Unilateral / Bilateral Tolerances for dimensions with Double Plus or Minus Tolerance

D

David E. Wagher

#1
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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M

M Greenaway

#2
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.
 
#3
I would question the logic of using a tolerance that - XXX to -XXX. Appears to add nothing but confusion.
 
R

Ravi Khare

#4
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.
 
R

Ravi Khare

#5
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.
 
S

sjvasudevan

#7
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 Ø25.40-Ø25.55. 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.
 

Jim Wynne

Staff member
Admin
#8
sjvasudevan said:
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 Ø25.40-Ø25.55. 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.
 
M

manpreet

#9
Dear All

When your are working on unilateral dimensions, you must go for single tail results instead of two tail results. Let there is specification on roundness and spec is 0.05 mm max. We all now zero is the best. lower the roundness variation better the results. in this case your target should be Zero & USL is 0.05 Max. All calculations must be based on Single tail. For implementation see attached xls file. if any doubt you can mail me at my personal ID "[email protected]".

regards
manpreet
 

Attachments

X

x_imran

#10
Re: Unilateral / Bilateral Tolerances for dimensions with Double Plus or Minus Tolera

Taking this point further i have a specific example for which i intend to have a clarity.

One of the product charecteristics for turning operation on CNC machine is 20.00 mm -0.00 & +0.1. This In this dimension the designer has communicated to keep the components on lower side. so all dimensions between 20.00 and 20.04 is good for the designers view point.

Now during manufacturing if we consider 20.05 as mean and then calculate capability for those components which are intentionally kept on lower side indicates very low capability index. in this scenario the customers requirement of capability index to be more than 1.33 fails to be achieved. what we have been following is that the Cpk higher i.e. the one w.r.t USL is considered and proposed to the customer.

Is this statistically valid.

Pls coment.
 
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