Multiple Patterns of Features

[R1chJC]

Hi All,

So I'm detailing a part much like the one in Fig.7-37-2009.

Could someone explain how the basic dimensions would work in this case? Since each large hole and the 4 smaller ones each reference ABC, does this mean I have to have basics going back to datum for every hole?

Thanks.

 

[pmarc]

No. You would just need basic dimensions to define true position of each large hole center in the main view and then in the detail view you could just show two linear basic dimensions (one horizontal and one vertical) between centers of smaller holes or use basic diameter that would go through the centers of smaller holes.

 

[R1chJC]

Hi pmarc,

Thanks for the reply. I can see what you mean, and that makes complete sense - as the larger hole moves the smaller ones move with it.

However, if I look at Fig 7-38 each hole can be tracked back to a datum through basic dimensions.

In Fig.7-37 how do you verify the position of the 4x smaller holes when you don't have there true position with respect to ABC as per the first line of the FCF?

I'm having one of those weeks where GD&T suddenly seems to make no sense!

Thanks,
Rich.

 

[gabimo]

I'm having one of those weeks where GD&T suddenly seems to make no sense!

Good for you....only weeks ...... I have months and years of nightmares.

when you don't have there true position with respect to ABC as per the first line of the FCF?

I don't think you have to. See previous replies by Evan
copy-paste

"The basic dimensions only apply to the theoretically exact geometry, and could be described as dimensions of the model. The basic dimensions need to define the theoretically exact relationship between the datum features and the toleranced features, which then defines the relationship between the datum feature simulators and the tolerance zones. The datum feature references determine how the actual part gets lined up to the datum feature simulators - and hence how the datum reference frame and tolerance zones are established on the actual part."

" ....the basic dimensions don't need to apply to features. .... Another advantage of basic dimensions is that if you don't like the ones that were annotated on the drawing, then you can calculate others and use those instead. As long as the relationship between the considered feature(s) and the datum feature(s) is fully defined, then you can use a different scheme and it doesn't change anything..." end of copy-paste

See attached another page from Paul D book. MBD definition does not use basic dimensions as you can take them from the CAD model. Now the question become how you can take them, which way, (referencing the other thread) from the model?
Whcih is "the best way" to get your basic dimensions from the CAD?
Does is really matter or no? More favorably disposed toward to say no than yes.

 

[pmarc]

In Fig.7-37 how do you verify the position of the 4x smaller holes when you don't have there true position with respect to ABC as per the first line of the FCF?

You do have true position of the smaller holes relative to ABC. It is a sum or difference of the basic dimension defining TP of large hole and the basic dimensions shown in the detail view.

 

[Tunalover]

This may be off-topic, but maybe not. I recall a rule that states that features located with positional tolerance FCFs having identical datum reference frames (same datums in the same order), identical tolerance zone shapes (e.g. either rectangular or cylindrical, not both) and identical material condition modifiers by default specify a group of features with simultaneous requirements. Is Figure 7-37 one of those cases?

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993

https://www.youtube.com/watch?v=BKorP55Aqvg

 

[3DDave]

ASME Y14.5-2009 Fig 7-37 does depend on simultaneous requirements.

I am uncomfortable with the small holes being controlled to two differently based datum reference frames at the same time. The long standing prohibition against double dimensioning was actually about double tolerances - that one tolerance chain could show the feature was acceptable while the other would show it was not. In this case the small holes could meet their dia 1.4 tolerance to A|B|C and fail to meet the 0.4 tolerance to A|D(M) even though the apparent mating part would only share the A|D(M) datum reference frame.

 

[chez311]

The long standing prohibition against double dimensioning was actually about double tolerances - that one tolerance chain could show the feature was acceptable while the other would show it was not. In this case the small holes could meet their dia 1.4 tolerance to A|B|C and fail to meet the 0.4 tolerance to A|D(M) even though the apparent mating part would only share the A|D(M) datum reference frame.

I could understand having reservations about wanting to double tolerance a feature with two different DRF's however as far as it was explained to me there is no reason it can't work in theory and there is no restriction on the datums that can be used - either what combination or precedence (it could be A|B|C and D|A). The only requirement is that all the tolerances applied to a feature must be met - as in your example that part would fail inspection because it does not fall within both tolerance zones, correct?

In practice I could see this being a major issue though as it would probably prove difficult to actually measure, gauge, and would likely result in production issues and a lot of scrapped parts unless one of the tolerance zones was relatively loose.

 

[pylfrm]

ASME Y14.5-2009 Fig 7-37 does depend on simultaneous requirements.

How do you figure?

I am uncomfortable with the small holes being controlled to two differently based datum reference frames at the same time.

Isn't that the case for essentially any example of composite or multiple single-segment positional tolerancing?


pylfrm

 

[axym]

Hi All,

I agree with chez311's and pylfrm's comments. I don't see the application of simultaneous requirements in Fig. 7-37. The A|B|C datum reference frame referenced in two different FCF's and so sim reqts would apply, but A|BC is derived from three planar datum features so the datum feature shift would be minimal. The A|D(M) datum reference frame is derived using a different datum feature D for each pattern, so there is no simultaneous requirement there either.

Also, controlling holes (or other features) with two or more different FCF's (with different datum reference frames) is a very well established practice. Y14.5 is full of examples of this, and it is one of the best methods of capturing fit and function requirements.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca