---
type: process
name: "CNC Turning"
category: "Cutting"
subcategory: "Mechanical"
materials: ["Metal", "Plastic"]
tolerances: "±0.005 in (±0.13 mm) standard; ±0.001 in (±0.025 mm) on diameters; ±0.0005 in (±0.013 mm) achievable; concentricity within 0.0005 in (0.013 mm) TIR in a single chucking"
volumes: "1–10,000 parts; bar-fed production runs well beyond 100,000"
lead_time: "3–10 business days for prototypes; 2–4 weeks for production quantities"
url: https://manufacturingprocesses.org/processes/cutting/cnc-turning
---

# CNC Turning

CNC turning rotates the workpiece against a stationary tool, producing round parts with excellent concentricity and short cycle times.

- **Category**: [Cutting](https://manufacturingprocesses.org/processes/cutting.md)
- **Family**: Mechanical
- **Materials**: Metal, Plastic
- **Typical tolerances**: ±0.005 in (±0.13 mm) standard; ±0.001 in (±0.025 mm) on diameters; ±0.0005 in (±0.013 mm) achievable; concentricity within 0.0005 in (0.013 mm) TIR in a single chucking
- **Surface finish**: 125 µin Ra (3.2 µm) as-turned; 32 µin (0.8 µm) with a finish pass; 16 µin (0.4 µm) achievable
- **Typical volumes**: 1–10,000 parts; bar-fed production runs well beyond 100,000
- **Lead time**: 3–10 business days for prototypes; 2–4 weeks for production quantities

## Overview

CNC turning spins the workpiece in a spindle while a single-point tool traverses along and across it, generating everything round: shafts, pins, bushings, spacers, hydraulic fittings, connector bodies and threaded studs. Bar-fed lathes take stock roughly 0.25–3 in (6–75 mm) in diameter and cut a complete part in seconds to a few minutes; chuck-fed machines take castings, forgings and slugs a foot or more across.

Diameters hold **±0.001 in (±0.025 mm)** as normal work, and features cut in the same chucking are concentric within about **0.0005 in (0.013 mm) TIR** — the strongest single reason to turn a part rather than mill it round.

Adding live tooling, a Y axis and a sub-spindle makes the lathe a mill-turn center that finishes cross-holes, flats and back-end features without a second machine, eliminating both the setup cost and the tolerance stack a transfer would introduce.

## How it works

1. **Stock feeding.** A bar feeder pushes stock through the spindle for parts within bar capacity; larger or non-round blanks are held in a chuck or collet. Bar work is what makes turning cheap per part — the machine reloads itself.
2. **Facing and rough turning.** The end is faced and the OD reduced in passes of roughly 0.050–0.150 in (1.3–3.8 mm) depth of cut at feeds around 0.008–0.015 in/rev in steel. Constant surface speed control keeps SFM steady as the diameter shrinks.
3. **Finish turning.** Depth of cut drops to 0.005–0.020 in (0.13–0.50 mm) and feed to 0.002–0.006 in/rev. Finish is governed mostly by feed and tool nose radius — theoretical roughness approximates **Ra ≈ f²/(32r)** — so halving the feed cuts roughness roughly fourfold.
4. **Centerline hole making.** Center drill, then drill, then ream or bore. Boring is what holds an ID to a tight tolerance and true roundness; a drill alone will not.
5. **Grooving, threading and knurling.** Retaining-ring grooves and thread reliefs are plunged with form tools; threads are single-pointed over multiple passes or cut with a die head.
6. **Live-tool operations.** Cross-holes, flats and keyways are cut with driven tools while the main spindle indexes to an angular position, avoiding a separate milling setup.
7. **Part-off and back working.** A parting blade roughly 0.080–0.125 in (2–3 mm) wide separates the part. On a sub-spindle machine it is caught, its back end faced, chamfered and drilled, and the finished piece drops out complete.

## Design guidelines

### Length-to-diameter ratio
Held in the chuck alone, keep unsupported length under **3× diameter**; with a tailstock center, 8:1 is comfortable. Beyond that, deflection and chatter take over and the part needs a steady rest — or a [Swiss machine](/processes/cutting/swiss-screw-machining), which supports the bar right at the cut and handles 20:1 routinely.

### Wall thickness on turned tubes
**0.020 in (0.5 mm)** is the metal minimum, but below about 0.040 in (1 mm) the chuck jaws themselves distort the part while it is cut. Soft jaws, an expanding mandrel or reduced clamping pressure are what make thin-wall work possible, and roundness rather than diameter becomes the limiting characteristic.

