Runout

[Madhu454]

I have question on the runout.

Please see the attached snap shot of ASME 1994 std, figure 6-49.
Please refer to the figure, the datum feature C & the same surface used for the simulation of datum C, is controlled with a runout control and refered to the same datum.

I have been told that , the feature that is used to simulate the datum, should not be controlled with the same datum.

Here they have used compond datums? is this allowed. Can anyone please help me

 

[pmarc]

I have been told that , the feature that is used to simulate the datum, should not be controlled with the same datum.

That is true. But the figure you are showing is not saying this. It is saying that the runout of a surface of datum feature C has to be measured relative to common axis established by simulators of datum features C and D. This is not exactly self-referencing.

The other side of the story is how to verify this requirement if theory says that datum feature simulator (gage) for C should be the smallest possible cylinder that can be circumscribed about the datum feature C - there will be no possibility of using the dial indicator. But if you use V-block to simulate datum axis C then the verification may be possible.

Side note: the method presented on this figure is one of possible ways to define mutual relationship between datum features C and D. This relationship had to be defined, otherwise nothing controlled axis offset between C & D.

 

[Madhu454]

Hi pmarc,

Thanks for your valuable post, still curious to know few more things on the measurement of the runout you have explained above.

I have understood the meaning of compound datums in this way, the datum feature simulators used for establishing the compound datum C-D should be coaxial. (Example -Two conecetric collect or chuck to hold the shaft at the two respective ends or any simillar arrangement - in our case this may not be possible as you explained in you post. we need to go for a v-block for a line contact)

Even if you go for a variable height V-blocks for this case, How we can ensure that, both the axis of the datum feature simulators (V-block)is alligned.

If my understanding is wrong, Please correct me

 

[axym]

Madhu454,

I think you are correct to question the meaning of Fig. 6-49. I don't like the idea of a self-referencing feature control frame either. This example has been in Y14.5 for decades, but I don't think it makes complete sense.

As pmarc pointed out, the relationship between C and D needs to be defined. In order to function well as a multiple datum feature, their mutual coaxiality needs to be controlled. Or, preferably, their "mutual runout" needs to be controlled. In my opinion, the runout of C and D needs to be controlled relative to a common axis. But that common axis is arbitrary - it does not need to be established using C and D as datum features. Unfortunately, Y14.5 does not allow runout to be controlled relative to an arbitrary axis - datum features need to be specified. So they chose to reference C and D as datum features, which poses major practical difficulties as you point out.

Y14.5 allows a Position tolerance to be specified with no datum features, to control the mutual coaxiality of a pattern of features. The same thing is needed with the runout tolerances, to control the "mutual runout" of features like C and D in Fig. 6-49. This would overcome the problem of the self-referencing FCF.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[pmarc]

Madhu454,
Yes, your understanding of my post is correct.
And on the other hand I understand your concerns about proper alignment of V-blocks. As Evan mentioned this can cause quite serious practical difficulties, but IMO it is possible at the end - unfortunately it would probably require iterative playing with different heights and angles of V-blocks to assure that 0.01 circular runout of C relative to C-D is finally met.

Regarding mutual relationship of C and D - position without datum reference is one of the other methods possible, but please remember that the position is not controlling form of a toleranced datum feature surface. In applications like the one shown in fig. 6-49 form of datum feature surfaces (i.e. straightness, circularity or cylindricity) is equally or even more important than the axes relationship. Datum features can be part's features where the bearings are mounted therefore they need to be of precise form quality. I can only guess the intentions of Y14 committee regarding this figure, but in my opinion circular runout is there because, apart of controlling positional relationship between features, it also controls their form.

 

[Madhu454]

Hi Pmarc & Evan,
Thanks for the post, I agrre with you the runout control is required to control the form parameters as well.