Irregular Features of Size

[mkcski]

I recently came across a design that incorporates an irregular features-of-size (FOS) as defined in paragraph 1.3.32.2 of ASME Y14.5-2009 Standard. The feature is a width type (two parallel planes) with the lower plane being (4) coplaner "pads" and the upper plane being (10) coplanar "pads" with a toleranced size between them. Both sets of pads are in a rectangular pattern and the lower and upper pads do not oppose each other at any location.

I understand the opposing elements (surfaces) concept and the "caliper test" as related to the inspection of the actual local size regular FOS, as well as how Rule #1 controls form if no form controls are specified. Also, the Standard, in paragraph 2.7.1, clearly says that Rule #1 only applies to regular FOS.

My question is: how do you inspect the actual local size of an irregular feature of size when there are no opposing elements to "mic-across"? The Standard discusses being "contained" by an actual mating envelope. Does this imply using a hard-gage of some kind that expanding/contracts and contacts the surface(s) of the irregular FOS and then measure the hard-gage with a micrometer? Does anyone have any experience with measuring the size and form of these types of size features that they could share?

 

[pmarc]

The dimension across pads simply should not be toleranced.
It should be basic with profile of surface callout applied to all 14 pads similar to what has been shown in fig. 4-33 (Y14.5-2009).

 

[gabimo]

Pmarc (and anybody else for that matter),

Since you are known as an expert here on eng-tips (see other comments), may I ask you how would you to comment a little bit on the linkedin thread about the irregular datum features applicable RMB.
Default requirement (Y14.5 draft) single solution to minimize the separation between the feature and the simulator
Default stabilization procedure (Y14.5-2009): candidate datum set.

There is any difference between them? Why this change then? Usefulness?

I would like to know your opinion.

Thank you
Gabimot

 

[mkcski]

Pmarc:

Your prompt response is much appreciated.

I told the Design Engineer exactly the same thing. I suggested he use profile to control coplainarity between/of the bottom (4) pads and specify them as a datum (plane) with targets. I also recommended making the 11.34 width "size" dimension BASIC and use composite profile to control width "size", parallelism and coplainarity of the (10) pads on the top.

It appears that the Standard, by stating "contained by an actual mating envelope", is indirectly "focusing" on use irregular FOS as datums where a MMB, LMB of RMB boundary would apply for datum simulation. Consequently, the size, form, orientation and location characteristics the datum feature(s) should be controlled so they are independent of a size dimension requirement. For example: suppose there are three .250 diameter pins, shown equally spaced 120-degrees apart on a circle with an inscribed and toleranced (size) diameter that makes tangent contact with each pin. This would be an incorrect use of size to control the location of the pins. The pins should be located by Position which would indirectly provide the inscribe circle. Do you agree?

 

[mkcski]

Gabimot:

Could you please expand on the first of your two questions? I am not sure what you mean by "(Y14.5 draft)" and the "single solution". The last draft of the Y14.5 Standard I commented on was in 2008 (for the 2009 release). Is there a new one out for the next release?

Regarding you second question: Jim Meadows has a published a very comprehensive book that includes a discussion on candidate datums. My approach for "rocking" datums, has always been to shim for stabilization and record shim thickness and location for a "repeatable" datum simulation (if necessary)

 

[jflongwell]

Using profile rather than a toleranced size dimension prevents the use of a material modifyer and allowing bonus tolerance.

 

[patdh1028]

For example: suppose there are three .250 diameter pins, shown equally spaced 120-degrees apart on a circle with an inscribed and toleranced (size) diameter that makes tangent contact with each pin. This would be an incorrect use of size to control the location of the pins. The pins should be located by Position which would indirectly provide the inscribe circle. Do you agree?

I disagree. My interpretation is such that an FOS can be dimensioned by size even though it may be irregular. It is still an FOS; just because you can't use a standard caliper for a two-point size doesn't mean you can't establish a size control, although it may be undesirable if you are limited on the inspection side. But that's a decision that you make organizationally, not from the standard IMO.

Given that, though, I'm not quite understanding your preference for controlling surface profile versus using a size control (with a Continuous Feature symbol if this is Y14.5-2009, of course). It doesn't seem to me to be any easier to inspect, which was implied by OP as a weakness of irregular FOS.

The way I think about it, you could use a hard gauge (long enough to engage the furthest-apart "pads") to establish an AME, and then feeler-gauge between the gauge and any non-touching surfaces to measure local size deviation of individual pads. In many ways this can be simpler than the inspection requirement for many profiles of a surfaces, especially if you are talking about composite controls. If you bring it to the CMM, either control can be established and measured.

Both OP and pmarc are content, with information presented, to say that a surface profile is more appropriate. I'm not convinced, given the information presented, why that would be any better in absolute terms. And to some degree it may be limiting, as jflongwell noted, as you could not use MMC or MMB to reference that feature (which may or may not be applicable anyways, but still if we're trying to get an absolute comparison it's worth mentioning). I would definitely like to hear your reasoning - could be I am not thinking about this correctly.

 

[powerhound]

My question is: how do you inspect the actual local size of an irregular feature of size when there are no opposing elements to "mic-across"?

You don't. This is why it's considered an irregular feature of size.

Does this imply using a hard-gage of some kind that expanding/contracts and contacts the surface(s) of the irregular FOS and then measure the hard-gage with a micrometer?

