CNC / machinist calculator
Tapping Feed Rate Calculator (RPM and Pitch)
In tapping the feed rate is not a free choice the way it is in drilling or milling. The tap is a screw, so it must advance exactly one thread pitch for every revolution of the spindle. That single fact fixes the feed: once you know the spindle speed and the thread pitch, the feed rate is determined. This calculator turns your RPM and thread, given as threads per inch for an inch tap or a pitch in millimetres for a metric tap, into the exact feed to program, in both inch and metric units so it matches whatever the control expects.
How it works
A tap cuts a helix, and the lead of that helix is the pitch: the axial distance the tap travels in one full turn. Because the tap is constrained to follow the threads it is cutting, the axial feed per revolution equals the pitch exactly. Multiply that by the spindle speed and you have the feed rate. For an inch tap the pitch is one divided by the threads per inch, so the feed in inches per minute is simply the RPM divided by the TPI. For a metric tap the pitch is already in millimetres, so the feed in millimetres per minute is the RPM times the pitch.
This is why rigid, or synchronous, tapping matters. The control must hold the exact ratio between spindle rotation and Z axis feed for the whole cycle, including the reversal at the bottom of the hole. If the feed and the spindle drift apart by even a little, the tap is either stretched or compressed against the thread it has already cut, and a small tap snaps. On a machine without rigid tapping a tension and compression tap holder absorbs that mismatch mechanically instead.
Tapping also runs much slower than drilling the same hole. The surface speed table below gives conservative starting speeds by broad material group; pick the RPM from those, then let this calculator lock in the matching feed.
Worked example
A 1/4-20 tap at 500 RPM: feed = 500 / 20 = 25.00 in/min. The feed is fixed by the pitch, so the only way to slow the feed is to slow the spindle. A coarser 13 TPI thread at the same RPM feeds faster because the tap advances further per turn.
Starting tapping surface speed
Tapping runs far slower than drilling the same hole. These are conservative starting surface speeds by broad material group; convert to RPM with the surface speed calculator for your tap diameter, then let the feed above lock to the pitch. Form (roll) taps tolerate somewhat higher speeds than cut taps.
| Material group | Starting speed (SFM) |
|---|---|
| Aluminum | 70 |
| Mild / low-carbon steel | 35 |
| Cast iron | 40 |
| Stainless steel | 18 |
Frequently asked questions
Why can I not choose the tapping feed freely?
The tap is a screw and cuts a fixed pitch, so it advances exactly one pitch per revolution. The feed is therefore locked to the pitch and the RPM and cannot be set independently like a milling feed.
How do I calculate tapping feed from TPI?
Divide the spindle speed by the threads per inch. A 20 TPI tap at 500 RPM feeds 500 divided by 20, which is 25 inches per minute. Coarser threads feed faster at the same RPM.
What is the metric tapping feed formula?
Multiply the spindle RPM by the pitch in millimetres. An M6 tap with a 1.0 mm pitch at 400 RPM feeds 400 times 1.0, which is 400 millimetres per minute. A finer pitch feeds slower.
What is rigid tapping and why does it need the exact feed?
Rigid tapping synchronises the spindle and the Z axis so the feed matches the pitch continuously, including at the reversal. If they drift apart the tap is stressed against its own thread and can break, so the ratio must be exact.
Why does tapping run slower than drilling?
A tap has many cutting edges engaged at once and reverses at the bottom of each hole, so heat and torque build quickly. Conservative surface speeds, well below drilling speeds, keep the tap alive, especially in stainless.
Related calculators
Sources
- Tap and die - threading and tapping basics (Wikipedia)
- Threading (manufacturing) - rigid tapping (Wikipedia)
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.