Makita HR4500C Instruction Manual page 10

If the bit cannot be pushed in, remove the
bit. Pull the chuck cover down a couple of
times. Then insert the bit again. Turn the
bit and push it in until
it
engages.
After installing, always make sure that the
bit is securely held in place by trying to pull
it out. To remove the bit, pull the chuck
cover down all the way and pull the bit out.
10