width of pattern slots as datum

[bxbzq]

Hi,

A simplified sketch attached. My question is, how do I call out the widths of the two rows of slots as datum feature B, which establishes the center plane of the distance 30 as datum. I looked through the '09 standard and could not find similar case. I guess I should not attach the datum feature symbol B to the basic dim 30, so is the datum feature B callout shown right way to go?

Thanks

 

[ewh]

I guess I should not attach the datum feature symbol B to the basic dim 30

Why not?

“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV

 

[CheckerHater]

You can attach datum feature symbol to Position FCF to indicate that all 6 slots form datum (Feature pattern datum)

You can attach it to basic dimension 30 as ewh suggested.

You can also do it the way shown on the picture.

All three will have slightly different meaning, but will refer to “centroid” of the pattern.

 

[bxbzq]

Because the basic dim 30 is not a touchable feature.

 

[bxbzq]

ewh, checkerhater,
Attaching a datum feature symbol to basic dim 30 is like attaching a datum feature symbol to a basic dia. of bolt circle. I don't see it is supported by para 3.3.2 in '09 std.

checkerhater,
what is the difference between the first and second case as you suggested?

 

[CheckerHater]

@bxbzq:

I see both as example of Patern of features, which isn't exactly "touchable feature".

The 1 is more conventional, but doesn't make it clear, what is the direction of datum.

The 2 gives the sense of direction, but looks "unusual".

About "bolt circle": when dealing with pattern of touchable features it is not explicitly forbidden to refer to line, connecting said features. Look at Fig. 4-23 in '09. So why not bolt circle?

 

[Belanger]

Fig 4-23 is still clearly addressing physical features, so it passes muster with paragraph 3.3.2 (although that doesn't explicitly say "phantom line").
A bolt circle through the axes of holes, however, doesn't seem to comply with 3.3.2.

For the OP's sketch, I suppose what you have already does what you want. Grabbing the inner edges of the slots is not the same as making a datum from their widths.

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

 

[Tunalover]

Under what functional scenario is it beneficial to make the pattern of slots a single datum? For that matter, when would it be beneficial to make a pattern of holes into a single datum? Where would the origins of those datums be? Sorry, but I've never fully understood the how and the why of these "pattern" datums.


Tunalover

 

[3DDave]

When I look at multiple features acting as a single datum I think of the fixture that would simulate that datum. If it's a pattern of holes, the fixture will have a pattern of pins. It isn't necessary to have any particular location or orientation derived from the holes; as long as the reference to the datums can be seen as related to the datum simulator, that is good enough. The confusion is that the datums don't move, the part does; it only looks like datums follow the parts around.

For a clear example of deriving, look at the figure where there is a sloped feature that is met with a sloped datum simulator, but the datum itself is not sloped.

The problem that comes from multiple features are from two sides.

First, if the features are referred to as RFS, it used to be indistinct if they are to be simulated with exactly simultaneously expanding pins that stop expanding once location and orientation are set or if some other condition should happen. I dislike the use of this case because I haven't seen a real application where multiple simultaneous expanding features would mate with a part.

Second, if they are MMC or MMB or whatever the new terms are, then the location and orientation of the part relative to the datum simulator are not unique, which presents a problem with CMM software looking for a single answer from which to derive an exact reference frame. This case is similar to the case when only a single hole is used, but CMM software can be written to handle the special cases of a single hole or a single slot; randomly located and shaped features requires a more sophisticated approach.

 

[axym]

Hi All,

Several interesting issues brought up here. Here are some thoughts:

First, I would say that the first drawing is at least well defined. The simulators would be four sets of parallel planes that grow and shrink simultaneously, and whose centerplanes are basically located. This would be difficult to accomplish physically, and it seems unlikely that the mating part would interface in this way. The datum isn't exactly the centerplane of the basic 30 distance - it's a centerplane derived from the four simulators.

I agree that the datum feature symbol should not be attached to the basic 30 dimension. When I think of situations like that, or like the bolt circle example, I try to think of what would be done if the basic dimension was not present (as in the case of an annotated model in which the CAD data is basic). The datum feature symbol would have to be associated with particular features. This is why the original drawing works - the pattern of four width features is clearly defined. For a bolt circle example, the datum feature symbol would be associated with a pattern of holes or pins.


Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[bxbzq]

Glad I'm not alone.

 

[fsincox]

tunalover,
Anytime a pattern of clearance holes is actually the sole locator of a part in an assembly, I feel it truly represents the "functional" requirement to state the pattern as a datum (usually a secondary, at MMB). However, I am not saying it is easier for manufacturing or inspection. It has, therefore, been been a very traditional practice to compromise function for manufacturing/inspection convenience, particularly due to the effect of "the datum virtual condition rule" and the "simultaneous requirement".
Frank

 

[Tunalover]

Frank,
Thanks for shedding light on that! I'll stick to using surfaces, edges, and the occasional hole as datum simulators for hole patterns.
TL


Tunalover