tolerance pin, diamond pin 1

[rhmeng]

I think this is right but I am hoping to get some input. I have two parts (part1 and part2)that mate to eachother via a pin and hole (datum B) and a diamond pin and hole (datum C). Each part is aluminum and I am using tightly toleranced bushings for the holes in part1, and a tight toleranced pin and diamond pin press fit into part2. Datum B on each part will mate, and Datum C on each part will mate. The remaining features of each part will be referenced from Datums A, B, C.

I have attached screen shots of relevant views. My question: is it correct to not have a Feature Control Frame associated with Datum B? The only Datum I would be able to reference would be Datum A, which is the face, and would not help at all in locating Datum B. The Numbered Notes that are called out just tell the machinist which pin/bushing/diamond pin to insert. My thought is that as long as Datum B on each part go together (and at their best condition there is minimal slop between the bushing and the pin) then I can use virtual conditions/fixed fastener method to geometrically tolerance Datum C so that they will always go together also. Then the rest of the part is dimensioned off of Datum A, B and C.

To save time I just included the tolerances of the pin/bushing/diamond pin in parentheses. Our optics guys say we need 50 micron centering between the parts so I have to figure out how to give them that. With Datum B lined up/mated, looking at Virutal Condition of Datum C: the max diameter of diamond pin (Dat C) on part 2 is (.1772 - .0002 = .1770) and the min bushing (Dat C) diameter of part 1 is (.1772 + .0006 = .1778). So .1778 - .1770 = .0008, divide by 2 for splitting the tolerance = .0008/2 = .0004. There will be a little slop in Datum B which I added in to give me .0005 tolerance for each part.

The Datum B locating, which is basically non-existent, is messing me up. I think its fine as this will located everything else. Thanks for the input.

 

[pmarc]

3DDave,
The entire pattern does not have to be located from anything if it is a secondary datum feature (it even does not have to be a secondary datum feature in certain cases). See fig. 4-26 or 4-28 or 4-39 in Y14.5-2009.

 

[3DDave]

pmarc - if a pattern is related only to the primary, then the pattern has no true location, just like a single hole in the same circumstance. It doesn't matter if it is used as a reference in another FCF.

 

[pmarc]

3DDave,

AndrewTT already nicely and clearly explained why the position symbol should be applied in this case, so I can only repeat after him - position is the correct symbol because the callout, apart from controlling perpendicularity of the pattern to the primary datum plane, controls location (distance) between the features in the pattern.

And adding to that, it is regardless of the "shape" of the pattern - it doesn't matter whether the pattern is rectangular, circular or of any other crazy shape one can imagine. The positional requirement creates a pattern of tolerance zones that (apart from being perfectly perpendicular to the primary datum plane) are disposed relative to each other as defined by (explicit or implied) basic dimensions "within" the pattern, but can be located anywhere on the primary datum feature. As long as all actual axes of the pattern features simultaneously lie within respective tolerance zones, the pattern is considered conforming to the positional requirement.

 

[AndrewTT]

PMARC, a few questions for you.

1) Is it possible to have the pattern of features (which is indicated as the secondary datum feature) create an axis that is not centered in the pattern? For example, on the attached image, can I make the axis created by this pattern of holes centered in-between the two lower holes, and in line with them?

2) If yes, how do I indicate that this is what I want on the drawing? Or, is this already indicated on this drawing, based upon how the basic dims have been applied, and/or how the center lines have been applied?

3) If the axis does reside here, instead of at the center of the pattern, have the two perpendicular planes, that get created along with the datum axis, already been clocked? Asked another way, do I need a third datum feature to clock this part?

 

[Belanger]

AndrewTT,

Of course I'll let pmarc answer for himself, but I wouldn't get too worried about where the datum axis is located. You could actually define the datum axis anywhere you want, because the datum reference frame is derived from the datum feature simulators that plug into your 3 holes.

