Feature of Size definition (1994)

[dtmbiz]

Would like input as to examples and comments for suggested use of the definition for feature of size regarding

ref para 1.3.17 ASME Y14.5M 1994

" ...a set of two opposed elements..."

 

[SeasonLee]

You may find out what had discussed recently here.

http://www.eng-tips.com/viewthread.cfm?qid=257385

SeasonLee

 

[TheTick]

In a simplified sense, any feature that can be measure with an outside caliper or an inside caliper.

 

[dtmbiz]

Thank you SeasonLee, I read the post you directed me to however it appears that the thread mainly addresses parallel and opposing planes.

TheTick, thank you also, however I used to think of a cone as a feature of size and found that it is not according to the 1994 def. I could find the axis (measureable with calipers) but isnt a feature of size.

I also have had discussions were a radius was called out as a feature of size. I have concluded it can be if it is more than 180 degrees.

I still dont know what this "2 opposing elements" is referring to.

I am tasked with dimensioning a fairly simple looking more or less square cover that is injection molded. Since the sides now have draft, I dont believe I can use them as a feature of size. I want to center several features and am having a difficult time because all features have draft. This makes a fairly simple part relatively hard to define now. I am looking for a simple method to do this.

Also, I wonder what this "Derived median plane of a feature" is (ref 1.3.15 1994)

 

[KENAT]

Ah, the joy of draft and its impact on usable datum features/surfaces. Datum targets may be usefull, ASME Y14.5M-1994 section 4.6, though I admit they somtimes seem OTT for simple shapes with just a slight draft.

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[dtmbiz]

Kenat,

I am familiar with targets, however if I pick the cover flat bottom as primary, and define 2 planes using 2 surfs at 90 deg's to each other from the outside walls, then my DRF origin isnt at the center of the part, but at a corner which defeats my intent of center relating features based on functionality.

I had been thinking that if I did use targets, that the part (which is 20mm deep) with a 2deg draft (as modeled) the difference between the bottom edge (start of draft) to top edge (end of draft) is approx. 4.5mm. If I use a target point (e.g. A3) inset from the bottom targets (e.g. A1 &A2) by the 4.5mm to establish the primary Datum; then if the part comes in with minimum draft (e.g. .5 deg) the part orientation when placed against the targets could hose up good dimensions. Comments?

 

[dtmbiz]

I see bad numbers for trig in my last post.... long day... Looks like no one can give me input to the use of "...2 opposing elements..." Only thing I can think of is maybe "line elements" vs. the entire surface.

 

[KENAT]

dtmbiz, I did spend a little time wondering how/if you could get datum targets to get you your part 'centerline' but it's not something I've done.

On the line elements thing, I'd be inclined to agree with you, which would be my justification for why a conical hole/feature could be a feature of size. I'm not sure I got a satisfactory explanation in one of the other recent threads on why some of the more 'expert' folks on here claimed cones couldnt' be FOS. Don't get me wrong, I can see they are a bit problematic but I'm not sure the 94 std really says they aren't FOS.

Based on our understanding of the opposing element thing, you could use the overall width/length of your part as datums. At the end of the day you could set it up in blocks/datum simulators.

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[dtmbiz]

Kenat,
I tend to agree with your input. Thanks for taking time to reply.

 

[SeasonLee]

Dtmbiz

Here is a good example of opposed elements, the first fig with only a half circle, its hard to find an opposed points so we will not consider it’s a FOS, however the second fig with the feature always contain opposed points, therefore we will say it’s a FOS.

http://www.tec-ease.com/tips/august-99.htm

SeasonLee

 

[KENAT]

dtmbiz, there was thread not long ago where I and a one or more others questioned if a feature had to have opposing elements to be a FOS, or whether less than 180° cylindrical surface could be a FOS given that the opposing element requirement is an 'or' in the standard not an 'and'. However, that's maybe best left to that or other threads.

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[axym]

dtmbiz,

Sorry I can't give any clarification on what constitutes a "set of opposed elements". There are different interpretations of the size definitions in Y14.5, even among "learned" GD&T specialists and the committee members themselves. We can debate the meaning of the feature of size definition but I don't think there's one right answer.

At the end of the day, you still have a part to dimension. Let's forget about what Y14.5 says for a minute and approach it another way. You said that you want the DRF to be at the "center" of the drafted sides. We want the datum referencing to simulate how this center is found during assembly. If there were a fixture or holding device to simulate your part's interaction with its mating parts, what should it look like? Are the features on the mating part parallel, or do they have draft as well? Will there be clearance with the mating part, or some sort of press fit?

Once we know what the fixture elements (datum feature simulators) should be, we can try to define datum referencing that will give that result. It might be possible using Y14.5M-1994 or Y14.5-2009, or it might not. Sometimes the required simulators need to be described in the form of a note.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[fsincox]

The circles in both seasonlee's examples have opposing elements (opposing line elements) just not the ones "caliper guys" want to use.

 

[dtmbiz]

Thanks to all !!! I appreciate everyone's time to comment.