Rounding or not of tolerance in FCF with Dual Dimensioning 1

[cadman1964]

Just a general question

When dual dimensioning a print that has GD&T. When converting the metric tolerance in the FCF, do you round if the 4th number is over 5? example - .4mm = .0157 would you round up? or leave it at .015?

Solid Edge V20

 

[weavedreamer]

What do your tolerance stacks permit?

 

[cadman1964]

this one is .1mm total tolerance

Solid Edge V20

 

[SeasonLee]

Here is a good reference

http://www.tec-ease.com/tips/september-05.htm

http://www.tec-ease.com/tips/august-05.htm

SeasonLee

 

[KENAT]

Often it's a good idea to add a 'significant figure' to reduce to effect on tolerance range. Since inches are an order of magnitude bigger than mm.

KENAT,

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

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

What is Engineering anyway: faq1088-1484

 

[PeterStock]

One rule of thumb I would use is that the tolerance of the secondary unit is to be equal or less than the primary. If the tolearance is rounded to a number that is greater than the primary tolerance, it would be possible to pass a part on the basis of the secondary tolerance that would not pass per the primary tolerance.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

 

[drawoh]

cadman1964,

I think we have had this discussion a couple of times.

Do not generate dual dimension fabrication drawings. Use one set of units to show requirements. Provide the alternate units for reference, only.

On a 3D CAD package like SolidWorks, you can get away with some stupidity, since the computer is working out the dimensions and the conversions. The numbers in your FCF almost certainly are typed, and you are manually calculating each conversion factor. What is the shop allowed to do when the numbers do not agree?

JHG

 

[powerhound]

This is the problem with dual dimensioned prints. I worked at a place that insisted on them despite my objections so I had a note in the title block that said inches were the manufacturing units and the mm were for reference only. I think you will have to settle on one set of units as inspectable and the other as reference. If the mm units become reference, then I believe rounding up is fine. That's what I did. The other thing I would do is change my inches to match mm like instead of applying a flatness of .005 to a surface, I would make it .004 and that would give me 0,1 to put in the FCF.

Powerhound, GDTP T-0419
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Inventor 2009
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Taji, Iraq OIF II

 

[cadman1964]

I think we have had this discussion a couple of times. Do not generate dual dimension fabrication drawings. Use one set of units to show requirements. Provide the alternate units for reference, only.

Yes i do know that's the better practice. but, this is customer generated and sometimes, even knowing were not product designers, we have to draw there part up to there standard.



Solid Edge V20

 

[ctopher]

See page 89 (10.2.2.2)

http://mscweb.gsfc.nasa.gov/543web/files/GSFC-X-673-64-1F.pdf

Chris
SolidWorks 08, CATIA V5
ctopher's home (updated Aug 5, 2008)
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[fcsuper]

Dual dims where killed in 1982. You'd think this corpse would be buried by now.

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

 

[cadman1964]

See that's the problem, just like going all metric, it will never die. So that's something i'll never understand why the took that out of the ASME standard

But, Ty everyone for everyones post's. I do agree with rounding up to the rules that i learned as a young man and stated by other's.

Solid Edge V20

 

[fcsuper]

I don't know why it was taken out, but my opinion is that it should've never have been included in the first place.

By definition, dual dimensioning is a form of soft conversion, which is explicitly disallowed by ANSI/ASME Y14.5 (regardless of the release).

Also, when converting tolerances, you create two different (though similar) tolerance ranges which allows for for more than one interpretation of each specification, which is also explicitly disallowed by ANSI/ASME regardless of the release.

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

 

[dgallup]

Some CAD programs (Pro/E is one) round the secondary units so that their tolerance band is always inside (smaller than) the primary units. This way, people that insist on using secondary units will not produce parts that get rejected when inspected in the primary units. Independently adjusting the number of decimal places in each display is also possible.

I hate it but we do make some dual dimension prints this way.

 

[SDETERS]

I thought the to be Dimnensioned dimension was the one on top, that was the dimension and tolerance to be met. Then the dimension in the brackets was for reference only. How, where and when I picked this information up I do not recall.

 

[cadman1964]

I thought the to be Dimnensioned dimension was the one on top, that was the dimension and tolerance to be met. Then the dimension in the brackets was for reference only. How, where and when I picked this information up I do not recall.

This is a very true statement. But, like in my shop, the guy's will look at the non metric thats in the brackets.

Solid Edge V20