# Speed and Feed Calculator for Tapping

Speed and feed calculator for tapping. Get RPM, feed rate in IPM or mm/min, and thread lead for UNC, UNF, and metric taps in aluminum, steel, stainless, and more.

## What this calculates

Tapping is the one CNC operation where feed rate is not a free parameter. The feed must equal RPM times thread lead so the tap advances exactly one thread per revolution, or the tap breaks, snaps, or strips the thread. This calculator returns the matched RPM, SFM, and IPM feed rate for common UNC, UNF, and metric tap sizes in every shop metal, using separate SFM tables for HSS cutting taps, solid carbide taps, and form (roll) taps.

## Inputs

- **Thread Size** — options: 1/4-20 UNC, 1/4-28 UNF, 5/16-18 UNC, 5/16-24 UNF, 3/8-16 UNC, 3/8-24 UNF, 1/2-13 UNC, 1/2-20 UNF, 5/8-11 UNC, 3/4-10 UNC, M3 x 0.5, M4 x 0.7, M5 x 0.8, M6 x 1.0, M8 x 1.25, M10 x 1.5, M12 x 1.75, M16 x 2.0 — Pick the tap size. Pitch is looked up from the ANSI / ISO tables.
- **Workpiece Material** — options: Aluminum (80/250 SFM), Mild steel 1018 (35/90 SFM), Alloy steel 4140 (20/60 SFM), Stainless 304 (15/50 SFM), Cast iron (35/80 SFM, no form tap), Brass (70/150 SFM), Copper (50/120 SFM), Titanium (10/30 SFM)
- **Tap Type** — options: HSS cutting tap, Solid carbide cutting tap, Form (roll) tap (no chips) — Cutting taps shear chips; form taps cold-form threads by displacing material.

## Outputs

- **Spindle Speed** (RPM) — Spindle RPM for the selected thread size and material.
- **Tapping Speed** (SFM) — Surface feet per minute at the tap pitch diameter.
- **Feed Rate** (IPM) — Feed rate = RPM x thread lead. One revolution advances one thread.
- **Feed Rate** (mm/min) — Feed rate in millimeters per minute.
- **Thread Lead (IPR)** (in/rev) — Axial distance per revolution = 1 / TPI (imperial) or pitch / 25.4 (metric).

## Details

The tapping feed equation

A tap threads the workpiece by cutting one thread pitch per revolution. So feed rate is locked to RPM through the thread lead:

Feed rate (IPM) = RPM x (1 / TPI) for imperial, or Feed rate (mm/min) = RPM x pitch for metric. TPI is threads per inch; pitch is the axial distance between threads in millimeters.

For a 1/4-20 tap at 600 RPM: feed = 600 x (1/20) = 30 IPM. For an M6 x 1.0 tap at 500 RPM: feed = 500 x 1.0 = 500 mm/min.

Tapping SFM by material and tap type

  
    MaterialHSS cutting tapCarbide cutting tapForm (roll) tap
  
  
    Aluminum 606180 SFM250 SFM150 SFM
    Mild steel 101835 SFM90 SFM65 SFM
    Alloy steel 414020 SFM60 SFM40 SFM
    Stainless 30415 SFM50 SFM30 SFM
    Cast iron35 SFM80 SFMN/A
    Brass70 SFM150 SFM100 SFM
    Titanium10 SFM30 SFM20 SFM
  

Form taps cannot be used in cast iron or brittle materials because the metal must plastically deform without cracking. Aluminum, mild steel, stainless, and copper all form-tap cleanly.

RPM from SFM for taps

The same cutting speed equation applies: RPM = (SFM x 12) / (pi x D) with D in inches. For a 3/8 inch HSS cutting tap in mild steel at 35 SFM: RPM = (35 x 12) / (pi x 0.375) = 356 RPM. Feed at 1/16 thread lead (16 TPI) = 356 x 0.0625 = 22.3 IPM. A rigid-tapping spindle cuts this cleanly; a floating tap holder needs a bit less rigidity tolerance in the feed-to-RPM match.

Rigid tapping vs tension-compression

Modern CNC mills with encoder-synchronized spindles (Haas, Mazak, Okuma, Tormach PCNC with Path Pilot) tap rigidly: the feed follows RPM exactly through a G84 cycle. Older machines use a tension-compression tap holder that allows a small mismatch between programmed feed and actual RPM. Either way, the calculator output is the programmed value. Reduce RPM 20-30 percent for blind holes under 1/4 inch diameter where chip evacuation is marginal.

## Frequently Asked Questions

**Q: How do I calculate feed rate for tapping?**

A: Feed rate (IPM) = RPM x thread lead, where thread lead = 1 / TPI for imperial or pitch in mm for metric (divide by 25.4 to get inches). For a 1/4-20 tap at 600 RPM: 600 x 0.05 = 30 IPM. For an M6 x 1.0 at 500 RPM: 500 x 1 mm = 500 mm/min. This calculator does the match automatically.

**Q: What RPM should I run a 1/4-20 tap in mild steel?**

A: HSS cutting tap in 1018 mild steel runs 35 SFM. For a 1/4 inch tap: RPM = (35 x 12) / (pi x 0.25) = 535 RPM. Feed at 1/20 thread lead = 535 x 0.05 = 26.7 IPM. Use lots of cutting oil (tapping fluid) and rigid-tap mode on a CNC mill. Back off 25 percent for blind holes with poor chip evacuation.

**Q: What is the difference between a cutting tap and a form tap?**

A: A cutting tap shears chips out of the thread. A form tap (roll tap, fluteless tap) cold-forms the thread by displacing material inward. Form taps are stronger and make no chips, but they need a ductile material (aluminum, steel, stainless) and a larger tap drill (typically 70 percent engagement instead of 75). Cast iron and hardened steel cannot be form-tapped.

**Q: Why does titanium tap so slow?**

A: Titanium has extremely poor thermal conductivity and chemically welds to the cutting edge at high temperatures. HSS taps in Ti-6Al-4V run only 10 SFM; any faster and the tap picks up titanium and breaks. Carbide taps run 30 SFM. Use a continuous flood of oil-based tapping fluid, pulse the cycle with peck tapping for holes deeper than 2 x diameter.

**Q: Can I use this calculator for metric taps?**

A: Yes. The select includes M3, M4, M5, M6, M8, M10, M12, and M16 at standard ISO pitches. The feed rate comes out in both IPM and mm/min. For an M8 x 1.25 tap in aluminum at 600 RPM: feed = 600 x 1.25 = 750 mm/min, or 29.5 IPM. The calculation is just as clean in metric because the thread lead is a direct pitch value.

**Q: How do I tap stainless steel without breaking the tap?**

A: Stainless work-hardens if the tap dwells or rubs. Run cutting taps at 15 SFM HSS or 50 SFM carbide, use a sulfurized tapping fluid, and never peck-break chips inside the hole. For blind holes, use a spiral-point tap that pushes chips ahead of the cutter. Feed rate must match RPM through the thread lead; any mismatch tears the thread or snaps the tap.

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Source: https://vastcalc.com/calculators/physics/speed-and-feed-tapping
Category: Physics
Last updated: 2026-04-08