electric brake/miter saw

[LCHIEN]

I have a Craftsman CMS, it has electric braking, when you release the switch it stops quickly, in a about a second. This is of course a safety feature.

For a while now, it has been intermittent in this function. Most of the time it stops quickly but ocassionally, it coasts to a stop (several seconds). No other problems.

Can any tool guru describe exactly how the braking works, and if there's a switch or contact that needs to be cleaned? Is the motor itself shorted out when the switch is released and the back EMF is shunted thus braking the saw?

[TheRic]

If Tom Slick doesn't see this post check with him. He use to repair power tools in his former job.

[LinuxRandal]

Not a tool expert here, but a question. Have you removed the blade and checked to see if the brake pad is wore down in an area, that it doesn't always make contact.

Maybe I'm mistaken, but I thought the pad was supposed to be exposed so it could easily be replaced (not internal).

[LCHIEN]

Not a tool expert here, but a question. Have you removed the blade and checked to see if the brake pad is wore down in an area, that it doesn't always make contact.

Maybe I'm mistaken, but I thought the pad was supposed to be exposed so it could easily be replaced (not internal).

mechanical brake? Really? I thought they were electric brakes.
Does the pad drag on the blade or the arbor? What releases the brake pad to press against whatever it presses against when you release the trigger?

Edit - a quick search of the internet shows that most CMSes advertise electric brakes as a feature. That means that the motor itself is used for braking, not some brake pads through mechanical means. I think the trigger is a double-throw switch that removes the line power and shorts the motor when the switch is released.

[LinuxRandal]

OK, did some searching, I guess they are electric.

I always figured the little pad/bump stop, behind the blade was used on the models with the brakes. Figured you pulled the switch and it popped in, released and it popped out.

Just seemed like a simple solution to me, needless to say mine doesn't have an electric brake.

[Sawatzky]

My Delta saw does the same thing. The electric brake does not seem to always work. The brushes are fine. I too am very curious to the answer to this problem.

[ragswl4]

I noticed the same thing on my Delta CMS and noticed the following. When I make a cut and hold the saw in the down position while releasing the power switch it justs coasts to a stop. If I return the saw to the up position and release the power switch, the brake engages. Must be some kind of actuator in saw that senses the position, switch, spring, etc....

[Tom Slick]

I didn't work on miter saws but I believe it's an electronic brake and it is controlled by the switch/trigger. cleaning or replacing the switch could solve your problem. I say could because It could be a loose wire, etc, but more then likely it's the switch.

[Lonnie in Orlando]

My DeWalt circular saw's electronic brake acts as you describe. The problem started after I ripped some glued-up 2X6's along the glue line. Saw got really hot before I knew it. The DeWalt service center told me that I had probably damaged the motor, which caused the brake to fail. Rather than pay for a repair, I will continue to use it as is until it finally stops running.

- Lonnie

[LCHIEN]

CMS schematic w/brake

So when all fails, go to the manual.
The attached schematic was in the manual. It shows a double-pole, double throw trigger switch.
In the operate position it energizes two field wndings in the motor in series with the armature (Through the brushes).
In the off position, the input Hot (black wire) is disconnected so no outside power is used. A brake winding (grey wire) not used in operation, is placed in series with the two field windings and the armature. I assume by the arrow markings its field is opposite to the field generated y the two "operate" windings, thus providing a retarding force when the armature/rotor is still spinning and generating back EMF into the windings.
This causes the motor to slow down much faster than just coasting with the generator under no load.

The manual troubleshooting chart says, if the blade takes longer than 6 seconds to stop, you may have a brake problem. Check the arbor nut to make sure the blade is secure to the arbor and not spinning (I did that once) and also check the commutator brushes, the switch and the motor brake winding. I would guess, like Tom suggested, it's most likely the switch.

Somewhere I also read that the braking may cause sparking at the brushes, that's normal.

[p8ntblr]

Loring, when your brake was working the blade stop spinning in under a second? My cms has a brake but I was timing it yesterday and mine was taking more like 3 secs. Not sure if it's broken or that's how it's supposed to be. it's a DW708. Anyone know how fast it's supposed to stop for this saw?

[LCHIEN]

Loring, when your brake was working the blade stop spinning in under a second? My cms has a brake but I was timing it yesterday and mine was taking more like 3 secs. Not sure if it's broken or that's how it's supposed to be. it's a DW708. Anyone know how fast it's supposed to stop for this saw?

yeah, I don't think it was under one-second, really.
The manual says if it stops in under 6 seconds, it's working.

[TheRic]

So when all fails, go to the manual. .....

Say it isn't so, you didn't really look in the manual!!! You could lose your title as engineer!!! You should have tore the CMS apart before looking in a manual!!!

[Andrew Benedetto]

My Dad was an aeronautical EE for the Navy for 35 years, had a sign on the desk when all else fails read the manual! Actually he would be the type to really read them.
I seem to recall the brushes contact w. the copper has an effect. Notice the spark there when the brake is activated, adds to the carbon buildup. I clean the copper on the commutator? and the braking improves.