Note: Machine as a pair 7

[leeekim]

I've come across this note several times before. My understanding is that they would like to hold a dimension to be identical such as a pair of rails for assembly. It there anything in the GD&T callouts that would do something along the lines of this note? I was thinking about using profile of a surface with a datum reference, but that would be a tighter tolerance if I want better match between the two parts. Any thoughts?

 

[Belanger]

My only thought would be that GD&T doesn't care how the part is made. So to say "machine as a pair" isn't really within the rules of Y14.5 paragraph 1.4(e).

Perhaps I'm being too much of a stickler

Profile of a surface sometimes has a similiar connotation, such as with coplanar surfaces (compared with a flatness callout). And there is also the rule of simultaneous gaging. Other than those ideas, I'm not totally sure what you're trying to do.

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

http://www.gdtseminars.com

 

[dgallup]

We do something called match grinding where one part is automatically gaged and another is ground to size to control the fit. The two parts are male & female. The two parts must be kept together ever after.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MechNorth]

leekim,
If the parts are machined from separate stock, the there is nothing in Y14.5 to relate the two separate parts. If they are parted from one machined piece, then you may be able to use <CF> (Continuous Feature) or Profile of a Surface to control the workpiece before parting. The drawing would have to show the multiple machined part joined with phantom lines to show the relationship. After parting, there is no relationship between the parts though, so the controls only apply pre-parting.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[KENAT]

We had a 2 part (female) dovetail which we determined needed to be made as a pair.

We drew the 2 parts on one drawing as I recall. We gave information on which face they were to be mounted for 'inspection' and indicated which dimensions applied to the pair.

We kind of did it like you would a non rigid part.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[CheckerHater]

leeekim,

The “drafting standards” are not limited to Y14.5
What you are talking about is called MATCHED SET.
You may find description of this technique in ASME Y14.24

 

[leeekim]

Thank you all for you insights. Thank you CheckerHater for the point about Y14.24

 

[fsincox]

Wow,
Thanks CH, some around here will not like a radius as a datum feature, I like it though.
We did a lot of this "matched part" kind of thing, too.

 

[CheckerHater]

I feel like brandy

 

[MechNorth]

Frank,
I (for one at least) have no problem with the radius being a datum feature. It's not a feature of size, but that's not a requirement; a mathematically defined surface makes an effective datum feature. Of course, +/- tolerances on the radii is bad, but that is not the point of the drawing.

tks for the reference, CH.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Jim,
Thank you for your reply, so we agree that due to ASME rules it is not a FOS. I am on board with that, now, as it is the "law" (for my land anyway). A radius as a datum is OK? But, using +/- tolerancing of the raduis is questionable? The tolerancing is hard because it forces you to find the center?? Doesn't its use as a datum do that also? Sorry, I have been having a hard time with this and have applications just like this part only not actually split, otherwise it is just like it, partial circular plate with a radial hole pattern emanating from the center point of the theoretical partial circle. Profile of the whole thing is frowned upon, although, I have made progress with it.

 

[MechNorth]

That's a frequent question, Frank. A datum doesn't automatically have a center, much less a reproducible one. If the radius was more than 180-deg, it would have a reproducible center. Unfortunately, the '94 and preceding standards, while allowing mathematically defined datum features, don't in any that I can recall show you how to use them. Thus the recurring questions of "where's the center" and "where do I measure from". '09 improved on things a bit by allowing MMB and LMB modifiers to non-features of size, as well as BSC and [stated values].

In the figure CH posted, what is the relationship between the radii segments? There are 4 of them that need to be related back to each other or to datums A/B/C. There is an IMPLIED coaxiality of the features, but no control provided. If they were FOSs, then position could be used. For better or for worse, the only geometric control that locates surfaces is Profile of a Surface. As the radial "size" dimension is based on a center point (i.e. center location), it must be repeatable to have any kind of functional meaning. By the Principle of Simultaneous Requirements, if the two outer radii (R9 & R11) were controlled with a profile of .02/A/B/C, and the two inner radii (R9.938 & R10.062) were controlled with a profile of .01/A/B/C, then they would all be gauged/inspected together, making the actual center irrelevant. That would, however, establish legitimate controls for the implied condition that I read into the drawing. The datum feature simulator for radius segment datum feature B, establishing Datum B, is of "perfect form & size" within gaugemaker tolerances, and therefore it can be used to establish a center that would be used to locate the four holes controlled with position.
From a Y14.5 perspective, this drawing is rather poor. Some controls are evidently missing, and the two surface profiles (2 SURFACES on each) should be wrt the DRF as well.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

Jim,
Thank you, again. I also thank others that have tried to help me struggle with these concepts.
Frank

 

[MechNorth]

Glad I could help, Frank. Was afraid I'd gone off on a tangent for a minute.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com