Rule 1, Concentricity, and Runnout

[Mechdesignron]

Hello,

I would really appreciate some clarification on something.

-Rule 1: With no GD&T, a cylinder can be any size or shape as long as it fits within the maximum material condition and the least material condition.

-Concentricity, Controls the center of two diametrically opposite points relative to the radial distance an axis.

-Runnout, Total distance from the specified axis.

If I have a print of a pin with two diameters, Diameter A 1.001/0.999 (.002 total tolerance) and Dia B 0.505/0.495 (0.010 total tolerance). With no GD&T how far off center can these two diameters be and the part be technically correct?

I would think that calling dia A as a datum and adding a Concentricity tolerance of .005 to dia B would be of no use because this is implied by rule 1. To make the Concentricity useful, I would state less than .005 to keep the center points within a tighter circle or if it didn't matter how much the two diameters are in line, I could say .020 concentricity, keeping the diameter tolerance within .010.

I would also think that adding a Runnout tolerance of .010 would be pointless for the same reason.

Please confirm my logic or correct my ignorance. If there is a source to demonstrate this, I would appreciate knowing what that is.

Thank You,
Ron

 

[axym]

Ron,

Your definitions are close, but I would add a few clarifications:

Rule 1: The cylinder must fit within a perfect cylindrical boundary of MMC size. The local sizes (all 2-point opposed diameters) must be within the size tolerance (i.e. between the MMC size and LMC size). Rule #1 applies separately to each feature.

Concentricity: controls all "opposed point centers" relative to the datum axis. Opposed point centers are the centers of diametrically opposed points.

Runout: I wouldn't describe this as the total distance from a specified axis. The runout tolerances (Circular Runout and Total Runout) relate to the total indicator reading that results when the part is rotated 360 degrees around the datum axis. Circular Runout controls the readings on individual cross-sections and Total Runout controls the reading for the entire surface. For a cylindrical feature, this roughly equates to the consistency of the distance between the datum axis and points on the surface.

Regarding the pin with two diameters, with no GD&T nothing is controlling how far off center they can be. Rule #1 controls each feature individually and does not impose a relationship between them. So the Concentricity and runout tolerances you suggested would make a difference.

All of these things are defined in the ASME Y14.5 standard for Dimensioning and Tolerancing, and various third-party textbooks and web sites.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Rule 1 applies to your cylindrical features independently. By no means Rule 1 controls relation between the features.
So I guess your diameters can be about .75 off-center and still be technically correct.

 

[KENAT]

If I have a print of a pin with two diameters, Diameter A 1.001/0.999 (.002 total tolerance) and Dia B 0.505/0.495 (0.010 total tolerance). With no GD&T how far off center can these two diameters be and the part be technically correct?

Infinitely. I mean to be a single part implies they have to intersect somewhere but without adding some kind of control there is no defined limit on coaxiality of the diameters - at least to ASME Y14.5M-1994, section 2.7.3 addresses this.

To maintain coaxiality a number of controls can be used or some global note about perfect coaxiality at MMC. Position, runout(s) and concentricity can be used - be aware that most folks consider concentricity to have limited applications because it deals with theoretical centerlines rather than actual surfaces.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
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[CheckerHater]

Here you can find some insight on maintaining coaxiality of two cylinders:

http://www.mechsigma.com/newsletters/2003_10_Newsletter.pdf