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DRILL AND COUNTERBORE SIZES FOR INCH SOCKET HEAD CAP SCREWS
Note 1
Close Fit: Normally limited to holes for those lengths of
screws threaded to the head in assemblies in which: (1)
only one screw is used; or (2) two or more screws are
used and the mating holes are produced at assembly or
by matched and coordinated tooling.
Note 2
Normal Fit: Intended for: (1) screws of relatively long
length; or (2) assemblies that involve two or more screws
and where the mating holes are produced by conven-
tional tolerancing methods. It provides for the maximum
allowable eccentricty of the longest standard screws and
for certain deviations in the parts being fastened, such as
deviations in hole straightness; angularity between the
axis of the tapped hole and that of the hole for the shank;
differneces in center distances of the mating holes and
other deviations.
Note 3
Chamfering: It is considered good practice to chamfer or
break the edges of holes that are smaller than "F" max
mum in parts in which hardness approaches, equals or
exceeds the screw hardness. If holes are not chamfered,
the heads may not seat properly or the sharp edges may
deform the fillets on the screws, making them suscepti-
ble to fatigue in applications that involve dynamic load-
ing. The chamfers, however, should not be larger than
needed to ensure that the heads seat properly or that the
fillet on the screw is not deformed. Normally, the cham-
fers do not need to exceed "F" maximum. Chamfe
exceeding these values reduce the effective bearing area
and introduce the possibility of indentation when the
parts fastened are softer than screws, or the possiblity of
brinnelling of the heads of the screws when the parts are
harder than the screws. (See "F" page 6
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