Calculate effective feed rates, chip thinning, and MRR for trochoidal (peel milling) toolpaths. Optimize stepover and axial depth for maximum productivity.
Based on chip thinning theory for low radial engagement HSM toolpaths. Results update in real time.
A machinist running a 12mm end mill in a steel pocket with conventional roughing takes 0.5 mm radial passes at 4 mm axial depth. The tool lasts 90 minutes, the pocket takes 45 minutes to rough, and the machine rarely exceeds 60% spindle load. Switching to a trochoidal toolpath with 10% stepover (1.2 mm radial) and 12 mm axial depth (full flute length), the same pocket roughs in 14 minutes. The tool lasts 120 minutes. The difference is not incremental — it's structural.
Trochoidal milling works because it keeps the tool's radial engagement low enough that the cutting edge never fully engages with the material. The chip load is distributed across multiple teeth in a constant chip thickness profile. Heat is carried away by the chip rather than conducted into the tool. The machine spindle stays below 80% load even at 3× the MRR of conventional toolpaths. These numbers are not theoretical — they are measured daily in production shops running high-speed machining strategies.
Conventional feed rate formulas assume the tool is engaged across a significant portion of its diameter. Trochoidal milling uses radial engagement of 5-15%, which creates a chip thinning effect — the chip produced at low radial engagement is thinner than the programmed feed per tooth. To maintain the recommended chip load, the feed rate must be increased by the chip thinning factor.
Chip thinning factor = 1 / sin(engagement angle). For a 10% stepover with a 12mm tool, the engagement angle is roughly 37°, giving a thinning factor of approximately 1.65×. This means the programmed feed per tooth can be 65% higher while maintaining the same actual chip load as conventional milling. Combined with the full axial depth typical of trochoidal toolpaths, the effective MRR increase is substantial.
Aluminum 6061: 2-3 flute tools with stepover of 10-15% and full axial depth (1-1.5× diameter). Feed rates can reach 2-3× conventional values. Chip evacuation becomes the limiting factor at high MRR — use compressed air or coolant through the spindle. MRR improvements of 4-6× over conventional are typical.
Mild Steel 1018: 4-5 flute tools with stepover of 8-12% and axial depth of 1-1.5× diameter. Feed increase of 1.5-2× over conventional. The limiting factor is machine rigidity, not tool life. MRR improvement of 2-4× is achievable with a rigid machine.
Stainless Steel 304: 5-6 flute tools with stepover of 5-10%. Axial depth limited to 0.75-1× diameter due to work-hardening at the depth-of-cut line. Feed increase of 1.3-1.8×. MRR improvement of 1.5-3×. The Milling Force Calculator can help verify that the reduced radial engagement keeps cutting forces within safe limits.
Titanium Grade 5: 4-5 flute tools with stepover of 5-8%. Axial depth of 0.5-1× diameter. Feed increase is modest — 1.2-1.5× — because chip load is limited by thermal considerations. The main benefit of trochoidal milling in titanium is consistent tool life, not MRR. Through-spindle coolant at 50+ bar is essential.
The three parameters that define a trochoidal pass are stepover (radial engagement), axial depth, and the trochoidal step (the forward advance per loop). Stepover is the most critical — it determines the engagement angle, which in turn determines the chip thinning factor and the cutting forces.
The engagement angle θ for a given stepover ae with tool diameter D is: θ = arccos(1 - 2×ae/D). At 10% stepover, θ ≈ 37°; at 20%, θ ≈ 53°; at 40%, θ ≈ 79°. Below 10% stepover, the chip thinning effect becomes extreme — a 5% stepover gives a thinning factor of 2.5×, allowing very high feed rates but with diminishing returns on MRR because the tool spends most of its time in air. The sweet spot for most materials is 8-15% stepover.
Trochoidal toolpaths generate constant acceleration and deceleration in the machine axes. Machines with poor acceleration (below 3 m/s²) cannot maintain the programmed feed rate on the tight trochoidal loops — the actual average feed drops to 40-60% of programmed values, negating the MRR advantage. For these machines, conventional roughing with moderate radial engagement (30-40%) produces better results.
Short tools (under 3× diameter length) benefit most from trochoidal milling. Long tools (over 5× diameter) introduce deflection and chatter that limit the achievable MRR regardless of toolpath strategy. In these cases, reducing axial depth and using conventional stepover is more productive than struggling with trochoidal instability.
What is trochoidal milling? A high-speed machining strategy where the tool follows a looping (trochoidal) path with low radial engagement (5-15%) and high axial depth. This maintains constant chip load and allows much higher feed rates than conventional roughing.
How do you calculate feed rate for trochoidal milling? Feed rate = RPM × flutes × programmed fz × chip thinning factor. The chip thinning factor compensates for the reduced chip thickness at low radial engagement. This calculator does all the math automatically.
What stepover should I use for trochoidal milling? 8-15% of tool diameter for most materials. 5-10% for difficult materials like titanium and stainless. Below 5% the tool spends too much time in air; above 20% the chip thinning benefit decreases and cutting forces rise sharply.
Can I use trochoidal milling on any CNC machine? No. Machine acceleration must be at least 3 m/s² to maintain programmed feed on the looping toolpath. Machines with acceleration below 2 m/s² will see reduced benefit. Newer machines with linear motors or dual-ball screws achieve 5-10 m/s² and maximize trochoidal performance.
How does trochoidal milling affect tool life? Trochoidal toolpaths typically improve tool life by 20-50% compared to conventional roughing at the same MRR because the reduced radial engagement prevents thermal shock and distributes wear evenly across the cutting edge. The constant chip load also eliminates the impact loading that causes micro-chipping.
What is chip thinning in trochoidal milling? At low radial engagement, the chip produced is thinner than the programmed feed per tooth. The ratio of programmed to actual chip thickness is the chip thinning factor. Feed rate must be increased by this factor to maintain the recommended chip load. This calculator computes the thinning factor for your specific stepover.
For end mills designed for trochoidal HSM toolpaths, check our High-Performance End Mills