LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
(b) When measuring a Unified National Fine thread, the same method and
formula is used except that the constant is 1.732 instead of 1.5155. Too
much pressure should not be applied when measuring over wires.
(5) The optical comparator, an instrument based on the magnification
principle, is used for obtaining a very accurate comparison between an
enlarged template and the threaded part.
(6) The Acme Screw thread was designed to overcome the difficulty of
cutting square threads with taps and dies.
The Acme thread form is
classified as a power-transmitting type of thread.
The Acme thread form,
when used on lead screws or similar parts, has a distinct advantage. This
is because the 29 included angle at which its sides are established (which
are under the load.
Further, because of the wide root and crest, this
thread form is strong and capable of carrying a heavy load. The formulas
used in determining the depth of Acme and worm threads was previously
presented in figure 41 on page 108.
The 29 worm and the Acme thread
cutting tools are both ground so that their side cutting edges are at a 29
However, these two threads should not be confused.
are different in the depth of the thread, width of tooth at the top, and
width of the tooth at the bottom. The Acme tool has a wider nose, while the
29 worm will cut a deeper thread than the Acme.
Cutting External Acme Threads.
(1) To machine an external Acme thread on the lathe, the first step would
be to grind a threading tool to conform to the 29 included angle of the
thread. The threading tool is first ground to a point, with the sides of
the tool forming the 29 included angle.
This angle can be checked by
placing the tool in the slot at the right end of the Acme thread gage
(figure 50 on the following page).
Be sure to grind this tool with
sufficient side clearance so that it will cut. Depending upon the number of
threads per inch to be cut, the point of the tool is ground flat to fit into
the slot in the Acme thread gage that is marked with the number