Single-Point Thread Cutting Depth
The cut depth for single-point threading a 60-degree external (Unified/UN) thread on a lathe. Pitch = 1/TPI; the full-form external thread height is 0.6134 x pitch (the UN external truncation). A 1/2-13 has a 0.0769 in pitch and a 0.0472 in radial depth; a 3/4-10 is 0.0613 in deep. Because most machinists feed with the COMPOUND set to 29.5 deg (just under the 30-deg half-angle, so the tool cuts mainly on the leading flank), the compound travels farther than the radial depth: compound infeed = depth / cos(29.5), so the 1/2-13 needs 0.0542 in of compound travel. Feed in over several passes with lighter finishing cuts, and verify with a thread gauge or the three-wire method rather than the depth alone. A setup aid; the thread standard (UN/UNJ/metric), the tool geometry, and a thread gauge govern the finished part.
Formula and source
pitch_in = 1 / tpi; single_depth_in = 0.6134 x pitch_in (60-degree UN external thread height); compound_infeed_in = single_depth_in / cos(29.5 degrees).
Single-point 60-degree (Unified/UN) external thread cutting depth and 29.5-degree compound infeed (Machinery's Handbook threading practice), by name; the thread standard, the tool geometry, and a thread gauge govern the finished part.
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