Runout referenced with a datum of pattern of holes

[Madhu454]

Hi All,

Recently I saw a drawing where the runout control is referenced with axis of pattern of holes as the datum. I doubt whether it is a legal specification for runout?

Please see the attached drawing.
The four holes dia 10-11 are the clearance holes, the disk will be mounted to a spinning part using the fasterners.

I understood that dia 9.8 gauge pins located on a PCD of 100mm can be used to varify the position of the 4 holes. Also I understood that using M modifier is a must when we use pattern of holes as a datum. I have no doubt on this.

The question is, the outer diameter of the disk is controlled with respect to the 4 hole pattern using runout , How is this possible?
because the datum B is the axis derived from the pattern of 4 fixed gauge pins of size 9.8. How to simulate the datum B here? will there not be any play between the holes and the datum simulator? obviously there will be a play - How to measure runout for this.

Can any one help me to understand this doubt.

 

[pmarc]

Technically there will not be a play between datum feature simulator B and datum feature holes, because datum B within runout FCF is called out at RMB (regardless of material boundary). This means that datum feature simulator pins are not fixed in size at 9.8, but have to expand from 9.8 till each of them gets maximum contact with its corresponding hole surface.

 

[ewh]

I've never seen it done that way. Maybe there is a reason we don't know of that makes it the best solution, but I would have controlled the OD with circularity or cylindricity, and referred the hole pattern back to that.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[Belanger]

I too have never seen that. But I agree with pmarc: it really isn't practical to use a pattern of features as a datum unless they are going to be taken at MMB. But of course, the datum references for runout must by RMB, so it seems to be a Catch-22. Why not go with position (with MMB on B) and circularity on the OD?

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

 

[dingy2]

Madhu454:

That drawing is flat wrong.

9.3.1 Datum Feature for Runout Tolerances (page 180 ASME Y14.5-2009) states "The datum axis for a runout tolerance may be established by a cylindrical datum feature of sufficient length, two or more cylindrical datum features having sufficient axial separation or a cylindrical datum feature and a face at right angles to it."

I think the use of positional tolerances would be more appropriate on this drawing.

Dave D.

http://www.qmsi.ca

 

[Madhu454]

Hi Pmarc,

you mean to say, we have to use a variable gauge pins but the center of which has to be exactly located on its true position (PCD basic 100 and 90 deg apart as shown in drawing.

I was under impression that, whenever we use a patten of holes as datum - even at RFS it has to be simulated with a fixed gauge. because the datum here is the axis derived from the pattern-not from individual holes.

In that case, assume we have 10 holes (or even more) insted of 4 holes, then do we need to use gauge pins for all the 10 holes? I hope we can choose some 3 - 4 holes in that case, does all the hol surface have to be in contact with the gauage pins- I think it does not require.

Please correct me if I understood wrong.

 

[Madhu454]

Hi,
I saw this drawing in " Tolerance stackup analysis by james meadows" chapter- stackup analysis for 5 part assembly.

 

[Madhu454]

Hi JP and Dave,
I think we can use position as you suggested. I to beleive the example i have posted is illegal specification, as dave quoted on selection of datums.

"9.3.1 Datum Feature for Runout Tolerances (page 180 ASME Y14.5-2009) states "The datum axis for a runout tolerance may be established by a cylindrical datum feature of sufficient length, two or more cylindrical datum features having sufficient axial separation or a cylindrical datum feature and a face at right angles to it."

 

[Belanger]

Yep. Dave nailed it; it's right there in black and white from the standard.

That said, I do see the intent. We can certainly envision something mounted on those 4 holes and then rotating, and then we may be concerned about the wobble.

But that specific usage is not in line with ASME.

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

 

[MechNorth]

The position control for the pattern of 4 holes is correct per '94 as it references at MMC as required. That requirement disappeared for '09. The problem that I see is that datum-B is called out RFS (RMB '09) in the runout control (as expected/required), but conflicting with the definition of datum-B.
If '94 is invoked, then you can't remove the MMC from the position control. If '09 is invoked, then I'd suggest doing away with the MMC on the position control and then you'd be ok. As for reasons to do this, it seems to me that the outer cylinder is being controlled wrt the center of the pattern; I have seen a need for something like this when you don't want too much overhang on mating pieces when centered up. Not a great way to do it, but achievable.

