Machining Time — Turning Pass

Length divided by how fast the tool advances, and the tool advances by one feed per revolution. That is all this is, and it is the building block of every cycle time estimate, every quotation, and every argument about whether a job should be on this machine or that one.

The important thing about it is what it leaves out. This is cutting time only, and on most real parts it is the smaller half of the cycle. Loading and unloading, indexing the turret, rapid moves between features, gauging, deburring, and the tool change when an insert reaches the end of the life Taylor's equation predicted all sit outside the equation. A quotation built on cutting time alone is short by a factor that grows as the batch shrinks — on a one-off it can be short by five.

Include the approach and overrun in $L$. The tool has to start clear of the workpiece and finish past it, and on a 30 mm part those few millimetres at each end are a large fraction of the pass. Add the number of passes, too: a 6 mm depth of stock removed 2 mm at a time is three of these, plus a finishing pass, plus the retract and reposition between each.

Where a CNC lathe is running constant surface speed, this equation stops being exact, because the spindle speed changes continuously through a facing or contouring move and so does the feed rate in millimetres per minute. The control integrates it properly; the answer here is an estimate taken at one diameter, and for a facing cut the honest thing is to take the mean diameter rather than either end.

Run backwards, the equation tempts you into a real trap. Given a cycle-time target it will hand back the feed or the rpm that meets it, and both bills come due elsewhere: the feed is what decides the surface finish and what the chipbreaker was designed around, and the rpm is what Taylor's exponent charges against the tool. Buying cycle time with spindle speed is the most expensive way to buy it. Buying it with depth of cut — fewer, heavier passes — is nearly free, and is almost always the answer when a job is running long.