Anyone build an electric brake for their RAS?

[Jerome Hanby]

My RAS takes a looooong time to spin down. It's a 120 Volt Dewalt GWI. Found a PDF documenting an add on brake. It basically uses a high current rectifier diode and a length of electric heater wire with a push button. When you flip the toggle switch to kill the power, it swaps the motor leads over to this circuit and the push button lets you place the heater wire across the motor and diode as a load. basically the same idea as regenerative braking on an electric car except the heater wire is getting all the generated power. The instructions indicate that you adjust the length of the wire to get a load that will stop the motor in no less than about 3 seconds.

This information in this PDF file looks like it's been around awhile. At this time and date, I'm not sure where to get the heater wire. I see replacement heaters fro clothes dryers, but I hate to blow $20-$30 for something I'm just going to cut a piece out of. Think I can just find some discarded stand alone electric heater and cut the heating element up into pieces?

Also wondering about making the power switch a double pole, double throw switch and once I have the brake worked out wire that in so the motor is on the throw leads with the AC on one side and the brake circuit on the other. Think that would be an issue?

[David Brimm]

My RAS takes a looooong time to spin down. It's a 120 Volt Dewalt GWI. Found a PDF documenting an add on brake. It basically uses a high current rectifier diode and a length of electric heater wire with a push button. When you flip the toggle switch to kill the power, it swaps the motor leads over to this circuit and the push button lets you place the heater wire across the motor and diode as a load. basically the same idea as regenerative braking on an electric car except the heater wire is getting all the generated power. The instructions indicate that you adjust the length of the wire to get a load that will stop the motor in no less than about 3 seconds.

This information in this PDF file looks like it's been around awhile. At this time and date, I'm not sure where to get the heater wire. I see replacement heaters fro clothes dryers, but I hate to blow $20-$30 for something I'm just going to cut a piece out of. Think I can just find some discarded stand alone electric heater and cut the heating element up into pieces?

Also wondering about making the power switch a double pole, double throw switch and once I have the brake worked out wire that in so the motor is on the throw leads with the AC on one side and the brake circuit on the other. Think that would be an issue?

I haven't built one but heater wire is also referred to as "resistance wire" and you can get that at Jamco for fairly cheap here: http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2119371_-1

You get the resistance you want by varying the length of wire used, if you need it thicker than 16ga you can twist multiple pieces together.

[Jerome Hanby]

That was the term I was missing. Thanks!

Hmmm, PDF says about 7 inches of B & S 20-ga 1.1 ohm heater element wire. I don't know if that 1.1 ohms is per inch, per foot, or what. That wire from Jamco is about 1/4 ohm per foot.

[Anthony Whitesell]

I'm not sure, but if RAS arbors are like tablesaw arbors then stopping the motor too fast may cause the momentum of the blade to unscrew the arbor nut. (one of the reasons SawStop stops the blade and not the motor)

[Alan Schwabacher]

I suspect that the reason this is only in old articles is that electric brakes have been built into saws made more recently. The instructions I found on the web say you want the blade to stop in no fewer than 8 seconds, not 3. As the article states, you need to adjust it to get the motor to stop at the right rate, since too fast could cause loosening of the saw blade, or motor overload and burnout. It also says in the parts list at the end of the article that you want 1.1 ohm/in heater element wire.

Is this the article: http://home.comcast.net/~greg.bruce/Radial_Arm_Saw_Brake.PDF (http://home.comcast.net/%7Egreg.bruce/Radial_Arm_Saw_Brake.PDF) (which seems to be taken from the March 1966 Popular Mechanics on Google books?)

[Larry Edgerton]

I had an Dewalt 16", and that was one of the things that I did not like about it. Kind intimidating....

I have an OMGA now and it has a blade brake, takes about a second to stop. You can use pinned blades on the saw but I don't, and I have never had the blade loosen up.

Well, there was that time I wired it up backwards........

Larry

[Rod Sheridan]

My RAS takes a looooong time to spin down. It's a 120 Volt Dewalt GWI. Found a PDF documenting an add on brake. It basically uses a high current rectifier diode and a length of electric heater wire with a push button. When you flip the toggle switch to kill the power, it swaps the motor leads over to this circuit and the push button lets you place the heater wire across the motor and diode as a load. basically the same idea as regenerative braking on an electric car except the heater wire is getting all the generated power. The instructions indicate that you adjust the length of the wire to get a load that will stop the motor in no less than about 3 seconds.

