Calculating Positional Tolerance 7

[JLang17]

I am familiar with using the X,Y tolerance range to calculate a diametrical positional tolerance (the square with a circumscribed circle method) using the SQRT(X^2+Y^2) equation.

I have a very simple drawing of a plate with a bunch of holes through it and all dimensions are +-.01. I need to apply GD&T to the drawing. Is it correct to simply use the above method to calculate all the positional tolerances? (in which case the tolerance would be the same for every hole since all dimensions are +-.01) It seems too easy.

I have no knowledge of the pin/shaft to go through the holes, or if it's a hole on another plate to be alligned. Is this information needed to apply a positional tolerance to the hole?

 

[fcsuper]

So, something is not adding up between the PDF chart and the download from KENAT. .0035 coord tol produces .0049 in the spreadsheet, but .010 on the PDF chart, unless I'm reading something wrong somewhere.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[JLang17]

KENAT's spreadsheet uses tolerance RANGE, so be sure not to multiply it by 2.

 

[KENAT]

fcsuper, I think JLang found your issue, take a look at the last example on the spreadsheet and I think it explains.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[fcsuper]

Got it. Thanks. Sometimes I just need to be spoon-fed everything.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

 

[JLang17]

If datums A|B are referenced in a positional tolerance FCF, on the drawing does there need to be a basic dimension from the feature to Datum B?

Of course the feature must be dimensioned to establish a location, but what if it's dimensioned from non-datum edges and with simple math the distance from Datum B is easily figured? Is that acceptable?

 

[KENAT]

JLang17, take a look at section 5.2 of ASME Y14.5M-1994.

5.2.13 "It is necessary to identify features on a part to establish datums for dimensions locating true postioing."

My understanding is that yes the location dimensions must be tied back to the datums, however this may be achieved directly from base lined dimensions or from chain dimensioning of basic dimensions per 5.2.2.

If you try and put a basic dimension from a non datum, that itself has a tolerance on it's location you'd be adding more tolerance than that in the FCF and so it's not allowed as I understand it.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

 

[NormCrawford]

Kenat,
You are correct in your understanding. The BASIC dimensions must always find their way back to the datum either directly or by a chain of BASIC dimensions.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.

http://www.GDandT.com