External Thread Symbol

Because they are designed to be assembled together, thread dimensions are different for internal and external threads. To distinguish between the two, the letter A is used to designate external thread and B is used to indicate internal threads. The A or B symbol is actually part of the class symbol. See thread class.

Size limits for standard external thread tolerance classes 6g and 4g6g apply prior to coating. The external thread allowance may thus be used to accommodate the coating thickness on coated parts. The designation of a screw thread gives the thread symbol, the nominal size, the pitch, and the thread tolerance class Ex M6 x 1 - 4g6g.

Types of Threads Before understanding representation, let us quickly see the types of threads commonly shown in drawings External Thread Found on bolts, screws, studs, etc. Internal Thread Found inside nuts or tapped holes Right-Hand Thread Most common, tightens when rotated clockwise Left-Hand Thread Less common, used in special applications Each thread is defined by certain

M20 The major nominal diameter of the thread.Appendix 13 in textbook lists metric screw thread series. 1.5 is the pitch of the thread. 4g6g is the tolerance class.It is a combination of tolerance grade and tolerance position. It indicates the allowance and tolerance for the pitch and major diameters of external threads, and pitch and minor diameters of internal threads.

height of the MJ internal thread to prevent interference at the external MJ thread root. M external threads are compatible with both M and MJ internal threads. M10 x 1.5-6g means metric fastener thread series M, fastener nominal size nominal major diameter 10 mm, thread pitch 1.5 mm, external thread class fit 6g.

There are three methods of representing screw threads on a drawing. Detailed Schematic Simplified . Screw thread representation present in this chapter is in accordance with the ASME Y14.6-2001 standard. Detailed Representation . A detailed representation is a close approximation of the appearance of an actual screw thread.

A 38 - 16 thread would be considered a coarse thread, a 38 - 24 thread has the same nominal diameter however, it has more threads per inch, making it a fine thread. Class Fit. When the male and female threads are mated, the two parts assemble together with a degree of tightness dictated by the fit of the two threads.

The standard method for designating a screw thread is by specifying, in sequence, the nominal size, number of threads per inch, thread series symbol, and thread class symbol, optionally supplemented by pitch diameter and its tolerance. An example of an external thread designation and what it means would be 14 - 20 - UNC - 2A

Learn how to specify and dimension threaded fasteners using the English and metric systems. See the symbols, notes, and conventions for external and internal threads, as well as rolled threads.

Learn how to designate and specify Unified Inch screw threads, including thread form symbol UN, series, class, and external or internal. See examples of thread notes and standard series for graded and constant pitches.