I don't really think it implies this but this is an acceptable way to determine the envelope. What you're concerned with here is the actual mating envelope or the boundary.

I disagree. My interpretation is such that an FOS can be dimensioned by size even though it may be irregular. It is still an FOS; just because you can't use a standard caliper for a two-point size doesn't mean you can't establish a size control, although it may be undesirable if you are limited on the inspection side.

The very definition of a regular feature of size requires opposed points. If no opposed points are present then it is not a regular feature of size.

I'm not quite understanding your preference for controlling surface profile versus using a size control

It's not preference, it's how the standard say to do it when it's an irregular feature of size.

But that's a decision that you make organizationally, not from the standard IMO.

If you are not using the standard and just making up your own stuff then I guess this statement could be valid; however, if you are making prints in accordance with the standard then your statement is incorrect. Decisions should always be made based on what's in the standard if that's what you're drawing to.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

 

[patdh1028]

While I thank you for your input, John, it would be more helpful if you could specifically cite in the standard where it says

It's not preference, it's how the standard say to do it when it's an irregular feature of size.

Because my copy of the standard (Y14.5-2009) says, in the definition of irregular feature size, that it is directly toleranced (1.3.32.2). Furthermore, scanning through Section 8, I don't see anything regarding application of surface profile to irregular features of size. Hell, the standard is a sizable document; you may be correct. But if you're going to say something is in the standard, make all our lives easier and tell us where please.

The very definition of a regular feature of size requires opposed points. If no opposed points are present then it is not a regular feature of size.

That's why I said irregular. An irregular FOS is still an FOS, even though it doesn't necessarily have opposed surfaces. Even though it is irregular you can treat it like a regular FOS in some cases i.e. material condition/boundary mods. And you can directly tolerance it (size) but like mkcski pointed out, rule #1 doesn't apply.

 

[patdh1028]

If you are not using the standard and just making up your own stuff then I guess this statement could be valid; however, if you are making prints in accordance with the standard then your statement is incorrect. Decisions should always be made based on what's in the standard if that's what you're drawing to.

Well that seems a little crazy, frankly; the standard does have "holes", it doesn't cover all cases, and there are often many ways to arrange a tolerance scheme that can accomplish functionally the same thing. If you are drawing parts and inspecting them in-house, wouldn't it be an organizational decision to choose one tolerance scheme over another if both schemes adequately control the acceptable variation, but one scheme is easier to inspect with available equipment? Start throwing a bunch of symmetry and concentricity controls around and see if your organization makes a decision about that

 

[JNieman]

Speaking less of 'ASME standard' practice and more of a practical standard, I don't see why measuring across pads like the initial example is out of the question. There are simple and functionally practical ways to do this with basic tools. I would be very surprised if the standard simply does not ALLOW for this.

I do understand the application of 'profile of a surface' and that it does adequately define the requirements, but I do not understand why it would tie our hands more than necessary with no obvious reason. Could someone explain? Maybe it's just not obvious to me, which is why I ask.

 

[CheckerHater]

@JNieman: Imagine simple irregular FOS with non-opposing surfaces.
You may or may not be able to measure it in simple and practical ways.
Standard is not forbidding you (in fact, part of Y14.5 about irregular FOS is full of "maybe this" and "maybe that"), but giving you a fair warning, that you may have to take serious consideration on how part will be measured and how to present it in less ambiguous way.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[dtmbiz]

Prior to 2009 the pads described could not be used as an FOS.

Why do we want to define an FOS or Irregular FOS?

For simplicity of conversation, in this case, it seems to be to establish a center plane relative to the pads on one side of the part and the pads on the other side of the part.

Hopefully there is a part function that is the reason relative for that center plane.

The thread discusses measurement by possibly using a hard gage.

Considering the description of the OP, if this was my part and inspection needed further explanation, I would tell them to first inspect coplanar requirement of one set of pads and then the other.

A fixture could be an alternative for a more expensive hard gage by possibly using surface plates.

After the coplanar requirements are satisfied then a surface plate contacting one set of pads
and another surface for the other set of pads could be used to establish the center plane.
From there the relative part features can be inspected, including the boundary limits for the irregular FOS.

powerhound rightfully points out that there is no local size involved for an irregular FOS.

patdh1028 points out that the standard's definition does sinclude that a directly toleranced feature(s) is required.
I agree with patdh1028’s logic on this which is the advantage of a material condition.

Pmarc wants to make use of a simple basic dimension instead of toleranced size dimension.
It was long ago that there was a thread that centered around whether or not a basic dimension
Could be used in an irregular feature of size. The standard does not state that you can, however it uses the figure 4-33 with strong implication that you can.

For what it is worth….
The 2009 Nuemann GDT training manual takes the position that an irregular FOS is an advanced concept and not common.
It would be advantageous to see a picture of the part described.

 

[mkcski]

Fellow posters:

The Engineer applied what was suggested in my 22 Feb 16 13:49 post - repeated below. He felt this best described his design intent. So I have my answer and consider more responses unnecessary. Thanks so much for your input to this "confusing" topic, which obviously needs more clarification from the Y14.5 Committee.

"I told the Design Engineer exactly the same thing. I suggested he use profile to control coplainarity between/of the bottom (4) pads and specify them as a datum (plane) with targets. I also recommended making the 11.34 width "size" dimension BASIC and use composite profile to control width "size", parallelism and coplainarity of the (10) pads on the top.