Miter Saw: Excessive Sparks From the Motor

The armature is the power-producing unit, located inside the field of the motor. If the armature is shorting out, it will produce a lot of sparks and, if not fixed, can cause other problem.
To inspect the condition of the armature, remove the belt cover and walk the belt off the pulleys. Some models will have screws holding a sliding bracket in place – loosen these screws. Loosen the belt tensioning screw at the back and remove the belt. Remove the motor end cap and slide the tension spring over.
Now, take out the brushes. Some models have brush caps on the side, which can be removed with a screwdriver, to take out the brushes. Remove the dust bag bracket, followed by the armature housing from the saw. Lightly tap the end of the armature free of the housing only with a rubber mallet, or brass hammer, to avoid damaging the armature shaft.
Clean the armature only with electrical contact cleaner and wipe dry. If the armature has burn marks on it, replace it. Burn marks indicate sparking and a bad armature. If the armature looks good, you can test it to verify its condition.
There are three test to use when checking an armature. The first is a continuity test. With the armature removed from the unit, stand it up on a bench. Attach one end of the ohmmeter to the shaft, for ground, and the other to the bar on the commutator. Move the probe to each bar on the commutator, checking all of the bars. If the ohmmeter shows a reading on one of the bars, this will indicate a short and a bad armature.
The second test is a bar-to-bar test to determine the resistance. Attach one probe to a bar on the commutator and the other probe on the bar right beside it. There will be a specific reading for this, depending on the design of the armature. Most importantly, you do not want a large fluctuation here between the values which would indicate a fault. If there is an increase in resistance, this will indicate there is a broken, or burned out, wire in the coil. If the resistance drops, then there is a short. Move around the commutator, checking each bar.
The third test is the 180-degree test. Attach the two ends of the ohmmeter on the commutator bars, directly across from each other. There will be a specific set of readings for this, depending on the design of your unit. Once again, what is most important is that there is not a large fluctuation in values, indicating fault. If there is an increase in resistance, that would indicate a broken or burnt wire. If the resistance drops, that would indicate that there is a short. Continue this test on all the bars. If an armature fails any of theses test, it is recommended that you replace it.
Armatures are sold as a complete unit, with the bearings and fan. You will have to transfer the pulley over, though. Heat up the pulley on the old armature and remove it. With the pulley still hot, install it onto the new armature shaft. Make sure to remove and reinstall the key way with the pulley. Insert the armature shaft into the armature housing. Install the pulley screw. Tap on the pulley screw, only using a hammer to avoid damaging the armature shaft to seat the lower bearing into the motor housing. Reattach the motor assembly to the miter saw and reinstall the brushes and the motor end cap. Reinstall the dust bag bracket, followed by the belt and belt cover. 

If a bearing is seized, it will put a strain on the motor and cause it to excessively spark. Repair procedures on bearings will vary from model to model, as they are different, but repair procedures will be similar.
To check and replace the armature bearings, remove the belt cover and walk the belt off of the pulleys. Some models will have screws holding a sliding bracket in place – loosen these screws. Loosen the belt tensioning screw at the back and remove the belt. Remove the motor end cap and slide the tension spring over and then pull out the brushes. Some models will have brush caps on the side, which can easily be removed with a screwdriver, to access and pull out the brushes. Remove the dust bag bracket and then the armature housing from the saw. Lightly tap the end of the armature free of the housing only with a rubber mallet, or brass hammer, to avoid damaging the armature shaft. Spin the bearings on the armature shaft to determine which one is bad.
For the bearing beside the fan, remove the pulley screw. Heat the pulley up with a heat gun to remove it from the shaft. Take out 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 entire shaft. If there is no room between the bearing and the fan for a bearing puller to be used, you will have to score the bearing with cut off wheel on both sides and then insert the bearing into a vice and tighten, until it breaks off.
With the outer race off, use needle-nose pliers to remove the cage and balls. Install the bearing separator onto the inner bearing race and then install the screw back into the armature shaft, to prevent damage from the bearing puller. Install the bearing puller onto the separator and proceed to remove the inner race of the bearing. Wipe the shaft clean and install the new bearing onto the shaft. Make sure to install the keyway with the pulley. Install the pulley screw.
To replace the other bearing on the armature shaft, use a bearing puller to remove it and then install the new one on the shaft, using a socket that only makes contact with the inner race of the bearing. Tap it in place using a hammer. Slide the armature back into the housing. Tap on the pulley screw, only with a hammer to avoid damaging the armature shaft to seat the bearing into the motor housing. Reattach the motor assembly to the miter saw. Reinstall the brushes and the motor end cap. Reinstall the dust bag bracket and then the belt and belt cover.

The carbon brushes supplies electricity to the armature by riding on the commutator. Over time, the brushes will wear out. When this occurs, they will produce more sparks than normal and will need to be replaced. To check the condition of the carbon brushes, remove the end cap of the motor. Release the tension on the springs by pulling the spring to the side with needle-nose pliers. Pull out the carbon brushes. Brushes must be replaced if they are 1/8 of an inch, or less. If the brushes are worn out, it is also a good idea to clean the commutator. Undo the screws securing the armature housing to the miter saw and pull it back. Using 600-grit aluminum oxide sandpaper only, clean the commutator in the direction of travel only. Clean in between the commutator bars with plastic scribe. Clean only with electrical contact cleaner and wipe dry. Reattach the armature housing to the miter saw. Install new brushes and reapply the tension and reattach the motor cap.

If the field is shorting out, it will produce more sparks than usual. The field will have to be repaired immediately as it can cause multiple problems, if left unfixed.
To access the field for inspection, remove the belt cover and walk the belt off the pulleys. Other models will have screws holding a sliding bracket in place – loosen these screws. Loosen the belt tensioning screw at the back and remove the belt.
Remove the brushes from the miter saw by taking off the motor end cap and sliding the tension spring over, pulling out the brushes. Some models have brush caps on the side, which can be easily removed with a screwdriver for easy access for brush removal. Remove the dust bag bracket and then the armature housing from the saw. Remove the hold-down screws that secure the field in the housing. Take a photo of the electrical connections for when you reassemble.
Disconnect all electrical connections and pull the field out of the housing. Clean the field only with electrical contact cleaner and inspect the field for burn marks or anything else out of the ordinary. If there are burn marks on the field, it needs to be replaced as it is shorting out.
You can also verify the condition of the field with a multimeter, if it looks good to verify its continuity. Set the multimeter to ohmmeter and place a lead on each end of the coil in the field. You will have to perform this test for each coil in the field. If the reading is infinity for one, or more, of the coils, then the field is bad and requires replacement. If there is a major variance between the coils, then the field needs to be replaced.
To install a new field, connect the wire connections referring to the picture you took. Install into the housing and secure it in place. Reinstall the armature assembly to the miter saw and tighten the screws. Reinstall the brushes, the motor end cap and the dust bag bracket. Reinstall the belt, reapply the tension and lastly, put the belt cover back on.

Some models have brush caps that hold the brushes in place on the commutator. Brush caps are used for easy access to the brushes and can simply be removed with a screwdriver. If a brush cap is broken, it will not be able to hold the correct amount of pressure on the commutator – leading to excessive sparking. Visually inspect your brush caps for anything out of the ordinary and replace them.