Question

Problem 2: Identify the different components of the following Unified National thread note. (10 points) 3/8-20 UNF - 2A - LH 3/8 20

Answer 1

3/8:- This component is the major diameter of the thread. Thus 3/8-inch thread will have a 3/8 standing at the left most of the thread specification. The dimensions usually come at margins of an inch (1/2, ¼, 1/8 ,1/16 etc). It must be noted that threads with a diameter smaller than ¼ have been given special numbers, from 0 to 10.

20:- This component stands for number of threads per inch. Thus, -20 means that there are 20 threads in each inch along the thread.

UNF (Unified National Fine):- External threads of this Fine Series have greater tensile stress area than comparable sizes of the Coarse series. The Fine series is suitable when the resistance to stripping of both external and mating internal threads equals or exceeds the tensile load carrying capacity of the externally threaded member. It is also used where the length of engagement is short, where a smaller lead angle is desired, where the wall thickness demands a fine pitch, or where finer adjustment is needed.

2:- 1 is the most loose thread, meaning there is less contact between threads and more clearance. 2 is the most common class and 3 is the most precise fit, used in accurate applications.

A:- A or B is used to specify whether it is an external or internal thread. External thread is designated A and internal – B.

LH:- LH is used to specify the unusual left hand – used sometimes for locking against rotational force or other special applications.

(3/8-20 UNF-2A-LH)

means this is a 3/8 inch diameter thread with 20 thread per inch, manufactured in accordance with Unified National Fine Standard. The thread is internal and precise and also left handed.