T. O. 1-1A-9
TABLE 2-13. TAPPING ALLOWANCE (HOLE SIZE TO SCREW SIZE) CONT'D.
DRILL SIZE
THREAD DEPTH
THREAD/SCREW SIZE
MAJOR DIA.
MINOR DIA.
DECIMAL & NR
PERCENT
1/4-24
0.2500
0.2143+0.003
0.1960-9
100
2031-13/64"
86
0.2090-4
75
0.2130-3
68
1/4-28
0.2500
0.2193+0.002
0.2090-4
88
0.2130-3
80
0.2187-7/32"
67
5/16-24
0.3125
0.2708+0.0032
0.2610-G
95
0.2656-17/64"
86
0.2720-1
75
0.2770-J
65
3/8-24
0.3750
0.3278+0.002
0.3281-2 1/64"
86
0.3320-Q
70
0.3390-R
66
1/2-24
0.5000
0.4579+0.003
0.4531-29/64"
86
0.4687-15/32"
57
2-106. In addition to the above, the tap basically should have a taper/chamfer of about 9°0 with center line on the
starting end to facilitate entry into hole.
The taper should be held short (1st thread) for blind holes, and on through holes, it may extend over 3 or 4 threads.
2-107. Due to high strength and poorer cutting quality of the stainless series steels, holes for tapping are usually made
as large as possible consistent with fit specified by drawing or other data. Actually due to the higher strength of this
material less thread area or engagement is required in comparison to most other metals. Due to the above and the fact
that less cutting is required, 75% thread depth is generally used as maximum unless otherwise specified. Higher
percentages of thread depth are necessary in material when stock is not thick enough to permit the required number of
thread. For tapping allowances of some size screws/bolts see Table 2-13.
2-108. The decreased thread depth also reduces tendency to gall and seize, power required to drive tap, tap wear, and
effect of swelling in soft material.
2-109. Tapping Speeds Corrosion Resisting Steel.
Tapping speeds used for stainless should be slower than those used for carbon steel. The 18-8 (300 series) are usually
tapped at 10-25 SFPM except for the free machining types which are tapped at 15-30SFPM. The straight-chromium 400
series generally is tapped at 15-25 SFPM, except the free machining grades, which are tapped at 15-35 SFPM.
2-110. Lubrication for Tapping. The lubrications recommended for tapping are sulphurized mineral oils with paraffin and
lard oil. The lubricant serves to prevent overheating as well as lubrication, and if applied under pressure, aids in chip
removal. Oil flow/application should be applied before tapping commences to prevent initial congestion of cuttings.
2-77
