Perpendicularity of an axis to an axis.

[Medmaker]

Hey everyone,

I'm having a conceptual problem with some language in the new 2009 standard regarding perpendicularity. The 1994 standard stated that a perpendicular tolerance zone can specify "a cylindrical tolerance zone perpendicular to a datum plane within which the axis of the considered feature must lie" (6.6.4.1). No problems there. In the 2009 standard, in addition to being perpendicular to a datum plane, it can also be perpendicular to a datum axis (6.4.2).

My logic tells me that calling out a perpendicularity tolerance of a hole to a datum axis (think of a radial hole in a cylindrical shaft) would drive the location of that hole to lie on the datum axis. Is this correct? Although I understand that orientation tolerances don't control location, in this case it seems unavoidable. Theoretically, it doesn't seems like two axes would be perpendicular without also being coplaner.

I'm interested to hear your thoughts.

Mike

 

[CheckerHater]

Imagine the shortest possible line connecting the 2 axes A.K.A “distance”.
Now “look “in direction of that line.
You will see axes being either perpendicular or not.

I could be wrong, but I think ISO used to have special symbol for that.

 

[Medmaker]

Thanks, CheckerHater. From what you are telling me I should be looking at projected perpendicularity? Sounds reasonable and would fit the "orientation only" model, I just haven't seen anywhere in the Y14.5 that describes that.

 

[CheckerHater]

So far I am just saying that 2 lines in space can be perpendicular.
I am trying to find some meaningful illustration, but without much luck.
I looked thru my copy of 14.5-2009 and I am not very happy with examples they provided.
Looks like 14.5.1M-1994 limits itself to 2 axes in the same plane.
If you have any particular part in mind, maybe some solution can be found that will make you comfortable.

 

[Medmaker]

I have a very common example in my business where we cross-pin shafts into a bore. The location of the hole along the axis isn't critical, but the orientation to the axis of the shaft and the axis of the hole passing through the axis of the shaft is critical. I know I can get there with position, but was interested in the interpretation of the new language.

I do disagree though that non-coplaner lines can be perpendicular in the true sense of the definition. I think the better definition would be skew lines. However, if that's the intent of the standard, so be it!

Thanks.

 

[pmarc]

Medmaker,
Two axes may be perpendicular to each other but do not have to cross each other. See attached graphic.

http://files.engineering.com/getfil...-8984-f284cd9a88c2&file=axis_to_axis_perp.JPG

 

[CheckerHater]

Medmaker,
Do you have access to a copy of ASME Y14.5.1M-1994?
Half of the book is dedicated to describing all possible cases of angularity/perpendicularity/parallelism.
You may not find direct answer to your question there, but it will definitely broaden your horizons

 

[TheTick]

Go to the supply cabinet and grab two pencils. If the walk back is far enough, you should have your answer before you sit down again.

 

[Medmaker]

Thanks, pmarc for the drawing. Makes it a bit more clear when the part isn't cylindrical. Checkerhater, I looked at the Y14.5.1 standard and of course they show my exact application, but with the hole passing through the axis with not much clarification in the text! However, if I look at the case example with angularity, they show the two axes skew to one another and then it's only a short jump to apply it to perpendicularity. I guess I got caught up in what the definition of perpendicular is. I'll concede to everyone's interpretation although my 8th grade geometry teacher might not! Thanks again.

 

[TheTick]

This is also simple to illustrate in 3D CAD (e.g. SolidWorks). Model a cylinder, throw it into an assembly twice over, and constrain the axes perpendicular. Then you can see how much freedom of movement such an arrangement has.