The Basic Machining Time Formula
When operators and shop owners ask, "how to estimate CNC machining time?", the fundamental answer always stems from a simple core equation:
Time (T) = Length of Cut (L) ÷ Feed Rate (F)
- Length of Cut (L): Total distance the tool travels while engaged in material.
- Feed Rate (F): The speed at which the tool moves through the material (e.g., Inches Per Minute or mm/min).
How to Calculate Machining Time for Milling
Milling calculations require knowing your Feed Rate. Feed rate isn't guessed; it's derived from spindle speed (RPM), the number of cutting flutes on your end mill, and the desired chip load per tooth.
Feed Rate (IPM) = RPM × Flutes × Chip Load
Example: You are milling a 20-inch slot with a 4-flute end mill. Your RPM is 4,000 and your chip load is 0.002".
Feed Rate = 4,000 × 4 × 0.002 = 32 IPM.
Machining Time = 20" ÷ 32 IPM = 0.625 minutes (37.5 seconds).
How to Estimate Machining Time for Turning
For CNC lathes, "Feed Rate" is typically programmed in Feed per Revolution (IPR or mm/rev). The mathematical premise is identical, just translated to rotational physics.
Time = Length ÷ (RPM × Feed Per Rev)
Example: Turning a 100mm shaft at 1,500 RPM with a 0.2mm/rev feed rate.
Table Feed = 1,500 × 0.2 = 300 mm/min.
Machining Time = 100 ÷ 300 = 0.33 minutes.
The Hidden Trap: Cycle Time vs. Machining Time
A common mistake in quoting is confusing machining time with true cycle time. The formulas above only account for the time the tool spends physically cutting metal. They do not include:
- Rapid approach and retract movements (Air Cutting)
- Tool change times (often 3-10 seconds per change)
- Part loading and unloading (Handling time)
Automating the Math
Calculating L ÷ F manually for a complex 3D toolpath with 12 tool changes is practically impossible. While CAM software provides accurate times post-programming, you often need rapid cycle time estimates during the quoting phase.
To solve this, we built a dedicated tool that automatically buffers basic machining time with standard industry variables (rapid rates, tool changes, and setup overhead). Stop guessing your margins—automate your time estimation.