Dewalt impact gun shaft doesn't spin straight

Removing Stuck Bit From Dewalt Impact Driver
Seems to be a common problem. There are several suggestions at the above link. among them were drill and use an easy out tool to twist counter clockwise. soak in oil and extract with vice grips. Heat the bit and chuck to expand the metal.

Mick, this could be a sign of a worn out/faulty motor, mine did this, ripped it apart and it was the motor. Yours may still be under 3 year warranty if you can find your receipt.
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DeWALT DCF885C2 Type 1 20V Impact Driver Parts
http://www.mmtoolparts.com/store/dewalt-impact-driver-parts
http://www.dewalt.com/en-us/legal/warranty/general-warranty-information
DEWALT ServiceNet

Dewalt nailers have some vinyl orings that fail. Usually found within the trigger valve, headvalve, and between the cylinder and cylinder spacer (white plastic piece). A dewalt nailer with failing orings may not leak air however. The vinyl orings tend to start to crumble and the tacky-crumbling pieces tend to create a seal. Easy way to check condition of the vinyl seals in a Dewalt is to remove the trigger valve and look at the orings on the firing pin/stem (2mmx1.5mm orings). If these two orings are soft and sticky, then orings inside of gun most likely the same. Check band on the outside of the cylinder (part # 390159-00) is what causes piston to return to top. But also check to too tight or too loose piston oring, bent shaft of driver and worn/ broken lower bumper. If all is well with internal parts and piston does not return to top, too much air is leaking past nose of gun. Oring kit sold by dewalt will contain most of the orings needed. The kit may not have orings for the trigger. You will at need two orings (2mm x 1.5mm) for the trigger valve stem or just replace the valve. Use white grease or silicone paste lube on all the orings. Good luck.

Remove the cutting head spool and see if you have old grass stuck around the shaft area that could be causing to cease your shaft..

I'm assuming you have a DCD970 as DeWalt doesn't have a DC970 and the DW970 is an impact driver. The chuck on your drill is a new design that doesn't have a lock screw inside the chuck, they are put on very tight with lock-tite to hold them on. They are made to be taken off with a special chuck holder and vice jig that only service centers have. The allen wrench method used on older chucks won't work as the chuck will just loosen when you hit the allen wrench. I have on occasion removed one by doing the following: Turn the chuck fully open then secure it in a vice with the drill straight up. Holding the drill body and using the handle for leverage, turn the drill counterclockwise, as you are looking down on it, with steady, strong pressure. If the chuck breaks free it will spin off easily. If you are at all afraid you may break the handle you may have to take or send it to a service center to have it removed.

The chuck on your drill was made to be taken off with a special jig and holder that cost $175+. Depending on how tight and how much lok-tite the factory used some chucks can be removed without the jig. You need a hex shaft at least four inches long and 3/8" to 1/2" wide. Lock the hex-shaft tightly in the chuck, then put the shaft in a vice with the drill straight up and down. Using very firm and steady pressure, turn the drill using the handle for leverage until the chuck breaks free. Don't bounce the pressure or use an impact wrench as this is very ******* the transmission. When you put the new chuck on, use a few drops of lok-tite and put it on as tight as you can without breaking the case. Let the lok-tite set up for at least one day.

sound like the throttle shaft cant find a pdf manual on line

Look at eReplacementParts.com and the parts diagrams

Those gears are only held on the motor with their tight fit, if yours is slipping that easily the motor will have to be replaced. The new motor will have that gear on it.

problem could be a broken clutch housing assy. you can purchase the part in assy.