### Thread reliefs and shoulder undercuts
Give every external thread a runout groove at least **1.5× the thread pitch** wide, and every shoulder that must seat flat an undercut. Without one, the incomplete thread and the tool-nose fillet hold the mating part off its face.

### Corner radii and chamfers
Every shoulder carries the insert's nose radius, typically **0.008–0.031 in (0.2–0.8 mm)** — draw it rather than implying a sharp corner. Put a 0.010–0.030 in × 45° chamfer on every entering edge; it costs nothing on the lathe and removes a deburring operation.

### Holes, threads and fits
Centerline holes should use stocked drill sizes ([drill size chart](/charts/drill-size-chart)) and tapped bores standard tap drills ([tap drill chart](/charts/tap-drill-chart)). Use standard thread sizes from the [thread size chart](/charts/thread-size-chart), since a non-standard pitch needs a custom insert. For press, slip and running fits, call out an [ISO 286 fit class](/charts/iso-286-fits-tolerances) rather than inventing a bilateral tolerance.

### Tolerance and finish
±0.005 in (±0.13 mm) is the default, ±0.001 in (±0.025 mm) on diameters is routine, and ±0.0005 in (±0.013 mm) needs a dedicated finish pass and stable temperature. Finish runs 125 µin Ra (3.2 µm) as-turned and 32 µin (0.8 µm) after a finish pass; below 8 µin (0.2 µm), grind or roller-burnish ([surface finish chart](/charts/surface-finish-chart)).

| Material | Turnability | Watch for |
|---|---|---|
| Brass C360 | Excellent | The free-cutting benchmark: best finish, longest tool life |
| 12L14 / 1215 steel | Excellent | Chips break cleanly; ideal for bar work |
| 6061 / 2011 aluminum | Excellent | 2011 chips better than 6061 in high-volume bar work |
| 303 stainless | Good | Use over 304 wherever corrosion requirements allow |
| 304 / 316 stainless | Moderate | Work hardens; long stringy chips |
| 4140 / 4340 alloy steel | Good | Turn before hardening; hard turning needs CBN |
| Ti-6Al-4V | Difficult | Low speeds, flood coolant, fire risk from fine chips |
| Acetal, PEEK, PTFE | Excellent | Sharp positive tooling; PTFE creeps after cutting |
| Nylon | Moderate | Absorbs moisture, so diameters drift after machining |

| Feature | Recommended | Limit | Why |
|---|---|---|---|
| L:D, chuck only | ≤ 3:1 | ~4:1 | Deflection and chatter |
| L:D, with tailstock | ≤ 8:1 | ~10:1 | Center supports the free end |
| Wall thickness | 0.040 in (1 mm) | 0.020 in (0.5 mm) | Clamping force distorts thin tubes |
| Shoulder radius | 0.015 in (0.4 mm) | 0.008 in (0.2 mm) | Sharper inserts chip and wear fast |
| Thread relief width | ≥ 1.5× pitch | 1× pitch | Tool must run out cleanly |
| Bore depth | ≤ 4× bore diameter | ~6× with carbide bar | Bar deflection scales with length cubed |
| Entering chamfer | 0.020 in × 45° | — | Removes burrs, eases assembly |

## Cost drivers

Turning is priced on cycle time, and cycle time on a bar-fed machine is short — which is why turned parts are usually the cheapest machined parts you can buy.

**Bar capacity is the first cost cliff.** A part that fits through the spindle runs from bar with automatic reload. One diameter larger and it becomes chuck work: manual loading, one part per cycle, several times the cost per piece.

**Tools and operations.** Each groove, thread, cross-hole and back-side feature is another turret position and more seconds per part. A design that fits inside a standard turret load runs unattended.

**Secondary operations.** A cross-hole that could have been cut with live tooling but instead goes to a mill adds a setup, a fixture and a tolerance stack.

**Tolerance and material.** Diameters at ±0.001 in (±0.025 mm) come free with a finish pass; below ±0.0005 in (±0.013 mm) you pay for thermal stability and in-process gaging. Free-machining bar costs more per pound and repays it in cycle time and tool life.

Four ways to take cost out:

1. Keep the largest diameter inside standard bar capacity so the job runs bar-fed.
2. Consolidate features onto one end so the part finishes without a second chucking.
3. Choose 303 over 304 and 12L14 over 1018 wherever corrosion and strength allow.
4. Call out [ISO 286 fits](/charts/iso-286-fits-tolerances) only on diameters that mate, and leave the rest at ISO 2768-m.

