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Lathe Operations - Left-Hand Thread Cutting
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A left-hand thread is a thread that when viewed axially (from the end) winds in a counterclockwise direction (Figure 15-3-1).

Figure 15-3-1

Left-hand threads are designated with an LH at the end of the thread designation, such as 3/4-10 UNC LH. Left-hand threads are used in applications where the nut may loosen because of the rotation of the motor or spindle. The procedure for cutting left-hand threads is the same as that used for cutting right-hand threads with two notable differences: The compound must be set at 29 degrees to the left of the operator (Figure 15-3-2), and the leadscrew direction is reversed so that the threading pass is made from the left to the right (Figure 15-3-3).

isn15-3-3.jpg (11757 bytes)
Figure 15-3-3

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Figure 15-3-2

Left-hand threads typically require a recess at the thread start location. Keep in mind when grinding the left hand threading tool that the side cutting edge angle on the left-hand thread cutting tool is opposite that of the right-hand thread cutting tool (Figure 15-3-4).

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Figure 15-3-4