Rule #1 of Y14.5 1

[OmarEn07]

Hi Guys,
I have a question, I hope you could help me:
Rule #1 of Y14.5 states that the limits of size of a FOS also control its form (UOS). But what happens when a FOS has only one limit of size? For example, a pin diameter that is called out at 10.00mm Max.

Thanks in advance for your help.
Best Regards.

 

[3DDave]

Rule#1 only controls one side of the tolerance. Typically that is the outside of external features (such as pins) and the inside of internal features (such as holes.)

In your case I can ship you a box of Zero diameter pins for the price of postage. How many would you like?

(Edit - Just to add - I am not making an offer to sell anything. It's a thought experiment. Don't Report this.)

 

[powerhound]

To add to what Dave said, if something is specified as MAX, that means there is no minimum so if this is really how that pin is called out, it's wrong. Rule #1 states that the limits (not LIMIT) of size control its form. There needs to be an upper limit and a lower limit.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[greenimi]

I am trying to save this thread and also expand my level of knowledge in this particular area (Rule#1 of Y14.5) so, I will provide a text and I will like you to criticize as much as you can and be stickler to details and also go into the semantics of the English language if possible if in any conflict with GDT language

Pretend we have 10mm ± 1mm cylindrical pin dimensioned with direct toleranced dimension ±. (Ø10±1)

Here is the text:
“The interpretation is that size tolerance imposes an equal form tolerance unless otherwise specified. This pin’s form must be perfect if the pin’s actual local size everywhere is Ø11. It can only then be malformed (e.g. bent) as that local size gets smaller, but then only up to the amount that it departed from the Ø11 size. So, for the example, a pin whose actual size was Ø10 exactly would be able to be bent 1mm. In other words, this pin must always fit inside a perfect Ø11mm cylinder”

Please do nit-pick on wording and terminology !!!!

 

[semiond]

No portion of the pin will be allowed to violate a boundary of a perfect form cylinder of 10mm diamter size.
Perferct form at LMC is not required, but you still need to specify a minimum diameter limit to fully define the part, and it's form. Otherwise, if there is no Min. limit, you could make a 1mm dia. wire, wind it to form a coil spring, and it will pass a check in a 10mm internal cylinder gage, as a good part. Would you accept a pin with a form of a coil spring?

 

[powerhound]

No portion of the pin will be allowed to violate a boundary of a perfect form cylinder of 10mm diamter size.

This is incorrect. The MMC size of this pin is 11mm so everywhere you said 10mm, change it to 11mm and you'll be correct.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[powerhound]

greenimi. I think your statement is correct and the english is fine as well.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[semiond]

powerhound,
You're talking about the wrong pin. greenimi's pin has a minimum limit. The one that hasn't is 10mm Max.

 

[OmarEn07]

Hi guys,
Thank you very much for your answers.
Now I have clear that Rule 1 does not apply for one-sided toleranced FOS.

Best Regards.

 

[chez311]

Omar,

I hope I'm not beating a dead horse but I'm just stating this because of the way you worded your answer and the example you chose of the OD of a pin - aside from the inability to apply rule#1, one sided tolerancing as shown creates the same issues that others have mentioned that are still a problem outside of the Y14.5 standard. Mainly this is that the dimension, without any other limiting factors, will theoretically be allowed to approach zero or infinity. There are certain cases where this may be acceptable (ref. Y14.5 paragraph 2.5 - corner radii that can approach a sharp corner or min thread depth whose max is limited by the hole depth) but I don't think this applies to the OD of a pin whose minimum is not limited by anything else and could theoretically be zero.

Edit: removed a link that confused the issue. Moral of the story is be careful applying one-sided tolerances to ensure it truly has the result you want.

 

[semiond]

OmarEn07,
I would word a conclusion otherwise:
Rule #1 applies to regular features of size with the exceptions noted in the Y14.5 standard. One sided tolerancing isn't recommended for cylindrical features, because of poor product definition.

 

[greenimi]

The gist of my question is: what the local size has to do with the longitudinal deformation of this cylinder? How to connect these two together: straightnes and the ACTUAL local size. The important word here I think is ACTUAL.
Or in another words, how to connect the individual cross sections to the longitudinal area of the cylinder?


Again, here is the text:
“The interpretation is that size tolerance imposes an equal form tolerance unless otherwise specified. This pin’s form must be perfect if the pin’s actual local size everywhere is Ø11. It can only then be malformed (e.g. bent) as that local size gets smaller, but then only up to the amount that it departed from the Ø11 size. So, for the example, a pin whose actual size was Ø10 exactly would be able to be bent 1mm. In other words, this pin must always fit inside a perfect Ø11mm cylinder”

 

[semiond]

greenimi,
If I understand correctly your question, the answer might lie here:

Para 2.7.1:

"(b) Where the actual local size of a regular feature
of size has departed from MMC toward LMC, a local
variation in form is allowed equal to the amount of such
departure."

I think this is pretty much self explanatory.

 

[greenimi]

"(b) Where the actual local size of a regular feature of size has departed from MMC toward LMC, a local variation in form is allowed equal to the amount of such departure."

Well, still not understanding how an actual measurement taken in one section (cross section) could affect the variation in form in another section (longitudinal) (such as straightness)?

 

[semiond]

greenimi,
The most simple answer to the question "how" would be:
Indirectly, but inevitably.
The more comprehensive answer you can provide yourself by making a couple of sketches, rembemring that the longtidual and the cross sectional areas belong to the same piece of solid material

 

[greenimi]

semiond,
Could you go a little bit deeper into details: it is still not clear in my head how the relationship (between the cross sections measurements -actual ones- and the longitudinal form error - straightness) works ?

 

[semiond]

This is not as good as the sketches suggestion but I can try with words:
You could look at the cross sections not as on circular areas but as lines in a view where you see whole length of the pin. If the pin is bent and you look at the middle of it, you will notice that the start point of the line is adjacent to the MMC boundary simulator, and the other end is pretty far from reaching the opposite side of the simulator. The collective effect of all the cross sections; their area sizes and where they are located in relation to the simulator, is what eventually dictates the form, and the straightness of the pin in the longtidual direction.

 

[Belanger]

Well, still not understanding how an actual measurement taken in one section (cross section) could affect the variation in form in another section (longitudinal) (such as straightness)

The longitudinal form is by definition the relationship of those many cross-sections. So if one actual local size deviates from another actual local size, it certainly could affect the variation in how those cross-sections align with each other.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[greenimi]

That is exactly what I am questioning: the relationship (or lack thereof) between longitudinal form error and cross sectional form error.
Or in other words the relationship between straightness and circularity when rule#1 is in place.

I am not sure what do you mean by "the longitudinal form is by definition....."

 

[greenimi]

In the layman terms how a measurement done as point to point (let's say with a caliper or a micrometer) in a cross section can determine and be a factor for the straightness (bent) of the cylinder?
So, someone measure the actual local size in a cross section and conclude that the cylider can be bent...? Why?

“The interpretation is that size tolerance imposes an equal form tolerance unless otherwise specified. This pin’s form must be perfect if the pin’s actual local size everywhere is Ø11. It can only then be malformed (e.g. bent) as that local size gets smaller, but then only up to the amount that it departed from the Ø11 size. So, for the example, a pin whose actual size was Ø10 exactly would be able to be bent 1mm. In other words, this pin must always fit inside a perfect Ø11mm cylinder”