Measuring runout on a shaft relative to two datum surfaces 1

[PetkovStoyan]

Hi everyone,

I am struggling with one practical issue how to measure shaft runout, when two datums are specified simultaneously, having different diameter size. Can this be achieved only on a lathe with dial indicator? Is it possible to align both diameters this way, without using custom made jigs?

And in general, is it ok to use the lathe and dial indicator to align along a datum, even if it is single datum diameter? Is this practice legit and valid?

Drawing attached

https://files.engineering.com/getfi...-4246-9516-1899c1f8f3da&file=GD_T_Problem.jpg

 

[3DDave]

The callout is fine.

You could try a pair of 90 degree v-blocks with the smaller diameter block on spacers/gauge blocks to make up the difference. Check the datum features at the v-blocks with a pair of dial indicators at 90 degree spacing to ensure the shaft is running true/has no unexpected lobes on the datum features.

I think the smaller diameter gets a 1.414 * (99-87.32)/2 mm booster => 8.26 mm, but make a drawing and check the calculation.

 

[PetkovStoyan]

Thank you for your reply.
I was thinking something else - shouldn't the datum surfaces be also constrained via runout tolerance, or any tolerance of form/position for that matter? There is none on the drawing, which in theory means any runout (or other deviation) of one of the surfaces relative to the other is permissible. What would you advise?

 

[3DDave]

Often, as implied about checking runout at the v-blocks, the datum surfaces will get a total runout requirement to their mutual/shared axis. If there isn't a requirement you cannot reject the part for excessive, but proceeding if there is much variation in the datum features with the v-blocks is a problem.

If there is a large amount on the datum feature surfaces then you will need to make precision collets to fit only the high points.

Discuss this possibility with the supplier of the drawing.

 

[PetkovStoyan]

I suppose that by "the datum surfaces will get a total runout requirement" you mean that it should be specified on the drawing, right?
And also, can you give me some guidance on how to align the two v-blocks to each other - do you use machine table, or dial indicator, some common groove, or what?

 

[3DDave]

The alignment is supplied by the shaft - it is in the directions to use those surfaces to align the part to make the measurement. Move the v-blocks to maximize the contact.

Total runout should have been specified. It wasn't, so no need to check it. Just note that if it is terrible on the part measurements made using those surfaces as a datum will also be terrible.

 

[PetkovStoyan]

Would you say that this was a designer's mistake, or there are such cases in which it is not required by design to constrain datum features with form/position tolerance? Or in other words, how often have you seen similar cases in your practice?

 

[pmarc]

PetkovStoyan,
The relationship between the datum features should have been specified, as 3DDave said. If it's not, then it's a drawing error.

On the drawing provided, there are other diameters that are missing similar tolerances as well. Is it possible they are invoked by a general tolerance callout defined near the drawing title block or in the general notes?

 

[PetkovStoyan]

pmarc,

Thank you for joining the conversation. Can you specify exactly what other tolerances are required?


No, there is nothing near the title block.

 

[pmarc]

In the provided view, I see 3 diameters that have no defined relationship to the datum axis A-B. Depending on the design requirements and the standard, these could be runout, position, concentricity or coaxiality tolerances.

 

[greenimi]

So IF this drawing is per ISO GPS and (again AND) runout is deemed to be the functional requirement, I would like to ask:
What would be the difference between runout (circular and total) with and without CZ modifier?
Since A-B is the common axis between those two datum features and due to the fact independecy principle is by default implied, what would be the subtle differences between those two type of specifications (w and w/o CZ)

 

[pmarc]

greenimi,
I don't think CZ modifier in the runout tolerance indicators would add value in this case.

 

[greenimi]

Pmarc,
And that is because of common datum A-B datum system? Or because of the geometry of the part discussed? Or maybe neither?

 

[pmarc]

It's actually both.

 

[greenimi]

So, if that's the case, could you please indicate a figure or post a picture where CZ would make sense in the A-B configuration? (Would add value).
I am still narrowing the cases to A-B scenario to limit the scope of my addition and inquiry to the OP thread

 

[pmarc]

The shaft in the illustration given by OP has a key slot at one end. If the item attached to the shaft at its other end had a similar feature and there was a design requirement to assure that the two features are "clocked" to each other, then the common tolerance indicator controlling location of the features relative to datum axis A-B would need to contain CZ modifier.

For features nominally coaxial with the datum axis A-B, I am not sure I see a good example. If, by any chance, you are implying that CZ could be used to transform multiple tolerance zones into a single tolerance zone extending across multiple features of the same nominal size, then I would say UF (United Feature) instead of CZ should be used.

 

[PetkovStoyan]

Can someone explain what is the difference between measuring the tolerances via v-blocks and collets? I suppose the collets should be adjustable to acommodate the high points in each case. Will you get the same readings by both methods? Is one of the methods preffered to the other, and what is the common practice?

 

[greenimi]

Can someone explain what is the difference between measuring the tolerances via v-blocks and collets? I suppose the collets should be adjustable to acommodate the high points in each case. Will you get the same readings by both methods? Is one of the methods preffered to the other, and what is the common practice?

All measurements done for any reasons are just estimates of the actuals.
The differences are the level of uncertainty acceptable for your product.

 

[Belanger]

PetkovStoyan,
A datum is defined as a theoretically perfect axis (or sometimes plane, point, etc.) which is to be created from the minimum circumscribed cylinders of A-B.
Thus, the preferred way would be the collets instead of V-blocks. If datum feature A or B were made slightly out-of-round, they might ride up and down as you rotate the part in the V-blocks.
V-blocks aren't prohibited, and the addition of some tolerance on the datum features (as suggested above) would minimize the error, but they aren't the best way to simulate the datum according to theory.

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

 

[3DDave]

Collets are the best, but then one needs bearings for the collets to run in and some structure to hold the bearings and assurance there is little runout between the contacting surface of the collet and the bearing surface of the collet and, further, in the bearing. In the case of the compound or common RFS datums then there needs to be a single collet with both diameters closing at some coordinated rate on both surfaces simultaneously.

I'm not a fan of v-blocks, hence the suggestion to add dial indicators to verify the datum feature surfaces are of sufficient quality. The alternative of collets? Probably cost 10-50X what it cost to machine this part to make it and is far more expensive to inspect than the part itself, especially given the internal details that are tougher to reach.

This leads to the question of how the mating assembly actually functions and is toleranced/varied that this is a suitable description of the required control.