Interaction between Size , Circularity and Staightness 2

[mkcski]

The attached drawing is the portion of a shaft where a combination guide and thrust bearing interfaces. The bearing surface is Babbitt and is oil lubricated.

I am trying to determine where the conflicts, if any, are between the size, circularity and straightness tolerances. The sketch is fuzzy: the tolerance on the 290mm diameter is -0.291mm to -0.323mm, or 0.032mm total on the diameter. This drawing is from an European company and there is no ISO GPS standard referenced, so for discussion, assume Rule #1 is NOT in effect.

Note: Please ignore the problems with the "parallel to A" FCF - a datum feature "A" cannot be parallel to itself. Also I am not concerned with the runout and flatness FCF's - it should be total runout.

Issue 1 - the circularity tolerance of 0.06mm is radial error, so the diameter can vary twice this - 0.12mm - and still meet the form requirement. But this is 4 times the size tolerance of .032mm. From my understanding, even without Rule #1 the form error must be at least half of the size error. Comments please.

Issue 2 - Given the independent and two-dimensional nature of the circularity and straightness tolerance zones, the 290mm diameter could be a conical shape and still meet the form tolerances. But, given the 0.032mm size tolerance, the 0.60 circularity and the much tighter 0.01 straightness tolerance, how do these interact? I suspect a conflict. My understanding: given the (radial) circularity error of 0.06mm, a line on the surface can "jump" 0.06 from one cross section to the next adjacent one. This would allow a surface straightness of 0.06mm, violating the 0.01mm requirement. So ...how does the 0.032mm size tolerance overlay with the two form errors, which appear to conflict each other? Given the surface is a bearing journal, I am thinking of recommending cylindricity as a more appropriate control. Comments please.






Certified Sr. GD&T Professional

 

[pmarc]

pylfrm,

For my question #1, how about adding AVG modifier to the diameter dimension together with loose circularity tolerance?

For question #2, I was thinking about associating DML straightness callout with size requirement modified by a note like PERFECT FORM AT MMC REQUIRED.

 

[mkcski]

pmarc:

You are absolutely correct about the 12 circularity error. Oh..and yes..half of the 120 degree pitch is 60 degrees
I really need to pay more attention to details and not just concepts. Sorry

Second, I agree that if the sections are rotated this way (by 60 degrees) the measured straightness error would be equal the circularity error. However, I am not sure I understand why you find "unconventional" the forcing of the lobes to align to obtain a lesser straightness error. For this specific (as described by you) "incarnation" of actual circularity error certain additional conditions must be met to satisfy straightness tolerance. But this is no different to what can happen in ASME. Using one more time the example of a shaft with total size tolerance of 0.1, straightness tolerance of 0.01 and circularity tolerance of 0.06, you would also have to satisfy pretty much the same conditions for the shaft produced like an egg (within the 0.06 circularity tolerance). In other words, it also would not be possible to have a shaft looking like a vertically oriented egg in one cross section and like a horizontally oriented egg in the other cross section.

From you last paragraph ,it appears we agree on the interaction between straightness and circularity in this scenario. I still have problems trying to understand why someone would use this approach to form control and not apply cylindricity. Oh well. I might get a chance to talk with the customers engineer and ask a few questions.

I really appreciate you taking the time to discuss this with me and helping me understand the "little" things.

Certified Sr. GD&T Professional

 

[pylfrm]

I still have problems trying to understand why someone would use this approach to form control and not apply cylindricity.

If you want to replace the current combination of tolerances with a single cylindricity tolerance that is no less restrictive, then you would need to use 0.010 for the tolerance value. That would substantially tighten the limit on circularity and taper, possibly increasing cost.

For my question #1, how about adding AVG modifier to the diameter dimension together with loose circularity tolerance?

That would indeed allow the geometry shown in your image, but it would also allow things the ISO size tolerance does not. Shapes with an even number of lobes, for instance.

pylfrm

 

[gabimo]

Per ASME Y15.4.1M-1994, the surface must fall within an envelope generated by sweeping a ball (having diameter equal to the upper size limit) along a spine. If we assume the curvature of the spine is not too extreme (as I suspect i

Is this true? What is the connection between the envelope requirement and the sweeping ball?

I don' think pylfrm got that concept correctly. Sweeping ball is in regarding the actual local size measurement and it is about the LMC of a shpere that is travelling within the material and is not allowed to protrude out.

Pmarc,
Am I correct? Or pylfrm quote is correct?

 

[greenimi]

ASME Y14.5.1M-1994


2.3.1 Variation of Size
(a) Definition. A size tolerance zone is the volume between two half-space boundaries, to be described below. The tolerance zone does not have a unique form. Each half-space boundary is formed by sweep­ ing a ball of appropriate radius along an acceptable spine, as discussed below. The radii of the balls are determined by the size limits: one ball radius is the least-material condition limit (rl.Mc) and one is the maximum-material condition limit (rMMC).
A 0-dimensional spine is a point. and applies to spherical features. A I-dimensional spine is a simple (non self-intersecting) curve in space, and applies to cylindrical features.



(c) Actual value. Two actual values are defined.
The actual external (to the material) size of an external (respectively, internal) feature is the smallest (respectively, largest) size of the ball to which the feature conforms. The actual internal size is the largest (respectively, smallest) size of the ball to which the feature conforms. The size may be expressed as a radius or diameter, as appropriate to the application.

 

[pmarc]

Is this true? What is the connection between the envelope requirement and the sweeping ball?

I don' think pylfrm got that concept correctly. Sweeping ball is in regarding the actual local size measurement and it is about the LMC of a shpere that is travelling within the material and is not allowed to protrude out.

Pmarc,
Am I correct? Or pylfrm quote is correct?

In my opinion pylfrm is correct. Per the 2.3.1 definition from Y14.5.1M-1994 in each cross section actual surface of external cylindrical feature of size cannot violate a circular boundary of MMC size.

For a scenario where Rule #1 is not in charge, the line along which the circular MMC boundary is swept does not have to be perfectly straight. For a scenario where Rule #1 is in charge, the line along which the circular MMC boundary is swept has to be perfectly straight. But this means that in absence of Rule #1 in ASME, unlike in case of independency principle in ISO, the perfect-form-at-MMC requirement is nullified only in axial direction. In each cross section the feature of size's contour still must be perfectly round when produced at MMC.

That would indeed allow the geometry shown in your image, but it would also allow things the ISO size tolerance does not. Shapes with an even number of lobes, for instance.

True, but my question was rather general - in ASME, with Rule #1 overriden how to allow a cylindrical feature of size to violate the MMC boundary not only along its length, but also in each cross section?