LATHE OPERATIONS - OD1645 - LESSON 1/TASK 3
(2) To determine the helix angle of the square thread (angle B, figure 51
on the previous page) draw the base line AC2 equal in length to the
circumference of the thread to be cut. Draw the line C2C perpendicular at
C2 and equal in length to the lead of the thread to he cut. Complete the
triangle by drawing line AC. Angle B (angle CAC2) in the triangle is the
helix angle of the thread.
(3) The tool bit should be ground at the same angle as angle B. Observe
the end view of the cutting tool figure 51. Note that the sides E and F of
the tool have been ground to give clearance, while the helix angle has been
The centerline of the tool face-is inclined as shown by
centerline KL, with clearance at each side as shown at I and J.
(4) For cutting the thread, the cutting edge of the tool should be ground
to a width exactly one-half that of the pitch (P). For cutting the nut, it
should be from 0.001 to 0.003 of an inch larger to permit a free fit of the
nut on the screw.
(5) The cutting of the square thread form presents some difficulty.
Although it is square, this thread, like any other progresses in the form of
an helix, and thus assumes a slight twist. Some operators prefer to produce
this thread in two cuts; the first with a narrow tool to the full depth, and
the second with a tool ground to size.
This procedure relieves cutting
pressure on the tool nose and may prevent springing the work. The cutting
operation for square threads differs from cutting threads previously
explained in that the compound is set parallel to the axis of the workpiece
and feeding is done only with the crossfeed.
The crossfeed is fed only
0.002 inch or 0.003 inch per cut.
The finished depth of the thread is
determined by the following formula:
The width of the tool point is determined by this formula also and will
depend upon the number of threads per inch to be machined. It is measured
with a micrometer, as square thread gages are not available.