Six Marks per Revolution
A 4140 PH (30 HRC) block is face milled with an 80 mm, 6-insert face mill on a 40-taper VMC: 600 rpm, 0.15 mm/tooth, 1.0 mm depth, 50 mm radial width, dry. The finish shows a regular pattern of six evenly spaced marks per cutter revolution, about 0.02 mm deep, with no audible squeal and a steady 55% spindle load. A dial indicator on the insert pockets shows one pocket 0.03 mm lower than the rest, with 0.035 mm TIR across the cutting edges. The operator wants to drop to 400 rpm to "get rid of the chatter."
1. Is this forced vibration or regenerative chatter? Show how the evidence decides it.
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Forced vibration from insert runout - the arithmetic is conclusive.
- Marks per revolution = 6 = inserts per revolution. A defect locked to the tooth count is driven by the teeth.
- Frequency: 600 rev/min × 6 teeth = 3600 impacts/min = 60 Hz.
- Spacing check: feed per revolution = 0.15 × 6 = 0.9 mm/rev; 0.9 / 6 = 0.15 mm mark pitch. Measure the pitch with a magnifier or a profilometer - if it is 0.15 mm, forced vibration is confirmed.
- TIR = 0.035 mm against a 0.15 mm chip load → the high insert carries ~23% less load and the low pocket ~23% more, every revolution, at the same angular position.
Contrast with regenerative chatter: marks would not be an integer multiple of the tooth count, the depth would grow with radial engagement or depth of cut, the amplitude would beat against spindle speed, and you would normally hear it.
2. Evaluate the operator's plan to slow down to 400 rpm.
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It will change the pitch and the chip load, but not the source of the marks.
- 400 × 6 = 2400 impacts/min = 40 Hz - still six marks per revolution, still one low pocket, still the same 0.035 mm TIR. The pattern remains; only the spacing changes.
- If the feed rate is programmed in mm/min and unchanged, lowering rpm raises feed per tooth from 0.15 to 0.225 mm, thicker chips, higher cutting force, more spindle load, and possibly more deflection. Slowing down makes it worse, not better.
Correct sequence:
- Fix the runout. Reset the low pocket with the axial adjustment (or replace the insert/seating screw). Target TIR ≤0.01–0.015 mm on a 0.15 mm chip load. Check the body for a bent pocket or trapped chip under an insert, and clock the body in the spindle taper.
- Verify with an indicator before cutting.
- Re-cut a test surface and measure the mark depth. It should drop below 0.005 mm.
Judgement point: speed changes are the last adjustment after geometry and rigidity are proven, not the first.
3. Suppose runout is corrected to 0.008 mm and the marks persist - now they are non-integer per revolution and louder at low speed. What do you do next?
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You have now unmasked true chatter; treat it as a stability problem, not a runout problem.
Diagnosis: non-integer marks per revolution, amplitude that grows with engagement, and speed dependence are the signature of self-excited (regenerative) chatter in a flexible mode - likely the tool overhang, the holder, or the part/fixture.
Adjust in this order (cheapest and most reversible first):
- Change spindle speed by a step, ±10% (e.g. 600 → 660 rpm), and listen/measure. Stability is lobed; a modest speed change can move you out of an unstable region. Do this as a controlled trial on scrap, one variable at a time.
- Reduce radial engagement (50 → 35 mm) and compensate with feed per tooth so chip thickness is maintained - this is the single most effective geometric lever on a face-milling operation.
- Increase rigidity: shorten overhang, use the shortest gauge-line holder, check insert seating and holder/taper contact, verify the part is supported under the cut and the vise/fixture is not the compliant element (push-test or tap-test the setup).
- Check insert geometry and edge condition - a worn or chipped edge raises cutting forces and shifts the stability boundary.
Record the rpm/engagement combinations that run quiet so the process window, not a single point, becomes the standard.