Thread: Electronic Braking on Makita SCMS?

I've had mine for about 8 years and the electronic braking has just started playing up in the last few weeks. It's only annoying when you want to do several cuts in a row and you want to place the blade ona mark before cutting. Otherwise, it doesn't bother me. Probably happens about every 1 in 10 cuts.

Haven't noticed any significant lateral play but then I haven't really checked for it either. I haven't touched the blade since I got it. Probably long over due for a sharpen but it still cuts pretty clean after thousands of cuts.

I had identical symptoms on my SCMS. Posted asking for help and got the advice I'm about to give.

Check that the blade is securely attached to the drive shaft.

In my case the nut had come loose and the braking was working fine, but the blade just kept spinning. At first it span for a few seconds and was up to 1 minute freewheeling before I fixed it.

I felt a bit silly.

I have the Dewalt version and have also been caught out with a loose arbor nut. What is with these machines? My table saw blades never come loose!:eek:

Given it's a very intermitent problem on mine, I doubt it's a loose nut. Well, the one on the blade, anyway. Could be a loose nut holding the handle.

Anyway, the motor on mine makes a noise while it's slowing down. Have either of the loose nuts guys noticed that, or is it silent after you let go of the trigger?

I check my "loose nut" regularly these days... (doesn't that sound a bit funny )

As regardling the electronic braking, my DeWalt also fails to stop around once every 10 goes. It sounds as if the electronic brake has failed to engage (no sound). But I assume that it has something to do with the trigger switch because when it fails to stop if I squeeze the trigger again, the brake always engages. Mild annoyance -- out of warrantee -- I figured I would take it apart and see what's going on, but it is not annoying enough yet for such drastic measures! Regards

The brake on these machine is an electrical brake not a mechanical brake.

A lot of the problem can be bad contact with the brushes on the armature. If the machine gets a lot of constant use everything heats up and the brushes bind up slightly in there brush holders not allowing the brushes to contact the commutator on the armature properly so not allowing the circuit to work properly so no brake or a very weak break.

Another part that could be causing the problem can be the switch if it does not work properly then the circuit is open and the brake will not work.

I've got exactly the same problem with a Makita drop mitre saw (not a slide one)... intermittent braking. I'll try the brushes.
Early on I had some serious "play" in the blade, and thought the Jesus nut had come loose. When I checked it was much worse, the shaft with blade and bearing had fallen out!:eek: Two tiny phillips head screws were all that retained the bearing, much smaller than I would have engineered into the thing. I put them back with a good dose of Loctite and so far its kept the shaft where it belongs.

Cheers,

The actual "brake" on electric saws is electrical not mechanical. It uses the spinning of the blade (motor) to generate a current after the switch is released. This is called back EMF (electro magnetic force) and is applied to the windings as a reverse current and stops the motor really fast. The most likely cause of not working has been already mentioned. ie. brushes and the switch. It is also quite a common failure due to the inherent design.

regards
Ingin

I'd like to take a stab at the braking thing if I may...

The brake does work off EMF as mentioned, simply put when the motor is spinning but without power applied, it's a generator.

If you short out the motor across it's brushes, then all that power is kinda backfed into the motor to power it backwards like, but since it's running forwards, it all comes to a screaming halt very quickly.

On the saw, the braking is not a dead short, it will be a short with some resistance built in to slow the blade qucikly, but not right now. If it was a dead short, it would stop nearly instantaneously, and with a kilogram of steel and tungsten coming to a stop that quick is just asking for shrapnel.


With the intermittant braking, it is possible that the braking device is getting warm and not working as well as it could or has a thermal cutout. After many cuts one after the other, this is not an unreasomable explanation. It's also possible the braking contacts in the switch are a little scruffy, since they are auxilliary contacts and are not likely to be as hefty as the main contacts.

Very likely for an intermittant brake to be electrical, especially if all mechanical causes (loose blade!) have been taken care of.

Thanks everyone for your replies.

I bought Makita because I thought that brand would
give me many years of trouble-free use.
So this is all a bit disappointing, since my machine
is brand new, and had only been used for a couple
of hours (around 80 cuts through 42x42 blackbutt).

I've checked the nut on my blade and was indeed able to
tighten it a little bit further, by about a 1/8-1/4 of a turn.
I didn't notice any obvious difference in lateral play of
the blade after doing this.

"Andy Mac" mentioned:

Early on I had some serious "play" in the blade, and
thought the Jesus nut had come loose.

Pardon my ignorance... what is a "Jesus" nut?

When I checked it was much worse, the shaft with
blade and bearing had fallen out! Two tiny phillips head
screws were all that retained the bearing, much smaller than
I would have engineered into the thing. I put them back with a
good dose of Loctite and so far its kept the shaft where it belongs.

I had a look for these screws, but didn't see them. Does one have to
dismantle the whole thing to find them? (I only moved the blade guard
out of the way, as described in the manual.)

Hi strangerep,
I wouldn't worry about it too much, but the blade has to be off before you'll see the retainer. Jesus nut is used to describe the fitting that holds helicopter rotors where they belong, because you'll be screaming for him if it lets go... old RAAF speak.
I agree with the comment that Makita is bought for reputation...you shouldn't expect anything less than quality equipment.

Cheers,