arm saw motor hums - Woodworking Talk - Woodworkers Forum
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post #1 of 8 Old 09-27-2019, 11:18 AM Thread Starter
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arm saw motor hums

(just some context, I am a high school student with no job, drivers license, car, or money)
my Dewalt 7749 radial arm saw belonged to my grandfather. it worked at first but after my parents brought it from my grandparents to our house,( a 3 hour, 170-mile drive one way) I had to take the saw apart to clean it (it sat for 30+ years inside a barn/workshop that was built in the 70s)

now it just hums and doesn't rotate when you turn it on. I can't separate the stator and rotor from the frontal motor housing because I can't get a bearing of the rotor at the back end of the motor.

I don't have a bearing puller and I think the motor brake is keeping the rotor from turning so I will take any tips on how to remove the bearing or even tips on how to fix the motor without taking the stator and rotor off
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post #2 of 8 Old 09-28-2019, 10:14 AM
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Check over on the vintage machine forum.
Aside from that take it to an electric motor shop.

I know that there is a centrifugal brake on some motors, but I doubt on this one. But my knowledge is limited.
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post #3 of 8 Old 09-28-2019, 10:38 AM
where's my table saw?
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I took one apart here....

This one was a Craftsman and without any photos of your it's difficult to know what to advise you to do. Typically when a motor just hums it needs a starting capacitor. See You Tube for more information.

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #4 of 8 Old 09-28-2019, 02:19 PM
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It could have two types of motor. One type has brushes and you will find them under little round screw-in plastic caps on the opposite sides of the motor. Be carefull when removing them, since there is a spring behind the cap. You can remove these to check their length replace them if the length between the spring and the end is less that about 1/4 inch. Motor repair shops and better hardware stores have generic brushes, so take your's with you to match them up with what's available. Does the motor spin freely? Maybe the bearings need attention. It brush replacement and bearing lubrication doesn't solve the problem, take the motor to an electric motor repair shop for further diagnosis/repair.

In the second type of motor there are no brushes. There is a capacitor connected to the motor wiring and hidden inside the case somewhere. It is usually a round black plastic cylinder with two wires. You likely need to replace this. The physical size as well as the electrical size is important, since it won't likely fit if it isn't the same size. The electrical measurements should be printed on the side and should say something like "70 mfd 270 vac". You need to buy a replacement using the electrical measurements from the side of your capacitor as well as it's physical size. You can look for one online, or locally at an electric motor service shop, Grainger, Johnstone Supply, or other industrial supply company near you. Fastenal might have what you need too.

Now the second part of the problem. In one end, usually the non shaft end of the motor, there is a switch that opens it's contacts as the motor spins up to speed, and closes it's contacts as the motor slows down. Clean the contacts of this switch by sliding a piece of emery or crocus cloth sand paper folded in half between the switch contacts. You want to slide it back and forth to clean the oxides off the contacts to expose shiny metal on both. Be careful not to bend the switch contacts as they need to go tightly together when the sand paper is removed. Now look at the centrifugal mechanism on the motor shaft that operates this switch. There are two weights with springs that swing out as the motor spins. They slide a plastic collar on the motor shaft as they spin out moving the collar and the collar moves to open the switch. For this, you need to make certain that the surface of the motor shaft, where this collar slides, is clean and shiny, so the collar can slide freely. After cleaning, a tiny bit of light machine oil, like 3-in-one oil can be applied, but it should be a very light coating as you don't want it dripping or flying off the shaft as the motor spins.

After you replace the capacitor and clean the switch contacts and the shaft area where the collar needs to slide, put the motor back together and make certain that the motor spins freely and maybe give the bearings a little oil before testing it.

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post #5 of 8 Old 09-28-2019, 02:42 PM
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Can't help you from here, and can give you a link to the manual:

Wise men speak because they have something to say; fools because they have to say something -Plato


Last edited by FrankC; 09-28-2019 at 04:10 PM.
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post #6 of 8 Old 11-22-2019, 08:56 PM
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Some motors have a starting circuit. Once it starts spinning, it sort of shift gears (electrically) and runs at normal speed. There are contacts in the motor that might have some dirt in the contacts. As the motor spins up to speed, there are some weights that fly out and that is what causes the contacts to switch to other contacts.
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post #7 of 8 Old 11-22-2019, 10:53 PM
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Here is a link to the original manual

The question that I have, did you disconnect any of the wires during cleaning? It sounds like to me that the stator and commutator are in the wrong phase relationship.


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post #8 of 8 Old 11-23-2019, 04:18 PM
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It is an induction motor so there are no brushes, but their is a start capacitor and potential/start relay, if there is a small box somewhere on the motor they would be inside of it, otherwise they will be inside the motor case, just guessing I would guess the relay, caps don't open/short during a trip, a relay could because it is getting jostled around

Don't get your hopes up real high though, if they are bad, you probably won't be able to find a replacement that will fit where the OEM part was a lot of those parts were proprietary

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arm saw motor hums, dewalt, electric motor, radial arm saw

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