Miter Saw: Doesn't cut Square

If you have a bent or warped fence, you will not get square cuts. Visually inspect your fence to see if it is bent or warped. If one is not visually noticeable, use a straight-edge across the entire length of the fence. If it is bent, or warped, replace it to get square cuts again. To replace the fence, remove the screws securing it to the base and remove the damage fence. Install the new fence and secure it with the screws. 

The miter saw has a bushing on the shaft that the upper portion of the saw attaches to at the base. The saw rotates up and down on this shaft, as you move it to perform cuts. When this bushing wears out, the entire upper portion of the saw will move side to side, giving you inaccurate cuts. To check for this, grab the handle and try to move the blade side to side, looking for play – there should be none. If there is play, this bushing requires replacement. To replace this bushing, remove the stop screw at the back so you can move the saw all of the way back. Remove the set screws, holding the shaft in. Tap the shaft out with a hammer and punch and pull the top of the saw away. Remove the worn bushing and install a new one. Position the saw back into place and hammer the shaft back in. Reinstall the set screws then lower the saw down and reinstall the stop screw, preventing the saw from going back too far.