Acme and Square-Threaded Taps Machinery's Handbook, 31st Edition
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Acme and Square-Threaded Taps These taps are usually made in sets, three taps in a set being the most common. For very fine pitches, two taps in a set will be found sufficient, whereas as many as five taps in a set are used for coarse pitches. The table on the next page gives dimensions for proportioning both Acme and square-threaded taps when made in sets. In cutting the threads of square- threaded taps, one leading tap maker uses the following rules: The width of the groove between two threads is made equal to one-half the pitch of the thread, less 0.004 inch (0.102 mm), making the width of the thread itself equal to one-half of the pitch, plus 0.004 inch (0.102 mm). The depth of the thread is made equal to 0.45 times the pitch, plus 0.0025 inch (0.064 mm). This latter rule produces a thread that for all the ordinarily used pitches for square-threaded taps has a depth less than the generally accepted standard depth, this latter depth being equal to one-half the pitch. The object of this shallow thread is to ensure that if the hole to be threaded by the tap is not bored out so as to provide clearance at the bottom of the thread, the tap will cut its own clearance. The hole should, however, always be drilled out large enough so that the cutting of the clearance is not required of the tap. The table, Dimensions of Acme Threads Taps in Sets of Three Taps , may also be used for the length dimensions for Acme taps. The dimensions in this table apply to single-threaded taps. For multiple-threaded taps or taps with very coarse pitch relative to the diameter, the length of the chamfered part of the thread may be increased. Square-threaded taps are made to the same table as Acme taps, with the exception of the figures in column K , which for square-threaded taps should be equal to the nominal diameter of the tap, no oversize allow ance being customary in these taps. The first tap in a set of Acme taps (not square-threaded taps) should be turned to a taper at the bottom of the thread for a distance of about one-quar ter of the length of the threaded part. The taper should be so selected that the root diameter is about 1 ⁄ 32 inch (0.794 mm) smaller at the point than the proper root diameter of the tap. The first tap should preferably be provided with a short pilot at the point. For very coarse pitches, the first tap may be provided with spiral flutes at right angles to the angle of the thread. Acme and square-threaded taps should be relieved or backed off on the top of the thread of the chamfered portion on all the taps in the set. When the taps are used as machine taps, rather than as hand taps, they should be relieved in the angle of the thread, as well as on the top, for the whole length of the chamfered portion. Acme taps should also always be relieved on the front side of the thread to within 1 ⁄ 32 inch (0.794 mm) of the cutting edge. Adjustable Taps.— Many adjustable taps are now used, especially for accurate work. Some taps of this class are made of a solid piece of tool steel that is split and provided with means of expanding sufficiently to compensate for wear. Most of the larger adjustable taps have inserted blades or chasers that are held rigidly but are capable of radial adjustment. The use of taps of this general class enables standard sizes to be maintained readily. Drill Hole Sizes for Acme Threads.— Many tap and die manufacturers and vendors make available to their customers computer programs designed to calculate drill hole sizes for all the Acme threads in their ranges from the basic dimensions. The large variety and combination of dimensions for such tools prevent inclusion of a complete set of tables of tap drills for Acme taps in this Handbook. The following formulas (dimensions in inches) for calculating drill hole sizes for Acme threads are derived from the American National Standard, ANSI/ASME B1.5-1997 (R2014), Acme Screw Threads. To select a tap drill size for an Acme thread, first calculate the maximum and minimum internal product minor diameters for the thread to be produced. (Dimensions for general purpose, centralizing, and stub Acme screw threads are given in the Threads and Thread ing section, starting on page 2054 .) Then select a drill that will yield a finished hole some where between the established maximum and minimum product minor diameters. To reduce the amount of material to be removed when cutting the thread, consider staying close to the maximum product limit in selecting the hole size. If there is no standard drill size that matches the hole diameter selected, it may be necessary to drill and ream, or bore, the hole to size, to achieve the required hole diameter.
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