GD&T Fixed Fastener using 3 plates

[Worker7]

I have 3 alum plates bolted together using two bolts several inches apart. I have clearance holes in each of the two top plates. They are then placed down on top of the third plate, which has two tapped holes for the bolts. Is this a valid equation for determining the size of the smallest clearance hole in either of the two plates? H = T1 + T2 + T3 + F Where the T values are the associated geometric circular tolerance zones, F is the largest fastener and H is the minimum hole size.

 

[3DDave]

No. Each plate is just Hole Diameter MMC - Fastener Diameter MMC - Projected position tolerance of tapped hole = Largest allowable Position tolerance diameter of clearance hole. There is no relation based on the number of plates being bolted whether it is two or two hundred.

 

[Worker7]

Thank you 3DDave. So if I am solving for the Hole Diameter MMC in Plate 1. Is this correct? Hole Diameter MMC in Plate 1 = Largest allowable Position tolerance diameter of clearance hole in Plate 1 + Projected position tolerance of tapped hole + Fastener Diameter MMC.

and then for Plate 2 would this be correct? Hole Diameter MMC in Plate 2 = Largest allowable Position tolerance diameter of clearance hole in Plate 2 + Projected position tolerance of tapped hole + Fastener Diameter MMC.

 

[Tunalover]

3DDave gave you the floating fastener case. This fastener scenario is actually the fixed-fastener case since the fastener does not float in a loosely assembled joint:
If "1" represents to bottom plate, "2" represents the middle plate, and "3" represents the top plate:

H2=F+T1+T2
T1=H2-F-T2

H3=F+T1+T3
T1=H3-F-T3

so H2-F-T2=H3-F-T3
H2-H3=T2-T3

where
F=fastener major diameter
T1 = p.t. of the holes in plate 1 at MMC
T2 = p.t. of the holes in plate 2 at MMC
T3 = p.t. of the holes in plate 3 at MMC
H2 = hole diameter in plate 2 at MMC
H3 = hole diameter in plate 3 at MMC

You can take it from here. You didn't say if the clearance holes are within your control with regard to sizes, size tolerances, and positional tolerances. If they are then this is pretty simple.


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993

https://www.youtube.com/watch?v=BKorP55Aqvg

 

[Tunalover]

Correction:
Rule of thumb is if the middle and top plates together are greater than F in thickness then projected tolerance zones should be used and the formulas tweaked accordingly.

ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993

https://www.youtube.com/watch?v=BKorP55Aqvg

 

[3DDave]

I gave the fixed fastener case.

 

[Tunalover]

You did. My apologies 3DDave.


ElectroMechanical Product Development
(Electronics Packaging)
UMD 1984
UCF 1993

https://www.youtube.com/watch?v=BKorP55Aqvg

 

[Worker7]

Tunalover and 3DDave thanks for your assistance. I greatly appreciate it.