the blade on my gks18v-25cn wobbles a bit, makes a weird humming noise and the cuts aren't straight

Diagnosis

The wobble and accompanying humming noise in the Bosch GKS18V-25CN are classic symptoms of axial runout. This occurs when the blade is not rotating on a perfectly perpendicular plane relative to the motor shaft. This is typically caused by a misalignment between the inner and outer flanges, or a buildup of sawdust and resin trapped behind the inner flange, which prevents the blade from seating flush against the arbor. When the blade is tilted even a fraction of a millimeter, it creates a centrifugal imbalance, leading to the "humming" vibration and "wandering" cuts that aren't square.

Another common cause is a warped blade or a damaged arbor bolt. If the blade was dropped or pinched in a deep cut, the steel can develop a slight bend that isn't visible to the naked eye but manifests as a wobble at 3,000+ RPM. To confirm, unplug the battery and rotate the blade slowly by hand while watching the teeth against a fixed reference point (like a workbench edge). If the teeth move side-to-side rather than just orbiting, the blade is warped or the flange is obstructed.

How to Fix It

To resolve the wobble and restore cutting precision, follow this precise sequence to ensure the blade is seated perfectly flat.

1. Remove the 18V battery pack immediately to prevent accidental startup.
2. Press the spindle lock button firmly to immobilize the arbor.
3. Using the provided wrench, loosen the arbor bolt. Note that the GKS18V-25CN uses a standard right-hand thread for the bolt; turn counter-clockwise to loosen.
4. Remove the outer flange and the blade. Use a compressed air canister or a stiff nylon brush to thoroughly clean the inner flange and the motor shaft. Even a single wood chip trapped here will cause a wobble.
5. Inspect the inner flange for any burrs or scoring. If the surface is pitted, the blade will not sit flat.
6. Reinstall the blade, ensuring the teeth are facing the correct direction (downward and forward).
7. Place the outer flange back on. Ensure the flange is centered perfectly. If it is skewed, the blade will wobble regardless of how tight the bolt is.
8. Tighten the arbor bolt firmly. Do not over-torque to the point of stripping the threads, but ensure it is snug enough that the blade cannot be moved by hand.
9. Reinsert the battery and perform a "dry run" (without cutting material) to check if the humming noise has subsided.

If That Didn't Work

• Replace the Blade: If the wobble persists after cleaning the flanges, the blade itself is likely warped. Swap the current blade for a high-quality carbide-tipped alternative. If the new blade rotates smoothly, the previous blade was structurally compromised.
• Inspect the Flange Washers: Check if the outer flange has become warped or bent. If the flange is not perfectly circular, it will apply uneven pressure to the blade. Replace the flange set if you see any signs of deformation.
• Check the Base Plate Squareness: If the blade is rotating smoothly but the cuts are still not straight, the base plate may have shifted. Loosen the mounting screws on the base, use a machinist square to align the plate with the blade, and retighten the screws firmly.
• Bearing Inspection: If the humming noise continues and you feel a vibration in the handle even with a brand-new blade, the motor bearings may have failed or developed "play." This requires a teardown of the gear housing to replace the ball bearings.

Heads Up

• Resin Buildup: Periodically clean your blades with a dedicated pitch remover. Resin buildup on the teeth creates uneven drag, which can mimic the feeling of a wobble during a cut.
• Blade Selection: Always ensure the arbor hole of the replacement blade matches the GKS18V-25CN specifications exactly. Using a blade with a slightly oversized center hole will cause severe vibration and potential bolt failure.
• Tension Warning: Never use a hammer to "straighten" a bent blade. This creates internal stress points in the steel that can lead to the blade shattering at high speeds.
• Battery Safety: Always remove the battery before any adjustment to the blade or flanges to avoid accidental trigger engagement.