Calculate threading RPM from cutting speed (SFM or m/min) and work diameter, or solve for speed or diameter; add thread pitch to get feed and cutting time.

Single-start threads (lead = pitch). Feed and cutting time appear when you add pitch and length below.

Thread feed, cutting time and RPM limit

Blank means 1. Whole passes, each cutting the full entered length at the same feed.

Comparison only; this does not cap the answer or approve a machine setting.

Threading RPM Formula

The following formula is used to calculate the threading RPM (Revolutions Per Minute). Variables:

RPM = (CS ร— 12) / (ฯ€ ร— D)
  • RPM is the threading speed in revolutions per minute
  • CS is the cutting speed (surface speed) in feet per minute
  • D is the workpiece diameter in inches

To calculate the threading RPM, multiply the cutting speed by 12 to convert feet per minute to inches per minute, then divide by (ฯ€ ร— D). (If using metric units, the equivalent form is RPM = (CS ร— 1000) / (ฯ€ ร— D) with CS in m/min and D in mm.) These are circumference-speed conversions, as described in Mitsubishi Materialsโ€™ turning formulas; they do not select a suitable cutting speed.

What is a Threading RPM?

Here, threading RPM means spindle speed for single-point thread turning on a rotating workpiece. The user supplies cutting speed; the calculator converts it using diameter. Its feed and time plan assumes a single-start thread, where lead equals pitch. Sandvik Coromant distinguishes this from multi-start threads, whose lead is pitch multiplied by the number of starts. Multi-start planning and thread-milling toolpaths are outside this model.

How to Calculate Threading RPM?

Select RPM, cutting speed or diameter. For feed and cutting time, also enter the pitch, thread length and passes under the optional settings. Axial feed equals RPM ร— lead; for this single-start model, lead is entered millimeter pitch or 25.4/TPI millimeters. Ideal cutting time in seconds is 60 ร— thread length ร— pass count รท axial feed, using consistent units. Every entered pass is assumed to traverse the same length at constant feed. Time excludes approach, return, retract, dwell, acceleration, synchronization and tool changes; additional passes count only when included in your pass total.

  1. First, determine the cutting speed (CS) in feet per minute.
  2. Next, determine the workpiece diameter (D) in inches.
  3. Next, use the formula RPM = (CS ร— 12) / (ฯ€ ร— D) to calculate the Threading RPM.
  4. Finally, calculate the Threading RPM.
  5. The machine-limit result only compares the calculated or entered RPM with your entered limit; it does not cap RPM or approve an operating setup. The 75% RPM scenario is an arithmetic comparison, not a speed recommendation. Verify tooling, workholding, actual machine capability and suitable cutting data separately.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Cutting speed (CS) = 120 feet per minute

Workpiece diameter (D) = 1 inch. RPM = (120 ร— 12) / (ฯ€ ร— 1) โ‰ˆ 458.37 RPM.