Changing the chuck of a 6408 makita electric drill

SOURCE: chuck change

After removing the screw and inserting the allen wrench shift the drill to the LOW position and turn the clutch to the drill only position. Use an allen wrench about the size of the chuck capacity so that it will not flex when hitting it with a mallet. Sometimes they are stubborn because of the torque but this should work. Put the allen wrench at about a 30deg angle and hit the wrench. I support the chuck on a workbench or a block of wood will do fine. The chuck has standard threads.

SOURCE: chuck removal

First open the chuck jaws and remove the LH chuck retaining screw. After that find a bent handle allen wrench about the size of the chuck. Put the shorter end in the chuck and clamp down on it. Put chuck on a block of wood or workbench with the long end at about a 30 degree angle so you can hit it counterclockwise with a hammer. It might take a couple hits but it will come off.

SOURCE: makita 6408 drill motor. can't see how to replace the chuck.

If you have the 6408 as listed the spindle is threaded and you are correct there is no retaining screw. What you would need to do is find a bent handle allen wrench about the size of the chuck capacity 3/8" and insert the shorter end in the chuck and tighten. put the chuck on a block of wood or workbench and position the long end at about a 30 degree angle in the counter clockwise position and strike it with a hammer, this might take a couple hits. If this is not a 6408 let me know and I will look at the tool for you and see what you got.

SOURCE: Broken Makita Drill 6408k chuck

If the chuck itself slips, you can tear down the outer layers (yes it is a bit brutal)
of the chuck until you reach the inner metal core. You can then try several things;
1 loosen it with a pipe wrench, provided you can get a grip on the axle of the chuck to counter the torque
2 Weld an allen key to the metal core of the chuck and use a firm rapid blow by a hammer to rotate it counterclockwise
3 Dismantle the final shaft with the chuck core, and set it in a vise with aluminium plates to protect it, while you turn the chuck with a wrench or pliers.
4.If there is no senter lock screw inside the bottom of the chuck it may be fitted with a thread locking compound. It will loosen easilier if heated with a heat gun or moderate gas heat.Only supply just enough heat to loosen it, or the hardening of the steel may be compromised.

SOURCE: Makita HP 1640 hammer drill chuck loosens grip on

Sounds like the chuck is worn, happens frequently when used often. Here's the link to Makita, their contact info should get you some answers as to what part number is needed as well as the closest location to you for service if required. Hope this helps.

http://www.makita.com/en-us/Modules/Tools/ToolDetails.aspx?ID=316372