Positional Tolerance

[squarkman]

Are my assumptions correct below? I'm new to GD&T:

Positional tolerance frames specifies a 3 dimensional zone concentric with the true position axis, inside which, the real manufactured hole's axis must lie for the part to meet the specifications.

The true position axis uses three basic (theoretically perfect) dimensions from GD&T specified planes or surfaces labeled at the end of the frame?

This saves money because the hole can then be more loosely toleranced. (I would wonder if the savings from looser tolereances isn't eaten up in increased costs due to using a more complex system. time to do such etc)

 

[MikeHalloran]

You need to read some more. You can't control the position of an axis, because there is no such thing. GD&T deals with things (features) that you can actually touch and measure.

It only saves money if everyone involved has a common understanding of it, e.g. everyone gets trained at once.




Mike Halloran
Pembroke Pines, FL, USA

 

[squarkman]

I'm not saying that it controls the position of an axis. But that the axis lies inside the zone.

How would you modify my statement to make it true? That would surely assist me.

Thanks

 

[MechNorth]

Squark, you have the right basic idea, though some serious reading/training is in order. The tolerance doesn't necessarily get looser (that's a common management mistake based on reading an article or two on the subject while sipping a scotch and watching a bad movie while flying somewhere), but by using the same radial value (i.e. from the basic position to the outer corner of a regular +/- tolerance square), you do gain 57% more tolerance zone. Also, if you use an MMC modifier, you may be able to trade off size & position tolerances...then you're reducing cost by potentially reducing scrap & rework.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[squarkman]

Thanks. I think I get the circumscribed square concept wrt tol zone.
But what accurately places the tol zone?? It's nice to say that such and such should be inside the zone, but unless you know where the zone is, what good is it all?

I guess I don't get how the dimension to a datum of the true position axis can be basic (theor perfect). What value to a manufacturer is a basic dimension? Can't the true position or zone center, then be off? If the hole true position axis' basic dim is 10 inches from DATUM A, what guarantees that the zone will be place accurately?
Thanks

 

[MechNorth]

Well, to start BASIC means THEORETICALLY PERFECT. So the tolerance zone is centered on that BASIC location. The quality of that location in an inspection setup is based on the inspector's and equipment's capabilities and inherent accuracies / inaccuracies. The BASIC location cannot be out of position because it is perfect. During an inspection setup, the actual location is measured as a deviation from the BASIC location. The ACTUAL position (i.e. the axis) of the feature (hole) needs to be within the tolerance zone centered at the BASIC location.

Does that help?

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca

 

[squarkman]

Thanks Jim,

You said...

The ACTUAL position (i.e. the axis) of the feature (hole) needs to be within the tolerance zone centered at the BASIC location.

To reject the part based on that, does inspection scribe a circle the size of the zone on the part exactly at the BASIC dim value and compare that to the ACTUAL hole? Does the real hole have to fit within the zone?

Does this mean that the scribed circle has to be larger than the actual hole since you can't compare axes positions located in zones on real parts?

So the zone has to be a circular area larger than the hole to make a comparison? If the ACTUAL hole is completely within the zone hole, then is passess. Is that how this works?

Thanks

 

[MechNorth]

The sequence for verification would typically be:
Inspect the feature (hole) for size and form. Use the largest pin gauge you can fit into the hole while meeting any orientation constraints (e.g. perpendicular or parallel to a specific face); the axis of this pin simulates the axis of the hole. Compare the cylinder which circumscribes the axis of the pin with the tolerance zone.

It's a little different for MMC, but that's another story for another day.

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development

http://www.profileservices.ca