Circular runout control; understanding potential surface effects

[dtmbiz]

Would anyone like to comment on the following?


Would the following be accurate in understanding the potential surface effects for a cylindrical feature as a result of only a circular runout control?


1. A circular runout tolerance does not need to be less than the dimensional size limits.

True?


2. Surface elements can have “steps” along the cylindrical feature axis as a result of the feature’s actual size, which may have varying circular element diameters, along with those circular element center points being displaced within the allowed circular runout tolerance.

For example, a 1.0” dia. ±.02” external cylinder with a circular run out control of .02”, could be produced within size limits and the .02”runout control, yet still allow “surface steps” as much as .04”.

True?




3. Whereas, a total runout control of .02” for the feature would only allow maximum surface element deviation of .02” regardless of the feature’s size limits.

True?

 

[Belanger]

All are true.

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

 

[dtmbiz]

Thank you for commenting John Paul

Much appreciated

~dtm~

 

[MechNorth]

I may be having a "late Friday" moment, but if the runout control exceeds the size control, then are you not violating the limits of size?

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

Jim, I don't think any of the 3 questions implied that the size of the part itself can ever exceed the maximum allowed by the size tolerance.

I suspect you are thinking about question #1, but that merely asked if the runout tol (for example, 0.6 in the FCF) can be larger than the size tol (which might be ±0.1). That is allowed; it just means that the circularity aspect of the diameter will be caught by the ±0.1, and the runout's job then consists of only orientation and location.

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

 

[dtmbiz]

John-Paul, your interpretation of my question is exactly what I intended to ask.

Mech North touches on why I asked these questions. In the past I have been lead to believe that the runout control tolerance must be less in value than the size tolerance for the feature it controlled. (and instructed to apply tolerances less than the size tolerance)

I haven’t used this callout much in the past. Now with my current employer, the circular runout is the default control in this company’s general notes.

I reviewed the control in ASME Y14.5m 1994 and concluded what my questions posed as true. Noteably that the circular runout control is in addition to a feature's size limit; not a refinement of a cylindrical feature's MMC.

The main problem that I have been concerned with is that the circular runout is not only used widely here on virtually every drawing, but that it is the only control used for sealing features. Typically the sealing feature in these products are o-rings. I have tried to point out that circular runout does not control the sealing surface in a way that IMO is sufficient.

 

[CheckerHater]

Think of a run-out as sum of your out-of round and out-of-center combined. (This is not entirely thru, but helpful)

When you tighten-up runout, you tighten-up roundness and position together, as they have to split the amount of run-out tolerance between each other (and then some, but I won't go there right now)

O-ring grooves are often tight crumped spaces and run-out usually is both economical and powerful enough to control them.

 

[MechNorth]

Tks J-P. I'm so used to using restrictive runout controls that I tend to forget that the tigher of the two controls (size or runout) restricts the form of the feature. For some reason I keep slipping into the mindset that runout behaves like straightness of a derived median line in that it overrides Rule #1, which is not the case for runout.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Belanger]

This brings up an interesting point about how the ASME standard sort of explains runout differently than all the others. It's almost as if they ran out of gas near the end of the book! The runout pictures never show size dims (which would have helped answer this question) and the descriptions are all based on the inspection method (TIR, oops I mean FIM), whereas the other chapters have detailed pictures of the imaginary tolerance zones. Just an idle thought...

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

 

[MechNorth]

Not so idle, J-P. Been raised many times, including in-committee. Disappointed to see it's still done this way with no improvement in documentation.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dgallup]

dtmbiz - this is off the subject of runout but addresses your primary concern which is the adaquate specifying of o-ring sealing surfaces.

The other vitally important control for o-ring sealing surfaces is surface texture. You need to specify lay as well as the roughness parameters. The measurement is perpendicular to the direction of lay. So if you have a revolved groove with circular lay the roughness trace is parallel to the axis. This will prevent steps or grooves in the sealing surface. The only problem is when you get to really small o-rings. Then the tracing length available forces you to use short cutoff lengths that will filter out the steps.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[axym]

J-P,

The runout tolerances are still explained in terms of readings on an indicator, instead of in terms of tolerance zones. But I don't think it's because they ran out of gas near the end of the book (I know you're kidding anyway). I think it's just because the indicator explanation is so straightforward, and tolerance zone explanations for runout get very complex. The indicator definitions were considered by many people to be not broke, so they weren't fixed.

