Diagnosis
The wobbling you are experiencing is known as "runout." In the Bosch PS31-2A, this occurs when the concentricity of the chuck's internal jaws is compromised. Because this is a compact 12V Max model, the spindle is relatively thin, and any slight misalignment in the jaw mechanism is magnified at the tip of the drill bit. This results in "ovaling," where a hole intended to be 1/4" actually becomes an oval shape because the bit is orbiting slightly off-center during rotation.
The root cause is typically mechanical wear in the jaw guide or a deformed bearing race within the chuck assembly. This happens most frequently if the tool has been dropped while a bit was clamped, or if the user has applied excessive lateral pressure while drilling, forcing the spindle to flex. You can confirm this by inserting a precision test arbor or a perfectly straight drill bit and rotating the chuck by hand while watching the tip; if the tip describes a circle rather than staying on a fixed point, the chuck is internally damaged and cannot be calibrated manually.
How to Fix It
Since the internal jaws of the PS31-2A chuck are not individually adjustable, the only permanent solution is a full replacement of the chuck assembly. The correct replacement part number is 2 607 001 017.
- Clear the workspace and remove the 12V battery pack from the base of the drill to ensure the trigger cannot be accidentally pressed.
- Open the chuck jaws fully. Look deep inside the throat of the chuck. You will see a small set screw. Note that this screw is **left-hand threaded** (counter-clockwise to tighten, clockwise to loosen).
- Use a 2.5mm or 3mm hex key (depending on the specific production run of your unit) to loosen this internal set screw. You do not need to remove it entirely, just break the tension.
- Insert a 10mm Allen key (or a compatible hex bit) into the chuck jaws and tighten them firmly onto the key.
- Rotate the entire chuck assembly clockwise (right-hand thread) to unscrew it from the drive spindle. If it feels stuck, apply a small amount of penetrating oil and wait five minutes.
- Clean the spindle threads with a dry cloth to remove any metal shavings or old lubricant.
- Thread the new 2 607 001 017 chuck onto the spindle by hand to avoid cross-threading.
- Tighten the chuck firmly using the 10mm Allen key method.
- Re-insert the internal set screw and tighten it securely to prevent the chuck from spinning independently of the spindle.
- Reinsert the battery and test with a known straight bit to verify the runout is gone.
If That Didn't Work
- Check the Spindle Alignment: If a new chuck still wobbles, the spindle itself may be bent. Remove the chuck and rotate the spindle by hand. If the shaft of the spindle is visibly oscillating, the internal gearbox is damaged, and the entire motor/gearbox assembly must be replaced.
- Inspect the Bit Quality: Low-cost or worn-down bits often have "bent" shanks. Test the tool with a high-precision cobalt bit. If the wobble disappears, the issue was with the consumable, not the tool.
- Verify Collet Seating: Ensure there is no debris, sawdust, or metal chips trapped in the tapered seat where the chuck meets the spindle. Even a single metal chip can throw the alignment off by a fraction of a millimeter.
- Check for Housing Play: Ensure the screws holding the motor housing together are tight. If the gear case is loose, the spindle may be shifting slightly under load, mimicking a chuck wobble.
Heads Up
- Never use the PS31-2A as a hammer or use it to "punch" through material, as this is the primary cause of spindle misalignment and jaw deformation.
- Avoid over-tightening the chuck jaws; excessive force can warp the internal guide rails over time.
- Periodically apply a tiny drop of dry PTFE lubricant to the chuck threads to prevent seizing, which makes future replacements much easier.
- Always use a magnetic bit holder if you are frequently switching between driver bits to reduce the wear and tear on the chuck's clamping mechanism.