Parallelism, location, dimensions, etc.

[bxbzq]

Hello guys,

My confustion originates from the training book from by Alex Krulikowski. On Fig. 7-15, a toleranced dimension (coordinate dimension?), highlighted (20.5, 20.8), is applied to a hole on a block, then a parallelism control is applied to the hole. See attached picture. While there are ways to apply parallelism control to control the parallelism of this hole, here are three I have in my mind:
Option 1, as the picture shows.
Option 2, locate the hole by a basic dimension, say 20.65, then apply a Position control, followed by a parallelism control to refine.
Option 3, locate the hole by a basic dimension, then apply a parallelism control. This is my first thought actually.

My first question: is the callout in option 3 legal? If it is legal, it would mean the parallelism can control location, which is not supported by the Standard. If not, what is the problem by doing it? Same question came to my mind when I looked back couple of pages in Alex's book, a parallelism control is applied to a surface that is one of the two opposed surfaces of a simple block. A tolerenced dimension (size dimension of FOS?) is applied to these two opposed surfaces. You can see Fig. 6-2 in the Standard, it is similar for this matter. So, what if the two opposed surfaces are controlled by basic dimension? Legal or not? Why?

Actually in option 1, my understanding is the parallelism control is a refinement to the toleranced dimension. This brings up another question. Section 2.7.4 in the Standard states:
"The limits of size do not control the orientation or location relationship between individual features. Features shown perpendicular, coaxial, or symmetrical to each other must be toleranced for location or orientation to avoid incomplete drawing requirements."

Interestingly the parallelism is not listed there, but included in subpara (a). Now back to fig. 6-2 in the Standard, the parallelism of the surface is obviously (to me) controlled by the size dimension. A parallelism control can be applied only if you want to refine the parallelism controlled by the size dimension. Something in the Standard seems to me not consistent.

I tried to search in the forum but did not find discussion specifically to this topic.

Please comment.

 

[DeanD3W]

bxbzq,
While Y14.5's wording doesn't support this nearly as well as I think it should, all explanations of orientation or location tolerances benefit if basic dimensions are thought of as locating and/or orienting tolerance zones, rather than features.

A basic dimension cannot locate a feature. A basic dimension can locate a tolerance zone, then the tolerance zone limits the imperfection of a feature's location and/or orientation.

I admit that this may seem to be a minor point, but I think a change in wording makes things more clear.

A hole can be located by applying a Position tolerance, then basic dimensions are required to locate the Position tolerance zone. Once the location and orientation are controlled by the Position tolerance an orientation tolerance can be added if a refinement of orientation is needed.

Basic angular dimensions are needed for all orientation controls also, but for Perpendicularity or Parallelism, the angle should be 0 degrees or an increment of 90 degrees, so it does not need to be shown.

Dean

http://www.d3w-engineering.com

 

[bxbzq]

Dean, I got your point. It is not minor to me. Thank you.

 

[SeasonLee]

Its same parallelism question from Alex Krulikowski training material, case 1 quoted from an exercise, please ref to the attached for deatils, I just want to know what are the differences between them and their application.

SeasonLee

 

[fsincox]

Season,
Good luck, I have been trying to get at this, too. They do need to clarify it in the standard, Evan please tell them.

thread1103-314840

thread1103-271079

I wish we could do polls here!
Frank

 

[CheckerHater]

They do need to clarify it in the standard...

They made it very clear in Y14.5.1M Para. A.3.5 Case 17.

And then they declared Appendix A "not part of the standard". Go figure... :-(

 

[fsincox]

CH,
Naturally, I agree, and would use that until they choose to elaborate further. After all it came from ASME! As I understand it the law is determined not just by the “written law” but it is also determined by "established judicial precedent”.
Frank