I had this problem as i was using the motor to tighten it by holding the chuck and spinning the motor, when I started tightening while it was not running it's fine and never comes undone.


Vince

If the problem is rounding of the jaws, as happens over time especially once skidding has started, then the chuck can normally be changed - open it up wide and remove the lock-screw whose head is exposed when you look in where the drill bit would go, (normally a left hand thread so don't go mad in the normal loosening direction making it tighter)

Then close the chuck onto the short leg of the chunkiest Allen key you can fit in the chuck, and strike the long end of the Allen key with a modest hammer or mallet to drive it round to undo it. (when the lock screw is left hand thread, the chuck is normally threaded, again pause and think about diorection before hitting the wrong way.) For anyone wanting to use this trick on drills with gears, engage the lowest speed, so the hammer blow is using the inertia of the motor to work against.

New chucks are fairly cheap on-line, but be aware there are several thread pitch and diameter variations to chose from and you need the right one for that model.



Makers exploded diagrams for that model  exploded diagram  show a screw as item 1 and chuck as item 2.

I suspect this is a design problem allowing interference from vibration. I have two types of keyless chuck: one lockable, the other not. As you can guess, the non-lockable one vibrates loose whereas the lockable one does not. However, there's a problem in that while the non-lockable one is as yours is, part of the drill (a 12v Bosch), the lockable one is a separate chuck that can replace a drills existing key type chuck, typically associated with 110/230v drills. So, until they design lockable built in chuck's, we're stuck with the problem.


F

I have a similar problem with a Milwaukee, usually last about 2 or 3 holes on hammer action before becoming loose but one tightened again it usually lasts much longer. I tighten by hand so not the problem mentioned above. It has happened since new.  I mentioned it to a rep at a show and he said it was the first time he had heard of it and offered to change it under warrantee but I never bothered as it would be too much messing around.

I have the same problem with the Makita,and have put it down to the sudden braking action if the trigger is released quickly.

Have noticed if I let the motor slow down gradually,the chuck stays tight.

                                                                                          Regards,Hz

You called!


There is a phenomena machinists call 'chuck walk'.  If the chuck is off axis to the work piece rotation (drill bit) as it turns the radial pressure increases and decreases on the tip of jaw 1 as it turns.  Similarly the pressure at the base of jaw 1 decreases and increases (out of phase).  This effect also happens to the other jaws at 120 degree phase.  The result is the work piece 'walks' out of the jaws or the threaded ring 'walks' against the jaws as it is an inclined plane it's easier for it to loosen than tighten.  Pretty soon something unpleasant happens.


The solution is to re-grind the jaws (easy if you have a lathe or tool grinder, doesn't everyone?) or buy a new chuck.


Dave Newton.