What Is Cycloid Milling? A Complete Guide to Trochoidal Cutting

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  Key takeaways
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  What is cycloid milling and how does the path work?
• 3
  Chip thinning, radial engagement, and heat
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  Where the cycloid curve comes from
• 5
  When cycloid milling earns its keep
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  Machine, holder, and tool requirements
• 7
  How to set up a cycloid milling pass
• 8
  Cycloid milling vs conventional slot milling

Cycloid milling is a milling method in which the tool center follows a series of small circles or loops while advancing along the slot. The name comes from the cycloid curve: the path traced by a point on a rolling circle. In a CNC program the motion is a blend of circular interpolation and a slow linear feed along the part.

The cutter is always smaller than the feature it is cutting. A 6 mm end mill can open a 12 mm slot this way. Instead of plunging into full radial depth, the tool takes a light radial bite and steps over, so the arc of engagement stays under roughly 90° instead of the 180° you get in a full-width straight cut.

That single change drives most of the benefit. A smaller engagement arc means fewer teeth in the cut at any instant, lower cutting force, and a thicker chip per tooth at the same feed. The chip carries heat away with it, which is why the method is common on titanium, Inconel, and hardened tool steel.

The trade-off is path length. The tool travels further to remove the same volume, so the program runs longer and the machine spends more time in motion. For a shallow open pocket in 6061 aluminium, that extra travel usually costs more than it saves.

When the radial depth of cut drops below about half the cutter diameter, the chip no longer forms at the programmed feed per tooth. The chip thins. If you keep the same feed, the edge rubs instead of cutting. Feed per tooth must be raised to compensate, often by 1.5 to 3 times, depending on the engagement angle.

This is the part most shops get wrong. They copy a trochoidal path from a template, keep the original feed, and wonder why the tool squeals and wears on the flank. The corrected chip load is what keeps the edge in the material and the heat in the chip.

Radial engagement is usually held between 5% and 15% of the cutter diameter for hard alloys. Axial depth can go much deeper, sometimes 1× to 2× diameter, because the radial load is so low. That is the real advantage: you cut deep in one pass instead of stepping down in many shallow layers.

Spindle speed also rises. With less of the cutter in contact, the tool runs cooler, so surface speed can be pushed higher than a conventional full-width cut. On a 5-axis machine the same logic applies to contoured walls, where the tool loops along the profile instead of burying itself in a corner.

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  Radial engagement5–15% of cutter diameter on hard alloys; up to 30% on aluminium.
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  Axial depth1–2× diameter on rigid setups with a stub or shrink-fit holder.
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  Feed correctionRaise feed per tooth 1.5–3× to restore the intended chip load.
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  CoolantHigh-pressure through-spindle coolant helps clear chips from deep slots.

A cycloid is the curve traced by a point on the rim of a circle as that circle rolls along a straight line. If the point sits outside the rolling circle, the curve is a prolate cycloid, sometimes called an extended cycloid. If it sits inside, it is a curtate or shortened cycloid.

Machine tool builders borrowed the term because the tool path looks like a stretched loop. Each loop overlaps the previous one, and the overlap is what leaves a clean floor. Step-over distance controls the scallop height left on the floor; on a finishing pass it is often set to 5–10% of the cutter diameter.

You do not need to hand-code the curve. CAM systems generate it from a trochoidal or dynamic milling operation, where you set the engagement angle and the software solves the loop. Manual programming is possible with G02 and G03 arcs, but the point spacing and overlap are tedious to keep consistent.

The curve itself is not the goal. The goal is a controlled engagement angle. Any path that holds a small, steady arc of contact will behave like cycloid milling, whether the CAM vendor calls it trochoidal, dynamic, or adaptive.

Deep slots are the classic case. A slot deeper than about 2× the cutter diameter traps chips and heat in a conventional cut. The looping path gives each chip a short, open exit and keeps the tool from recutting them. On a 4,000 mm travel machine, long channels in a mold base or a structural extrusion benefit most.

Hard and heat-resistant alloys are the second case. Titanium, Inconel, and 17-4PH stainless have low thermal conductivity, so heat stays at the edge. A small engagement arc and a thick chip move that heat into the chip instead of the tool. Tool life often improves by a wide margin, though we do not quote a fixed number because it depends on the alloy and the holder.

Thin-wall parts are the third. Low radial force means less deflection, so a wall 1 mm thick is less likely to spring away from the cutter. This matters on medical housings and aerospace brackets where wall thickness is set by weight, not by stiffness.

The method is a poor fit for shallow open pockets in soft aluminium, for roughing where a large-diameter face mill can clear the same volume faster, and for jobs where the CAM license does not include a trochoidal option. In those cases a conventional pass is cheaper per part.

The path puts constant direction changes on the machine. A control with a fast block-processing rate and look-ahead keeps the feed steady through the loops. Older controls may stutter, which shows up as chatter marks on the floor and a shorter tool life.

Rigidity matters more than raw spindle power. A stub-length carbide end mill in a shrink-fit or hydraulic holder is the usual choice. Long reach tools flex under the looping load and the engagement angle drifts, which defeats the purpose. If the slot is deeper than the available stub tool, a necked cutter with a relieved shank is the next option.

Chip evacuation sets the practical depth limit. Through-spindle coolant at high pressure is common on deep slots. Without it, the loops can pack chips into the bottom of the cut, and the tool will rub on the recut material. Air blast works for shallow passes in aluminium.

At GreatLight, cycloid paths run on 16 simultaneous 5-axis centers, 12 four-axis mills, and 27 three-axis machines, with a maximum processing size of 4,000 mm and a Ø400 mm rotary table for round parts. Tolerances hold at ±0.005 mm and finishes reach Ra 0.2–0.8 μm on a fine pass.