Quick Confirmation--Total Runout or Position for Cylindrical feature?

[cratechallenge1]

Hello all,

Quick question: I've been reading the forums here for a few hours and I'm pretty sure I have my answer already but I'd like to confirm with the community to make sure I'm thinking about this in the correct manner.

On a tubular part, I have two cylindrical features that I'm trying to control the coaxiality of (relatively tight)--one is a simple locator feature (for assembly) and the other is the internal cylinder bore, with the central axis of the internal cylinder bore being the primary datum feature. I don't mind if the locator feature is slightly out of round (as long as it falls within the feature tolerance limits), I only want to make sure that the central axis of the locator is coaxial with the central axis of the internal cylinder bore.

I've been debating back and forth between using either total runout or position to control the locator with respect to the primary datum, but since I mainly care about the axes of the features lining up would it make the most sense to use position here?
For reference, this part is essentially a long tube so the manufacturing will be done on a lathe.

Thanks for your help!

-cc1

P.S. as a side note, how would an inspector get setup to measure the positional tolerance of this?

 

[greenimi]

ISO or ASME drawing?

I agree I would use position.

Runout = position + form error (circularity / cylindricity)
("=" and "+" not in the mathematical sense)

 

[cratechallenge1]

Thanks for the quick reply @greenimi--this is an ASME drawing. Currently using Y14.5-2009 as I haven't yet sat through to review the changes that were made in the 2018 version.

 

[drawoh]

cratechallenge1,

The important thing about ASME[ ]Y14.5[‑]2018 is that concentricity has been removed. It is only depreciated on the 2009 standard.

What are your datum features? If your datum feature is an end face and then your internal cylinder, you should use position. If the internal bore is your only datum feature, I would use runout. Think hard about how you will fixture this thing for inspection.

My very crude assumption about positional tolerances is that I have an feature, however accurate, located somewhat sloppily. Runout gives me accurate concentricity, assuming I can inspect everything. I cannot imagine why I would place a tight runout specification on a sloppy diameter. Maybe you can.[ ]

--
JHG

 

[cratechallenge1]

@drawoh I like that explanation

Attached is what I have for the mating end sections (this is the female groove side). The part itself is a long (think feet) honed out tube section and I need the ends to mate up concentrically with each other. Datum A is the primary and datum B is the male groove face (not pictured, other side of tube), but I've added a datum C to try and make it more clear that this positional tolerance only applies to this female groove face. The locator itself is quite tight, but I guess I meant ovality in terms of the lovely drawings in the dimensioning and tolerancing books (illustrated crazy out of shape to show worst case scenarios).

Ultimately I need the locator to be tight/concentric with the primary datum axis to minimize the ID overlap between mating tube sections as much as possible.

Since this is a long tube, I am honestly not sure how this would be set up for inspection, as most of my experience has been in the realm of relatively small component parts that you can put on a granite table or something--that being said I'm unsure which avenue (runout or pos. tol.) I should take.

-cc1

 

[drawoh]

cratechallenge1,

I don't like how your datum featire[ ]C is part of your measured diameter. I think I would use run-out to control your diameter. If your tube is several feet long, you may have to think through your stiffness. If you have the actual standard ASME[ ]Y14.5[‑]2009, read section[ ]4.20 on Restrained Condition.

--
JHG

 

[cratechallenge1]

@drawoh thanks so much for the note on sect. 4.20 this is exactly what I was looking for!

In terms of drawing practice, if I were to stick with positional tolerance in this case would it then make sense to have the pos. tol. directly back to A only, so that datum feature C has no effect and the location is only controlled relative to A?
edit: nvm just saw this clarified in fig. 7-56

Either way I think the run-out may be easier to inspect (dial indicator with restrained condition on tube ID ends) compared to pos. tol. with the CMM, right?

-cc1

 

[Basa SD]

Hi,

I totally agree with drawoh!!

 

[drawoh]

cratechallenge1,

Positional tolerance from a single diametric feature of size, is meaningful and fairly inspectable. Can you insert a CMM into a 3"[ ]diameter hole several feet long, far enough to establish a datum?

--
JHG

 

[cratechallenge1]

@drawoh thanks for the clarification--in the case of using CMM could I put a datum target on like the last inch of the cylinder ID since I'm most concerned about it being concentric specifically at the tube ends rather than concentric along the entirety of the tube and then call the positional tolerance on the locator back to that target? (concentric in the actual sense of the word not the geo tol)

thanks,
-cc1

 

[drawoh]

cratechallenge1,

If you just use an end of your tube ID as a datum, you will need a perpendicular face as an additional datum. How about specifying datum targets at each end of your tube?

--
JHG

 

[pmarc]

If I may chime in... I am not sure I see a connection between the appropriate selection of the datum features and the selection of the geometric tolerance to control the feature in question. I believe the key information is this:

[...]but since I mainly care about the axes of the features lining up would it make the most sense to use position here?

This clearly indicates that what you are looking for is position, not runout.