# Aluminum pocket wall chatter only on left side, toolpath or frame stiffness?

**URL:** <https://cnc-routers.org/t/aluminum-pocket-wall-chatter-only-on-left-side-toolpath-or-frame-stiffness/51>\
**Category:** Troubleshooting & Support\
**Created:** [October 5, 2026, 3:07pm UTC](https://cnc-routers.org/t/aluminum-pocket-wall-chatter-only-on-left-side-toolpath-or-frame-stiffness/51 "2026-10-05T15:07:03Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![dnovak](https://cnc-routers.org/user_avatar/cnc-routers.org/dnovak/32/50_2.png) [@dnovak](https://cnc-routers.org/u/dnovak)\
**Post date:** [October 5, 2026, 3:07pm UTC](https://cnc-routers.org/t/aluminum-pocket-wall-chatter-only-on-left-side-toolpath-or-frame-stiffness/51/1 "2026-10-05T15:07:03Z")

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Before anyone says routers can’t cut aluminum, here are the numbers.

Machine: 3kW spindle, 20mm ballscrews, steel-welded frame. Job: 6061-T6, 10mm deep pocket, 6mm 2-flute carbide, 12mm stickout.

Parameters: 18,000 RPM, 1,200 mm/min feed, 0.5mm stepdown, 1.2mm stepover (20%), climb, mist. Chip load 0.033mm/tooth, dead center of the manufacturer’s range. Floor finish is mirror-clean, so the toolpath is sound.

Symptom: walls show a 2 to 3mm pitch wave on the left side only, worsening below 6mm depth. Right wall is fine. Identical toolpath both sides, so don’t blame my G-code.

My suspicion: climb milling on the left wall pushes the tool into stock, and with 12mm stickout the deflection shows. But I want measurements, not guesses. Has anyone dial-indicated spindle runout and gantry deflection on these frames under lateral load? If your router holds Ra 1.6 on aluminum walls, post your exact RPM, feed, stepdown, and stickout. I’d take measured numbers over “works fine for me” any day.

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**Author:** ![jess](https://cnc-routers.org/user_avatar/cnc-routers.org/jess/32/52_2.png) [@jess](https://cnc-routers.org/u/jess)\
**Post date:** [October 6, 2026, 12:33am UTC](https://cnc-routers.org/t/aluminum-pocket-wall-chatter-only-on-left-side-toolpath-or-frame-stiffness/51/2 "2026-10-06T00:33:52Z")

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Hang on. On that left wall, is it being cut along X or Y? And when you climb mill the left wall, does the tool get pushed toward the frame or away from it?

Broke a 4mm endmill on a 6061 pocket last week off a bad work offset, and the feed felt wrong on one side the whole time. If the axis explains that, a lot suddenly makes sense.

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**Author:** ![mike1958](https://cnc-routers.org/user_avatar/cnc-routers.org/mike1958/32/49_2.png) [@mike1958](https://cnc-routers.org/u/mike1958)\
**Post date:** [October 6, 2026, 8:23am UTC](https://cnc-routers.org/t/aluminum-pocket-wall-chatter-only-on-left-side-toolpath-or-frame-stiffness/51/3 "2026-10-06T08:23:54Z")

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Do you actually have gage readings for the spindle runout and the lateral gantry deflection, or is that still a hunch?

Every “only on one side” symptom I have chased came down to the axis, not the tool. If the left wall chatters and the right one does not with the same program, the machine is telling you the two sides are not equally stiff. On one steel-frame router the left gibs were torqued noticeably looser than the right — no cutter deflection at all, just the axis giving under the climb-mill push.

Dial-indicate the gantry while you cut that wall. Lock the far side, put a dial against the near leg, and watch it move under load. That number settles it — not the runout, the deflection.