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1848 ALTERNATIVE CENTRALIZING ACME SCREW THREADS 60-Degree Stub Thread A clearance of at least 0.02 × pitch is added to depth h to produce extra depth, thus avoiding interference with threads of mating part at minor or major diameters. Basic thread thickness at pitch line = 0.5 × pitch p; basic depth h = 0.433 × pitch; basic width of flat at crest = 0.25 × pitch; width of flat at root of screw thread = 0.227 × pitch; basic pitch diameter = basic major diameter − 0.433 × pitch; basic minor diameter = basic major diameter − 0.866 × pitch. Square Thread.—The square thread is so named because the section is square, the depth, in the case of a screw, being equal to the width or one-half the pitch. The thread groove in a square-threaded nut is made a little greater than one-half the pitch in order to provide a slight clearance for the screw; hence, the tools used for threading square-threaded taps are a little less in width at the point than one-half the pitch. The pitch of a square thread is usually twice the pitch of an American Standard thread of corresponding diameter. The square thread has been superseded quite largely by the Acme form which has several advantages. See ACME SCREW THREADS. 10-Degree Modified Square Thread: The included angle between the sides of the thread is 10 degrees (see accompanying diagram). The angle of 10 degrees results in a thread which is the practical equivalent of a “square thread,” and yet is capable of economical production. Multiple thread milling cutters and ground thread taps should not be specified for modified square threads of the larger lead angles without consulting the cutting tool manufacturer. 5˚ Machinery's Handbook 27th Edition Screw Clearance (See Note) 0.25p p 2 0.4563p p 2 h 0.25p G 1 Pitch 2 Diameter Allowance Clearance (See Note) In the following formulas, D = basic major diameter; E = basic pitch diameter; K = basic minor diameter; p = pitch; h = basic depth of thread on screw depth when there is no clearance between root of screw and crest of thread on nut; t = basic thickness of thread at pitch line; F = basic width of flat at crest of screw thread; G = basic width of flat at root of screw thread; C = clearance between root of screw and crest of thread on nut: E = D − 0.5p; K = D − p; h = 0.5p (see Note); t = 0.5p; F = 0.4563p; G = 0.4563p − (0.17 × C). Note: A clearance should be added to depth h to avoid interference with threads of mating parts at minor or major diameters. Copyright 2004, Industrial Press, Inc., New York, NY Nut
Machinery's Handbook 27th Edition BUTTRESS THREADS 1849 BUTTRESS THREADS Threads of Buttress Form The buttress form of thread has certain advantages in applications involving exceptionally high stresses along the thread axis in one direction only. The contacting flank of the thread, which takes the thrust, is referred to as the pressure flank and is so nearly perpendicular to the thread axis that the radial component of the thrust is reduced to a minimum. Because of the small radial thrust, this form of thread is particularly applicable where tubular members are screwed together, as in the case of breech mechanisms of large guns and airplane propeller hubs. Fig. 1a shows a common form. The front or load-resisting face is perpendicular to the axis of the screw and the thread angle is 45 degrees. According to one rule, the pitch P = 2 × screw diameter ÷ 15. The thread depth d may equal 3 ⁄ 4 × pitch, making the flat f = 1 ⁄ 8 × pitch. Sometimes depth d is reduced to 2 ⁄ 3 × pitch, making f = 1 ⁄ 6 × pitch. f P f P f P NUT 45˚ 45˚ d 5˚ 50˚ 50˚ d d1 d g e 3˚ 33˚ 33˚ SCREW f f Fig. 1a. Fig. 1b. Fig. 1c. The load-resisting side or flank may be inclined an amount (Fig. 1b) ranging usually from 1 to 5 degrees to avoid cutter interference in milling the thread. With an angle of 5 degrees and an included thread angle of 50 degrees, if the width of the flat f at both crest and root equals 1 ⁄ 8 × pitch, then the thread depth equals 0.69 × pitch or 3 ⁄ 4 d 1 . The saw-tooth form of thread illustrated by Fig. 1c is known in Germany as the “Sägengewinde” and in Italy as the “Fillettatura a dente di Sega.” Pitches are standardized from 2 millimeters up to 48 millimeters in the German and Italian specifications. The front face inclines 3 degrees from the perpendicular and the included angle is 33 degrees. The thread depth d for the screw = 0.86777 × pitch P. The thread depth g for the nut = 0.75 × pitch. Dimension h = 0.341 × P. The width f of flat at the crest of the thread on the screw = 0.26384 × pitch. Radius r at the root = 0.12427 × pitch. The clearance space e = 0.11777 × pitch. British Standard Buttress Threads BS 1657: 1950.—Specifications for buttress threads in this standard are similar to those in the American Standard (see page 1850) except: 1) A basic depth of thread of 0.4p is used instead of 0.6p; 2) Sizes below 1 inch are not included; 3) Tolerances on major and minor diameters are the same as the pitch diameter tolerances, whereas in the American Standard separate tolerances are provided; however, provision is made for smaller major and minor diameter tolerances when crest surfaces of screws or nuts are used as datum surfaces, or when the resulting reduction in depth of engagement must be limited; and 4) Certain combinations of large diameters with fine pitches are provided that are not encouraged in the American Standard. Lowenherz or Löwenherz Thread.—The Lowenherz thread is intended for the fine screws of instruments and is based on the metric system. The Löwenherz thread has flats at the top and bottom the same as the U.S. standard buttress form, but the angle is 53 degrees 8 minutes. The depth equals 0.75 × the pitch, and the width of the flats at the top and bottom is equal to 0.125 × the pitch. This screw thread used for measuring instruments, optical apparatus, etc., especially in Germany. Copyright 2004, Industrial Press, Inc., New York, NY h r
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