Where in ASME 14.5 does it state we can add & subtract Basic Dim's. 6

[Brandy7]

Where in ASME 14.5 does it state we can add & subtract Basic Dim's. without tol. accumulation? I know we can, I just can not find a direct statment in the Std. (Venders say no).

Thanks All,

 

[EngJW]

I might be confused by the question, but if you add or subtract basic dimensions you would just get a ref dimension.

 

[TheTick]

The reason it is safe to add and subtract basic dimensions is because there is no applied tolerance and they are not affected by physical nonconformities. The dimensions are absolute with respect to the idealized model or drawing. It is not until a tolerance frame is applied to a feature that any variance is introduced.

 

[PaulJackson]

I thought like you, that I had seen a statement like that somewhere in the standard but after looking again I do not think that it is stated directly.

There are definitions and explanations that when considered in concert with one another support the outcome of the statement.

1.3 Definitions > 1.3.9 Dimension, Basic … used to describe the theoretically exact...
1.4 Fundamental Rules (a)… (or indirectly in the case of basic dimensions), …
5.2 Positional Tolerancing (a) … (theoretically exact) …
5.2.2 Application to Base Line and Chain Dimensioning … For positional tolerancing, unlike plus and minus tolerancing …

 

[powerhound]

I'll just chime in to say that I am in agreement with Paul and Tick. You can add and subtract basic dimensions because they are theoretically exact. If you have a feature control frame that calls out a tolerance WRT a datum that is not directly tied to the feature, as long as you can get from the feature to the datum by adding or subtracting other basic dimensions, you are okay.

Powerhound, GDTP-T
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[drawoh]

Brandy7,

I do not think there is a mention anywhere in ASME Y14.5M-1994 about adding and subtracting basic dimensions. Why should there be? The standard explains what they mean, and you can apply this to your arithmetic.

True position tolerances and profile tolerances usually relate each feature to common datums. It is bad practise to show feature to feature dimensions on such a drawing, but the tolerances are from the datums, regardless. Your addition and subtraction of the basic dimensions is exact. Tolerances are not accounted for.

If you have a bunch of holes located to true position, the distance between any two holes varies by the true position tolerance. This can be verified by simple geometry.

JHG

 

[ewh]

Where is it stated anywhere that you are allowed to add and subtract any dimension? Yet it is (un)common sense that you can, as long as tolerances are considered. This is a necessity of design. To do so with basic dimensions doesn't even require that, as they are absolute, as TheTick points out. Being able to add and subtract basic dimensions was one of the very first things that I learned about GD&T, and not being able to can make for a much more complex drawing.
It sounds as if your vendors want to get out of a situation that they find uncomfortable, and are grabbing at straws.

 

[WHITMIREGT]

If the above is true, then a basic dimensions could be repeated without making them reference, just like datum symbols can be repeated without making them reference.

Gary, I will be 72 on October 10th using GDT since 1953.

 

[ewh]

"If the above is true, then a basic dimensions could be repeated without making them reference"

I don't see the connection. I think that would still be considered double dimensioning, whether it is a basic dimension or a toleranced dimension. It would only serve to obfuscate the drawing if it weren't a reference dimension, datum or not.

 

[WHITMIREGT]

ewh
We had a magnified detail on a printed circuit board where the basic dimensions showed up again and we could have made them reference for double dimensioning, or not.

 

[ewh]

I would have made them reference. It would make the drawing just a little more clear.

 

[KENAT]

Tick & Paul Jackson pretty much caught it, 1.3.9 of ASME Y14.5M-1994.

They are theoretically exact. The tolerance comes from the position, profile or other tolerance of the feature/datum target itself.

If you are basically talking about relating one hole to another or that kind of thing then you can add and subtract them all day and the resultant value is theoreticaly exact.

It is sometimes necessary to add/subtract them when doing tolerance checks, designing mating parts etc. Normally for inspection I'd hope you were measuring the actual dimension but I suppose this may not always be possible/easy, however, your measurement accuracy would take a hit.

In what context is the Vendor saying you can't add/subtract them, is it related to inspection?

KENAT, probably the least qualified checker you'll ever meet...

 

[Brandy7]

Where in ASME 14.5 does it state we can add & subtract Basic Dim's Question. The actual callout is a Profile of 2 surfaces (Coplanar Surfaces without Datum). It is not directly dim. to a Datum but is indirectly dim. to a Datum. Vender states he should get Tol. accumlation. I continue to state that Basic Dim's. have no tol. and thus no Tol. accumulation.

Thanks all for your comments.

 

[ewh]

"Coplanar Surfaces without Datum"

This callout does not refer to a datum to hold the profile relative to?

 

[ewh]

Ask the vendor which tolerances accumulate.

 

[desertfox]

Hi Brandy7

Are you saying you have a profile which as a basic dimension to it ie a radius for instance, if so there should be a tolerance given for the radius of that profile,
you cannot ask a manufacturer to produce a component to an exact dimension and not give a tolerance.

regards

desertfox

 

[drawoh]

Brandy7,

What is the context of your question? If you are calculating the nominal distance between two features controlled by basic dimensions, there is no tolerance. If you are calculating the distance between two as-measured features controlled by basic dimensions, there is a tolerance. Each feature is controlled from the datums you specified.

If you are specifying profile tolerances, datums are mandatory.

If you have a copy of ASME Y14.5M-1994, take a look at Figs_6.20 and_6.21. These show methods of calling up nominally co-planar surfaces.

JHG

 

[PaulJackson]

There are many instances in component designs where basic dimensions are declared indirectly for displacement or orientation with respect to the DRF (datum reference frame) specified for the geometric tolerance callout. For the sake of appearance, expedience, process reference, or congruity with mating component details the basic dimensions are specified anomalous to the geometric DRF’s that they are controlled from.

A simple circular pattern of features may be specified 5X with a basic diameter of xx.xxx and an angular displacement of 5/360 with one feature shown coincident with the centerlines… or displaced xx.xxx degrees from it. That specification would yield X and Y basic coordinates for measurement that are not directly specified.

On angled or compound gun-drilled passages in structures designers often detail the end-points with basics normal to the plan views where the passage intersects other features (spot faces, ODs, IDs, surfaces, etc.) rather than rotating and specifying the basics normal to the gun-drilled hole itself albeit for expedience sake. Inspectors must figure the basic displacements normal to the specified feature with respect to the DRF to measure the hole’s position deviation and then convert the axial displacements at each end of the hole where they would intersect the basic value of the intersected feature just so their “basic value displacements” match up with those specified by the designer. All of this is to satisfy an edict from an STA that the measurement results relate directly to the “ballooned” basic dimensions pet peeve. The same goes for the profile displacement of the spot face surface or counterbore depth itself! Lazy designers…just kidding.

There are so many other reasons why the basics are specified in ways other than the way they must be figured with respect to the specified DRFs (if there are position requirements to multiple DRFs to limit stack accumulation, etcetera, etcetera, etcetera) but they all describe the exact theoretical form, location and/or orientation relative to the basic contour and/or DRFs they are scrutinized to so they can be manipulated mathematically all day long to accomplish the scrutiny that one is interested in performing!

Paul

 

[PaulJackson]

Brandy7 are you Quality_Control on CMMtalk.com?

For the profile question see:

http://www.cmmtalk.com/welcome.htm

> GD&T > profile_surface_coplanar.

Paul

 

[desertfox]

Hi Brandy7

Have a look at this site:-

http://www.roymech.co.uk/Useful_Tables/Drawing/draw_geom_ex.html#Form