But the philosophical disadvantages of an inspection-based definition have been recognized, and there is a desire to make the definitions for the runout tolerances consistent with those of other characteristics. A while back, a few of us from various Y14 subcommittees informally looked at converting the indicator-based runout definitions into corresponding tolerance zone definitions. I did some of the figures and believe me, it gets complicated. I had to model the surface-indicator interactions in CAD in order to puzzle out what the resulting zones would be and how they would have to behave. But I will say that I learned a quite lot about GD&T from studying this.

I'm cautiously optimistic that the next revision of Y14.5 will have tolerance zone definitions for the runout tolerances.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Belanger]

Yes, Evan. I agree that the inspection-based explanations are the easiest. So I guess I wasn't complaining, but just noticing a paradigm shift or whatever you want to call it in how the approach to the symbols changes when we get to runout.

Another thing is that when we do tolerance stacks, most of us (at least this is how I teach it) will treat the runout tolerance as residing around the axis. It makes the stack easier by just thinking of its error as being entirely due to location. But the actual tolerance zone resides around the circumference.

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

 

[CheckerHater]

Think of it this way - Circularity has simple "proper" definition.

And then there is 27-pages book - ASME B89.3.1 - that explains how exactly to measure it.

You decide which is worse. I will take inspection-based definition anytime.

 

[Belanger]

You'll take the 27-page book over the simple ASME handling of circularity?

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

 

[axym]

I'm not sure I understand the Circularity comment either. The Y14.5 definition of the tolerance zone is relatively straightforward, but evaluating Circularity in a repeatable way on an actual feature can be very difficult. This is especially true if the feature is warped or bent.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Belanger, axym,

You both get it right.

When you have "simple" definition of control that is difficult / impossible to gauge / measure, why do you consider it valuable?
What is so "simple" about it?

Unless the part in question is extremely expensive / important, I will take run-out over concentricity or roundness thank you very much.

 

[axym]

CH,

Ok, I think I see your point.

There are several Y14.5 characteristics that are relatively simple to define and specify, but are very difficult to measure repeatably and cannot be gaged. Circularity is one of them. Surface line Straightness, derived median line Straightness RFS, Concentricity, Symmetry, and Profile of a Line are others.

The problem that we have is that giving more tolerance usually makes the inspection more complicated. The comparison of Circularity and Circular Runout is a good example. Circularity is less restrictive than Circular Runout, but is more difficult to inspect. The lack of a datum axis means that the location (sorry) of each Circularity zone can be individually optimized.

So if the real requirement is just for the cross sections to be circular, what should the designer specify? Should they specify Circularity in order to give the most tolerance? Or should they specify Circular Runout so that the inspection will be straightforward?

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Evan,

This (and your other examples as well) depends primarily on design of the part and secondary on the manufacturing process you anticipate.

Let say, you have round part turned and/or ground in centers.
Checking runout to same centers will give you (almost) as good results as roundness.

Sometimes difference between "best" and "good enough" gives you huge advantage.

This doesn't mean we have to abandon controls we don't like.

 

[pmarc]

Taking advantage of the fact that the circular runout is debated here I'd like to ask 2 questions related to fig. 9-2 of Y14.5-2009:

1. When applied to a cylindrical feature, circular runout controls each cross-section's circularity and axis offset. But when applied to a conical feature, circularity in its standard meaning (according to paragraph 5.4.3 and right picture in fig. 5-10) is not controlled because the measurement is not taken in a plane perpendicular to feature's axis. Is that right?

2. According to para. 9.4.1 when verifying circular runout, the indicator is fixed in a position normal to the toleranced feature. Now let's assume that conical feature in fig. 9-2 consists of 2 segments where one is at an actual angle 43[°] to datum axis while the other is at actual 47[°]. Does this mean that indicator has to be oriented at right angle to the surface individually for each segment of the cone?

Thanks for a feedback.