Guides · 23 October 2026

How to estimate CNC turning cycle time

For a quote you do not need a CAM simulation, but you do need a cycle time you can defend. For most turned parts, four numbers get you close: the cutting speed, the feed, the depth of cut and the lathe's top speed. Here is the method, with a worked example.

Spindle speed comes from cutting speed, until the lathe runs out of rpm

Insert makers give a cutting speed, Vc, in metres per minute. The spindle speed that gives it on a diameter D (mm) is:

n = 1000 × Vc ÷ (π × D)

On small diameters that number gets large quickly. Ø10 mm at 200 m/min needs 6,366 rpm; on a lathe that tops out at 3,500 rpm you only reach about 110 m/min, and the cut takes nearly twice as long as the formula suggests. Always cap n at your machine's maximum.

Cutting time for one pass

The tool moves at feed × rpm, so one pass along a length L takes:

t = (L + approach) ÷ (feed × n)

Add a couple of millimetres of approach and overrun to the length of cut. Feed is per revolution (mm/rev), straight from the insert's recommended range.

Roughing passes, then one finishing pass

Take the stock to remove per side, leave a finishing allowance (0.2 to 0.5 mm is common), and divide the rest by your depth of cut, rounding up. Each roughing pass runs at its own diameter, so the rpm rises a little as the part gets smaller. The finishing pass usually runs at a higher speed and a lower feed.

Facing an end is a special case: with constant surface speed the rpm climbs as the tool moves toward the centre, until it reaches the machine's limit, so the last few millimetres run at maximum rpm.

A worked example

Ø40 mm bar turned to Ø32 mm over 120 mm, steel, on a lathe with a 3,500 rpm limit. Roughing at 200 m/min and 0.25 mm/rev with up to 2 mm a side, 0.3 mm left for a finishing pass at 250 m/min and 0.10 mm/rev, 2 mm approach, one end faced.

Notice that the finishing pass takes longer than either roughing pass: its feed is less than half.

The time that is not cutting

Loading the bar or the blank, closing the door, measuring, rapids and tool changes all add up. In this example, half a minute for loading and measuring and 0.2 minutes for rapids and tool changes bring the cycle to 1.81 minutes, which is 63% more than the cutting time alone. On short parts the non-cutting time is often the larger share. Time it on a similar job you have run.

From cycle time to batch time

Add the setup once per batch. With a 45-minute setup and 50 parts:

45 + 50 × 1.81 = 135.7 minutes, about 2.3 hours

That is the machine time to price, before labour, overhead and margin. At 10 parts the setup is more than two thirds of it, which is why small batches cost so much more per part.

Common mistakes

• Ignoring the rpm limit on small diameters, which makes the estimate far too short.
• Using the top of the insert's feed range. Use what you actually run on that material, not the catalogue maximum.
• Leaving out non-cutting time. It is real machine time, and you pay for it.
• Never checking. Log the actual cycle on the first batch and compare. A few comparisons tell you whether your estimates run fast or slow.