This information in this PDF file looks like it's been around awhile. At this time and date, I'm not sure where to get the heater wire. I see replacement heaters fro clothes dryers, but I hate to blow $20-$30 for something I'm just going to cut a piece out of. Think I can just find some discarded stand alone electric heater and cut the heating element up into pieces?

Also wondering about making the power switch a double pole, double throw switch and once I have the brake worked out wire that in so the motor is on the throw leads with the AC on one side and the brake circuit on the other. Think that would be an issue?

Jerome, the motor isn't being used as an alternator to supply power to the resistor.

What's happening is that the diode and resistor are being placed in series with the motor and the 120 volt source, the resistor is needed to limit the current drawn by the motor.

When you push the braking switch, it applies half wave DC to the motor, which causes the motor to act like an alternator. The rotating energy is converted to heat inside the rotor shorting bars in the rotor. Don't start and brake the motor frequently unless you have a great desire to melt the rotor bars.

I personally would start with a 25 ohm 100 watt resistor and see where that takes you.

Regards, Rod.

P.S. My Hammer saw/shaper has DC braking, it's apparent due to the growling sound and the vibration. It uses a small electronic modile with a built in timer to feed DC to the motor.

[Bruce Wrenn]

I have a mid sixties Craftsman RAS that had an electronic / electric brake. When I taught school, we had a 14" Delta with electronic brake. It was a PITA, as you had to constantly adjust it to keep from spinning blade nut off. At one school, the teacher made his own brake. It was a spring loaded lever attached to the side of the guard, with a piece of leather that rubbed against the side of the blade. Worked like a disk brake. Push down on lever, and leather stopped the blade.

[Jerome Hanby]

Good point, I failed to see the AC also went to the push button switch. If 100 watt will work, buying several of those would be cheap, could parallel and/or series them to dial in the resistance. POn teh flip side, i found another document with the same schematic that also list sources!

[Rod Sheridan]

Hi Jerome, my saw has the DC brake, however it also has pins to lock the blade to the arbour.

I wonder if the blade nut would unscrew without locking pins?

Regards, Rod.

[Anthony Whitesell]

I think the likelihood of it unscrewing, or at least loosening, is very likely. It will depened on a few things.

How much the arbor nut was tightened.
How quickly the shaft was stopped.
The weight of the blade (ie., how much spinning mass and momentum).

[Neil Brooks]

Would blue loctite keep the arbor nut from backing out (while still allowing you to wrench it loose/tight) ?

[Mark Ashmeade]

Did any of the later DW saws have an electro-mechanical brake?

My 16" Delta has one built in, and so does my toy RIDGID.

The Delta wasn't wired up correctly when I got it, and although it would run, the brake didn't work. With a 16" blade, it took forever to spin down. I guess my motor bearings are OK! Having wired it correctly, the brake is super efficient and stops the 5lb blade in 3-5 seconds.

Perhaps it would be possible to get a later motor with brake, if available, and sell the original one?

[Neil Brooks]

Sort of related: If memory serves ... doesn't a LONG spin-down time ... tend to indicate bad (worn) bearings ?

When I changed out mine, the spin-down time went from something like forever to (I'd have to dig up the actual "test data") 14s.

While a brake might be better than waiting THAT long ... it does make me think about that correlation between bearing condition and spin-down times....

[Mark Ashmeade]

Well I guess it might be, but the flywheel effect of the blade will probably be the major culprit. I'd have thought smooth bearings would help though.

[Anthony Whitesell]

Maybe. But are you going to put loctite on the nut each time you change the blade?

[Jerome Hanby]

I thought the same about my bearings for the same reason, then I saw some forum post citing that as an indication of worn bearings. Not sure which I believe...

I like my saw as it sits (except for the blade spin down time). I'd hate to have to change motors...plus I get to build eh brake <g>.



Did any of the later DW saws have an electro-mechanical brake?

My 16" Delta has one built in, and so does my toy RIDGID.

The Delta wasn't wired up correctly when I got it, and although it would run, the brake didn't work. With a 16" blade, it took forever to spin down. I guess my motor bearings are OK! Having wired it correctly, the brake is super efficient and stops the 5lb blade in 3-5 seconds.

Perhaps it would be possible to get a later motor with brake, if available, and sell the original one?

