Unkown symbol on drawing

[robertadc]

I have a drawing that has a circle with a horizontal line across it when specifying a range of dimensions (see attached picture). This is done for a diameter as well as linear dimensions. Does anyone know what this means? It is from a GE drawing.

I'm sure if I stay within the limits, it will not matter. But I wonder if it is acceptable to go over by a small amount, such as 0.5531 in the example?

Robert

 

[MechNorth]

Robertadc,
Rule #1 of ASME GD&T indicates that the feature must have perfect form at its MMC size (largest size for a pin / smallest size for a hole), but does not require perfect form at its LMC size (smallest pin / largest hole).

A convenient way to remember MMC & LMC is that MMC is Maximum Material (MASS) Condition of the workpiece, whereas LMC is Least Material (MASS) Condition of the workpiece. Notice that it's "of the workpiece", not of the feature; what's the mass of a hole? The smallest hole will result in the greatest mass of the workpiece; the largest hole will result in the least mass of the workpiece.

So, if it's a pin, you can check the lower limits of size using directly opposed points on the caliper. You cannot check the upper limits of pin size using a caliper because it doesn't consider form, which is required by Rule #1.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[flash3780]

I work with a lot of GE prints. That symbol means "critical dimension". It's something that GE puts on their prints to point out that they "really mean it". I think that the quality guys look at dimensions with that symbol more closely during inspections.

 

[robertadc]

MechNorth,

Thanks for the clarification.

flash3700,

Thanks for the information. The .001 tolerance range on the dimensions pretty much gave their criticality away.

Robert

 

[Belanger]

The funny thing is, now that we know it means "critical dimension" or whatever you want to call it, it actually might change the discussion about what the acceptable part limits are (recall the posts above about the accuracy required of the inspection device).

It could be that the given range need not be met every time. This is because the eyeball might be specifying a statistical requirement, not one for every darn part. (I'm just thinking about Y14.5 paragraph 2.17 and Figure 2-24.)

But I can't say for sure. Any company that uses any sort of ST -- or eyeball, or KPC, or critical dim, or whatever -- should have internal standards or documentation that spells out the specific parameters such as Cpk threshold, sample size, etc.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[KENAT]

Urgh, not the concept of critical dimensions. This has been discussed in some detail before.

My concern is still that does this then imply other dimensions are not critical? If so, do I need to bother hitting the specified tolerance on them...

Posting guidelines faq731-376

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

 

[dgallup]

I also hate the concept of critical dimensions but they get forced down our throats by the standards organizations.

Basically, when there is a critical dimension then it must be addressed by your FMEA and control plan. It does not mean that the other dimensions can be out of print, just that being out of tolerance won't kill any one.

 

[PeterStock]

I just saw the same symbol on an old drawing for a different aerospace company. I believe it is also an indication of something being critical, but it does not seem to be defined on the drawing. I know what I will be checking into tomorrow.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[Belanger]

It simply means that for auditing purposes by your customer (or internal quality), those "critical" dimensions will be examined. Of course, every dimension on a print must be met, but the "critical" flag simply places those dimensions on a different level as to how often they must be checked, and it sounds the alarm if the bell curve starts to drift, whereas "non-critical" dimensions might drift off center (still within range) and not be noticed.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[KENAT]

thread286-189745 thread1103-165581 thread1103-165637

I’m still not convinced that details of what dimensions to inspect or how frequently etc. really belong on the drawing.

Surely it could change based on changing vendors, product maturity, changing processes…

At a previous employer, as I’ve mentioned before, there was a quality plan for each major product which addressed things like what dimensions to inspect and how frequently…

However, this is a bit off topic from the OP, sorry.

Posting guidelines faq731-376

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

 

[fcsuper]

We dropped the concept of "critical features" and "process control dims" a long time ago. The drawing is the feature spec, regardless to all that. Inspection is then a separate set of criteria, which can be ID'd on the drawing or in a separate document.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper

 

[PeterStock]

The symbol was refered to in a spec called out on the drawing. The spec is inactive for new design so it would not be used on new drawings. I does mean that the dim is critial to safety. I believe it was a common practice in the past but may not have been per any industry standard. The spec called out on the drawing is a company spec.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[mfgenggear]

.001 or less tolerance is in my opinion automatically crittical.