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Such an appropriately sized drill is called a tap drill for that size of thread, because it is a correct drill to be followed by the tap. Many thread sizes have several possible tap drills, because they yield threads of varying thread depth between 50% and 100%. Usually thread depths of 60% to 75% are desired. People frequently use a chart such ...
Unified Screw Threads with Tolerances; Unified Coarse/Fine diameters and tap drill sizes (U.S. units) Unified Coarse/Fine tap drill sizes (U.S. units) Imperial Metric fastening size conversion charts; International Thread Standards; Conversion chart Whitworth/BSF/AF and metric Archived 2 May 2010 at the Wayback Machine
where is the tap drill size, is the major diameter of the tap (e.g., 10 mm for a M10×1.5 tap), and pitch is the pitch of the thread (1.5 mm in the case of a standard M10 tap) and so the correct drill size is 8.5 mm. This works for both fine and coarse pitches, and also produces an approximate 75% thread.
From 1.0 through 2.95 mm, sizes are defined as follows, where N is an integer from 10 through 29: N · 0.1 mm; N · 0.1 + 0.05 mm; From 3.0 through 13.9 mm, sizes are defined as follows, where N is an integer from 30 through 139: N · 0.1 mm; From 14.0 through 25.0 mm, sizes are defined as follows, where M is an integer from 14 through 25: M ...
ISO 965-2: Limits of sizes for general purpose external and internal screw threads. ISO 965-3: Deviations for constructional screw threads; ISO 965-4: Limits of sizes for hot-dip galvanized external screw threads to mate with internal screw threads tapped with tolerance position H or G after galvanizing
In the case of female threads, tap drill charts typically specify sizes that will produce an approximate 75% thread. A 60% thread may be appropriate in cases where high tensile loading will not be expected. In both cases, the pitch diameter is not affected. The balancing of truncation versus thread strength is similar to many engineering ...
M designates common V threads (like for instance metric threads); Sq designates square threads; Tr designates trapezoidal threads; The shank length is not always the same at the thread length, which is the case if the insert has a threadless portion (sub-shank)
A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is an inclined plane wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread.