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The Tolerancing Engineer Newsletter - April 2009
...by our client company personnel and James D. Meadows using our ‘GD&T HOTLINE’

Readers - As many of you know, I have no real schedule as to when a new newsletter comes out. It has a lot to  do with a mix of my time in the office and the batch of letters written by you with clear, important questions others might be interested in reading. Here is the latest installment of the continuing saga of The Tolerancing Engineer (serving the GD&T community since 1982).

~ James D. Meadows

The ASME Y14.5-2009 standard on Dimensioning and Tolerancing has finally hit the streets.  It represents a radical change from the ASME Y14.5M-1994 standard.  A wide variety of new rules and symbols are included.  For companies that want to adopt the new standard there will be a learning curve.  The Y14.5-2009 standard will allow designers to express their design requirements more clearly.  Product representations will be more specific in conveying tolerancing needs.  Products will be more easily manufactured, and appropriate inspection techniques clarified with these new tools.

 

As with all standards, the ASME Y14.5-2009 standard is written in such a concise way as to be difficult to learn from.  To support this learning process, I’ve written 3 new books and made a film series to explain the new standard.

 

One of the textbooks is also being sold by ASME as a dimensioning and tolerancing handbook.  It is entitled: Geometric Dimensioning and Tolerancing - Applications, Analysis & Measurement [per ASME Y14.5-2009].  It is almost 600 pages in length and covers virtually every facet of dimensioning and tolerancing including the new principles, rules and symbology.

 

The second book is called WORKBOOK AND ANSWERBOOK for Geometric Dimensioning and Tolerancing [per ASME Y14.5-2009].  This 367 page book has 5 sections.  Section 1 will provide information (rules, principles, symbols and changes) necessary for anyone wishing to progress to the rest of the book.  Section 2 provides worksheets for a wide variety of GD&T topics.  It includes applications problems from a wide variety of industries, tolerance analysis problems and general GD&T knowledge questions.  Section 3 consists of word problems that include questions about every geometric characteristic symbol, datum features, boundaries and rules.  Section 4 consists of questions and illustrations that specifically apply to the new rules in Y14.5-2009.  Section 5 is the answerbook.  It shows, in great detail, how each problem in the workbook is solved.

 

The third book is entitled ‘NEW RULES’ in GD&T [per ASME Y14.5-2009] is based on the ASME Y14.5-2009 Dimensioning and Tolerancing Standard.  This 160 page text and workbook/answerbook reflects new symbology, rules and basic principle revisions that took the Y14.5 committee almost 15 years to complete.  It was written to specifically highlight the revised concepts covered in ASME Y14.5-2009 as well as to explain some of the more difficult and advanced concepts allowed by older versions of Y14.5, but extended in the 2009 revision.

 

The ‘NEW RULES’ in GD&T book has available for purchase an accompanying DVD series wherein the concepts that are new to the ASME Y14.5-2009 standard are explained in great detail.  This DVD series will make clear the sometimes difficult to understand changes instituted in the revised Y14.5-2009 standard.  This film works in conjunction with the ‘NEW RULES’ in GD&T book and refers to pages from the book during the presentation.

 

Subject: GD&T Question about Cylindrical Tolerance Zones


James,
I have a question I would like to ask you (I took your class about 7 years ago).  Can you take a look at the attached print?  The circled area or DATUM C is the area in question. Our manufacturing facility is stating that the positional callout on Datum C should be not called out diametrically.  I am more along the lines of that it should be a diametrical callout.  Can you please let me know your expert opinion?

 



Thanks in advance for your help.
Regards,
Jason

 

Jason,

Both datum features B and C need a diameter sign in their feature control frames.  Perpendicularity to datum plane A of both holes is a critical issue for any part that mates with these holes and seats on A.  The holes’ axes may not violate a cylindrical tolerance zone (which the diameter sign signifies) and still generate compatible mating boundaries.  If the diameter sign is left out of either control, it would most commonly be viewed as a typographical error. 

Jim

 

Subject: Quick Question Regarding Angles

 

Jim,

I attended a GD&T seminar of yours a few years ago at UW Milwaukee.  Today, I am asking for a quick print interpretation (my first and only one as a prior seminar attendee).

 

The 16 +/-0.5 degree angle, as drawn, would one measure the angle to the "leg" directly under the angled tube (left most leg as seen in the view with the angle call-out) or to datum A?  Can one assume the left leg is in the same plane as datum A?

 

Thanks for the help,

Scott

Scott,

The fact that there is a datum plane A on the drawing has nothing to do with checking the 16 degree angle.  Datum features and the subsequent planes and axes they generate are only used for what we tell people to use them for in feature control frames with geometric tolerances.

 

The fact that datum feature A seems to be in the same plane as what the 16 degree angle is measured from or to (who knows which), is not pertinent to the measurement.

 

It does raise another question, and that is, "Did the designer mean datum plane A to be constructed from one surface, or two, or three?". 

