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.

 

[rhmeng]

part 2

 

[3DDave]

Maybe should have its perpendicularity to [A] controlled?

 

[rhmeng]

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

 

[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.

 

[rhmeng]

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

 

[rhmeng]

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?

 

[3DDave]

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

 

[rhmeng]

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?

 

[3DDave]

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.

 

[JNieman]

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.

 

[rhmeng]

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.

 

[3DDave]

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.

 

[AndrewTT]

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).

 

[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. It is allowable to have different positional tolerances on opposite ends of the same hole.

 

[Belanger]

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 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

 

[JNieman]

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.

 

[AndrewTT]

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).

 

[JNieman]

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.

 

[3DDave]

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.