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tolerance pin, diamond pin

tolerance pin, diamond pin

(OP)
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.

RE: tolerance pin, diamond pin

Maybe [B] should have its perpendicularity to [A] controlled?

RE: tolerance pin, diamond pin

(OP)
ohh yea thats a good point I should include that, that would be all I could do tho I think..

RE: tolerance pin, diamond pin

I often set up tolerances similarly for locating pins. Like 3DDave says, though, I also include a true positional tolerance on Datum B to A-alone, which controls its perpendicularity.

RE: tolerance pin, diamond pin

(OP)
cool so i think this is ok then, just seemed a little weird when I first did, thanks for the help

RE: tolerance pin, diamond pin

(OP)
actually real quick, to control perpendicularity it seems like the tolerances would have to be super tight for this to work. If Datum B hole is at a minimum .1772 + .0006 = .1778, and Datum B pin is at a maximum .1772 + .0005 = .1777, then the perpendicularity has to be controlled to a a tenth right?? .1778 - .1777= .0001. That seems super tight is is not?

RE: tolerance pin, diamond pin

It depends on how hard you want to push to get it installed. It's only small if you never want any load applied.

RE: tolerance pin, diamond pin

(OP)
I dont really want a load applied to it, each part is housing optics, and they will be assembled and disassembled many times during the build. I think I get what you are saying tho, if there is a slight interference you will just have to push harder to mate the two parts, and it will deform accordingly?

RE: tolerance pin, diamond pin

Probably the bolts holding the parts together would bend the pins if the parts don't sit flat. You can see if your stress people have any suggestions about how much strain is acceptable.

RE: tolerance pin, diamond pin

Yes, the tolerance for perpendicularity will have to be quite well controlled. You have to have some reasonable manufacturing limitations/expectations. While it's not unreasonable to achieve .0001" perpendicularity, it is not a practical or efficient task. This topic gets more into the engineering than the GD&T literacy, though.

How many are you making? If it's one, you can specify that the parts be located within the accuracy required, and then match-reamed and pinned in place. After all, if they are matched pairs, not expected to have interchangeable parts; the true location of the pins doesn't matter so much as Part 1 can be taken off Part 2 and be put back in the same place again.

If you have to be able to take Part 1 off, and have -any other- Part 1 go back in the same place, then you must have more control, certainly. In that case, your diamond pin is where you gain your breathing room. It has a little 'wiggle room' in the vertical axis, obviously, and left/right on your drawing, you have to decide how much you can allow. That's where you get your true position tolerance. I believe your expectation of .0005 is a bit tight. If that's what you need, that's what you need, though. It raises a flag with me, though.

The perpendicularity of Datum B does not have to be .0001 imo. The pin will not bend. It'll crack before it bends any significant amount. Your part will deform before the hardened tool steel pins bend. Unless your part is some kind of much-harder material, which is doubtful. Pins are going to be 50+ HRC.

However, if they are not replaceable parts, I would look at allowing for the manufacturer to match-drill/ream those holes after they are assembled in place accurately. That way, they can snug up the bolts that fasten them, bump the one part around until it's dialed-in right where it needs to be, tighten the bolts further, double check position, and machine as needed.

RE: tolerance pin, diamond pin

(OP)
JNieman, thanks for the detailed response, these will not be made in high quantities, probably only 3 or 4 at a time. They also do not have to be interchangeable and we can have matched pairs of mating parts. We should be able to work with the mfgr to tell them this.

I came up with perpendicularity of .0001 because that is what the tolerance stack up required for there to be no interference for Datum B (and I think it is actually even less). The pin and bushing of Datum B (at best case) can essentially be the same size so any deviation from perpendicularity will cause them to interfere..

Regardless, I think that making the drawings and then getting them to a shop to talk about them is the best way to do this as you have stated. I have been trying to come up with a good way to explain the design intent on a drawing of machining the datums together, but its getting messy, a call might be better, however.

RE: tolerance pin, diamond pin

Since the bearing load is small, you can improve your chances by limiting the engagement. Even .030 inches is plenty.

Also, if a pin breaks, it's probably from hydrogen embrittlement. Maybe JN is thinking of plastic deformation and not elastic bending.

RE: tolerance pin, diamond pin

Quote (JNieman)

I often set up tolerances similarly for locating pins. Like 3DDave says, though, I also include a true positional tolerance on Datum B to A-alone, which controls its perpendicularity.

