CNC Cycle Time Formulas
The mathematical foundation of quoting and scheduling. Precise formulas for Milling, Turning, and Drilling.
Don't Do This By Hand
These formulas are complex. Our Machining Time Calculator handles the math, including approach and over-travel distances.
Key Variables
Milling Formulas
Standard Side/Slot Milling
Where f = IPM (Inches Per Minute). Calculate IPM first using: RPM × IPT × Flutes.
Face Milling (with Approach/Overtravel)
Turning (Lathe) Formulas
Turning / Boring (Constant RPM)
- f_n: Feed per revolution (IPR), selected from tool/material recommendations.
- N: RPM.
Note: CSS (Constant Surface Speed) changes RPM as diameter changes, making this formula an approximation for facing cuts.
Drilling / Tapping
Drilling (Standard)
Peck Drilling (Deep Holes)
Deep-hole cycles require retract and chip-clear events. Model the multiplier from your actual peck depth, retract strategy, and machine acceleration behavior.
Threading & Tapping
Tapping (Rigid or Floating)
The × 2 accounts for both the cutting stroke (downward) and the reverse stroke (retract). For rigid tapping, the spindle reverses at full speed. For floating holders, retract is slightly slower.
Revolutions = 15 / 1.25 = 12 turns. Time = (12 / 500) × 2 = 0.048 min (2.9 sec).
Single-Point Threading (Lathe)
Single-point threading usually requires multiple passes based on thread pitch, depth, material, and tool condition. Include spring-pass logic according to your thread quality target.
Grooving & Parting Off
Grooving / Parting (Lathe)
- Radial Depth: Distance from OD to groove bottom (or center for parting).
- f_n: Feed per revolution selected from insert grade, geometry, and part rigidity.
Radial depth = 1" (to center). Time = 1 / (0.003 × 350) = 0.95 min (57 sec).
Industrial Case Study: Complete Part Estimation
Part: Aluminum 6061-T6 bracket, 6-operation machining sequence. Theoretical vs. actual time comparison.
| Operation | Calculated | Actual | Variance |
|---|---|---|---|
| 1. Face Mill (top surface) | 0.25 min | 0.30 min | +20% |
| 2. Rough Pocket (3 passes) | 2.10 min | 2.50 min | +19% |
| 3. Finish Pocket (1 pass) | 1.40 min | 1.55 min | +11% |
| 4. Drill 6× holes (#7 drill) | 0.15 min | 0.25 min | +67% |
| 5. Tap 6× M5 holes | 0.10 min | 0.20 min | +100% |
| 6. Chamfer edges | 0.30 min | 0.35 min | +17% |
| Subtotal (cutting only) | 4.30 min | 5.15 min | +20% |
| + Tool changes (5 × 6 sec) | — | 0.50 min | |
| + Rapid positioning | — | 0.40 min | |
| Total Cycle Time | 4.30 min | 6.05 min | +41% |
Key Takeaway:
In this scenario, actual cycle time was materially higher than cutting-only time because of non-cutting overhead. Use your own historical variance by operation type instead of fixed multipliers when quoting.
Frequently Asked Questions
Why is my actual cycle time 20-50% longer than calculated?
The T = L/F formula only calculates pure cutting engagement. Actual cycles include overhead from tool changes, accelerations, rapids, spindle state transitions, probing, and handling. These overheads dominate especially in short-cycle operations.
How do I estimate cycle time for 3D contouring / sculptured surfaces?
For complex 3D toolpaths, use CAM-generated time from the actual posted toolpath whenever possible. For pre-CAM quoting, use a structured proxy model and reconcile against historical jobs of similar geometry and tolerance.
What is a good cycle time buffer for quoting?
Use a buffer derived from your own historical ratio of actual cycle time to calculated cutting time, segmented by operation class and machine family. Avoid universal percentages and recalibrate the buffer periodically.