ASME B18.3 & ASME B18.2.8 clearance hole fits

[renasis]

Does anybody know how the recommended holes sizes for screws were established in the ASME B18.3-1986 & B18.2.8 tables? For example, the following recommendations are given for a 1/4-20 screw.

ASME B18.3 - Close Fit .266" (17/64 drill)
ASME B18.3 - Normal Fit .281" (9/32 drill)
ASME B18.2.8 - Close Fit .266"/.272" (17/64 drill)
ASME B18.2.8 - Normal Fit .281"/.290" (9/32 drill)
ASME B18.2.8 - Loose Fit .297"/.311" (19/64 drill)

Military standard AND10387 gives a +.006/-.001 tolerance on drilled holes between .251-.500. Given this, each of the hole sizes would give a min position clearance of the following:

Min hole - Max screw = Possible position error
ASME B18.3 - Close Fit .265-.250 =.015"
ASME B18.3 - Normal Fit .280-.250 =.030"
ASME B18.2.8 - Close Fit .265-.250 =.015"
ASME B18.2.8 - Normal Fit .280-.250 =.030"
ASME B18.2.8 - Loose Fit .296-.250 =.046"

In general, most clearance and threaded holes have ±.005 tolerances in the x and y. Position error for each hole would be .014", for both would be .028". So where does the .030" come from on a normal fit?

I obtained these values from references to the standards. I don't own them. Maybe there is a note in them about how clearance was determined for each of the fits?

 

[Doug Hunter]

The Machinery Handbook might give more background on the history you seek.

Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/

 

[drawoh]

renasis,

I would not bother with the standard. The clearances and the positional tolerances of your holes are mathematically related. The more accurately you can locate them, the smaller your clearances need to be.

Calculating Locational Tolerances

--
JHG