Datums and centerlines 2

[urkson]

My current argument with some of our older engineers is showing centerlnes as datums on their prints.I know it has been an accepted practice by some over the years even though frowned upon by some others. I have explained that under GD&T it is basically forbidden. As head of QC Mechanical I attend GD&T seminars and it has always been explained that a datum has to be something you can touch and you cannot touch a centerline.I am having this argument because I see the younger engineers are taking the lead from from their 'Mentors' and getting into this 'bad' habit. Do you think I right to press the point?

 

[ewh]

If your drawings are to be interpreted to ASME Y14.5, then yes you are right to press the point. One workaround that I have used when dealing with stubborn engineers on this particular issue is to label datum centerlines as "(CL symbol)DATUM X", denoting that the centerline is just that, a centerline of a datum.

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[Belanger]

Yes, you are 100% correct. And it is worth pushing the point. A brief word about the terms, though: A "datum" doesn't exist on the part, so the datum itself is not labeled. (A datum is a theoretically perfect plane/axis/point.) What is labeled on a drawing is the "datum feature," which is a touchable feature. The triangle symbol is actually called the datum feature symbol.

Getting those definitions across to your colleagues should be enough to show why a centerline shouldn't be labeled as a datum. Other reasons: it can be ambiguous since several items can often be shown as aligned to a common centerline; and also because labeling a centerline doesn't tell anybody in the real world what to actually grab onto.

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

 

[TheTick]

Just keep asking them, "Centerline of what?". The shoulder? The slot? The big part? The little part?

 

[KENAT]

Yes push the point.

Isn't there a court case about this very issue where the symbol was on the axis not a feature and because of this the definition was ambiguous and a company ended up paying for a bunch of bad parts or some such. I seem to recall this being one of the poster child cases for drawings being a legal document etc.

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[urkson]

Been a great help, now I have some quotes.

On the next post I want to discuss Basic Dimensions that will be later.

Thanks

 

[Belanger]

Ugh -- don't tell me that your co-workers think that a basic dim appeals to the title block tolerance...

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

 

[urkson]

This is a follow-up to show you what I am dealing with. Tell me if I am wrong but Item 1. 'Datum B' should be part of the upper box (concentricity). Item 2. (roundness)the callout should be, perhaps, .004 not +/-.005. Item 3.since this is a broached hole shouldn't the call-out be symmetrical?

To John-P on the Basic Dimension, yes that is one of the answers I get.

 

[ewh]

Are you sure this company is supposed to be following Y15.4?


“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[drawoh]

urkson,

You are correct. Concentricity requires a datum. Circularity does not. I don't think the ± means anything in an FCF.

All three concentricity tolerances, including the broach, can be replaced by positional tolerances. In the case of the broach, it should be replaced. Concentricity is not supposed to be used much anymore.

Note that while concentricity compares a round feature with another round feature, a positional tolerance requires a fully fixtured part. You need a full set of datums with your specification.

--
JHG

 

[ewh]

Your ±.005 tolerance on the Ø.825 will control the roundness which the FCF circularity tolerance value is attempting.

As an aside to your original question, terms such as "DRILL" and "BROACH" should be omitted from the drawing; per Y14.5 1.4 Fundamental Rule (e): "The drawing should define a part without specifying manufacturing methods. Thus, only the diameter of a hole is given without indicating whether it is to be drilled, reamed, punched, or made by any other operation. However, in those instances where manufacturing, processing, quality assurance, or environmental information is essential to the definition of engineering requirements, it shall be specified on the drawing or in a document referenced on the drawing."

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[Tunalover]

If the datum is a "width" datum associated with a dimension then a datum is created on the center plane of that dimension. If a center line is drawn there then dimensions can be taken from it. I often use this method to create hole patterns that are centered regardless of the tolerance on the width.

Tunalover

 

[powerhound]

Urkson,

This drawing is pretty atrocious. Sorry but I can't think of a nicer word. It's like someone just rewrote the standard and forgot to mention it to anyone.

On Item 11, both callouts are wrong for reasons already mentioned. ewh asked a valid question. This drawing is so far removed from the reality of ASME Y14.5 that it makes me think this person is used to working to some in-house standard and doesn't realize there's a standardized way to do things like this.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[urkson]

This drawing was produced by a young fella 2 years out of school and now being 'Mentored' by one of the older engineers (who doesn't care and will not be told about ASME standards) This young fella has also admitted to me that he doesn't know very much about GD&T which I thought was suprising since he is just out of school. Should this company be following Y14.5 you asked. This is actually a company that we bought and brought into our group about 5 years ago and has been allowed to continue to do it's own thing except on the QC side (my involvement). It's practices are pretty dismal. I am sure you know the story, small private company, made good and brought into the bigs. The engineers do not issue ECN's, change drawings without Revving them up and issue instructions to vendors and fail to document them. It's a continual fight on the QC side.

Thanks for all the feedback. Now review my post on Basic Dimensions.

 

[ewh]

Good luck!

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[JNieman]

That method of "cowboying" things can work when you have one guy doing things and the boss is comfortable gambling on the chance of the cowboy NOT keeling over from a heart attack any time soon, but when you get into a team environment and a manufacturing environment that demands accountability, repeatability, consistency, and redundancy of labor (I don't know about you, but I like knowing that I can take a vacation and have someone else handle the fires instead of having to work remotely when I should be vacationing). Sounds like there's a systemic venomous problem in the workforce that needs addressed but I'm sure I'm not telling you anything new.

This problem goes quite far beyond simple "adherence to standards" though at the LEAST you /should/ be pressing that. If you are going to be in the position to assure the customers of the quality of your products then you have to start with the quality of standards your team performs by. Our quality manager isn't very good at inspection. He understands the standards but he doesn't really do much of any hands-on inspection. He reviews the rules, guidelines, and standards by which we operate, verifies our adherence, and calls us out when we deviate, or at least works with us to figure out why, and how to immediately solve it in a satisfactory way that he could defend to a customer. When engineering, production, inspection are not adhering to satisfactory standards, he talks to us about it, and if he isn't satisfied with the resolution of that meeting, it goes to higher management.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[dgallup]

That print is just gibberish. There is no way a vendor can possibly meet it. I would no quote it if it were sent to me. Being small and private is no excuse for bad work. Garbage into the manufacturing process equals garbage out. Some training classes are clearly in order.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[TheTick]

I like to make good prints because I like to get good parts. If they do not like to get good parts, then, by all means, they ought to continue to make bad prints.

 

[KENAT]

"This young fella has also admitted to me that he doesn't know very much about GD&T which I thought was suprising since he is just out of school"

Why does this surprise you? I learnt nothing about GD&T (or even general good drafting practices) at school and as far as I can determine few others did either. I strongly suspect many drafting (or whatever the term is) classes these days focus on how to use a certain CAD package not fundamentally what you need to do with it to create a good drawing/MBD.

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[drawoh]

This drawing was produced by a young fella 2 years out of school...

I am on the executive of one of the Toronto chapters of OACETT. I am looking into setting up a GD&T clinic. I took my GD&T course at Ryerson University, in the evening, long after I graduated. The course is no longer available. The big local universities, University of Toronto and York University, both have good engineering departments, but they do not teach GD&T. Seneca College does. This is taught in the evening. An engineering technologist, graduating from Seneca with a three year diploma probably has not been taught this. I am talking to them about the clinic.

When I took my course at Ryerson, I recall that there were around six of us in the class, one of whom was a day student. This is not enough to keep them running a course.

GD&T is not being taught at colleges and universities in my area. Probably, it is not being taught in your area.

--
JHG