Question on total runout and circular runout in Y14.5-1994 standard

[Fortelas]

I don't know if this has been discussed in this forum before. In Y14.5-1994 standard, Fig 6-51 and Fig 6-52 show features with runout tolerance that use themselves as datum. I couldn't find any explanation about how this will be checked or even the reason to justify this practice. Can anyone help me on this?

Also, it seems to me runout tolerance is quite strange in GD&T standard. Unlike form, profile, or orientation, runout is more an inspection method than geometric tolerance. If circularity and cylindricity could refer to a datum axis, then they would be same as runout functionally on controlling circular surfaces. As for surface perpendicular to the datum axis, wobble can be controlled by flatness. So why not expand definition of form control and eliminate runout altogether? Did I miss something here?

 

[DeanD3W]

Hi Fortelas,
I think the point you make about it being strange to have runout tolerances in a GD&T standard may be valid... Yes, I've heard Circular Runout being referred to as "Circularity on a stick" and Total Runout as "Cylindricity on a stick" (at least when applied to a cylindrical feature anyway.

For a surface perpendicular to a datum axis, wobble can be controlled by Perpendicularity, which would be like "Flatness normal to a stick" possibly.

While runout could be replaced with a modified capability for form controls, or a modified capability for profile controls, I don't know how much benefit that would provide. I think we would have essentially the same tool, but possibly a different description of how that tool provides the desired effect (which may be the point you're getting at).

Regarding Figure 6-51, each datum feature has a runout tolerance applied relative to a datum axis created from that datum feature and also one other datum feature, so that is not the same as being relative entirely to itself.

In Figure 6-52 datum feature D is controlled relative to [C,D]... I think it is important to think of this case as a reasonably good illustration of how a datum feature differs from the datum that is established from its datum feature simulator. Datum axis D for this case is perpendicular to datum C because it is the axis of a perfectly perpendicular "True Geometric Counterpart" to use Y1.5M-1994 terminology ("Datum Feature Simulator" in Y14.5-2009 terminology). So, datum D is an axis that will be perpendicular to datum C, but that does not mean that the surface of datum feature D is perpendicular to datum C or that the surface of datum feature C is perfectly cylindrical. Since on a real physical part the datum features are imperfect, datum feature D's surface will definitely have imperfect runout with respect to a datum reference frame that is oriented to datum C and has its rotation centered about the point that the perfectly perpendicular datum axis D pierces datum plane C. Sorry for the wordiness, but our language requires this level of detail in order to fully make sense (assuming that what I just said made some sense ). While I may tend to rely more on the form control provided by a size tolerance and an orientation control provided by a perpendicularity tolerance, for the sake of illustration, I think Figure 6-52 is OK... I'd prefer to see a size tolerance added to datum feature D. I also prefer that runout tolerances not be associated with dimension lines and instead have a leader which points to the surface (since it is a surface control), but that's a personal preference since Y14.5 allows the application of runout tolerances as shown.

Dean

http://www.d3w-engineering.com

 

[axym]

Fortelas,

I also question the validity of 6-61 and 6-52. People have come up with methods to check this, with complex mechanisms in which the datum feature simulators contact the features to establish the datum axis and then move out of the way. But I don't agree with it.

Take Fig 6-51. The function of features C and D is to act together to establish a datum axis. They will be put into contact with two perfectly coaxial datum feature simulators, and to establish stable contact C and D should have good form and be coaxial to each other. One way of looking at this is that the "combined form" of C and D should be controlled. But there is no such tool in Y14.5 - form controls apply to each feature independently.

Another way of looking at it is to extend Dean's "stick" description. If Total Runout on a cylindrical feature is "Cylindricity on a stick", then Cylindricity is like Total Runout except that the stick is not chosen for us (i.e. no datum features). The "combined form" control we need would then be Total Runout applied to both C and D, with no datum features. But this tool doesn't exist in Y14.5 either - runout tolerances must always reference at least one datum feature to establish a datum axis.

