Mythical non-accumulative tol callout on patterns

[fcsuper]

A practice I've seen some engineers use for a patterned row of holes is to show the linear dimension between the first two with the added statement "NON-ACCUMULATIVE".

In my view, this is a poor practice, since the engineer needs to make a choice to apply the tolerence to the overall length of such a pattern or apply tol the spacing between the individual elements (for a tol stack-up). Stating "non-accumulative" is an under-specification or even a non-specification.

ASME Y14.5 does address patterns in paragraph 1.9.5. However, the methods shown seem to incur a tolerance stack up between the elements. What is the preferred ASME compliant method to apply a tolerence to the overall pattern, rather than incurring a tol stack-up (without using GD&T)?


Matt Lorono, CSWP
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[drawoh]

fcsuper,

Patterns of holes are the very first thing I apply GD&T to, largely because it is a mess to do anything else. What happens when two holes fall on the same dimension line you toleranced at ±.005"? What happens when your part is maximally out of square? Do you measure your holes from the angled edge, or from a corner? Remember, there are no datums specified. Your post above adds to the confusion.

A GD&T FCF is the neatest, simplest way to solve the problem.

The non-GD&T solution would be to apply ordinate dimensions. These would have absolute tolerances as per ASME Y14.5. In the absence of any standard, your notes mean whatever you choose them to mean at your end, and whatever I choose them to mean at my end.

JHG

 

[KENAT]

GD&T is the answer to your problem.

I don't think there is a robust not GD&T way to even try and do it without unnecessarily tightening tolerance.

Ordinates, or at least coming from common origin, as drawoh says may sort of achieve what you want, depending on your complete requirement.

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[fcsuper]

KENET,

As you well know, GD&T scares the average engineer.

Matt Lorono, CSWP
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[KENAT]

So do vaguely correct grammar & punctuation, stylish dress, and talking to even vaguely attractive women...

Are you suggesting we give up on all of those too;-).

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[BiPolarMoment]

The companies I've worked for have admittedly been fast and loose with their dimensioning on occasion but I can say the only time I've seen and/or used something similar would be to denote marked graduations where an ordinate scheme will clutter the drawing and the features are intended to have a common zero point. Not saying it's right, just that it was done.

 

[drawoh]

As you well know, GD&T scares the average engineer.

Challenge this engineer to make a functional, non-mis-interpretable drawing without GD&T and ASME Y14.5. He can put it up here, and we will see if we cannot mis-interpret it.

JHG

 

[fcsuper]

"This engineer"? It's about 90% of them, from what I've seen.

Matt Lorono, CSWP
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[carnage3]

Could you just use basic dimensions to create the pattern of true positions and specify the tolerance to true position?
BTW im not scared to talk to pretty girls or of GDnT, Just the rest of Kenat's list.