Positional Tolerancing 101 (or maybe even 001)

[Dawg2012]

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.

 

[KENAT]

Correct, the 2 parts being fixed in relationship to each other, using the formula correctly ensures fasteners will always fit.

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..

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Belanger]

But also notice that in the situation where we usually talk about the fixed fastener formula, the threaded hole and the clearance hole are toleranced back to other datums (i.e., the edges of the respective parts).

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

 

[Dawg2012]

The specific application I currently have is an interface with four fasteners and two locating pins going into a close tolerance hole and slot. The pins allow the interface to be taken apart and put back together with very repeatable locational precision.

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.