CNC / machinist calculator
Single-Point Threading Infeed Calculator
Cut a thread on the lathe one pass at a time without guessing the infeeds. Enter the pitch and how many passes you want and this calculator gives the total cutting depth for a 60-degree thread and an equal-area infeed schedule, so each pass removes a similar amount and the tool is not overloaded on the last cuts. It turns a thread callout into the depth numbers you dial in at the cross-slide.
How it works
A 60-degree external thread is cut to a total depth of about 0.6134 times the pitch. Plunging straight to that depth in one pass would overload the tool, so the cut is split into several passes of decreasing depth. The naive approach of equal depth per pass takes a bigger and bigger chip each time, because the thread is a V and the cut gets wider as it deepens.
The equal-area schedule fixes that by making each pass remove the same chip area. The cumulative depth after a given pass is the total depth times the square root of the pass number divided by the total passes, so the first passes are deeper and the last are light finishing cuts. This calculator lists the cumulative depth at each pass and the increment to add, ready to enter on the cross-slide or as Z infeeds in a threading cycle.
These are radial cut depths. If you feed at the common twenty-nine and a half degree compound angle, the compound dial reading is larger than the radial depth, so use these as the radial target and let the cycle or the compound geometry handle the infeed direction.
Worked example
A 20 TPI thread has a total cut depth of about 0.6134 x 0.05 = 0.0307 in. Over 8 equal-area passes the first cut is the deepest and the last are light finishing passes.
Frequently asked questions
How deep do I cut a 60-degree thread?
A 60-degree external thread is cut to about 0.6134 times the pitch. For a 0.05 inch pitch (20 TPI) that is roughly 0.031 inch total radial depth, reached over several passes rather than in one plunge.
Why use an equal-area infeed instead of equal depth?
Because a thread is a V groove, equal depth steps take a larger chip each pass as the cut widens, overloading the tool late. An equal-area schedule deepens less each pass so every pass removes a similar chip area.
How many threading passes should I use?
It depends on material and pitch, but coarse threads and tough materials want more, finer threads fewer. Many shops use six to twelve passes including spring passes; enter your count and the tool spreads the depth accordingly.
Is the depth radial or on the compound?
The depth here is the radial cut depth. If you infeed on a compound set near 29.5 degrees, the compound dial moves more than the radial depth, so treat these values as the radial target the thread must reach.
Does this work for internal threads?
The total depth and schedule are the same for an internal thread of the same pitch; only the cut direction reverses. Confirm the minor and pitch diameters with the thread dimension calculator so the bore is sized correctly first.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.