How Dimensions Tolerances & Profile 2

[Poeman]

I would like to thank everyone for their comments regarding my previous question on tolerances and basic dimensions. I now have a follow-up question on geometric tolerances. If a dimension is an absolute dimension from a datum, say 15.000 mm from datum C and the 15.000 dimension line has an associated profile tolerance of 0.1 mm relative to datum C, would the tolerance on 15.000 mm be effectively +/- .05? (1/2 of the profile tolerance on each side of the nominal dimension.)

 

[drawoh]

Poeman,

On an orthogonal part, the primary datum is taken from the three points of the surface extending the furthest out.

The secondary datum is the plane perpendicular to the first datum that contacts the extreme two points of the secondary datum surface.

The tertiary datum is the plane perpendicular to the primary and secondary datums that contacts one point of the tertiary datum surface.

This only sounds complicated. Try to imagine how you are going to jig this up on a reference table. It is a very practical standard.

Your profile tolerance will be with respect to the datums described above. The number in the profile tolerance box is the total variation, not the +/- value. Your 0.1mm profile is effectively a +/-0.05mm tolerance.

I suggest you buy a copy of ASME Y14.5M-1994 and read it. This is all explained very carefully, for obvious reasons.

JHG

 

[ewh]

Poeman,

drawoh's comments are right on the money. Also be aware of unilateral profile tolerances, where the tolerance zone is to one side of the profile. These are shown by a phantom line on the side of the profile to which they apply (in a true view).

See also ASME 14.41-2003 DIGITAL PRODUCT DEFINITION DATA PRACTICES, in which the unilateral profile has been updated to use a modifer instead of the phantom lines (does not have to be a true view).

Eric

 

[Poeman]

Thanks for your help drawoh and ewh! I really appreciate it!!! I obtained a copy of ASME Y14.5M-1994 and read through Section 6.5 on Profile Control and I think I now understand profile control and geometric tolerancing much better -- though I am still a long, long way from being an expert .

So now, to be certain I understand this correctly, am I correct that in my previous example of the 15.000 mm surface location (located by a coordinate of 15.000 from Datum C) the associated 0.1 mm profile requirement attached to that coordinate line effectively means 15.000 ± 0.05 mm from datum C. I believe this is consistant with both of your comments to this point.

 

[ewh]

Yes, you understand correctly.
Don't let the complexity of the standard bother you. The best way to learn it is to use it (correctly). I have seen more drawings with tolerancing applied incorrectly than not, and always try to investigate what the proper usage is. Few can sucessfully argue against the reasoning set forth in the standard.