2 hole pattern as datum 2

[cjccmc]

In attached sketch, I want to have the two .752 holes located +/- .004 dia to each other and perpendicular to datum A .001 dia. Then these two holes become datum B to locate the .252 dia hole. (We use model based definition where CAD file is queried to get dimensions that's why no dim are shown in attachment). Does what I show give me what I need in legal ASME 1994 terms? I was wondering if I need the ref to datum A in the TP frame. It doesn't seem to mean anything but I'ver never seen TP without ref to at least one datum. Thanks for the help.

 

[dingy2]

Yes, the positional tolerance should reference datum A as shown. The positional tolerance to qualify datum B reflects the positonal of the holes to each other and perpendicularity to datum A.

Your drawing, in my opinion, is correct to ASME Y14.5M-94.

Dave D.

http://www.qmsi.ca

 

[pmarc]

Technically your scheme is OK. From practical (functional) point of view, we might debate whether (M) modifier should be added to datum reference B or not.

Presence or absence of A datum reference in positional callout for smaller hole makes difference. With A present, axis of the hole cannot be out-of-square more than .004 cylindrical tolerance zone allows. Lack of A could result in the axis being tilted more than .004 to A. That is, .004 defined by positional tolerance to B plus additional tolerance allowed by perpendicularity callout for datum feature holes to A.

 

[cjccmc]

Thanks for your replies. Looks like I have all I need for practical purposes for this callout.

One point of curiosity. If I wanted the two .752 dia holes to be the primary datum A, would it be legal to call out a TP tol without reference to a datum? It would look like:

2X dia .752
TP/ dia .004/
A

 

[SeasonLee]

No, you can’t use the hole pattern as a primary datum.

When a hole pattern locates the part and is used as a datum feature, it is usually referenced as a secondary datum feature. When a hole pattern is used as a secondary datum feature, it restricts the part movement in three direction, it works as both secondary and tertiary datum feature, so tertiary datum feature is not required.

SeasonLee

 

[SeasonLee]

Some remarks on my previous post.

You can’t use the hole pattern as a primary datum on this case, but for some cases, the hole pattern can be used as a primary datum feature, so you are right on the latest post, a position control is used to control the spacing between holes if you want to choose the hole pattern as a primary datum feature.

SeasonLee

 

[MechNorth]

Agree with Dave, with Pmarc's addition that per Y14.5M-1994, the (M) is required when using a pattern of features of size to establish a datum. It's not required in the '09 release, which (to my thinking) is a good change.

SeasonLee, what's your reasoning that the pattern of two holes couldn't establish the primary datum? Picture this as a rubber (PU, or other) gasket, the primary purpose of which is to locate two tubes wrt each other; the outer profile having no functionality in application (I came across something very similar a few weeks ago for a client). As usual, it comes down to the functionality.



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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[greenimi]

Can we achieve the same results by using composite callouts? (question for academic education purpose only)
Composite position
Upper segment position of .004 wrt A. Lower segment .001 with no datum.
If what I wrote is not equivalent with the OP callouts, how would you make it to have the same effect but using composite, if all possible?
Thank you

 

[pmarc]

greenimi,
I am not sure I understand your question. Do you want to replace position + perpendicularity callouts on pattern of holes B by composite positional callout?

 

[greenimi]

Pmarc,
Yes, if all possible. I would like to use composite instead of position + perpendicularity.
If I cannot get the same results using composite, what about two single segment?
How the DRF would look like then?
Thank you pmarc

 

[pmarc]

In my opinion there is no way you can achieve the very same effect using composite position one one hand and position+perpendicularity on the other.

Keep in mind that lower segment of composite position (the one without datum references) would always control spacing between the holes, while perpendicularity callout would imply by nature that there is no locational relationship between toleranced holes in the group.

As for two single segment positional callouts, it will not work too. If you are thinking about second segment without any datum reference, this would result in the very same intepretation as for composite position, as far as I can imagine.

 

[SeasonLee]

Jim,
Your picture of non-rigid part is a good example to use a hole pattern as a primary datum, as I mentioned on my second post earlier, for some cases the hole pattern can be used as a primary datum feature, but for the OP I can't see any reason to switch the planar datum feature to the hole pattern as a primary datum feature.

greenimi,
If the datum reference frame already established, then you can use the composite position callout to control it, but here the DRF is not established yet, we are talking about it.

SeasonLee

 

[MechNorth]

Season, quite probably; the function wasn't described unfortunately. Agree with your comment re use of composite on a primary.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[powerhound]

Jim,

Do you believe that allowing the use of a hole pattern at RFS as a datum is a good thing because it's an extra thing you can do now or is there an application that you have in mind that it serves better? I'm not challenging the thought, I'm just curious as to how the actual datum could be established/simulated in such a case.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[axym]

I also have concerns about the use of datum feature patterns referenced RFS. I realize that Y14.5-2009 allows this practice, but there are some gray areas in how the simulation would work in practice. There is a question as to whether or not the simulators expand/contract independently of each other, to achieve stable contact with the datum features. There is still ongoing debate over this in the subcommittees.

My feeling is that applications in which patterns are specified at RFS usually involve some type of subtle deformation of the datum features and/or the gage elements.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

"There is a question as to whether or not the simulators expand/contract independently of each other, to achieve stable contact with the datum features. There is still ongoing debate over this in the subcommittees."

Paragraph 4.12.4 seems to answer the very issues that you say are still questionable. I too agree that it's not of much practical use, but the standard does say that they expand/contract simultaneously.

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

http://www.gdtseminars.com

 

[axym]

John-Paul,

Despite, what 4.12.4 states, the issues are still questionable. It does say that they expand/contract simultaneously, but it also states that the simulators make maximum contact with the extremities of the datum features. There are of course different interpretations of the meaning of "maximum contact". There are those who assert that if one simulator cannot expand/contract any further, that the other simulators continue to expand/contract until fully stable contact is achieved. I personally do not agree with this interpretation, but there are plenty of folks who do.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

I am amongst those guys, Evan. While I can imagine difficulties in interpretation of 4.12.4 in case of pattern of features of equal nominal size, I do not really see how the other approach might work in case of pattern of features of unequal size, like shown in fig. 4-25. Are you saying that if datum feature simulator B achieved maximum contact with its datum feature at diameter 27.98 for instance, datum feature simulator A shall stop at 22.98, even if size of actual mating envelope of datum feature A could allow the simulator to contract let's say down to 22.92? Would that be a simulation of datum regardless of material boundary of datum feature A?

 

[MechNorth]

A very real-world application, in fact. I used tgo work with large molds with 4 leader pins, moving in bushings. The position of everything else on each of the large plates was based on the leaderpins/bushings to provide alignment. Ideally, we would have used the 4 leaderpin mounting holes as the datum feature pattern (on the core plate, specifically), but we didn't want MMC as it would have skewed how the assembly actually functioned. RFS would have worked far better. We ended up using the center width & center height to establish the datums, but that isn't how the parts actually work.
On mating plates, aligned by the leaderpin bushings, the 4 leader pins at their actual size establish the relationships, not the MMC condition. I came across a couple others the last couple years in consulting, and unfortunately can't share them. There ARE valid reasons to go with patterns at RFS.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Evan, this topic seems not too far from the dilemma which was solved by the datum translation modifier. Without the translation triangle, the pins expand at their perfect location until stability is achieved. With the translation triangle, the pins could move within acceptable location tolerance while they expand.

Of course I'd have to think this out a bit more, but I'm just saying that your mention of the still-living debate about a pattern as a datum RMB made me think of the translation modifier.

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

http://www.gdtseminars.com