A 1/4-20 thread mill in a 20 mm deep hole passes the go gauge at the top and fails at the bottom. The tool is a single-profile mill, one pass, conventional climb. The hole was drilled to the minor the chart asked for.
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A thread that gets loose with depth on a single-profile mill is usually deflection or a tool that is not standing straight. Take the thread in two radial passes so the last pass is light, and check the holder for runout.
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Climb versus conventional changes the way the tool loads. If the bottom is the loose end, a spring pass at the same radius often brings the gauge back without a smaller drill.
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Confirm the minor is not tapered from the drill walking. A thread mill will follow a bad hole and the gauge will blame the thread.
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Questions and answers were written on this account when they were published.