So what we see in Fig. 6-51 is an approximation of the real requirement. The FCF's for features C and D reference C-D. But functionally this really isn't necessary - we should be able to choose the datum axis. It's true that C-D would probably be very close to the optimal datum axis, but it's an unnecessary restriction. If the whole part was machined when mounted on centers, the runouts of C and D would be very accurate to the axis of those centers.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[DeanD3W]

Evan,
For the part in Figure 6-51 one could also apply position tolerances to datum features C and D, with either no datum features, or if they preferred, referencing C-D... So what makes runout a less viable tolerance? I'd probably choose Position myself, but I'd like to hear what you say to this.

If this part was molded plastic, instead of machined how would that change what you say in your last sentence? Isn't the datum axis you would choose so close to C-D that they are essentially identical?

Dean

http://www.d3w-engineering.com

 

[Fortelas]

DeanD3@, thank you for detailed reply. Let focus on Fig 6-52 first. Read your post least twice but still am not able to understand how this part can be inspected. Obviously, it will sit on a flat surface to establish datum C, then how to establish datum axis D for inspection? A dial indicator will not work in this case as datum feature D will be fixed to a chuck to establish datum axis D, thus block part of surface. A CMM machine may scan surface and establish a mathematically calculated datum axis D, and use it to inspect other surfaces, but this process would be expensive. The point is how a surface can be referred to itself as a datum for tolerance control?

I attached a drawing to show an alternative I think, using perpendicularity instead of runout for datum feature D. Since the Y14.5-1994 book has been around for more than a decade and it was created and reviewed by some best people in GD&T field, I am sure there is a good reason for this annotation, hopefully someone will enlighten me on this.

 

[axym]

Dean,

Yes, Position tolerances could be applied to features C and D to control their coaxiality. I would prefer no datum features - referencing C-D is not exactly equivalent and causes the same underlyinbg problem of the simulators covering up the considered features. Surface Profile could also be applied to control their form, size, relative orientation and relative location (with no datum features of course). I'm not saying that runout would be a less viable tolerance, I'm saying that it would be nice to be able to apply it without datum features.

I agree that the datum axis I would choose would probably be essentially identical to C-D. But that doesn't mean that we should have to reference C-D in the FCF. I realize that this is a very subtle distinction in most runout applications, but it needs to be made. It's the principle of it, I guess. The C-D reference represents an unnecessary constraint that creates practical difficulties and would often be ignored. We also don't have to reference C-D if we apply Position or Profile so it shouldn't be a requirement for runout.

If the part was molded instead of machined, then there wouldn't be centers to line up on but the principle would still apply.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[DeanD3W]

Fortelas,
I agree that for the part shown Perpendicularity would be a more likely choice. Once you add a diameter symbol before the tolerance value in your figure then I think all would be well. It is still a valid figure in Y14.5 as it stands though. A CMM could be used for measurement.

Evan,
I agree with everything you're saying. Maybe a good approach would be to have modifiers for profile that would provide all the characteristics of runout. I've never thought of deleting runout from our options, but the rules may be simpler if we use profile and one new modifier and an existing additional modification. This seems worth thinking about.

Dean

http://www.d3w-engineering.com

 

[MechNorth]

The mechanisms for transferring from datum features to surrogate datum features aren't that terribly complex in most cases, and aren't that expensive for most moderately sized parts.

Functionally, total runout on a datum feature wrt a compound datum is reflective of how the part works, relating the individual datum features to the "net" datum as it were. When that is appropriate, I recommend to engineers that they include a set of tooling centers as the surrogate datum features; this adds significant value because the tooling centers are often the means of holding the workpiece during turning/grinding and therefore is highly repeatable. Also adds value in refurbishment and rework because the work-holding features are reused, and they usually aren't damaged during part use as compared with bearing surfaces which usually are damaged.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com