For instance, in Figure 4-26 of the standard they use a four-hole pattern to create the secondary datum. Then they show the datum axis at the center of the 4 pins. But that's not the only way to do it. I could place the intersection of the datum reference frame anywhere I want -- maybe at the center of one of the holes, or any other random location -- as long as I give basic dimensions back to at least one of the holes.

So for any pattern used as a datum at MMB (even if a pattern spaced out in some random shape), the "datum axis" isn't necessarily pinned down to any specific location; it's all based on the datum feature simulators.

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

 

[Belanger]

By the way, well done on that exam question, Andrew

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

 

[AndrewTT]

Thanks John-Paul, I was thinking/hoping that the datum axis could be placed anywhere. Most would probably assume that it would be centered to the pattern unless there was a clear indication that it was somewhere else. I believe that is what 3DDave was asking. Where would the datum axis be on an irregular pattern? Answer: anywhere you want, as long as you describe that location correctly with your basic dims.

 

[3DDave]

I wasn't asking where the center was. I was pointing out that a single hole is a pattern with 1 member, and whatever applies to a pattern(N) applies to a pattern(1).

Since the standard establishes that separate zones can be applied to a single hole along its depth, it is obvious that a single hole can have separate positions along its depth and that these positions are located relative to one another, requiring only a single orientating reference to make a sensible positional tolerance measurement.

That is, the position of each portion of a hole is located relative to other portions of the same hole and that these are located from an axis that is nominally oriented to the given datum.

I haven't seen any line of reasoning beyond 'because it says so,' which doesn't explain why it says so. I expect at the outset it was to advertise/market the position concept, much like 'geometric dimensioning'. I have a book by Lowell Foster titled "A Treatise on Geometric & Positional Dimensioning & Tolerancing" indicating that PD&T was considered as separate from GD&T (though it includes a reference to "JIMS-4 General Dimensioning and Tolerancing," so the muddying of the concepts goes way back.

 

[pmarc]

AndrewTT,
Here are my answers to your three questions:
1. Yes, it is possible to create a datum axis derived from a pattern of features that is not centered in the pattern. As you and J-P already stated, as long as there is a basic relationship between the pattern (pattern of datum feature simulators) and the new origin of datum reference frame, the origin can be put anywhere you want (see para. 4.12.3 and fig. 4-28). So what you have done in the attached example is perfectly fine.

2. You don't have to do anything else in your example (although you could, for clarity, show axes of coordinate system XYZ on both views, with the origin located at the mid-distance between the lower holes).

3. You don't need a tertiary datum feature to clock the part. You wouldn't need it also if there were only two holes instead of three.


3DDave,
Doesn't the logic you presented lead to a conclusion that all drawings that have a perpendicularity callout applied to a single secondary datum hole, instead of position, are wrong? After all, even for a perpendicularity tolerance "the position of each portion of a hole is located relative to other portions of the same hole and that these are located from an axis that is nominally oriented to the given datum".

 

[3DDave]

Not at all. In that situation they are equivalent and rather than marking an equivalent answer as wrong I'd accept either one.

OTOH I am in favor of eliminating Perpendicularity and Parallelism in favor of Angularity to simplify the symbol pallet.

 

[pmarc]

3DDave,

I just hope this logic does not lead you to a conclusion that Position and Perpendicularity are equivalent when applied to a pattern of two or more features nominally perpendicular to primary datum plane.

On the other hand - and forgive me if I am twisting your words - I will be suprised if you say they aren't, because then it will be like saying that for Perpendicularity applied to multiple features the statement: "the position of each portion of a hole is located relative to other portions of the same hole and that these are located from an axis that is nominally oriented to the given datum" suddenly does not apply.

The idea with eliminating Perpendicularity and Parallelism in favor of Angularity is another story, and from what I know has as many advocates as opponents. As usual, things aren't just black and white, but based on the draft of new version of Y14.5 it looks like Perpendicularity and Parallelism will not disappear quickly.

 

[3DDave]

Pmarc, what 'suddenly does not apply?' I think you have missed a step that contradicts your conjecture, which is where your surprise comes from.