Jim Sykes, P.Eng, GDTP-S
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[JMarv]

Dave, Jim, & J-P;

Assuming the "intent" is correct, would labeling the 4X Ø10.5 holes individually as datum targets satisfy the requirements?

What would be needed to make the 4 hole pattern fall under the criteria of "two or more cylindrical datum features having sufficient axial separation" ?



Joe
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[dingy2]

Joe:

Sorry but the holes need to be in alignment creating the axis. Stick with positional.

Dave D.

http://www.qmsi.ca

 

[CheckerHater]

I should probably change my handle to "Devil's Advocate"

The standard sais "may be established", not "should", not to mention "shall".
So your choices are not limited to "two or more cylindrical datum features having sufficient axial separation"

 

[Belanger]

Jim, there was never a requirement that position on a pattern use the MMC modifier. Even in 1994, see Fig. 5-10 as an example.

So If '94 is invoked, then you could indeed remove the MMC from the position control. But we still have the difficulty of establishing that pattern RMB when you get up to the runout tolerance...

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

 

[MechNorth]

J-P, I was dealing with the creation of a datum based on a pattern of FOS. In support of that, let's look at 4.5.8 Pattern of Features; "Multiple features of size, such as a pattern of holes at MMC, may be used as a group to establish a datum when part function dictates. See Fig. 4-22. ..." There is no support (text or illustration) in the standard for use of the RFS modifier in this context. You are correct that there is no requirement to locate a pattern of FOS at MMC.

An ongoing issue of terminology may be at issue here again. In "voluntary standards" (i.e. not mandated by law), terms like "must", "will", "shall", etc. that are EXPLICIT and RESTRICTIVE are generally avoided because they preclude all other options. Their use can open a large can of legal worms for the document creators and publishers. Seemingly wishy-washy / open words like "may", are used in their place. To me, this is a real issue because it allows significantly different interpretations and "reading into" of the standards.

The certification exam treats this particular item as a "must" (i.e. features must be at MMC for the simulator).

Don't agree with Dave re the axes having to be aligned. That's contrary to 4.5.8.

Joe, if you really want the outside cylinder to be located wrt the pattern of 4 holes, and cylindrical, you can go with position and a cylindricityh control (expensive), but I would probably invoke Y14.5-2009 for that particular feature and drop the MMC from the FCF. Need to consider though if the extra restriction on the perpendicularity and inter-feature position of the 4 holes is necessary or too restrictive.

Jim Sykes, P.Eng, GDTP-S
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[CheckerHater]

Some time ago I've started thread on similar, but simpler case:

http://www.eng-tips.com/viewthread.cfm?qid=278412&page=13

Runout to something that isn't cylinder.

Now, after 14.5 2009 being around for a while the idea of complex feature as a datum doesn't look so crazy.

What if we consider holes all together as "irregular shape feature" and create setup similar to Fig. 4-35 in ASME 14.5 2009?

 

[dingy2]

Hi Jim:

Nice to see that you are back.

See that you are using the 94 standard referencing 4.5.8 and it is true that a pattern of holes can be a datum. No doubt about that.

Now lets get to either circular or total runout using the 94 standard.

6.7.1.1 page 189 of the 94 standard Basis of Control - The datum axis is established by a diameter of sufficient length, two diameters having sufficient axial separation, or a diameter with a face at right angles to it." There is no mention of a pattern of holes used as a datum.

Yeah, they have to aligned.



Dave D.

http://www.qmsi.ca

 

[MechNorth]

Tks Dave. Interesting. I've always thought of an axis of revolution, as opposed to the specific definition in 6.7.1.1. I would then agree with your interpretation. ;~}
I've heard stories / rumors that the requirement to specify the pattern of FOS at MMC was to allow / require hard-gauging because nobody knew how to finitely measure and verify. Don't know if that's true or not. If so, it makes sense that they would have precluded a pattern of features as a datum axis for runout controls. Doesn't seem so difficult today to use a pattern at RFS (RMB), but who knows.



Jim Sykes, P.Eng, GDTP-S
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[Belanger]

Glad you clarified that, Jim! Your original post said that a position control for a pattern of must be at MMC (for 1994). Now I see that you were referring to using a pattern as a datum.

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

 

[MechNorth]

sorry.

Jim Sykes, P.Eng, GDTP-S
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