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Regarding the proportion of tap drill to thread major diameter, for standard V threads (ISO V thread and UTS V thread), there are several rules of thumb with strong predictive power: A good tap drill is 85% (± 2 pp) of major diameter for coarse threads, and 90% (± 2 pp) of major diameter for fine threads. [1]
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.
Parallels that only have 2 or 4 precision faces will often have the tool-marks from the machining on the non-ground sides. They are then paired and placed in a grinding machine, and each face is ground until the overall dimensions are correct - they are paired during this stage so that even if the dimensions are not correct, they are still ...
The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch.For diameters smaller than 1 / 4 inch, the diameter is indicated by an integer number defined in the standard; for all other diameters, the inch figure is given.
So a 1 / 2 in (12.7 mm) diameter drill will be able to drill a hole 4 + 1 / 2 in (114.3 mm) deep, since it is 9 times the diameter in length. A 1 / 8 in (3.175 mm) diameter drill can drill a hole 1 + 5 / 8 in (41.275 mm) deep, since it is 13 times the diameter in flute length. [3]
The drawbar thread is typically 7 ⁄ 16 ″–20 tpi . The angle of the cone is 16°51′ (16.85°) with an OD of 1.25″ and a length of 15 ⁄ 16 ″. [ 21 ] (source, Bridgeport Manufacturer) The diameter of the parallel locating portion is not a "fractional inch" size like the other dimensions and is 0.949″ to 0.9495″.