 

[pmarc]

3DDave,
I could have missed something - not denying that. So to help me understand what you are saying, could we go step by step?

If I understand correctly, you say that for a single feature (e.g. hole) nominally perpendicular to primary datum plane Perpendicularity and Position are equivalent callouts, and that there is nothing wrong in applying Position in this case because there is a positional relationship to consider - "the position of each portion of a hole is located relative to other portions of the same hole and that these are located from an axis that is nominally oriented to the given datum".

So my question is: In your opinion, are Position and Perpendicularity callouts also equivalent when applied to two or more features nominally perpendicular to primary datum plane?

 

[3DDave]

Pmarc, no. Perpendicularity only applies individually to features. It's a subset of the Position control.

 

[pmarc]

3DDave,

My apologies for very late response - last weeks have been really intensive for me and last few days were hopefully peak of that.

It is not that the use of Position instead of Perpendicularity for a single feature of size nominally perpendicular to the primary datum plane is on the top of my "common GD&T mistakes" list, but the argument you used to justify Position as valid choice in this application is really interesting (and somewhat new) to me, and that is why I decided to jump into the discussion.

I am glad to hear that you don't think that Perpendicularity and Position are equivalent when applied to multiple feautres (actually, I never thought even for a moment that you think otherwise). The reason I asked you this question was to point out that your argument/logic is not universal. You can apply it in case of a single feature of size, but cannot for multiple features even though in the latter case the positional relationship between single feature's axis and each portion of that feature still exists. That is my point.

Now, you can of course disagree with what I just said, but I would still like to present you my logic.

The reason I think Position is inappropriate choice for a single feature of size nominally perpendicular to the primary datum plane is because the Position callout (just like any other geometric callout) defines TOLERANCE ZONE, and in this very case the tolerance zone can't be located from anything. The only relationship that exists between the tolerance zone and the datum plane is perpendicularity, so Perpendicularity or alternatively Angularity should be used.

For multiple features, there is an option:
- either we may want to control perpendicularity of the features to the datum plane and the spacing between them - then we choose Position callout, which basically says that the TOLERANCE ZONES are perpendicular to A and spaced relative to each other;
- or we may want to control perpendicularity of the features only - then Perpendicularity or Angularity is a choice. With that option chosen, the callout says that the TOLERANCE ZONES are perpendicular to A, but do not have to be spaced relative to each other.

So even though that within each tolerance zone the positional relationship between each portion of the feature undoubtedly exists (even for all 4 form characteristics), the existence of such relationship isn't/shouldn't be an argument in selection of proper type of geometric callout.

 

[3DDave]

pmarc,

When applied to a pattern, where on the part is the pattern located when it is only related to the primary datum?

The pattern is located in some random spot and without relation to any other portion, just like a single feature. It's only after the first feature in a pattern is made that there is any sense to the location of the remaining features - the first feature, on its own, is sufficient and can be checked for conformity to the orientation and straightness limitations afforded by the position tolerance before any other features are added.

The argument seems to be like, if it's one hole, it has to be made with a drill press, but if there are two it can be a mill; that a mill can't make a single hole because it's far too capable. And I've seen shops run just like that, but it's convention, not inherent limitation.

 

[pmarc]

3DDave,

Theoretically, the pattern related only to primary datum plane (usually this is the case when the pattern is assigned secondary datum feature) can be located anywhere on the primary datum feature, but then then the remaining features of the part are located relative to it, so in fact it is not that the pattern can wander freely all over the part.

Again, the reason that the secondary datum pattern is controlled with position tolerance is simply because on a definition level a mutual positional relationship between the features/tolerance zones witin the pattern exists - and the relationship is spacing.

The sequence of making features of the part has nothing to do with it. As long as final product meets drawing requirements, the datum pattern can even be produced in the very last step of manufacturing process. Maybe in most cases it is not the smartest approach, but in some this works perfectly fine.