[Jerome Hanby]

Wonder if those locking pins can be retrofitted?


Hi Jerome, my saw has the DC brake, however it also has pins to lock the blade to the arbour.

I wonder if the blade nut would unscrew without locking pins?

Regards, Rod.

[Neil Brooks]

Maybe. But are you going to put loctite on the nut each time you change the blade?

I might.

It comes in stick form. Sitting next to my RAS ... wouldn't be a huge deal.

[Anthony Whitesell]

And RAS are not like a tablesaws where you change the blade constantly (or atleast I do). Is the arbor long enough for a second nut (jam nut)? Probably would not work with a dado blade, but for those select times you could just switch the brake off.

[Jerome Hanby]

My brake was probably going to be push button activated, so letting a dado blade spin down the long way would be no big deal...


And RAS are not like a tablesaws where you change the blade constantly (or atleast I do). Is the arbor long enough for a second nut (jam nut)? Probably would not work with a dado blade, but for those select times you could just switch the brake off.

[Stephen Yeates]

I considered all of the above to brake my DeWalt MBF motor. Finally solved it by picking up a DeWalt 925 for 80 bucks and sliding its motor and yoke into the MBF arm. It slid right in and didn't even require the roller bearings being adjusted. Now I have a brake on a slightly more powerful and bigger motor that stops VERY quickly. When I picked up the saw the seller told me his Dad had had the motor recently rebuilt. He didn't even put that in the ad! I expect a long life from this motor! I am going to part out the 925 I think. As a bonus the 925's yoke takes later DeWalt motors too. See Frankensaw posts for more info. Thanks to the Delphi forum for telling me the yoke of the 925 would fit BEFORE I bought it.

[david brum]

Stephen, that's a great upgrade. I picked up the same yoke for my MBF, then scavenged a 2hp B&D motor from a free craigslist saw. Works beautifully and I can spin 10" blades, so they're interchangeable with the table saw.

[John TenEyck]

Sort of related: If memory serves ... doesn't a LONG spin-down time ... tend to indicate bad (worn) bearings ?

When I changed out mine, the spin-down time went from something like forever to (I'd have to dig up the actual "test data") 14s.

While a brake might be better than waiting THAT long ... it does make me think about that correlation between bearing condition and spin-down times....

Exactly right. Worn bearings spin forever, new ones slow down pretty quickly. When I changed the bearings in my GWI the spin down time went from too long to remember to less than 20 seconds. That was about a year ago, and they are still in the same range, which is acceptable to me. I thought about the break idea and discarded it from a risk/reward standpoint. I don't want the blade getting loose, and I really don't want to risk frying the motor.

John

[Stephen Cherry]

I wonder why saws do not have eddy current brakes? They would put the blade in a magnetic field, and that would stop rotation. Plus, it would tend to tighten the nut.

http://en.wikipedia.org/wiki/Eddy_current_brake

[Edward Oleen]

just checked the Jameco site, and they say they don't have the resistance wire any more, but to call them during business hours.

Specifically, they said "We're sorry but the product you are looking for is no longer available.
If you would like a replacement suggestion for product number 2119371
then please call us at 800-831-4242 during normal business hours."

Motors have been built with dynamic braking for a long time. I have an electric push-type lawn mower with one: the switch IS double-pole, double throw. There is also made separate dynamic brakes for LARGE motors... multi-horse-power large. Also Multi-Large-Bucks, and Multi-Large-Weight and Multi-Large-Space-Required and several other Multi-Large things, like "contactors" instead of switches...

Several years ago now I did a lot of research (meaning I wasted a lot of time when I could be making sawdust...) into the matter. I heard from a product-liability lawyer I ran into (not literally... he was a lawyer, after all...) in Constantines in the Bronx (before they closed the shop) and he said he thought that they didn't have them on smaller wood-working machines for reason of product liability... It seems that there was a case in which the manufacturer of small airplanes installed new, improved brakes in a minor revision of their single-engine four-passenger aircraft. Someone who owned an older version of the plane ran off the end of a runway and killed himself and his passenger(s). The manufacturer was sued for negligence for not retrofitting all versions of the plane with the new brakes - for free, yet. The manufacturer LOST.

If rapid-stop motors are now on ww machines, then maybe OSHA stepped in?

[Rick Lizek]

Just buy an Ambi-tech retrofit brake for your saw. Braking too fast will loosen the arbor nut.