Miter Saw: Grinding or Squealing Sound

When bearings wear out, they will produce a grinding, growling or even a low rumbling sound and will usually be accompanied by a vibration. To check the condition of the bearings, you will have to access them. These repair procedures will vary on all different makes and models, but will be similar.
For armature bearings you will need to remove the belt cover and walk the belt off the pulleys. Some models will have screws securing a sliding bracket – loosen these screws but do not remove. Loosen the belt tensioning screw at the back and remove the belt. In order, remove the motor end cap, the brushes, the dust bag bracket and then the armature housing from the saw. Lightly tap the end of the armature with a rubber mallet, or a brass hammer, to free it from the housing. Do not use a regular hammer as this can damage the armature shaft.
Spin the bearings on the armature shaft to determine which one is bad – bearings should be smooth and quiet. A bearing gone bad will make a sound and feel rough. For the bearing located beside the fan, remove the pulley screw. Heat the pulley up with a heat gun and remove it from the shaft. Remove the retaining ring with a small screwdriver by walking it up the shaft.
Do not damage the fan. On some models, the fan is not sold separately and, if damaged, you will have to replace the armature. If there is no room between the bearing and the fan for a bearing puller (or separator), you will have to score the bearing with a cut-off wheel on both sides and then insert the bearing into a vice and tighten, until it breaks off.
Once you have broken off the outer race, you will need needle-nose pliers to remove the cage and balls. Next, install the bearing separator under the inner bearing race and then install the screw back onto the armature shaft, to avoid damage from the bearing puller. Install the bearing puller onto the separator and proceed to remove the inner race of the bearing. Clean the shaft and then install the new bearings onto the shaft using a socket that only makes contact with the inner race of the bearing. Then, tap it in place with a hammer. Reinstall the retaining ring. Heat the pulley back up and reinstall on the armature shaft, making sure to install the keyway with the pulley. Install the pulley screw.
To replace the second bearing on the armature shaft, use a bearing puller to remove it and then install a new one onto the shaft with a socket that only makes contact with the bearings inner race, to avoid damage. Tap it in place using a hammer.
Position the armature shaft back into the housing. Lightly tap on the pulley screw with a hammer to avoid damage to the armature shaft and to seat the bearing into the motor housing. Reinstall and secure the motor assembly to the miter saw, followed by the brushes and the motor end cap. Reinstall the dust bag bracket, the belt and lastly, the belt cover.
To replace the saw bearing, remove the outer flange, blade and inner flange. Take out the spindle/transmission assembly. With the spindle/transmission assembly removed, use a bearing separator, or bearing puller, to remove the bearing on the shaft.
Clean the shaft and tap a new bearing in place, using a socket that only makes contact with the inner race of the bearing to avoid damage to the bearing. For the bearing inside the bearing box, use a bearing puller to separate the bearing box (sleeve) from the spindle. Using a socket that makes contact with the outer race of the bearing, tap free with a hammer.
Install the new bearing with the same socket to avoid damaging the bearing and tap it in place with a hammer. Some may have roller bearings located inside the saw housing, which can be tapped out with a punch from the outside – it will look like a cap on the outside. You may need to slightly heat up the housing around the bearing.
Once out, install a new roller bearing using a socket and tap it in place from the inside. Apply some grease to the gears and then install a new gasket.
Reinstall the spindle/transmission assembly into the saw, followed by any washers. Reattach the inner flange, blade and outer flange.  

A worn drive belt will produce a squealing sound. When this occurs, the belt is worn and will need to be replaced or tightened, if loose. Try tightening the belt first – if your unit has an adjustment screw. Some models do not have an adjustment screw, therefore you must replace the belt as it is stretched. Remove the belt cover and walk it off the pulleys. Some models will have screws holding a sliding bracket in place. Loosen the screw on the bracket and the belt tensioning screw at the back and remove the belt. Inspect the belt for uneven wear along the sides, cracks, frays or anything else out of the ordinary that may indicate a worn belt. Install the new belt by walking it onto the pulleys or placing on the pulleys and tightening the tension screw to the tighten the belt. The belt should not deflect more than ¼ of an inch. Reinstall the belt cover.

The carbon brushes supplies power to the armature by riding on the commutator. Since they are in constant contact, they will wear down and produce a grinding or squealing sound. To check the condition of the carbon brushes, remove the end cap to the motor. Slide the tension spring to the side and remove the brush. Brushes under 1/8 of an inch are worn and need to be replaced. If the brushes look good, check them for broken-off pieces. If there are any broken pieces, brushes will also need to be replaced. Depending on the location of the broken-off piece, it can also produce a grinding or squealing sound. Make sure there are no broken pieces inside the unit. Install the new brushes, slide the tension spring onto the brush and reinstall the end cap to the motor. Some models will have brush caps conveniently located on the side of the unit, for easy access. Unscrew these with a screwdriver to remove and replace the carbon brushes.

Gears transfer power between shafts. Worn gears will produce a grinding sound as the gear teeth cannot mesh together. Repair procedures will vary on models.
To check and replace gears, remove the outer flanges, blades and inner flange, including any washers behind the inner flange. Drop the blade guard down and remove the gear box assembly. Clean out old grease.
Now, inspect the gears for broken, chipped or missing teeth. Damaged gears must be replaced. To do so, you may (or may not) need to heat up mounting plate for the gear assembly and then tap the shaft free from the assembly.
Gears may be mounted on the shaft in different ways. Some will be held on by a retaining ring, which you will have to remove. Once removed, the gear will come off – or you may need to tap it off the shaft. Remove the keyway with the gear.
When installing the new gear, make sure to install the keyway or the gear will not rotate. Reinstall the retaining ring on the shaft. Some gears may need to be removed with a hydraulic press, as well as reinstalled with one. Other gears are an all-in-one-unit, with the shaft.
Once the gear, or gears, have been replaced, reinstall them back into the housing. They may go back in by hand or you may need to add a little pressure with a vice. Apply new grease. Install a new gasket and reassemble back into the saw and secure it in place. Raise the blade guard and install any washers on the shaft, inner flange, blade and outer flange and tighten in place.