. Can any one help me out of this doubt.

I have a Limit Tolerance such as 25.40-25.55. The print says it is special as per customer. Can you suggest how do I fix parameters for capability studies? What is the design intent of MEAN?

Thanks in advance.
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Old 3rd June 2006, 09:27 AM
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Quote:
In Reply to Parent Post by sjvasudevan

Hi All,
It is just related to this topic. Can any one help me out of this doubt.

I have a Limit Tolerance such as 25.40-25.55. The print says it is special as per customer. Can you suggest how do I fix parameters for capability studies? What is the design intent of MEAN?

Thanks in advance.
The "parameters" are the upper and lower tolerance limits; the nominal is 24.475 (although in this case it does represent the mean of the upper and lower limits, it's still better to refer to it as "nominal" because sometimes the tolerances limits aren't symmetrical). Only the designer knows for sure what the design intent is; you have to assume that if the part doesn't meet the tolerance limits, it won't work.

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