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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

maintained.

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 thenut 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:DEPTH-1/2 P

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.

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