I don't believe you can use position on a single hole if you have only established one datum plane. You would have to relate the hole to datum feature A via. perpendicularity. However, if you call both holes a pattern (2X ø.1172) and use this pattern as your secondary datum feature then you could use position to relate this pattern to datum feature A. The position tolerance would orient the holes perpendicular to A and locate them to each other (see attached example).

RE: tolerance pin, diamond pin

It's always been a curiosity that a single hole can't have a position tolerance even though it controls the location of one end of the hole axis relative to the other end. It is allowable to have different positional tolerances on opposite ends of the same hole.

RE: tolerance pin, diamond pin

Quote (3DDave)

It's always been a curiosity that a single hole can't have a position tolerance even though it controls the location of one end of the hole axis relative to the other end.
How can one end of a hole's axis be relative to the other end?
I'm thinking of paragraph 1.4(n) as I ask.

Or maybe you're referring to Fig. 7-27 in the standard. But even so, that example still ties the hole's axis to datum B in a relationship of location.

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

RE: tolerance pin, diamond pin

Perpendicularity would possibly be more clear and concise - I'm open to changing to that, if it's truly confusing when using a single-datum reference in a true position control.

It is my understanding that if using a single datum to control a hole which is nominally perpendicular to the datum referenced... that it creates a cylindrical tolerance zone whose diameter is as specified in the FCF. This means the centerline of the hole must fall in that tolerance zone. Its location has not been controlled - its position just... it's wherever it is. It must simply be oriented correctly in relation to the datum referenced.

It does appear that the 2009 standard prefers using Perpendicularity for establishing a Secondary Datum Feature when establishing features for a DRF. At a quick glance, the standard only appears to use single datums with position-tolerance when they are parallel centerlines, such as the centerlines of bolt holes in a bolt-flange in relation to the shaft centerline. Odd that parallelism wouldn't be used there, but perpendicularity is used for holes perpendicular to a single datum. I suppose it could arguably be 'parallel' in that they would never intersect, but still be deviating from the basic condition quite a lot.

I think I developed this habit by taking a situation with multiple single-segment FCFs (Fig 7-47) and thinking I could separate the refining lower segment into other cases where I only wanted the 'perpendicularity' control. A quick google search of "true position with only one datum" brings up some 'spirited' forum discussions in various places. Looks like I may've been using faulty methods.

RE: tolerance pin, diamond pin

JNieman, I view it this way.

1) Position controls orientation and location.

2) A single hole cannot be positioned to the primary datum plane (in a perpendicular orientation) b/c, while the position tolerance zone will control the perpendicularity of the hole, what is it located from? Where is true position? I need to put a basic dimension describing true position, but from where? I don't have enough datums to do this yet.

3) I can use position on a pattern of holes (in a perpendicular orientation to the primary datum) b/c the location portion is the location from each other. The position will control the pattern of holes perp. to the primary datum and located from each other. I can even make this pattern of holes my secondary datum feature. If I do this it will yield a datum axis (and also two perp. datum planes) that is perp. to the primary datum. This datum axis will be located in the center of the pattern.

4) You can use position on a hole to achieve coaxiality if the hole and the primary datum are aligned to each other. In fact, the standard allows for position to be used independent of datums to achieve coaxiality. You see this in the lower segment of composite position controls (figure 7-45, 7-49, 7-51). See also fig. 7-59.

So there seems to be a difference in what is allowed with the use of position between coaxiality (zero, or 1 datum axis established) and location of features (full datum reference frame established).

RE: tolerance pin, diamond pin

I'll consider myself fortunate to have never had bad parts due to my incorrect requirements. I'll be using a more appropriate orientation control from now on! Thanks for the helpful tangential clarification. I can think of / find nothing that disagrees with what you say.

RE: tolerance pin, diamond pin

Quote (AndrewTT)

3) I can use position on a pattern of holes (in a perpendicular orientation to the primary datum) b/c the location portion is the location from each other. The position will control the pattern of holes perp. to the primary datum and located from each other. I can even make this pattern of holes my secondary datum feature. If I do this it will yield a datum axis (and also two perp. datum planes) that is perp. to the primary datum. This datum axis will be located in the center of the pattern.


Sure. And where is the entire pattern located? What's the true position of the pattern if there is no secondary datum? What if the pattern is irregular and has no 'center'? Doesn't each section of a feature need to be controlled for position with respect to the location of every other feature section? Everything that applies to more than one hole applies to only one hole for this case.

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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?

RE: tolerance pin, diamond pin

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

RE: tolerance pin, diamond pin

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

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

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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".

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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

RE: tolerance pin, diamond pin

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?

RE: tolerance pin, diamond pin

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

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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.

RE: tolerance pin, diamond pin

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.

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