## FAQ

### What tolerance can CNC turning hold?

±0.005 in (±0.13 mm) as a default, ±0.001 in (±0.025 mm) on diameters as normal precision work, and ±0.0005 in (±0.013 mm) with a dedicated finish pass and stable temperature. Features cut in the same chucking hold concentricity within about 0.0005 in (0.013 mm) TIR.

### How long can a turned part be relative to its diameter?

About 3× diameter held in the chuck alone and 8× with a tailstock center. Past that the part deflects away from the tool and chatters. Swiss screw machining supports the bar at the cutting point with a guide bushing and routinely runs 20:1 and beyond in the same material.

### What surface finish does CNC turning produce?

125 µin Ra (3.2 µm) as-turned, 32 µin (0.8 µm) after a finish pass, and 16 µin (0.4 µm) achievable. Finish comes mostly from feed rate and nose radius, approximated by Ra ≈ f²/(32r), so a lighter feed or a larger nose radius improves it directly. Below 8 µin (0.2 µm), grinding or roller burnishing is the right process.

### What is the minimum wall thickness for a turned tube?

0.020 in (0.5 mm) is the practical floor in metal, but below about 0.040 in (1 mm) the chuck jaws distort the part while it is being cut, so roundness rather than diameter becomes the limiting characteristic. Soft jaws, an expanding mandrel or reduced clamping pressure are what make thin-wall turning work.

### Do I need a thread relief on a turned thread?

Yes for any thread that runs into a shoulder. A relief groove at least 1.5× the thread pitch wide lets the threading tool run out cleanly and lets the mating part seat against the shoulder. Without one, the incomplete thread and the tool-nose fillet hold the assembly off its face.

### Can CNC turning cut cross-holes and flats?

Yes, on a machine with live tooling. Driven tools cut cross-holes, flats and keyways while the main spindle indexes to an angular position, so the part comes off complete. Without live tooling those features become a second setup on a mill, adding cost and a positional tolerance stack.

## Alternative processes

- [Swaging](https://manufacturingprocesses.org/processes/forming/swaging.md): Swaging hammers or presses the end of a tube or bar inward against a die to reduce or taper its diameter without removing any material.
- [Metal Injection Molding](https://manufacturingprocesses.org/processes/forming/metal-injection-molding.md): Metal injection molding molds a powder-and-binder feedstock exactly like a plastic, then debinds and sinters it into a dense small metal part.
- [Powder Metallurgy (Press and Sinter)](https://manufacturingprocesses.org/processes/forming/powder-metallurgy-press-and-sinter.md): Press-and-sinter powder metallurgy compacts metal powder in a rigid die and sinters it below its melting point into a finished net-shape part.
- [Metal Extrusion](https://manufacturingprocesses.org/processes/forming/metal-extrusion.md): Metal extrusion pushes heated billet through a shaped die, producing continuous aluminum or copper profile of constant cross-section.
- [Investment Casting](https://manufacturingprocesses.org/processes/forming/investment-casting.md): Investment casting builds a ceramic shell around a wax pattern, melts the wax out and pours metal into the cavity, giving fine detail in almost any alloy.

## Related processes

- [CNC Milling](https://manufacturingprocesses.org/processes/cutting/cnc-milling.md): CNC milling removes material with a rotating multi-flute cutter moved along programmed paths, producing prismatic parts from solid stock.
- [Swiss Screw Machining](https://manufacturingprocesses.org/processes/cutting/swiss-screw-machining.md): Swiss screw machining feeds bar stock through a guide bushing so cutting happens right next to the support, holding tight tolerances on slender parts.
- [5-Axis CNC Machining](https://manufacturingprocesses.org/processes/cutting/5-axis-cnc-machining.md): 5-axis CNC machining tilts and rotates the part or the head so the tool reaches every face and stays short and rigid on complex surfaces.
- [CNC Machining](https://manufacturingprocesses.org/processes/cutting/cnc-machining.md): CNC machining removes material from solid stock with a programmed cutting tool, producing accurate parts directly from a CAD model with no tooling.
- [Sawing](https://manufacturingprocesses.org/processes/cutting/sawing.md): Sawing separates stock with a toothed blade — band, circular or cold saw — and is the default first operation on bar, tube and plate.

---

*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/cutting/cnc-turning)*

*Last updated: August 11, 2026*