How to calculate required positoinal tolerance of a stack up?

[XaeroR35]

I am rusty with my GD&T and usually have the manufacturing guys do it for me, but I would rather know what I am doing at this point. Let's say I have 2 plates with matching hole patterns. Pins with a min/max diameter tolerance go through both plates. How do I actually calculate what my positional tolerance will be for each plate? Oh yeah: the pins pass through one plate and thread into the second plate.

The tolerance currently being built to does not look like it works:

Pin diameter: .120/.125
Through hole: .127/.133 with positional tolerance: |diam .005M|A|B|C|
Threaded hole: .112-40 UNC-2B with positional tolerance: |diam .005M|A|B|C|

It appears to me that a maximum pin diameter with a minimum through hole diameter may cause problems when trying to fit these together.

 

[drawoh]

XaeroR35,

The math of this is explained at the back of the ASME Y14.5 standard. I have an article on Calculating Locational Tolerances on my website.

This is easy to work out from first principles.

Your pin has a maximum diameter of .125", located within a Ø.005" circle.

You show your tapped hole position at MMC/B. I know this is done, but it makes no sense to me if the screw is tightened down. The thread centres your pin.

In addition, your tapped hole will have some perpendicularity error. We cannot calculate this without knowing the depth of your hole. You can specify your tolerance projected the length of your pin. If your pin's height equals the thread depth, allowable perpendicularity could triple your positional error.

Your pin occupies a space as much as Ø.125"+Ø.005"[×]3=Ø.140". This is is the diameter that must be clear at all times. Add your hole's positional tolerance to this, and you have a minimum hole diameter of Ø.145". If you ignore or control for perpendicularity, your minimum hole is Ø.135".

Your clearance hole is small enough that parts conforming to your drawings possibly cannot be assembled, at least, not without a big hammer.

--
JHG

 

[XaeroR35]

Thanks for the feed back. I found the basic formula for fixed fastener tolerance and it looks like we have negative tolerance! I have to assume the pins are small enough that they are bending to fit in place. Looks like a redesign is in order.