# Cutting Speed Calculator Lathe

Cutting speed calculator lathe turning: enter workpiece diameter, material, tool material, and roughing or finishing cut to get spindle RPM, SFM, and feed per revolution.

## What this calculates

This cutting speed calculator lathe tool uses the workpiece diameter (not the tool diameter) because on a lathe the workpiece is what spins. Enter the OD you are currently turning, the material, the tool material (HSS or carbide insert), and whether you are roughing or finishing. The calculator returns spindle RPM from the standard SFM-to-RPM equation, plus a recommended feed per revolution in IPR.

## Inputs

- **Workpiece Diameter** (in) — min 0.001 — Outside diameter currently being turned. As you take material off, RPM should increase to hold SFM.
- **Workpiece Material** — options: Aluminum (500/1000 SFM), Mild steel / 1018 (100/400 SFM), Medium carbon / 1045 (80/300 SFM), Alloy steel / 4140 (70/250 SFM), Stainless 304/316 (60/200 SFM), Tool steel A2/D2 (50/180 SFM), Cast iron (80/280 SFM), Brass (250/500 SFM), Bronze (120/300 SFM), Copper (180/400 SFM), Titanium (35/120 SFM), Plastic (600/1500 SFM) — Material on the lathe.
- **Tool Material** — options: HSS (high-speed steel), Carbide insert — Most modern lathes use carbide inserts. HSS is still useful for small hobby lathes, form tools, and interrupted cuts.
- **Cut Type** — options: Roughing (heavy material removal), Finishing (surface quality priority) — Finishing bumps SFM up 10-20 percent but drops feed 40-60 percent for a cleaner surface.
- **Depth of Cut** (in) — min 0 — Radial depth per pass. Roughing = 0.050-0.150, finishing = 0.005-0.020.

## Outputs

- **Spindle Speed** (RPM) — Spindle RPM = (SFM x 12) / (pi x workpiece diameter).
- **Cutting Speed** (SFM) — Surface feet per minute for the chosen material and tool.
- **Feed per Revolution** (in/rev) — Recommended feed per revolution for the chosen cut type.
- **Feed Rate** (in/min) — Linear feed rate = RPM x IPR.
- **Metal Removal Rate** (in^3/min) — MRR = SFM x depth x feed x 12 (approx). Shows how fast material comes off.

## Details

Lathe turning equations

  - RPM = (SFM x 12) / (pi x D_work) where D_work is the workpiece diameter in inches, not the tool diameter.

  - Feed rate (IPM) = RPM x IPR where IPR is the feed per revolution.

  - MRR (in^3/min) = 12 x SFM x depth of cut x feed (Sandvik turning handbook).

For a 2 inch diameter bar of mild steel turned with a carbide insert at 400 SFM: RPM = (400 x 12) / (pi x 2) = 764 RPM. Roughing feed = 0.012 IPR, so feed rate = 764 x 0.012 = 9.17 IPM. At 0.050 depth of cut, MRR = 12 x 400 x 0.050 x 0.012 = 2.88 in^3/min.

Turning SFM reference

  
    MaterialHSS SFMCarbide SFM
  
  
    Aluminum5001000
    Mild steel (1018)100400
    Medium carbon (1045)80300
    Alloy steel (4140)70250
    Stainless (304, 316)60200
    Cast iron80280
    Titanium35120
    Brass250500
  

Workpiece diameter changes as you cut

This is the gotcha that bites every new lathe operator: the SFM formula uses the diameter right now, not the starting diameter. As you turn a 2 inch bar down to 1.5 inch, the cutting speed at 764 RPM drops from 400 SFM to 300 SFM. On a CNC lathe with G96 constant surface speed, the control increases RPM automatically. On a manual lathe, you step the RPM up as the diameter decreases.

Roughing vs finishing

Roughing prioritizes material removal rate: deeper depth of cut (0.050-0.150), heavier feed (0.010-0.020 IPR), normal SFM. Finishing prioritizes surface quality: shallow depth (0.005-0.020), lighter feed (0.003-0.006 IPR), 10-20 percent higher SFM to keep the chip moving. Switching between them in this calculator changes both feed and SFM.

## Frequently Asked Questions

**Q: How do I calculate cutting speed on a lathe?**

A: Use RPM = (SFM x 12) / (pi x D), where D is the workpiece diameter in inches and SFM is the cutting speed for your material. For 1018 mild steel with carbide, SFM is around 400. A 2 inch diameter bar: RPM = (400 x 12) / (pi x 2) = 764 RPM. This cutting speed calculator lathe handles all the common materials.

**Q: Why does lathe RPM use workpiece diameter, not tool diameter?**

A: On a lathe, the workpiece spins and the tool is stationary. Cutting speed is the speed at the surface of the spinning workpiece, which depends on workpiece diameter. On a mill, the tool spins and the work is stationary, so you use the tool diameter. Same formula, different D.

**Q: What is G96 constant surface speed?**

A: G96 is a CNC lathe code that tells the control to hold SFM constant by increasing RPM as the workpiece diameter decreases during a face or profile cut. Programmed as G96 S400 for 400 SFM. Pair with G97 to switch to constant RPM for threading or drilling. A cutting speed calculator lathe gives you the starting RPM; G96 takes it from there.

**Q: What is a good lathe feed rate for finishing?**

A: Finishing feed is 0.003-0.006 IPR for steel and 0.005-0.008 IPR for aluminum. Combined with 10-20 percent higher SFM than roughing, it gives a mirror or near-mirror finish on the right material. For a 2 inch steel bar at 880 RPM (15 percent above 764) and 0.005 IPR, feed = 4.4 IPM.

**Q: How do I calculate lathe MRR?**

A: MRR in cubic inches per minute = 12 x SFM x depth of cut x feed per revolution. Example: 400 SFM, 0.050 depth, 0.012 IPR = 12 x 400 x 0.050 x 0.012 = 2.88 in^3/min. This is the number production lathes optimize for.

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Source: https://vastcalc.com/calculators/physics/cutting-speed-lathe
Category: Physics
Last updated: 2026-04-08