Positional Tolerancing 101 (or maybe even 001)
Positional Tolerancing 101 (or maybe even 001)
(OP)
I have reason to really understand the Fixed Fastener positional tolerancing formula given in Appendix B of 14.5.
The formula itself is pretty straight forward, as is the interchangeability concept, but what about the nominal relationship of one part to the other?
It would appear, after mocking up a very simple interface in a CAD program, that the formula produces interchangeability WITHOUT either part having to move nominally in relation to the other part. E.g., the part with clearance holes does not have to move nominally in order to achieve interchangeability.
Is this true? Am I understanding what I'm seeing correctly?
I'm sure this seems pretty basic, I've just never really thought about it until having to ensure interchangeability when the interface uses four fasteners and two pins, with the two pins going into a close tolerance hole and a slot.
The formula itself is pretty straight forward, as is the interchangeability concept, but what about the nominal relationship of one part to the other?
It would appear, after mocking up a very simple interface in a CAD program, that the formula produces interchangeability WITHOUT either part having to move nominally in relation to the other part. E.g., the part with clearance holes does not have to move nominally in order to achieve interchangeability.
Is this true? Am I understanding what I'm seeing correctly?
I'm sure this seems pretty basic, I've just never really thought about it until having to ensure interchangeability when the interface uses four fasteners and two pins, with the two pins going into a close tolerance hole and a slot.





RE: Positional Tolerancing 101 (or maybe even 001)
Just remember, when I say correctly I'm also talking about adding projected tolerance or oversizing hole as appropriate to guarantee 100% interchangeability at worst case..
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Positional Tolerancing 101 (or maybe even 001)
Those datums are what lock in the "nominal relationship" of the two parts. If we think that the threaded hole is what drives the nominal relationship, then it should be the datum feature, and the formula doesn't apply in that situation.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
http://www.gdtseminars.com
RE: Positional Tolerancing 101 (or maybe even 001)
One part has four clearance holes and two pins. The other part has four threaded holes, one close tolerance hole for the primary pin, and a close tolerance (in one direction) slot for the secondary pin.
In both parts the primary pin/hole is the secondary datum and the pin/slot (in one direction) is the tertiary datum for the positional tolerancing of the holes.
I believe tolerancing the holes in both parts from the primary pin/hole produces interchangeability while allowing the primary pin/hole to remain essentially fixed between the parts, given that I account for the worst case clocking of the parts to each other produced by the secondary pin/slot, which should be fairly easy to calculate.