True Position of coaxial diameters 1

[Runz]

If I have a drawing with two coaxial diameters with no true position callout, what is the assumed true position, if any?

Is it located positionally by the size tolerance?

Thanks,

 

[Heckler]

Runz,

Could you describ the part.....is it a shaft or block?

I you have ASME Y14.5M then check the Taylor Principle or comely known a Rule #1 - Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in it geometric form, as well as its size, are allowed.


If this is a shaft part....then I would make one hole a datum and apply runout to the other hole referencing the applied datum. Do not use TP on a shaft....your QA department will through a fit.

Best Regards,

Heckler
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[alexit]

Like hole with counterbore, features are assumed to be concentric to each other within size tolerance(s). Where holes are located to your other datum is depending on the dimensioning done to define their location.

 

[Heckler]

If it's a counter bored hole then you can apply the same TP tolerance for the hole and c'bore or individual TP tolerances to the hole and c'bore by placing the word INDIVIDUALLY under the feature control frame. So if you applied a single TP feature control frame to the hole and c'bore it would be understood that these features. There is a section in the spec that deals specifically with c'bore holes.

Best Regards,

Heckler
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[Runz]

The part in question is actually a stepped pin. If the centerline axis runs through both diameters, is there a default true position implied? If so, does a true position tolerance added to one of the diameters need to be a refinement of the size tolerance?

I have a drawing of a stepped pin and the centerline runs through the entire part. The size tolerance on the smaller diameter is ±.010 and the true position tolerance is ±.022.

Just wanted to know if a tighter true position was already being held by default, then the ±.022.

Thanks,

 

[Heckler]

Runz,

As I mentioned in my previous post......I would not use TP for this. I would make one diameter a datum and reference the other diameter to that datum via runout or total runout. The same would be true if you want to use TP one diameter would have to be a datum and the other diameter would be referenced to that datum.

A positional tolerance defines a zone within which the center, axis or cetner plane of a feature of size is permitted to vary from true (theoretically exact) position.

Circular Runout is a two dimensional, surface to an axis control. The tolerance is applied independently at each circular cross section. When applied to a surface constructed around a datum axis, circular runout will control cumulative variations of circularity and coaxiality. Unlike total runout , it doesn't control taper.

Total Runout is a three dimensional, surface to an axis control. Total runout provides a composite control of all surface elements. When applied to a surface constructed around a datum axis, total runout will control the cumulative variations of circularity, straightness, coaxiality, angularity, taper and variations in the surface.



Best Regards,

Heckler
Sr. Mechanical Engineer
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[alexit]

Part shows common axis so small diameter must run-out within +/-0.010 of large diameter by default (Taylor principal), your true position is redundant (and way too big). As Heckler wrote, do not use true postition for cylindrical parts, this is not good useage. Use runout or total runout (or roundness/cylindricity if you have machinery to measure this.)

 

[alexit]

Sorry, crosspost with Heckler...

 

[Runz]

Regardless of true position or Concentricity, section 2.7.3 (Relationship between individusl features) seems to disagree with Alexit's statement of run-out.

Am I misinterpreting this section.

Without any true position, concentricity, etc, how much can the center axis of these two coaxial diamter (each with a size tolerance of ±.010) vary?

 

[Runz]

I guess to put it another way, if I can live with a true position of .020 between the two coaxial diameters, do I need to add any controls to the drawing if the size tolerance is ±.010? If there are no GD&T controls, what is the positional relationship between the two diamters?

Thanks,

 

[Heckler]

"Runout tolerance control - where a combination of surfaces of revolution are cylindrical, conical, or spherical relative to a common datum axis, a runout tolerance is recommended. For this type of control, MMC is not applicable because the tolerance controls elements of the surface." quoted from the spec

Best Regards,

Heckler
Sr. Mechanical Engineer
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[ringman]

Runz,

You stated previously that the positional tol was +/- .022.

I respectfully suggest that GD and T by Y14.5 does not recognize negative tolerances.

 

[PhilBW]

I guess to put it another way, if I can live with a true position of .020 between the two coaxial diameters, do I need to add any controls to the drawing if the size tolerance is ±.010? If there are no GD&T controls, what is the positional relationship between the two diamters?

Rule 1 (The Taylor Principle) applies to individual features only, not to the relationship between two features. If there are no GD&T controls, there is no relationship between the two diameters.

The axis of one cylinder should be the dateum, and that datum should be used to control the runout of the second cylinder.

Phil

 

[wgchere]

Since Runz has stated this is a pin, I am assuming there are no surfaces of revolution. As Heckler's quote from ASME Y14.5M-1994 states, total runout is for surfaces of revolution. I would apply true position to the problem and make one of the cylinders the datum.

Without datums and GD&T... from an inspection point of view, one cylinder will be constrained and guaged, establishing the actual axis of the part, and the other cylinder's axis will have to fall within the second cylinder's size tolerance zone, with the established axis of the first cylinder as the axis of the size tolerance zone.

 

[caseynick]

If there no GD+T controlling the relationship of one diameter to the other the drawing is incomplete. There is no assumed relationship. The location of one to the other can be controlled by position or runout. Runout would also put a tighter constraint on the form as the runout tolerance would have to be smaller than the size tolerance.