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Unkown symbol on drawing

Unkown symbol on drawing

Unkown symbol on drawing

(OP)
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

RE: Unkown symbol on drawing

I am thinking, but not having worked at GE I do not know, it indicates that the dimension is critical to function, safety or customer in some way. It is likely that it is to some sort of GE spec. Ask for a clarification.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

RE: Unkown symbol on drawing

The symbol looks like incomplete datum target symbol.

According to ASME Y14.5 all numerical values are absolute, so if 0.553 is specified on the drawing, it means 0.5530....00. If the measurement is 0.5531, then the part is out-of-spec.
     

RE: Unkown symbol on drawing

Quick look through Genium Manual which is based on the GE didn't appear to have anything like that except for Datum Targets though I didn't look too deeply.

Assuming it conforms generaly to ASME Y14.5m-94 or similar (does it say on the print?) then .5531 would not be acceptable, the upper limit is .55300000000000000000000000000000000000000000000000000000... regardless of that symbol.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Unkown symbol on drawing

Perhaps just a goofed-up diameter symbol.  Was the drawing translated from another CAD program?

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

RE: Unkown symbol on drawing

I'm of a like mind with JP on this one.  I vaguely recall early versions of some CAD packages (V2 Autocad & an early CadKey?) used a symbol like that for diameter.   

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unkown symbol on drawing

It's not likely for diameter since the OP stated "This is done for a diameter as well as linear dimensions."

When uncommon symbols are used, they should be defined in the drawing's notes.  This is true even if they are a "company standard", since others are not likely to know what it means outside of the context of that company.  At my company, we use an eyeball to ID inspection points.  On each drawing where an eyeball appears, there is a general note that explains what it means. (yes, it is creepy when I put two eyeball symbols next to each other...thankfully there's no reason to ever do that on a real drawing.)

I guess a company could refer to their own standard within the drawing, but that would still have to be stated on the drawing, and copies of that standard would have to be made available to vendors and customers alike.   

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

&

RE: Unkown symbol on drawing

How old is the drawing from GE?
I am working at GE/Energy on NX and using some Inventor and I haven't seen that symbol in the year I have been here.

 

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

RE: Unkown symbol on drawing

(OP)
"Assuming it conforms generaly to ASME Y14.5m-94 or similar (does it say on the print?) then .5531 would not be acceptable, the upper limit is .55300000000000000000000000000000000000000000000000000000... regardless of that symbol."

Yes, ANSI Y14.5M applies (it doesn't say which version, I assume the newest).  

It is fine to say that .553 is an absolute limit for interpreting the drawing, but is it really the limit for a finished part?  If so, I guess I am actually restricted to some amount less depending on the accuracy of my measuring equipment.  

For example, if a linear dimension is 0.552-0.553 and I try to measure this with calipers with an accuracy of +/-.001, there is no way I can be sure the part is good.  If I measure with micrometers with an accuracy of +/-.0001, then I can only say the part is good if it reads between .5521 and .5529.  Is this right?

Robert

 

RE: Unkown symbol on drawing

(OP)
The drawing is from 1986.  It looks like it is hand drawn except for some pre-printed blocks and general notes.

RE: Unkown symbol on drawing

If ANSI Y14.5M applies and the drawing is from 1986, than it should be according to 1982 version.

Your statement that you are "restricted to some amount less depending on the accuracy of my measuring equipment" is correct. But you are not quite correct by saying that there is no way to be sure the part is good when measured with caliper with an accuracy of +/-0.001. You can always have reading 0.552 or 0.553 and then the part would be OK.

If you measure with micrometer with accuracy of +/-0.0001, then the reading can be somewhere between 0.5520 and 0.5530, not 0.5521-0.5529.

 

RE: Unkown symbol on drawing

I agree that it is not wise to assume that it is to the lastest version.  We currently design to the '94 version even though it isn't the latest.
This points out how important it is to include the date of the standard when specifying what standard to use for interpretation.

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

RE: Unkown symbol on drawing

Tks Matt.  I can see why it'd be a bit creepy with a pair on a drawing ... maybe add a maniacal monocle to one of them?  I think I'll do something similar to use as a stamp in Adobe when I want a client to pay attention to something.

You can't assume it's '82 compliant just because it's made after '82; there's still folks out there working to the '60s version.  If it has the "M" modifier on Y14.5, then by deduction it would be the '94 version; that's the only one that has the "M" modifier.  Though I'm not sure that would stand up in court.

As for whether or not you can live with 0.0001, which likely would be eaten up by your measurement system accuracy ... it's up to the user's risk aversion.  If the part could fail because it is 0.0001 or thereabouts out of spec, you likely will be risk averse and not accept it.  On the otherhand, if it's a clearance hole and you can live with more, then it's a fairly safe move to accept it.  

Now, as for the issue of how to measure it, per Rule #1; full form check at MMC and two-point check at LMC (Taylor's Principle).  Don't know if it's a hole or a pin, but a caliper measurement would only be valid (i.e. in compliance with Y14.5 ... any version that I'm aware of) for the LMC size.

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unkown symbol on drawing

(OP)
MechNorth,

I don't understand what you mean when you say that calipers would only be acceptable for measuring LMC.  Do you mean that they could only be used to verify that the part is not too small?  I should also point out that for measuring the width, the part is small enough that the entire part fits between the jaws of the clipers so they would be reading the highs of the part.

RE: Unkown symbol on drawing

(OP)
That should be calipers not clipers.  Is there no edit feature?

RE: Unkown symbol on drawing

Generally speaking, your measurement system must be able to resolve to 1/10 or better of your tolerance.    

RE: Unkown symbol on drawing

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  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

RE: Unkown symbol on drawing

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.

RE: Unkown symbol on drawing

(OP)
MechNorth,

Thanks for the clarification.

flash3700,

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

Robert

RE: Unkown symbol on drawing

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

RE: Unkown symbol on drawing

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Unkown symbol on drawing

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.

RE: Unkown symbol on drawing

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
www.infotechpr.net

RE: Unkown symbol on drawing

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

RE: Unkown symbol on drawing

thread286-189745: "Critical" or "Inspection" dimensions -- useful or waste of thread1103-165581: CRITICAL on drawings thread1103-165637: Inspection Dimensions

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Unkown symbol on drawing

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

&

RE: Unkown symbol on drawing

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
www.infotechpr.net

RE: Unkown symbol on drawing

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

 

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