GD&T Dimensioning

[ABBman]

I would like guidance on when to use direct dimensions as opposed Basic dimensions. For example, say you are starting out with a rectangular part. The part can have complex features but would you use direct coordinate dimensions as opposed to Basic dimensions with profile tolerancing??

I'd appreciate any help anyone can offer.

Thanks,

 

[ShaggyPE]

Basic dimensions are used when there is an external means of defining the tolerance of those dimensions. This is done with a feature control frame. In your situation this is with a profile tolerance.

Typically profile tolerancing is used when the shape is somewhat complex.

-Dustin
Professional Engineer
Pretty good with SolidWorks

 

[drawoh]

ABBman,

Questions like this are better asked in the Drafting Standards, GD&T & Tolerance Analysis forum.

According to ASME Y14.5, the perpendicularity of unspecified but apparently perpedicular features, is controlled by your tolerance note specifying nominal angle tolerances. An angle tolerance of ±1[°] or ±0.5[°] is remarkably sloppy over a distance of 10" or 250mm. I am now very reluctant to use ± tolerances to control outlines. They work fine from one well defined edge, but not from two otherwise uncontrolled ones.

Profile tolerances, and their resulting basic dimensions, provide far more control, and less ambiguity on your drawings.

JHG

 

[ABBman]

Thanks for the info and sorry about the incorrect posting of this email. It slipped my mind and was an unintentional error.

It seems to me that the angle and perpendicularity are constrained under the requirements of Rule #1 when using direct dimensioning??

But are you saying that direct dimensioning for example X.XXX +/- .XXX is never appropriate and should now be superseded by Profile of a surface?

Maybe you can expand on that just a bit.

Thanks again for your help

 

[KENAT]

ABBman,

This topic gets debated in great depth every now and then over in the GD&T forum, you may want to search there.

Some folks espouse an extreme where almost everything is surface profile or similar GD&T with no +- except maybe on features of size whose location is controlled by position tol.

Some folks go to the other extreme minimizing GD&T except perhaps where there's a 'very tight' tolerance.

To my mind, the optimum is somewhere between the 2 extremes, based on my understanding of 14.5.

Posting guidelines faq731-376

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

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[drawoh]

ABBman,

Rule #1 applies to features of size. Strictly speaking, your outline is not a feature of size.

± tolerances are perfectly legal under ASME Y14.5, and the standard explains exactly what they mean. There are lots of situations where they are useful. Without a standard, most tolerances are ambiguous.

The problem with a large rectangular outlines is that you must control angles. Linear ± tolerances control distance and parallelism, only.

JHG