Tolerance on implied 90 angles.
Tolerance on implied 90 angles.
(OP)
I have a question that is derived from a drawing I recently checked. There were two values in the title block for angles, one of +/-2 degrees if the angle was stated as a whole number and one of +/- .5 degrees if the angle showed a decimal. My question is which applies in the case of an implied 90 degree angle?
My thought is that if I was making the part I would assume +/- .5 degress, but if I was inspecting the part, I would use the +/- 2 degrees as my accept/fail criteria. I am thinking that the 2 degrees would be what it would be rulled as in court (if it came to that).
What do you think?
My thought is that if I was making the part I would assume +/- .5 degress, but if I was inspecting the part, I would use the +/- 2 degrees as my accept/fail criteria. I am thinking that the 2 degrees would be what it would be rulled as in court (if it came to that).
What do you think?
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net





RE: Tolerance on implied 90 angles.
Y14.5 says in paragraph 1.4.i that we are to assume a 90º angle. That sounds like a whole number to me, so I too would go with the ± 2º. But this is a lesson to all the kids out there to be extra careful with "assumed" things.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
RE: Tolerance on implied 90 angles.
Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion
RE: Tolerance on implied 90 angles.
I think the clarification needs to be in the general tolerance, not in the decades-old practice of 90º implied angles.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
It is a good idea to use GD&T profile tolerances systematically to control the outlines of your parts. It might be a surprise to people to find out just how inaccurate a tolerance of ±1° is linearly.
I asked a while ago (thread1103-261904: Plus Minus Tolerances) about holes dimensioned from edges controlled by the angle tolerances on the tolerance block. We did not really agree on anything except that outlines defined by linear ± dimensions are not a good idea.
RE: Tolerance on implied 90 angles.
Sorry should have been more clear. I'm not suggesting to label every angle, just the ones that there may be questions about. If you think an angle is implied, but not sure about it, just dim it and make it reference.
Chris
SolidWorks 10 SP4.0
ctopher's home
SolidWorks Legion
RE: Tolerance on implied 90 angles.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X4
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Tolerance on implied 90 angles.
RE: Tolerance on implied 90 angles.
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Tolerance on implied 90 angles.
You're checking the drawing. So isn't your job to point out errors and ambiguities to the designer so they can be corrected?
You've found an ambiguity. Point it out to the designer.
RE: Tolerance on implied 90 angles.
Rather than adding a 90.0° dimension to your drawing, just add a perpendicularity or profile specification.
The reason I usually apply profile tolerances around my outlines is that angle tolerances are sloppy when applied over long distances. If you have an edge 100mm long, a 2° error moves the top by 3.49mm. If you reduce the error to 0.5°, your error drops to 0.87mm. This may still not be tolerable. If your holes are dimensioned from the angled edge, they will be out of position by 0.87mm, and that almost certainly is not tolerable.
Option 1 is to apply a more accurate angle tolerance, of say 0.05° for an error of 0.09mm. Your fabricator will charge you an arm and a leg for that one.
Option 2 is to apply GD&T specifications to control the outline. A profile tolerance of 1mm allows ±0.5mm of wobble along the edge, making it equivalent to a ±0.3° tolerance, but it will not be as expensive. This is precisely the situation where GD&T works.
RE: Tolerance on implied 90 angles.
The ambiquity is not with the design, it is with the drawing format. This belongs to our customer and is not up to us to change. For the drawing I was working on, I did the dimensioning scheme, the ambiguity did not apply, controls were added that made the default angular tolerance unneeded.
Drawoh;
In fact on this drawing I did use profile and will as needed on others. But for some features, the angular tolerance may be more appropriate. I was just making the point that I would not be leaving any features that are implied 90 degrees that do need the tighter tolerance, refering to the looser tolerance by default. Profile will probably be one I use the most.
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Tolerance on implied 90 angles.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Tolerance on implied 90 angles.
Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net
RE: Tolerance on implied 90 angles.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Tolerance on implied 90 angles.
