True Position (Position) vs. Perpendicularity

Hello,

I have a drawing that has this TP callout: TP 0.05 A.
This is a through hole diameter (datum B) and datum A is the face of the part. (In other words, datum B diameter is "straight" through Datum A.
I interpret that (since there are no X/Y basic dims) to mean Diameter (datum B) s/b PERPENDICULAR to datum A w/in .05
I have an Inspector who insists on disagreeing, but have other trusted veteran Inspectors who agree with me.
Just trying to get some further feedback. Would've been much simpler if the engineer had just indicated Perp rather than TP.

Hi Gil,

(By the sounds of it) You're correct about the TP callout to 'A' effectively being sqaureness i.e. It's controlling the orientation of the axis with respect to face 'A'. Poor drafting though, perp / sq. would have been better as you say...

How can a TP callout have no basic dimensions, how is the diameter being located?

Glen

There should be a Ø symbol on the DRF frame to indicate axis control ideally... Maybe this is what your inspector is thinking what's missing / should be there... and it should be.

Sounds like this is an attempt at making the hole with only 1 datum call out the secondary datum for another hole?

EdenG said:

Hi Gil,

(By the sounds of it) You're correct about the TP callout to 'A' effectively being sqaureness i.e. It's controlling the orientation of the axis with respect to face 'A'. Poor drafting though, perp / sq. would have been better as you say...

How can a TP callout have no basic dimensions, how is the diameter being located?

Glen

Click to expand...

Thanks for the reply Glen,
There are basic dimensions on both the X & Y axis', however neither are datums. I believe this is what also contributed to the confusion. There is also a Diameter symbol in the control frame. The diameter is referenced to the face...seems cut and dry (to me anyway) that it's supposed to be perpendicular to face.

Gil R. said:

Thanks for the reply Glen,
There are basic dimensions on both the X & Y axis', however neither are datums. I believe this is what also contributed to the confusion. There is also a Diameter symbol in the control frame. The diameter is referenced to the face...seems cut and dry (to me anyway) that it's supposed to be perpendicular to face.

Click to expand...

Yeah, that's it then really. A TP callout to control squareness is actually quite progressive and a good thing, it's generally referenced to a primary, secondary and tertiary datum to get this effect (2 location, 1 orientation) but not always. The diameter symbol and a planar reference is the giveaway here...

Functionally the sqaureness is important and a good thing to be checking anyhow even if it wasn't called out... I wouldn't get too bogged down in the semantics of a callout with colleagues -- even if you're right Where's the draftsman / engineer when you need them huh?

EdenG said:

Yeah, that's it then really. A TP callout to control squareness is actually quite progressive and a good thing, it's generally referenced to a primary, secondary and tertiary datum to get this effect (2 location, 1 orientation) but not always. The diameter symbol and a planar reference is the giveaway here...

Functionally the sqaureness is important and a good thing to be checking anyhow even if it wasn't called out... I wouldn't get too bogged down in the semantics of a callout with colleagues -- even if you're right Where's the draftsman / engineer when you need them huh?

Click to expand...

LOL, part of the problem is not a lot of TP "experts" around here. I am the Quality Manager and don't claim to know it all when it comes to TP, however some things are fairly straight forward. The Inspector who was disagreeing with me (still is, I think, LOL) is a smart guy, but a bit stubborn where this was involved. He was convincing the CNC Manager and the Quality Supervisor that he was correct. I needed input from a good/reliable source, which is why I asked here (as well as previous, trusted colleagues). Thanks again for your input!

I have seen the same callout numerous times over the years, and personally I have always considered it 'lazy' tolerancing. I have treated it as perpendicularity of the hole in relation to Datum A, measuring/calculating the error in perpendicularity at the bottom and top of the hole.

According to US drafting standards, the datum of a design feature which is a hole is the virtual CENTERLINE, not the diameter. It is understood by convention that datum B is always perpendicular to datum A, because these are virtual constructs. What we think of as the cylinder of a physical hole is controlled by the diameter dimension, but its location is controlled by the position in 3 dimensions of the centerline. A tolerance of position of 0.05 on a hole states that everywhere within the confines of the part, the centerline of the physical hole will be within 0.025 of the ideal virtual centerline, since the cylindrical tolerance zone has a diameter of 0.05. In this way, tolerance of position provides a tolerance for non-perpendicularity as well as offset, as well as the case of a hole which is not straight (if the workpiece were distorted by clamping force when drilled, for example, and then clamping forces are released).

Many novices find tolerance of position confusing. I think a big part of the confusion is a GD&T drawing communicates both the ideal virtual geometry as well as geometry of physical features of the part, on the same drawing. It is important to understand this dual structure to understand what the symbols and dimensions refer to.