Unusual GD&T Callout similar to the MMC or LMC symbol

Ron Rompen

Trusted Information Resource
#1
I hope this is the correct forum; my apologies if I have selected the wrong one.

I am reviewing a print for a new part, and have come across a GD&T callout which I have never seen before.

The callout is similar to the MMC or LMC symbol (i.e. a single letter, M or L, encased in a circle), but it is 'E'.

Not just a typo, this appears in several places. However, I have been unable to find ANY reference to it. Unusually though, it appears in my AMGDT truetype font set (shifted square left bracket).

I have attached a copy of one of the callouts to this message, and would appreciate any assistance or guidance that could be provided.
 

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Jim Wynne

Staff member
Admin
#2
Re: Unusual GD&T Callout

I hope this is the correct forum; my apologies if I have selected the wrong one.

I am reviewing a print for a new part, and have come across a GD&T callout which I have never seen before.

The callout is similar to the MMC or LMC symbol (i.e. a single letter, M or L, encased in a circle), but it is 'E'.

Not just a typo, this appears in several places. However, I have been unable to find ANY reference to it. Unusually though, it appears in my AMGDT truetype font set (shifted square left bracket).

I have attached a copy of one of the callouts to this message, and would appreciate any assistance or guidance that could be provided.
My guess is that it's the customer's own symbol. If there's no legend on the drawing that helps, ask the customer.
 

Stijloor

Staff member
Super Moderator
#3
Re: Unusual GD&T Callout

The callout is similar to the MMC or LMC symbol (i.e. a single letter, M or L, encased in a circle), but it is 'E'.
Hello,

The "E" in a circle is an ISO GD&T symbol. It means "Envelope Principle."
This is similar to ASME Y14.5-1994 "Rule #1." Simple answer...

Expanded answer...

From the ISO GD&T Standards:

In ISO 286-1, ISO 2692, and ISO 8015, the following definitions relate to tolerancing principles and size: Principle of Independency: Each specified dimensional or geometrical requirement on a drawing shall be met independently, unless a particular relationship is specified. Therefore, where no relationshipis specified, the geometrical tolerance applies regardless of feature size, and the two requirements of size and of geometrical tolerance are treated as being unrelated.

Envelope requirement: The envelope of perfect form at MMC size of the feature shall not be violated. (Note that the envelope requirement violates the Principle of Independency.)

My comment: Note that the envelope requirement ( "E" in a circle) is similar to the ASME "Rule 1."

Stijloor.
 
Last edited:

Ron Rompen

Trusted Information Resource
#5
:thanx:

Thanks to both of you for your input....after looking closely at the drawing, I did notice that it references the ISO 2768 standard (for general tolerancing) so it would be safe to assume that it also uses the ISO standard for GD&T.

I will follow up on the web link that was provided, just for additional clarification. However, the cross-reference to 'Rule 1' did make sense, although I would have (personally) considered that to always be implied, even if not stated directly.

Thanks again
 
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