Edge distance rule of thumb: Allow 3 times a single thread height (1 pitch x 3) plus 1/2 the max thread diameter.
This will be the distance from the centerline of the hole to the edge of the part. For example, the edge distance for a 4(.112)-40 screw
would be (1/40*3)+(0.112/2)=.131 or a .262 diameter boss. If you are using a helicoil or keensert or would like to leave room for
possibly adding them later, use the max diameter of the insert instead.
In the table below:
Constant A: Shall be .005 to .015 larger than the major dia.
Constant B: Shall be .005 to .015 smaller than the minor dia.
Thread relief dimensions for tapped holes and
threaded shafts (pdf)
