LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
2 Class 2 Fit.
This fit represents a high quality of commercial
thread product, and is interchangeable screw thread work.
3 Class 3 Fit. This fit represents an exceptional high quality of
commercially threaded product and is recommended only in cases where the
high cost of precision tools and continual checking are warranted.
4 Class 4 Fit.
This fit is intended to meet very unusual
requirements more exacting than those for which class 3 is intended. It is
a selective fit if the initial assembly by hand is required. It is not, as
yet, adaptable to quantity production.
(2) Thread Designations.
In general, screw thread designations give the
screw number (or diameter) first, then the initial letter of the series, NC
(National coarse), UNF (Unified fine), NS (National special), etc., followed
by the class of fit. If a thread is a left-hand, the letters LH follow the
fit. An example of designations is as follows:
Major diameter of thread
.
Number of threads per inch
.
.
Thread series symbols
.
.
.
Class of thread .
.
.
.
.
.----External thread
.
.
.
.
.
Left-hand
.
.
.
.
.
.
3/4 10 UNC 2
A
LH
Two samples and explanations of thread designations are as follows:
(a) No. 12 (0.216) --24 NC-3. This is a number 12 (0.216 inch diameter)
thread, 24 National coarse threads per inch, and a Class 3 fit.
(b) 1/4-28UNF-2A LH.
This is a 1/4 inch diameter thread, 28 Unified
fine threads per inch, left hand thread, and a Class 2A fit.
e.
Thread Cutter Bits and Cutters.
(1) The basic forms of thread cutting bits and cutters were given in the
first part of the lesson.
110
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