 

If datum plane A was referenced in a feature control frame for something like Angularity within a tolerance measured in millimeters, then the 16 degree angle would have to become a basic dimension (a boxed dimension without a plus and minus tolerance next to it).  Then we would know what was measured from what and the only thing we would need clarified would be how many surfaces constitute datum feature A and should it or they be controlled for form (and possibly coplanarity) tighter than what is given in the size tolerance from the opposite side of the part?

Jim

 

 

 

Subject: Query on a Datum Feature That References Itself

 

Hi Jim,

Please clarify my doubts about the attached illustration.

 

The issue is a Feature of Size having a Positional tolerance that references datum B and the same Feature of Size establishes Datum B.

 

Does this make sense and is it legal?

 

Regards,

Janardhan G.

Janardhan G,

The control is just clumsy.  The two holes called datum pattern B could be positioned and only datum A referenced in the control.  That would mean the two holes are positioned to each other and perpendicular to datum A to within zero at MMC.  What has been done on the drawing you sent me was probably trying to do that, but got clumsy and it ended up as two controls with datum B mistakenly referenced in the position control.

Jim

 

 

 

Subject: Tolerance Stack-Up Question

 

James,

I took your tolerance stack-up analysis course several years ago.  I remember that you encouraged us to contact you if we have questions.  I like the approach you taught us, but I find that when I'm checking work of other engineers, I find mistakes aren't made on what's there but on what's not there.

 

For example:

If a print doesn't have a position callout on a hole, the engineer conveniently forgets to consider it.  This tends to happen when using prints from days past when we weren't as diligent with our prints.  It makes the analysis easier but it's wrong.  Engineers tend to use the print as a checklist of things to consider when performing the analysis.  This assumes that the print is correct.  (Bad assumption.)  Besides making sure prints are complete and correct, do you have any advice to avoid this mistake?

Steven

 

Steven,

I agree with everything you wrote.  I find the most vexing problem in doing stacks is incomplete or incorrect requirements.  I take the easy route and call the person responsible and try to fill in what is missing, so that I can continue the analysis.  I'm no better at analyzing incomplete or incorrect product requirements than anyone else.

 

I'm like you, in that, I always assume that something in the requirements will be missing or wrong.  I usually look for those before I do anything else, so that once I begin the analysis I'm not slowed down or stopped when I reach that point.

Jim

 

 

Subject: Question about Coplanarity

 

Hello Jim.

My old company laid me off. They are in the process of closing the plant. I am now employed at a new company and have a quick question that I am unable to answer (without trepidation). So I am deferring to you and your expertise.

 

I have a sheet metal bracket. The end view has an omega shape; 2 tabs on the bottom and a raised middle portion. The tabs form the primary base datum -A- and are used to locate features on the bracket.

 

I have 2 options:

Option 1: apply a 0.5 profile to the face of the tabs-both surfaces- and call it -A-.

Option 2: apply a 0.5 flatness callout on the 2 surfaces and call it interrupted surface -A-.

 

I know the power of profile. Is flatness legal for an interrupted surface primary datum?

 

I look forward to your answer.

 

Thank you in advance,

Ted

 

 

Ted,
Flatness won't put multiple interrupted surfaces into a single tolerance zone.  Use profile of a surface to control both coplanarity and flatness.
Jim

 


Subject: Print Interpretation

Jim,

I hope all is going well with you. I took one of your GD&T classes last April and was hoping you could help me clear up an interpretation issue we have been having internally here.

 

We have made the parts shown below and have run into an issue on how the true position of the sphere diameter. should be interpreted. I had initially made suggestions to make the .280 and the .10 dimensions as basic and only check the sphere true position from datum’s A and B. This would have to be checked in X, Y and Z axis, but internally it is being challenged that we can check the sphere true position in all 3 directions.

 

Is it proper to check a true position of a sphere in all three directions? Or can it only be checked in two directions say…. X and Y and then the .280 would have to be checked separately (which I feel should be .380 dimensioned from Datum A). I think some of the reason things are not adding up in regards to GD&T is because our customer is doing some value added operations after they receive the part from us and maybe they are locating from the surface opposite of Datum A.

 

I know you are a very busy person and I appreciate any help you can lend.

 

Regards,

Mark

Mark,
The deviation from true position of a spherical diameter's center point is equal to 2 times the square root of the deviation from the basic dimension on the X axis squared plus the deviation on the Y axis squared plus the deviation on the Z axis squared.
 
Hope this helps.
Jim

Subject: How to Control for Warp?

Hi James,
I'm not sure how is the best way to control the amount of warp for a part.   The attached drawing shows an example.   The thickness of the part must be controlled to a relatively tight tolerance of ±0.01.  However, it is acceptable for the part to be warped up to 0.10 in the unrestrained condition.  This is a unique situation where size limits cannot control surface form since the surface form is not controlled as tight as the size.  I don't think flatness or profile is appropriate because the amount of acceptable warp exceeds the size limits.    Any help would be appreciated.

