Diameter of Tap Drill Machinery's Handbook, 31st Edition
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Table 6. Unified Miniature Screw Threads—Recommended Hole Size Limits Before Tapping
Lengths of Engagement
Thread Size
Internal Threads
To and including 2 ∕ 3 D
Above 1 1 ∕ 2 D to 3 D
Above 2 ∕ 3 D to 1 1 ∕ 2 D
Minor Diameter Limits
Recommended Hole Size Limits
Pitch Max mm mm mm mm mm mm mm mm mm Min Max Min Max Min Max Min
Designation
0.30 UNM 0.080 0.217 0.254 0.226 0.240 0.236 0.254 0.245 0.264 0.35 UNM 0.090 0.256 0.297 0.267 0.282 0.277 0.297 0.287 0.307 0.40 UNM 0.100 0.296 0.340 0.307 0.324 0.318 0.340 0.329 0.351 0.45 UNM 0.100 0.346 0.390 0.357 0.374 0.368 0.390 0.379 0.401 0.50 UNM 0.125 0.370 0.422 0.383 0.402 0.396 0.422 0.409 0.435 0.55 UNM 0.125 0.420 0.472 0.433 0.452 0.446 0.472 0.459 0.485 0.60 UNM 0.150 0.444 0.504 0.459 0.482 0.474 0.504 0.489 0.519 0.70 UNM 0.175 0.518 0.586 0.535 0.560 0.552 0.586 0.569 0.603 0.80 UNM 0.200 0.592 0.668 0.611 0.640 0.630 0.668 0.649 0.687 0.90 UNM 0.225 0.666 0.750 0.687 0.718 0.708 0.750 0.729 0.771 1.00 UNM 0.250 0.740 0.832 0.763 0.798 0.786 0.832 0.809 0.855 1.10 UNM 0.250 0.840 0.932 0.863 0.898 0.886 0.932 0.909 0.955 1.20 UNM 0.250 0.940 1.032 0.963 0.998 0.986 1.032 1.009 1.055 1.40 UNM 0.300 1.088 1.196 1.115 1.156 1.142 1.196 1.169 1.223 Designation Thds. per in. inch inch inch inch inch inch inch inch 0.30 UNM 318 0.0085 0.0100 0.0089 0.0095 0.0093 0.0100 0.0096 0.0104 0.35 UNM 282 0.0101 0.0117 0.0105 0.0111 0.0109 0.0117 0.0113 0.0121 0.40 UNM 254 0.0117 0.0134 0.0121 0.0127 0.0125 0.0134 0.0130 0.0138 0.45 UNM 254 0.0136 0.0154 0.0141 0.0147 0.0145 0.0154 0.0149 0.0158 0.50 UNM 203 0.0146 0.0166 0.0150 0.0158 0.0156 0.0166 0.0161 0.0171 0.55 UNM 203 0.0165 0.0186 0.0170 0.0178 0.0176 0.0186 0.0181 0.0191 0.60 UNM 169 0.0175 0.0198 0.0181 0.0190 0.0187 0.0198 0.0193 0.0204 0.70 UNM 145 0.0204 0.0231 0.0211 0.0221 0.0217 0.0231 0.0224 0.0237 0.80 UNM 127 0.0233 0.0263 0.0241 0.0252 0.0248 0.0263 0.0256 0.0270 0.90 UNM 113 0.0262 0.0295 0.0270 0.0283 0.0279 0.0295 0.0287 0.0304 1.00 UNM 102 0.0291 0.0327 0.0300 0.0314 0.0309 0.0327 0.0319 0.0337 1.10 UNM 102 0.0331 0.0367 0.0340 0.0354 0.0349 0.0367 0.0358 0.0376 1.20 UNM 102 0.0370 0.0406 0.0379 0.0393 0.0388 0.0406 0.0397 0.0415 1.40 UNM 85 0.0428 0.0471 0.0439 0.0455 0.0450 0.0471 0.0460 0.0481 As an aid in selecting suitable drills, see the listing of American Standard drill sizes in the twist drill section. Thread sizes in heavy type are preferred sizes. Hole Sizes for Tapping Unified Miniature Screw Threads.— Table 6 indicates the hole size limits recommended for tapping. These limits are derived from the internal thread minor diameter limits given in the American Standard for Unified Miniature Screw Threads ASA B1.10-1958 and are disposed so as to provide the optimum conditions for tapping. The maximum limits are based on providing a functionally adequate fastening for the most common applications, where the material of the externally threaded member is of a strength essentially equal to or greater than that of its mating part. In applications where, because of considerations other than the fastening, the screw is made of an apprecia - bly weaker material, the use of smaller hole sizes is usually necessary to extend thread engagement to a greater depth on the external thread. Recommended minimum hole sizes are greater than the minimum limits of the minor diameters to allow for the spin-up de - veloped in tapping.
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