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ASME Y14.5 Figure 7-18

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3DDave

3DDave

Aerospace
May 23, 2013
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The more I look at that the less satisfying that figure is. A 1mm diametral allowance on a pair of what look to be flared tubing fittings that may be only, based on eyeball scaling, 8 mm of engagement? That's a 7 degree allowable deviation for each - they could have a 14 degree spread or convergence between them. With a 20mm thread, those tubes aren't bending easily to fit.

If it's just an example for the use of a pattern as a datum feature, there is no need to have the wonky position tolerance that is not explained. They could better have used a profile of surface tolerance symbol to control the boundary of the base and have it simply be a flat plate with no other features.
 
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To make it more realistic for a real-world problem, they could have made this a composite position and added a second segment refining the orientation relative to the same DRF. But guess what, this is just an example from the standard, and the drawing is incomplete.
 
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No one is going to design a special fitting that can only work with hoses - particularly since hose end fittings are huge and would run directly into each other if not parallel with a projected tolerance zone out, for this size, 150mm.

They don't need the complication when they are explaining exactly one concept.

The drawing is over-complete, not incomplete. They have unnecessary additional distraction and suggesting adding more distraction is going the wrong way.
 
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Maybe, but having another flat plate with some holes in it example would suck. Even now, it looks too much like a standard for dimensioning and tolerancing perforated plates.
 
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The special design is to have huge tolerances that can only work maybe with some hoses rather than a better controlled design that will work with hoses and large diameter semi-rigid tubing.
 
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Most likely the purpose was to create an example that looks like an application that justifies using a pattern of holes as a locating and clocking datum feature. Since this practically involves some level of float, reflected by the datum shift in the geometric definition, it already implies some looseness in control. It's definitely not a precision application. They continued with the intent of looseness to widen the position tolerance. Maybe they overdid it.
 
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The danger is that beginners will believe them. That's the point of the standard - to be believed - so it seems like a bad thing to do.

Estimating, they have a dia 0.4mm tolerance on a 50 mm spacing of the holes vs dia 1mm on an 8mm engagement, so about
+/- one half degree of rotation due to clearance in the holes vs the estimated +/- 7 degrees allowed for the related fitting. The exaggeration is of a worst case ratio of MMB to LMC hole of 92% but is depicted at roughly 66%.

So why do they emphasize the holes are LMC at true position? If the holes can be perfectly located then there's no need for a location tolerance. Why do they not show that the reason to allow displacement is to better align the related features to the DRF; they don't even show the related features.

All that is written is:

Where datum feature B is referenced at MMB, relative movement (translation and/or rotation) of the datum features is allowed when there is clearance between the datum features and their true geometric counterparts.
Click to expand...

 
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"...to be believed..."
Believed about what? Tolerance values? I’ve never met anyone who looked at a figure in the standard and said, 'Hey, that looks like my part, so that must be the right tolerance value. I’ll just put it on my drawing.'"
 
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Believe them that the interpretation and application is useful when the committee fails to explain the possible outcomes besides a single, misleading example, and ignores the reason why this particular outcome is worth mentioning.

That the values are not evaluated is icing on that sh*t cake.
 
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See the title of the thread.

In anticipation of the next question, which would have been the correct one to have asked, the possible outcomes of using a hole pattern as a datum feature.
 
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"the committee fails to explain the possible outcomes besides a single, misleading example"
"Besides" is supposed to imply that there is an example that does include an explanation of the possible outcome, but it is the only such example ("single") and even that example is "misleading". 7-18 does not include any explanation, misleading or not, of the possible outcome, and that's why I thought you were referring to some other example.
 
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I thought it was about the loose position tolerance. It seems you also believe that a pattern of holes is never a good datum reference, even when it locates and stops the rotation of the component in the assembly?
 
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The latter accusation is not an argument I made.

Please take some time and read what I wrote instead of imagining winning against straw men you have made?
 
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I don't accuse, make straw man arguments, or try to win anything.
I'm trying to understand what's wrong with the example other than the expected negligent assignment of tolerance values (because it is not the goal of the standard to give guidance on tolerance values selection).
You said that not analyzing the tolerance value is just the "icing on that sh*t cake", but you did a poor job at telling what the "sh*t cake" is.

The standard only defines the tools, other sources teach how to apply them based on design requirements.

Seems like I'm the only one trying to make sense of your grumblings, so no need to freak out at me.
 
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"the values are not evaluated" is the icing.
"fails to explain the possible outcomes" is the cake.

English idioms are difficult.
 
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"'fails to explain the possible outcomes' is the cake"

"'Means this' is called an explanation"

Hard to decide what the problem is? Or maybe you don't need to decide to grumble.
 
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