Joseph E. Shigley
The University of Michigan
Ann Arbor, Michigan
21.1 SCREW THREADS/21.1
21.5 TAPPING SCREWS / 21.35
This chapter is intended to cover the description, uses, materials, and sizes of
threaded fasteners. The amount of data available concerning this subject is
extremely large, so the intent here is to provide the information necessary for the
usual machine-design task of selecting such fasteners. The data contained in this
chapter have been compiled in part from the standards listed in Ref. [21.1].
27, T SCREWTHREADS
Standard screw threads consist of the Unified inch series and the metric series. Two
profiles have been standardized in the metric series; these are called the M and MJ
profiles. Figure 21.1 shows that both the Unified and metric M threads utilize the
The metric MJ profile has a rounded fillet at the root of the external thread and
a larger minor diameter of both the internal and external threads. This profile is used
for applications requiring a high fatigue strength and is also employed in aerospace
The Unified-series profile, shown in Fig. 21.1, is designated as UN. Another uni-
fied profile, designated as UNR, has a rounded root on the external thread.
Unified thread standards are based on the nominal size (major diameter) and the
number of threads per inch. The three standards coarse (UNC), fine (UNF), and
extra fine (UNEF) are listed in Table 21.1 and are called the standard series. Typical
specifications would be written
Both these designations specify a nominal size of 1A in and 20 threads per inch.
A constant-pitch unified series consisting of 4, 6, 8,12,16, 20, 28, and 32 threads
per inch has also been standardized. These are used mostly for sizes over 1 in, and 8
UN, 12 UN, and 16 UN are the preferred pitches.
FIGURE 21.1 Basic thread profile for unified (UN) and metric (M) threads (ISO 68).
D(d) = basic major