Subject: Profile of a Line Questions for you! Part 1
I hope everything is going well for you. If you don’t mind, I have a few
questions for you regarding profile of a line. In no particular order:
1) In the attached PDF, in which I’ve applied profile of a line to the threads
of a screw, what orients and/or locates the cross-sections that generate the
line elements/establish tolerance zones? I would assume the datums referenced in
feature control frames, do this, no? So taking for example “Scenario 1”, would
it be correct in assuming that multiple cross-sections could be taken that all
must pass through datum axis A?. If this is true, then how would this control
really differ from profile of surface? Take enough cross-sections and you would
basically end up generating a boundary/envelope that encompasses the whole part,
2) In “Scenario 2”, in which I’ve added datum C to the feature control frame, it
would seem to me that there is now only one possible cross-section that could be
located/oriented to datums A, B, and C. Is this correct?
3) If the orientation and/or location of the cross-sections are indeed dictated
by the referenced datums, then what’s up with Figure 8.27 in Y14.5-2009, which
appears to define the direction of the line elements based on the view in which
the profile control is applied? I can’t see anything in the standard which
Let me start with question 3 first. This figure was born out of a question the
Y14.5.1 math committee had about what orients the measurement of straightness of
a surface’s line elements. The answer we received from Y14.5 was that it was the
plane of the view that the straightness was shown in. We asked, doesn’t that
constitute an implied datum. They said, basically, “So what?”. We asked them to
allow straightness of surface elements to use datum references. They (Y14.5)
said no, and if you want to reference datums, use profile of a line. So, from
that history, they interpret this drawing with profile of a line which
references A and B, that all of the (straightness) profile tolerance zones
consist of two parallel lines 0.07 apart, that are oriented to both A and B.
Therefore, all measurements must be taken at that orientation.
Question 1 about the profile of a line used on the thread, unless the “means
this” drawing beneath the control is actually part of the part drawing, I would
surmise they were trying to control the outside diameter’s highest line element
(only) as a continuous curvature. I don’t think they meant the entire surface of
the thread and all of its line elements. And I don’t think they meant the radial
line elements, because if they did, they should have used a local note, such as
EACH RADIAL LINE ELEMENT.
Question 2. If my interpretation given about question 1 is correct, then I don’t
think the addition of datum C adds anything to the control.
This is what I think. But it’s been a long day.
I hope this helps.
Subject: Profile of a Line Questions for you! Part 2
As always, thanks for your insight. Your email just highlighted the fact that I
have a serious lack of understanding what profile of a line means when applied
to a revolved part! Would you mind taking a look at the attached PDF? On the top
half, I’ve laid out a sample drawing. On the bottom half, I’ve laid out three
different “options” of line elements that I think the above profile of line
specification could be controlling. Without adding some sort of clarifying note,
I’m at a loss as to which “option” would most correctly reflect the drawing
specification as I can’t find anything in Y14.5, Y14.5.1, Y14.41, etc. that
would help me understand this. If you could lend me your thoughts on this, it
would be most appreciated.
Option C is most correct, but because all dimensions on the feature being
controlled are basic, I don't see a difference between this and a profile of a
surface control. This controls the size and the form of the entire surface.
Hope this helps.
Many thanks for the quick response! Would your answer be any different were
larger OD of the part (represented by the basic 1.00 diameter) instead some sort
of threaded feature? I ask as many folks assume that when applied to a revolved
part, profile of a line automatically assumes that you are trying to control
only a single high-point/tangent line element (i.e. “Option A” from my drawing).
That's an interesting question, Joe. Line element profile applies to all line
elements of the surface to which the line profile points.
As in straightness of a surface, the view in which it is depicted often helps us
decipher which line elements are being controlled (straight lines or circular
On threads, if a basic dimension is only shown dimensioning the outermost
diameter, I would assume one was trying to tolerance only the size and shape of
that outer diameter with profile. And, in that instance, due to the basic
dimension, I would see no difference between profile of a line or profile of a
If the entire form of the thread (all aspects of it) were defined with basic
dimension and angles, I would assume, again, that they meant profile of a
surface, but for the 3D surface.
Got to start lecturing now. I'm in California.
Hope this helps a bit.
Subject: Re: Question Related to Projected Tolerance Zone
Dear Professor Meadows,
My name is Ayman. I was involved in conducting a tolerance analysis for one of
the components for a project I am involved in and I have been using your book
“Tolerance Stack-Up Analysis, 2nd edition” as a reference. Particularly I am
trying to understand the analysis you presented on pages 107 & 108 for the
projected tolerance zone. You mentioned in this analysis that screws and screw
holes will be treated as one, just as though they were shafts. As a consequence
to this statement, you used the LMC (.2408) and MMC (.264) of the screw in
addition to the positional tolerance (0.014) to determine inner and outer
boundaries of the screw mounted in its threaded hole. While this makes perfect
sense to me, I followed the same steps to analyze the control frame shown in the
I am having hard time explaining to my colleague how the screw tolerances (LMC
and MMC) can determine the inner and outer boundary of the screw in the case of
a projected tolerance zone. I was asked to remove the screw contribution from
the calculation of the inner and outer boundary for the screw mounted in the
screw hole. While removing the screw tolerances did not change the boundary
locations significantly, I am not convinced that they should be removed. Can you
please help me clarify this matter?
