The Golden Formula: Q = Ae × Ap × Vf
Material Removal Rate (MRR), often denoted as Q, is the volume of chip created per minute.
- Ae (Width of Cut): Stepover in inches or mm.
- Ap (Depth of Cut): Axial depth in inches or mm.
- Vf (Feed Rate): Table feed in IPM or mm/min.
Why cubic inches matter
A machine costs $100/hr whether it's removing 10 in³/min or 1 in³/min.
Goal: Maximize MRR until you hit the limit of (1) Horsepower, (2) Rigidity, or (3) Tool Holding.
Strategy 1: The "High Efficiency" Approach
Traditional roughing uses ~50% Ae and ~50% Ap. This is inefficient because it puts a high load on the bottom 0.25" of the tool.
Better Way (HEM): Use 10-15% Ae (small stepover) but 200-300% Ap (full flute length).
- Spreads wear over the entire flute.
- Low radial force reduces chatter.
- Allows for massive Feed Rate increases due to Chip Thinning.
Strategy 2: Balance the Triangle
Visualize MRR as a volume. If you can't go deeper (Ap is limited by part geometry), you must go faster (Vf) or wider (Ae).
Scenario: You have a 10HP spindle. You are only using 3HP.
Action: Increase Feed Rate until load meter hits 85-95%. Most modern tools break from runout/vibration, not spindle stall.
Calculating Horsepower Requirements
You can estimate required power using the Unit Power Factor (Kp):
HP = MRR × Kp
- Aluminum Kp ≈ 0.25 HP/in³
- Steel Kp ≈ 1.0 HP/in³
Example: Removing 20 in³/min of Aluminum requires ~5 HP. Removing 20 in³/min of Steel requires ~20 HP.