Question regarding starting point to measure based off BASIC dimensions 1

[brandnew1]

Hi All,

Is it right or safe to just begin at an arbitrary point (based on basic point) that is marked at a specific distance and measure all other values based on this beginning point?

Attached i have a blueprint for the part and i just want to make sure that i start measuring the part accurately. However these angles and radius' are BASIC which i understand to be 'theoretically' exact, however in manufacturing this isn't realistic and usually i see a FCF tied to these basic dimensions which allows for some tolerance.

So for example can i measure the outer diameter and create a point that is 8.8 from this diameter and then measure other radius' and angle based on this point?

thanks for the guidance

 

[Belanger]

While it's true that basic dimensions don't have any tolerance, the feature you're measuring has tolerance from the feature control frame (in this case profile of a surface). But to really answer your question, we'd have to see what datums A and B are. It's possible that the imaginary zero point could be derived from a datum. But if not, then yes you could pretty much start at any point and begin to construct the outline of the part (did you mean the OD of R10 rather than 8.8?).

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

 

[brandnew1]

Thanks for the reply, attached is an updated pdf showing where Datum A and B are (Datum A is the surface and B is just a diameter)

Just in addition, sorry for the not so smart question, but when a profile of surface is an Unilateral Tolerance (Outside) does this make the BASIC dimensions +.010 / -.000 (for the O.D.) & +.000/-.010 for more of the ID (8.5 & 8.6 radius')? i see this for just straight dimensions like the radius' however how does this work with angles? How does a tolerance get to the angles or radius to the holes found in the part (i'm just trying to wrap my thought about how the profile of surface is an unilateral tolerance which would make the part larger only in one direction but how the perspective that this is viewed seems to change.

 

[brandnew1]

here is another pdf of what i was curious about when it comes to tolerancing based on a profile of surface that is unilateral toward the outer portion of the part.

thank you again

 

[Belanger]

Yes to the question about unilateral profile. On the larger outer radius the given profile tolerance has the effect of +.010 and -0. But on the inner radius (the side closest to the imaginary center of the radii) it has the effect of being +0 and -.010.

When it comes to angles, you can really translate the profile's linear tolerance of .010 to an angular tolerance. So while a basic angle helps establish the perfect outline, it's usually better to just think of the tolerance as a bandwidth of .010 splashed all the way around. Keep your thinking in this linear mode and that avoids some of the pitfalls.

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

 

[brandnew1]

Hi Belanger,

thank you for your answer.

i just wanted to verify that you mean't "can't" instead of 'can' in the following sentence:

"When it comes to angles, you can really translate the profile's linear tolerance of .010 to an angular tolerance". i just want to make sure because the sentence says 'can' but the way it is explained sounds like can't but i could be wrong here.

thanks again

 

[Belanger]

Gosh yes, I meant can't!

And this is because a profile tolerance is in inches (or millimeters), not degrees.

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

 

[brandnew1]

i don't mean to bug you but i just have one more situation with this part and i was wondering if you could clarify, based on your expertise.

i found an article on the web written by you (Belanger) that indicated, "This means that there is a profile zone imposed around the entire perimeter of the part, but only in the left-hand view. [in the example there are two images and the all-around is pointed to the left image]" ([URL unfurl="true"]http://gdtseminars.com/blog/[/url])

So with the part drawing i'm dealing with, the profile zone is only dealing with the right-hand view (from previous pdf uploaded in original post) - however the example from the link did not tie the profile to datums.

Here is where i think i may be over analyzing this (based on a simple drawing i attached - this part is a small segment of a larger diameter with this unilateral profile tolerance)

Datum A is the plane with a flatness of .005 and datum B is diameter of .498 +/- .001. Diameter B is theoretically 8.8 from the center point of this large diameter which is only given on the drawing by the radius' which are basic as well.

So the main question i have, does this 8.8 BASIC dimension off of datum B have a tolerance? And also does the unilateral profile tolerance affect all portions of the part (including datum A and B?)

 

[Belanger]

It's true that the "all around" idea only applies to the perimeter in a given view. However, if a part naturally curves around in the "depth" direction, then the profile of a surface would carry into that depth also. For instance, if I look at a flat drawing of a perfect torus (doughnut shape), I could point to the view with a profile tolerance (surface profile) and not even say "all around." But the profile would indeed apply all around (all over, to be more precise) because the very nature of the true profile (the torus shape) already curls around.
I'm not sure if your part is a version of a torus, but let me know if I'm getting at the gist of your question.

The other question you had near the end was about the 8.8 basic dim. Based the snippets I've seen, I think the 8.8 goes FROM datum B to the imaginary vertex off to the right. From there the other basic radii travel back to the left to establish the perfect profile that the GD&T will then apply to. So there is no tolerance on the basic of 8.8. It's just a stepping stone to describe the rest of the part, and that's where the real tolerance resides.

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

 

[Gilmiril]

When dealing with profile tolerances, don't try to apply the tolerance to any of the dimensions. Instead, think of the basic dimensions as describing theoretical surfaces, and the profile tolerance then defines the allowable variation relative to these theoretical surfaces.