How to Estimate Milling Time for a Part
Estimating milling time isn't guesswork: you need three parameters (Vc, fz, MRR) and a method. Here's how to do it, with real numbers.
The three numbers that govern milling time
Machining time depends on three fundamental parameters: cutting speed Vc (m/min), characteristic of the material and tool; feed per tooth fz (mm/tooth), which determines table feed; the volume of material to remove, which combined with the first two gives the Material Removal Rate (MRR, cm³/min).
The 4-step method
1) Calculate the volume to remove: stock volume minus finished volume. 2) Determine roughing MRR: for aluminum 6082 on a modern machining center you can reach 300-500 cm³/min, for hardened steel 40-80 cm³/min, for Inconel 8-15 cm³/min. 3) Roughing time = volume / MRR. 4) Add finishing (depends on surface to machine and feed rate), drilling/tapping (per number of holes), and auxiliary times: rapids, tool changes (3-8 seconds each on modern machines), table rotations.
Example: flange in aluminum 6082
Stock 150×150×40 mm (900 cm³), finished 620 cm³ → 280 cm³ to remove. Realistic roughing MRR on a 15 kW 3-axis: 250 cm³/min → 1.1 minutes pure roughing. Perimeter finishing and facing: ~4 minutes. 8 M6 holes (drilling + tapping): ~3 minutes. Tool changes and rapids: ~1.5 minutes. Total estimated: ~10 minutes/part, plus setup.
Note how roughing, which psychologically seems like the long part, often weighs less than finishing and drilling: this is the most common perceptual error in eyeball estimates.
Why eyeball estimates miss by 30-50%
Those who estimate by experience tend to: remember parts that 'went well' and not the problematic ones; ignore auxiliary times (which account for 20-30% of the total); use optimistic cutting parameters that in production don't hold due to vibrations, weak fixtures, or end-of-life tooling.
The solution is to systematically compare estimate and actual: after 20-30 comparisons you'll discover YOUR real correction factor, machine by machine.
MachinePilot's deterministic estimation
MachinePilot calculates cycle time deterministically: it uses Vc, fz and MRR specific to each of the 32 catalog materials, adapts them to the power and characteristics of the selected machine (dual turret, sub-spindle, 5-axis simultaneous) and adds auxiliary times. By importing your actual Time Studies (Excel/CSV), the system calibrates to your machines achieving ±8% accuracy.