Regards,
Gary

 

Gary,
Use straightness of the derived median plane if it is per ASME Y14.5M-1994 or flatness of the derived median plane if it is per ASME Y14.5-2009.  Just put the straightness or flatness control below the size limits of the feature.  In this way, the straightness control can either be smaller or larger than the size tolerance.  It eliminates Rule #1 and allows the part to violate the envelope of perfect form at maximum material condition. 
James

 



Subject: Datum Target Areas

 

Hi Jim.

 I have a part that I need to hold flatness on Datum A but only in specific areas; is the way I have used datum target areas correct? I'm also using a constraint note due to the part wanting to bow after being machined. Thanks for your help.

Eric

 

 

Eric,
Given the note, I think this will be fine.  Normally, I would recommend Profile of a Surface to control the flatness and also the coplanarity between the areas.  But since they are all on the same surface, and you are inspecting it while restrained, it should be okay.  Just remember, for future situations, that surfaces can be flat, but not necessarily lie in the same plane.  For example, all could be flat and not parallel and be at different heights.  But, with this situation it's probably fine.
Jim



Subject: 1982 ANSI interpretation of MMC

Mr. Meadows, trivia questions for you (only because I not sure of the correct interpretation).

 

Would it be accurate to state that all drawings that were created & released prior to 1982 are allowed/granted MMC for the actual feature identified within a feature control frame due to the fact that MMC was ”implied” on all drawings per ANSI Y14.5? 

 

We continually challenge ourselves on this subject but my understanding is since MMC was implied, that the engineer(s) created such drawings without using the MMC modifier knowing it was implied. Would this be correct to assume?

 

Another note – We’ve seen some drawings which were created before 1982 that use MMC sporadically on the drawing which is interesting because if it was implied why would the engineer feel the need to use the modifier on certain feature control frames but not others? So even with the sporadic usage, we would still imply MMC on features where it was not identified just to be consistent with the ANSI standard.

 

Please feel free to reply at your earliest convenience as this information would be very helpful on future interpretations. If you have any questions or require any additional information, please feel free to contact me.

 

Best Regards,

Mark

 

 

Mark,
Prior to ANSI Y14.5-1982, only position was implied at MMC.  All other geometric characteristic symbols were implied at RFS.
Jim

 



Subject: Question on your Measurement Book

 

Hello Jim,

I was looking over your book preview (Measurement of geometric tolerances in manufacturing) at Barnes & Noble, but it only showed the first page of the list of contents.  And the other pages I could view without buying the book did not really show me what I was looking for.

 

We’ve got a bit of an argument going at my company between engineers and inspectors over the inspection of composite tolerances.  According to Lowell Foster’s book, for the top segment of the composite tolerance; we can use the standard X and Y coordinates deviations to calculate the actual positional tolerance (2 x square root of X2 + Y2).  But for the bottom segment it says we cannot use this method.  The inspector used the same method on both segments and is failing the part based on the part not meeting the lower segment.

 

I’ve asked them to use the paper plot method on the lower segment and it passes but they do not understand it.  I was wondering if your book gives a mathematical method so that I can order it, or you if can reference some software or calculator program that can do it.

 

Thanks,

John

 

John,
 
With two holes, it's easily done with simple math.  With more than two holes, it either has to be graphed, or you have to have a pretty involved software program to calculate compliance.  The problem in hole to hole relationships without location datums is the movement and rotation of the pattern of zones (or the collected variables data to see if they will fall into the tolerance zones).
 
You can always pick two of the holes and pretend they are datum features, using one to measure distance from and the other to clock your measurements.  This means the first hole is perfectly positioned and the second is out of position in only one direction.  Subsequent holes are seen as out of position in both the x and y directions established by the first hole and the common plane between the first and second hole.  The problem is that this method isn't perfect.  It doesn't allow the rotation and movement of the pattern to match the data to the tolerance zones.
 
If you are willing to graph the results, either on graph paper or on a computer screen, it's very easy to determine compliance and you have a visual display for doubters.  Yes, these things are addressed in my purple "Measurement" book and my yellow GD&T book. 
 
I can't believe your measurement people are pretending that the datums existing solely for the upper level control also apply to the lower level control and are rejecting parts on the failure of holes to meet their relationship to those datums, when (in the hole to hole tolerance level of control) none applies.
 
Good Luck.
Jim

 


Subject: Datum References within a Position Control

Jim,

I was wondering if you could shed some light on a question about a particular use of datums within a position control. I’m reviewing several drawings and most of them have multiple coaxial diameters where one is a datum (datum axis A) and the others are positioned to it. The question I have is about an appropriate use of a secondary datum (datum B) that is a perpendicular face to datum axis A. Just about every drawing contains at least one coaxial diameter that is positioned to A(M)|B| (as overly simplified below). Does the addition of B actually add any value to this control? I assume if they were trying to control orientation (perpendicularity) to B, they would make it the primary datum. There are no basic dimensions from B, so I don’t see what it adds. Am I missing something?

Thanks in advance,

Bob

Bob,
You aren't missing anything.  Unless datum feature B is used as a primary datum, it doesn't add anything to this control.
 
Jim

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