Machinery's Handbook, 31st Edition
2154 Diameter of Tap Drill of tap drills for these threads also may be based on the hole size limits given in Table 3, page 2160 , for Unified threads that take lengths of engagement into account. See Recommended Hole Sizes before Tapping on page 2170. The size of the tap drill hole for any desired percentage of full thread depth can be calcu lated by the formulas below. In these formulas the Percent Full Thread is expressed as a decimal; e.g., 75 percent is expressed as .75. The tap drill size is the size nearest to the calculated hole size. For American Unified Thread form :
1.08253 Percent Full Thread × Number of Threads per Inch – -------------------------------------
Hole Size Basic Major Diameter =
For ISO Metric threads (all dimensions in millimeters): Hole Size Basic Major Diameter 1.08253 Pitch × Percent Full Thread × ( ) – = The constant 1.08253 in the above equation represents 5 H /8 where H is the height of a sharp V-thread (see page 1940). The pitch is taken to be 1. Hole Sizes for Tapping Unified Miniature Screw Threads are given on page 2168 and Table 6. Theoretical and Tap Drill or Core Hole Sizes for Cold Form Tapping Unified Threads are given in Table 9, page 2175 and for ISO metric threads are given in Table 10, page 2176. Reaming Allowance.— Reaming is a secondary operation that slightly enlarges previously drilled or bored hole. Reamed holes are very accurate with a smooth surface finish. The reaming allowance is amount of material left in the hole after drilling. A properly sharpened drill produces a hole that is about 0.002–0.008 inch (0.050–0.203mm) over - size. For accurate diameters, a common practice is to rough out the hole with a smaller drill, then size the hole with the proper drill, leaving the correct reaming allowance. Use the following table to determine the drill size used prior to reaming the hole. Size of Reamer Reaming Allowance Size of Reamer Reaming Allowance inch mm inch mm inch mm inch mm 1 ⁄ 32 – 1 ⁄ 8 0.79–3.18 0.003–0.005 0.076–0.127 3 ⁄ 8 – 1 ⁄ 2 9.53–12.70 0.010–0.015 0.254–0.381 1 ⁄ 8 – 1 ⁄ 4 3.18–6.35 0.004–0.008 0.101–0.203 1 ⁄ 2 – 3 ⁄ 4 12.70–19.05 0.015–0.03 0.381–0.762 1 ⁄ 4 – 3 ⁄ 8 6.35–9.53 0.006–0.010 0.152–0.254 3 ⁄ 4 –1.00 19.05–25.40 0.03 0.762 Example: A finished bore of 1.000 inches after reaming is required. Select the drill size for predrilling that allows adequate allowance for reaming. Solution: According to the table above, a 1-inch drilled hole needs to be approximately 0.03 inch undersize to allow sufficient allowance for reaming, thus the hole size prior to reaming should be approximately 1.000 − 0.03 = 0.97 inch. A 31 ⁄ 32 drill can be expected to produce a slightly oversize hole, say 0.971 to 0.977 inch. The 31 ⁄ 32 drill provides a reaming allowance of 0.0 23 to 0.029 inch.
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