Perpendicularity of an Imaginary Line
Perpendicularity of an Imaginary Line
(OP)
This seems like a pretty straightforward design case if you're trying to orient but not necessarily locate two parts, but it's not something I've ever found a great example for.
I've got two holes. An imaginary line drawn between their centers would need to be *oriented* perpendicular to a flat surface Datum A. But the holes aren't *located* to anything with any precision (except each other).
http://files.engineering.com/getfile.aspx?folder=e...
I've got two holes. An imaginary line drawn between their centers would need to be *oriented* perpendicular to a flat surface Datum A. But the holes aren't *located* to anything with any precision (except each other).
http://files.engineering.com/getfile.aspx?folder=e...





RE: Perpendicularity of an Imaginary Line
Maybe little picture will help?
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Perpendicularity of an Imaginary Line
RE: Perpendicularity of an Imaginary Line
One possible work-around is to make the holes the datum and then have the surface be toleranced for perpendicularity from the holes; see attached. This bypasses the problem of creating a location tolerance between the holes and the surface. The holes probably need a datum reference yet I hesitate to use the back face as a datum because that brings in another element that wasn't asked for in the question.
Another possibility is to make the surface the datum feature and then use profile of a surface on the two holes, referencing that datum. Any location dims of the hole would not be basic but just toleranced dimensions. (After all, you still need to give some relationship there.) The downside to this method is that profile would also control the form and size of the two holes, which you aren't interested in.
Anyhow, there's some stuff to get the discussion going.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Perpendicularity of an Imaginary Line
If you apply perpendicularity without diameter symbol to both holes that will mean that axis of every one of them resides in plane, perpendicular to A.
As holes are controlled by the FCF, basic zero dimension between the two implies as well.
That brings us very close to the solution.
Ah, Friday
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Perpendicularity of an Imaginary Line
The
I am having trouble with avoiding the use of position control due to the lack of "precision". I suggest specifying more datums and use composite position and refine the perpendicularity to A in the lower FCF. The upper FCF could be MUCH larger than the lower - .500 vs .010 inch.
Certified Sr. GD&T Professional
RE: Perpendicularity of an Imaginary Line
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Perpendicularity of an Imaginary Line
Any comments on my post and sketch?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Perpendicularity of an Imaginary Line
How can you make the part if you don't locate the holes? Location needs dimension and tolerance.
Certified Sr. GD&T Professional
RE: Perpendicularity of an Imaginary Line
Treat it only as a thought experiment for how to control perpendicularity of the imaginary center plane of two holes relative to the surface (or vice versa), yet not touching the idea of location. Dicey, but fun!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Perpendicularity of an Imaginary Line
OK. If it's not a real part and just a case study, then I could accept your sketch - reversing the relationship between the holes and the surface.
Lee:
Just to be clear, GDT controls physical features and not theoretical construction (center) lines.
Certified Sr. GD&T Professional
RE: Perpendicularity of an Imaginary Line
RE: Perpendicularity of an Imaginary Line
Do you really need it?
Can you use a plier to drive the screws/ nuts? Yes. It is a good idea? No.
Can you use a caliper to hammer the nails? Yes. It is a good idea? No.
It is not a proper tool for the job. But we actually don’t know the job.
RE: Perpendicularity of an Imaginary Line
My asking if it would be considered cheating was tongue-in-cheek, and meant to subtly admit that it's a direct solution, but not an artful one.
RE: Perpendicularity of an Imaginary Line
Certified Sr. GD&T Professional
RE: Perpendicularity of an Imaginary Line
I agree that it's a bit of an academic discussion. Y14.5 doesn't have a tool to control the orientation of the features in a pattern without controlling their location (only the lower segment of a composite FCF will do it).
Here are some comments on your sketch:
-The perpendicularity tolerance on the planar surface would require it to be flat within the same tolerance
-I'm not sure that the tolerance on the planar surface could be called perpendicularity. The planar surface is nominally parallel to the axes of the two holes, so I think it would be called parallelism.
-Having the holes as the datum feature, referenced MMB, introduces datum feature shift. I believe this would allow more angular variation between the planar surface and each hole axis, than there would be if the planar surface was the datum feature.
Nescius,
I hadn't thought of the customized datum reference frame. That would do the job as well. The FCF would be |POS|dia 0.1|A[u,v]|
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Perpendicularity of an Imaginary Line
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Perpendicularity of an Imaginary Line
In addition to providing the hole pattern orientation control you desire, a composite position tolerance or a regular position tolerance with a customized datum reference frame will also control the distance between the holes to the same tolerance value. If this is undesired, then I would suggest using the bidirectional positional tolerancing method described in ASME Y14.5-2009 para. 7.4.4 and illustrated in Fig. 7-28. Since you are only concerned with control in one direction, you could skip the tolerance in the other direction. It might not technically be bidirectional anymore, but I think the principle should apply just the same.
Attached is a modified version of Fig. 7-28 illustrating what I propose.
pylfrm