I'm not advocating this practice, but in a GD&T "courtroom" I would say that the OP's scenario is covered. (But I guess that the question coming up is evidence that it should be more clear.)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
I think we have largely agreed on this.
"The part I have delivered to you is within 2°, therefore it conforms. Gimme my money!"
You know what happens next.
RE: Tolerance on implied 90 angles.
It is because the standard doesn't say it. The standard is in use in any non-standard way, then that non-standard use needs to be spelled out just the same an any other requirement not listed in the standard.
Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion
&
RE: Tolerance on implied 90 angles.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
So, if you're thinking of a simple cube, you'll have to apply datums to three sides to fully define it. The maximum angular error would be dictated by the size tolerance (i.e. the angle created when one corner is at the maximum size and the other is at the minimum size).
/--------/
/ /|
/--------/ |
|<--XX±X-->|
RE: Tolerance on implied 90 angles.
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|
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That's Rule #1, right? Perfect form at MMC.
RE: Tolerance on implied 90 angles.
Think about it: if you get out a measuring tape and just measure the height, that may be at MMC but that's OK. If you then use the measuring tape to check the width, it too could be at MMC and that's OK. But the angle could be 5º and you might never know it.
This is why there should always be a general note to control those implied 90º corners, since Rule #1 only controls form (flatness), not orientation (perpendicularity).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
Implied 90 deg. angle is basic only if it's angle between datum features, otherwise general tolerance applies.
Also (to pour some gasoline into campfire), if you invoke ISO 2768 part 2, implied Perpendicularity will override general angle tolerance. :)
RE: Tolerance on implied 90 angles.
Maybe this is where the ISO 2768-2 comes in? I'll have to look that up ... maybe that's what you are saying in your first statement.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
Nevertheless your comment is very interesting. In your opinion, on the enclosed picture which angle is implied basic 90: A, B, both, none?
RE: Tolerance on implied 90 angles.
In CHater's picture, I would say that angle A is an implied basic 90deg angle, and angle B defined by the basic dimensions leading back to the datums.
RE: Tolerance on implied 90 angles.
The datums planes are perpendicular to each other. It would be nice if the datum faces were, but not absolutely necessary.
If the three datums are nominally perpendicular faces, the primary datum is the three points of the first surface, that make contact with the reference face. The secondary datum is the two points of the second face. The tertiary datum is one point on the third face.
RE: Tolerance on implied 90 angles.
Exactly, the datums are perfectly perpendicular.
When the part is measured it is rested against datum A, then datum B, then datum C. So, in the parallelogram example above, the LHS (if that face is the datum feature) would be brought into contact with the datum and the RHS is defined by dimension XX±X relative to that datum (not from the datum feature).
RE: Tolerance on implied 90 angles.
And what exactly IS angle, defined by basic dimensions, providing that basic dimensions define the perfect shape?
RE: Tolerance on implied 90 angles.
Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X4
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Tolerance on implied 90 angles.
Here is how I see it...
Basic on the meaning of para. 1.4(j) of Y14.5-2009 where it is said that: "A 90 deg basic angle applies where center lines of features in a patern or surfaces shown at right angles on a 2D ortographic drawing are located or defined by basic dimensions and no angle is specified", I would say dimension B from your sketch is meeting the definition.
IMO dimension A can only be considered as implied 90 deg dimension (without word 'basic'). It could be 'basic' if geometrical tolerance was defining the relationship between datum features A and B (e.g. perpendicularity). Since there is no such callout general angular tolerance applies to this angle.
RE: Tolerance on implied 90 angles.
First, let me take a stab on just angle A:
We can't exactly say if that's a basic 90º because it all depends! If you are talking about the angle between datums A and B, then the answer is always, exactly, basic 90º (per ASME para. 4.1).
But if you are talking about the angle between the two surfaces on the part, then the angle is not basic... it's 90º but must revert to a general angle tolerance.
For angle B:
Yes it is a basic angle per ASME para. 1.4.j.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Tolerance on implied 90 angles.
I am trying to convince myself that things are the way you described them. Not everything is falling together, but unfortunately I am getting very busy right now.
Will be glad to discuss issue further on next appropriate occasion.