If you visualize the clearance hole passing over the screw (forget about the
screw head), the clearance exists between the hole and the body of the outside
diameter of the screw. Therefore the MMC and the LMC of either the major
diameter of the screw thread, or the ground portion of the screw's body (if it
has one) determines the clearance. Also, that clearance is affected by the
position tolerance on both the threaded hole that determines the location of the
screw, and the clearance hole.
I can't see how anyone would disagree with that. But, people sometimes can be
Thank you for your clarification. I have a follow-up question to check my
understanding if you do not mind.
So for a shoulder screw, the MMC and LMC of the shoulder would be considered in
the calculation of the clearance and not the LMC and MMC of the threaded
portion. Is this correct?
Thank you in advance for answering my questions.
Yes, that is correct. And the MMC of those would be used to determine the
available position tolerance. But don’t forget that once the position tolerance
on the threaded hole and the clearance hole are assigned, they will also affect
Subject: Drawing Interpretation question
Note to reader: Some views omitted.
Question on interpretation of BP#46 of attached drawing:
Interpretation #1: Threaded feature is positioned .065 basic from datum C
because the basic is given from datum C (datum B used for alignment only)
Interpretation #2: Threaded feature is positioned .097 basic (.162 basic - .065
basic) from datum B because datum B is secondary in control frame (datum B used
for alignment and mating – p. 508 of your GDT book).
Your opinion is valued.
The threaded feature is located from both B and C, but in different directions.
If B had been referenced after C, then B would only have stemmed the rotational
degree of freedom. It's unusual that the B surface is referenced before the C
hole, in that, usually when a hole is used as a location datum feature, one uses
it to locate in both the X and Y directions. But, in this drawing, B locates in
the up and down direction (call it the Y direction) and C locates in the left to
right direction (call that X).
Hope this helps.
Subject: Drawing Interpretation
I have attached a print. I know it’s not “right”, but I don’t know what to
suggest to product development to make it “better”.
I’d like to know about necessary datums? Some background info:
• All features will be printed on EOS (additive manufacturing/3D printing)
except for the tabs (which will be machined)
• Primary inspection methods will be CMM (all based on GD&T), hard gaging
possibly for position (based on final GD&T), and standard mics/calipers
• Position of the screw holes are important (technically appear in 3 planes, but
the print only depicts them in two)
• To add additional clarity, the majority of this part is porous (should that
make a difference on which face becomes a datum).
One face (C4) will be machined
flat (potential datum with flatness callout)?
Sorry to bother you, and thank you in advance! Feel free to show this e-mail.
Well, in general, it would be nice to stabilize the part to a datum reference
frame. It would be great if the datum features reflected how the part works, but
sometimes just stabilizing the part and relating everything to everything else
is the best way to go. If the surface at C4 is to be machined flat, then it
could, as you suggest, become a datum feature and receive a flatness control.
Then, it looks like someone thought datum feature B was important enough to make
it a datum feature, so if B is perpendicular to the surface in C4 (and I’m not
sure that it is), we could assign it a perpendicularity tolerance back to
whatever datum feature we call the surface at C4. Subsequent to that we could
make all location dimensions and angles basic dimensions. Then make the sizes of
the curved surfaces (that are not datum features) basic dimensions. At that
point, we could simply position all holes and profile all angled or curved
surfaces to the same datum references, with perhaps the machined surface at C4
as primary and datum feature B as secondary. This would create a simultaneous
requirement where all features are measured in one set-up from the datums (after
the datum features are qualified as meeting their requirements).
This would have
the advantage of everything being related to everything else to within the sum
of their tolerances to the datums they have in common. Everything is related and
everything is measured in one set-up. For the surfaces and holes that are
critical to function, the tolerances might have to be tight. For the surfaces
and holes that are not critical, perhaps the tolerances can be loose. Geometric
tolerances are not more expensive, unless they use tighter tolerances. So,
tighter tolerances should only be used when the tolerances that are looser are
not working properly. This type of complete control is unambiguous, makes
manufacturing aware of the relationship tolerances to be held between all things
and lets inspection in on how the part is to be fixtured when measured.
Functionality should then be better with every feature’s specificity of
tolerance and how it relates to every other feature.
I hope this helps.
Subject: Draft Angle
My name is Anthony. I am the Project Lead for our Product Improvement Program. I
received your business card from a colleague as a good resource to reach out to
you for a drawing interpretation.
See attached illustrations. There area in question is with regard to Note 10 and
the Implied 90 degree.
The drawing is for a molded part with a note that references a max allowable
draft angle on an (implied) 90 surfaces, no angular dimension.
The question is, can an implied 90 degree surface that also has a note about max
draft angle also include the general tolerances on the drawing associated with
The manufacturer is claiming the draft angle can be the max value per the note,
plus the general +/-2 degrees relative to any angular dimension (implied or not)
that doesn't have a tolerance. One additional point that I did not mention below
which may help for clarification...
The manufacturer is attempting to apply the +/- 2 degrees to Datum C.
Any feedback is much appreciated.
I don't see a problem with the way the original drawing is specifying the draft
angle. The ASME standard on Castings, Forgings and Molded Parts (ASME Y14.8)
states that the drawing must specify the draft angle as +DFT or -DFT or DFT
INCLUDED. This can be spelled out or abbreviated. This drawing uses the note and
the view to do the same thing. In the view, the draft angle is clearly shown as
minus draft and in the note it clearly states that the maximum draft is 3.5
I don't know how they could have stated the specification more clearly. The
general tolerance note of plus or minus 2 degrees is "UNLESS OTHERWISE
SPECIFIED". Since it has been specified as I described in my first paragraph,
the general tolerance note does not apply here.
I hope this helps.