Position Refinement

[dthom0425]

Hi all,

Sorry if this question is embedded somewhere in this forum. I searched for a little while - not sure I got an answer to my question.

If I have a fixed fastener situation and the threaded hole is controlled with position and refined via perpendicularity, which number tolerance (pos or perp) gets used in the virtual mating calculation to verify my screws will pass through?

Just making something up here:

1/4-20 bolt
.014 TP tapped hole
.005 perp tapped hole

.283+/-.005 clearance hole
.008 TP clearance hole

So is it:
.25 (bolt dia) + .014 TP = .264 Virtual Screw
.283 (clear hole dia) - .005 - .014 = .264 Virtual hole
PASS

Or is it:
.25 (bolt dia) + .005 perp = .255 Virtual screw
.264 Virtual hole
PASS

Thanks for the help

 

[Burunduk]

dthom, it depends on the constraints that act on the components. Does something constrain the location of the screw and clearance hole?
Perp. doesn't account for location.

Also, is it just one hole and one screw?
I ask because otherwise composite position should be more appropriate than a perp. refinement.

 

[3DDave]

It isn't quite any of those. The apparent diameter of the screw that is assembled in the hole depends on the depth of the hole and the thickness of the mating part.

If you had a copy of the standard, this is explained in the Appendix, "FORMULAS FOR POSITIONAL TOLERANCING"

Mainly you can derive it yourself - draw an exaggerated section view of the assembly situation with the screw thread at the extreme location and maximum allowed perpendicularity variations; remember to tilt both directions and position at both ends of the allowed diameter.

Project that axis up the thickness of the mating material and it will be obvious with grade school geometry what the required hole size minimum will be.

Notice that if only one hole is used to limit location movement that only the perpendicularity will matter as the mating part can move as required. But if some other features control the assembly location or if two or more holes do, that freedom will vanish and the tapped hole position tolerance will matter, as diagrammed per the above.