Corner Chatter and Wall Taper in a 6061 Pocket
On a 3-axis vertical CNC mill, a 120 mm × 80 mm × 12 mm deep pocket is roughed and finished in 6061-T6 using a 12 mm 3-flute solid carbide end mill with a 45° helix and 30 mm length of cut, held in an ER32 collet with 45 mm overhang. The program uses 300 m/min, 0.05 mm/tooth, 6 mm axial depth, 6 mm radial width, climb milling and flood coolant. The first part has chatter marks concentrated at the four internal corners, a 0.08 mm wall taper that is wider at the top, and Ra 3.2 µm. The operator wants to reduce feed to 50% and take a 0.2 mm finish pass at the full 6 mm depth to save time. The machine has a 12,000 rpm, 10 kW spindle, and a 16 mm end mill is available.
1. Calculate the programmed spindle speed and feed rate. Estimate whether the cut is within the machine's power and speed limits.
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Spindle speed: n = 1000Vc/(πD) = 1000 × 300/(π × 12) = 7958 rpm. This is below the 12,000 rpm maximum.
Feed rate: f = n × z × fz = 7958 × 3 × 0.05 = 1194 mm/min.
Material removal rate: MRR = axial depth × radial width × feed = 6 × 6 × 1194 ≈ 43,000 mm³/min, or about 716 mm³/s. Using a specific cutting force near 800 N/mm² for 6061-T6, cutting power ≈ 716 × 800 = 573,000 N·mm/s = 573 W, or about 0.7 kW at 80% efficiency. That is well below the 10 kW spindle rating. The parameters are therefore not limited by spindle power or top speed; the problem is tool and workpiece deflection plus corner engagement.
2. Diagnose the corner chatter and wall taper. Explain why simply halving the feed at full depth may not correct the problem.
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At a square internal corner, the radial engagement jumps from 6 mm to 12 mm, effectively a full slot for a short interval, so the cutting force can roughly double and excite chatter. The 12 mm cutter with 45 mm overhang has an overhang-to-diameter ratio of about 3.75, which is flexible. Wall taper wider at the top is consistent with tool deflection at the bottom of the pocket: the cutter tip is pushed away more, leaving the lower wall undersize and the top closer to programmed size.
Halving the feed lowers the chip load to 0.025 mm/tooth. That may reduce forced vibration, but it can also make the edge rub and raise the specific cutting force, and it does nothing about the corner engagement jump or the long overhang. It also does not correct the taper. Correct the cause by reducing overhang to 25-30 mm, using a tool with a 0.5-1 mm corner radius, programming a corner radius or trochoidal path that keeps engagement constant, and using separate roughing and finishing depths.
3. Propose a process plan for finishing the walls to hold ±0.05 mm and Ra 1.6 µm. Include measurable checks.
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Rough at 6 mm axial and 4-5 mm radial, leaving 0.4 mm on the walls. Semi-finish with 0.15-0.20 mm radial at 6 mm axial. Finish with 0.10 mm radial at full 6 mm axial, 0.03 mm/tooth and 8000 rpm, giving 720 mm/min. Use a new collet and the shortest safe overhang. Climb mill, keep flood coolant directed at the cut, and take one spring pass without changing the offset.
Measure the first wall at the top and bottom with a micrometer or bore gauge. Check tool runout at less than 0.01 mm TIR. If taper remains, split the finish into two 3 mm axial steps rather than programming a tapered wall. Inspect Ra against 1.6 µm and adjust speed or chip load based on the measured finish, not on sound alone.