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Unknown Symbol...

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rvenzon

rvenzon

Aerospace
Dec 28, 2007
19
I've come across a symbol that I have not seen and cannot seem to find any info on... Particularly the "Circle U" followed by .0000? Any help to decipher would be great.

See attached pic of the frame.

Thanks
 
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Yes, A circle U is used in ASME, ASME Y14.41-2003 to be exact.
It refers to the distance from the surface a profile tolerance is offset. A distance of 0.000 means that the tolerance is bilateral.
This does away with having to draw a phantom line to denote which side a profile tolerance is applied to.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]
 
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Just out of curiosity, ewh, doesn't leaving it blank also suggest a bilateral tolerance zone?

V
 
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Yes, 0.000 is a condition where the symbol does not need to be used. I can't find any reference to using in with a bilateral tolerance such as that in the standard, only unilaterally and unequally disposed applications.
I'm not sure how they are going to handle it in the new Y14.5 release, but it probably won't change much.
By the way, "U" stands for "up", or outward from the surface.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]
 
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Actually U stands for Unilateral and the notion that zeroes following the U negates it is incorrect. If the tolerance is bilateral then there is no U used. It is the same as is Y14.5M. The zeroes following the U mean that all the tolerance goes inward of the profile surface...as in 0 outward, .030 inward. See fig 10-18 on page 69 in Y14.41-2003. What would negate this is if the number following the U was .015. I've been told that the application of the U has been adopted for profile in the new GD&T standard.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
 
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Thanks, powerhound. You are correct. I didn't read the standard closely enough. It makes sense that if you use the symbol with 0.000, the tolerance would then have to apply into the surface (0.000 outward). Duh!

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]
 
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Actually, you can't use inward & outward as references because it would change the intent if the feature were internal (female) or external (male). And the "u" doesn't mean up or unilateral according to the standard. The "circle-U" as it's called out is an indication that the profile tolerance zone is not equal bilateral. The number preceding the Circle-U is the total width of the tolerance zone, and the number following the Circle-U is the portion of that total that goes to the side that adds mass to the part. So, a callout of 0.8(U)0 on the profile of a hole means that none of tolerance goes towards the smaller side, and all of the 0.8 goes towards the side that reduces the mass (i.e. the hole gets bigger. 0.8(U)0.8 means that the hole will get smaller than the nominal size. An unequal bilateral tolerance of 0.8(U)0.3 means that 0.3 of the 0.8 is on the side that increases the workpiece's mass, and 0.5 of the 0.8 is on the side that reduces the workpiece's mass.

The first time that I saw the Circle-U symbology was about 10 years ago in the GM Corporate Engineering Addendum. It has since been adopted into Y14.41, and is included in the draft of the upcoming Y14.5.

This symbol presumably came about because of the non-associative nature of the offset chain lines stipulated in Y14.5 to indicate unilateral and unequal bilateral zones. When the geometry was moved or changed, the offset curves didn't move and you could easily flip from a unilateral +ve to a unilateral -ve tolerance zone without ever noticing.

Unfortunately I don't think most cad packages are up to date on this symbology, so you may not even be able to use it yet. For those that can, and want to use the symbol, you can use the control and either directly below the FCF or in a note or corporate addendum invoke the Circle-U per ASME Y14.41-2003.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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I can tell that I gave a poor explanation.
the tolerance would then have to apply into the surface (0.000 outward)
Click to expand...
To clarify my thought pattern on this, I refer to "inward" as relative to the surface indicated, thus inward on the surface of a hole would make the hole bigger, because the direction is toward the material; same thing with "outward", which would make the hole smaller, being away from the material. Same intent, different symantics.
In my own defence, I did point out that the standard specifies the symbol for "only unilaterally and unequally disposed applications" (paraphrased directly from the standard).
I guess it's time to take a refresher course... on this and on Communication 101. ;)

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]
 
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Jim,
The definition given in Y14.41 actually says "outward". It makes no mention of adding mass or anything like that. Whether the feature is male or female makes no difference. I don't know what makes you think this. What's being controlled is the surface of the feature and inward of the surface means only one thing...into the part surface. Outward means away from the part surface. This is clearly illustrated in Y14.41-2003.
What you said about adding mass is not wrong but it complicates the issue when phrased like that. Maybe I'm just not that smart. ewh got it right with his simple illustration.





Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
 
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Hi Powerhound,

Yes it does say outward from the surface. I use the convention "the portion following the circle-u is the portion that goes to the side that adds mass" because I typically get people trying to say it makes the feature smaller, or it makes the feature larger, depending on what kind of parts they're used to thinking of. "Outward" from a surface could be taken to mean radially outward (i.e. the circular feature gets larger) which makes a difference then whether it is a male or female part. Figure 10-18 in Y14.41 actually has an error as it shows the offset to the inside (correct) while showing the dimension lines to the outside. I use the mass analogy for teaching MMC & LMC also, so it's a natural mental continuation for students. Most people that I've taught have pick up the idea of adding mass quicker than understanding what outward "from the surface" means, and using a common concept helps to get the basic idea across efficiently. It's too easy sometimes to get into the rut of communicating things the easy way rather than the correct way.

Tks for the corrections. Like I said, it's been a long day.

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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Tks for the Word of the Day, Chris. I looked it up on Wikipedia, and found another new word, phosphoribosyltransferase, the definition of which is not yet in Wiki. But used as in "... Uracil also recycles itself to form nucleotides by undergoing a series of phosphoribosyltransferase reactions."

Jim Sykes, P.Eng, GDTP-S
Profile Services TecEase, Inc.
 
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I guess I'm still not grasping why there's a problem with using "outward". If the part is a cylinder, adding material outward of the surface makes the cylinder bigger and thus adds mass. If you have a hole in a plate, then adding material outward of the hole surface makes the hole smaller and thus adds mass. If someone takes it to mean radially outward from the axis of a hole, then that's just incorrect and the same discipline and mindset that was used to overcome the tendency to incorrectly call the MMC of a hole it's max size needs to be applied here. If the zone extends "into" the part then it's inward, if it extends "out" of the part, then its outward. That's just too easy.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
 
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