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Forum topic by tvrgeek posted 02-10-2020 05:21 PM 587 views 0 times favorited 26 replies Add to Favorites Watch
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tvrgeek

621 posts in 2329 days


02-10-2020 05:21 PM

I was wondering while waiting for the table saw to spin down, why don’t most of the power tools have some sort of braking? There are several ways to use back EMF to slow an SC motor. I have seen mechanical brakes on band saws, but nowhere else. I wonder how many accidents happen because of not waiting for a tool to stop.

My old Delta chop saw has a brake, but it is a brush type motor. Induction motors are a little harder, but not much.


26 replies so far

View wildwoodbybrianjohns's profile

wildwoodbybrianjohns

1142 posts in 227 days


#1 posted 02-10-2020 06:06 PM

Just fyi, all table saws for european market must have electronic braking now by law.

-- Wildwood by Brian Johns: If you tell the truth, you dont have to remember anything (S. Clemens) Edit: Now where is that darn pencil/ tape measure!

View Mike_in_STL's profile

Mike_in_STL

1188 posts in 1214 days


#2 posted 02-10-2020 06:23 PM

My Laguna Platinum Dovetail cab saw has some kind of brake. There is a noticeable “thunk” as the motor spins down and the blade changes speed quite rapidly after the “thunk”.

You pose a good question. The RAS I inherited from the grandfather has a manually applied brake on the motor housing and it was manufactured in the late 70s/ early 80s.

-- Sawdust makes me whole --Mike in STL

View JohnMcClure's profile

JohnMcClure

956 posts in 1320 days


#3 posted 02-10-2020 06:42 PM

This would be interesting to try.
A relay connected across the motor’s terminals, that closes about 1/2sec after the power switch is opened, would probably do the trick.
Coastdown doesn’t bother me enough to find out, but it would be a neat experiment. If someone tries it I’d be happy to hear about it!

-- I'd rather be a hammer than a nail

View Lazyman's profile

Lazyman

4820 posts in 2067 days


#4 posted 02-10-2020 07:37 PM

The reason is probably mostly cost. Unless they are required to add it like they are in Europe, they most likely won’t add the cost and if ever are required, they will probably pass on double the cost to the consumer.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View MrUnix's profile

MrUnix

7838 posts in 2879 days


#5 posted 02-10-2020 07:40 PM

+1 to what Nathan sez…

Margins are so slim that even a few pennies can make a big difference with mass produced machines. I can do braking on my Unisaw, but it’s 3-phase. Not as easy on a single phase machine.

Cheers,
Brad

-- Brad in FL - In Dog I trust... everything else is questionable

View Harryn's profile

Harryn

86 posts in 3268 days


#6 posted 02-10-2020 08:31 PM

My 1960’s Craftsman radial arm saw has braking, it consists of two back to back diodes in the motor end caps. Have replaced them twice over the years.

View Scap's profile

Scap

116 posts in 607 days


#7 posted 02-10-2020 08:43 PM

On these items that have brakes, do the arbor nuts have any type of mechanical device to keep them from loosening on shutdown?

View tvrgeek's profile

tvrgeek

621 posts in 2329 days


#8 posted 02-10-2020 08:59 PM

Now that is a good question. My Delta chop saw does not, but the braking is not that hard.

Looking on WIKI about induction motor braking, three phase is easier than single phase.


On these items that have brakes, do the arbor nuts have any type of mechanical device to keep them from loosening on shutdown?

- Scap


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tvrgeek

621 posts in 2329 days


#9 posted 02-10-2020 09:01 PM

That would have been a brush type motor. Very different from an induction motor. You could pretty much just short the brushes and the back EMF would brake it.


My 1960 s Craftsman radial arm saw has braking, it consists of two back to back diodes in the motor end caps. Have replaced them twice over the years.

- Harryn


View PPK's profile

PPK

1670 posts in 1489 days


#10 posted 02-10-2020 09:04 PM

I guess spindown has never bothered me. Doesn’t even seem like a huge safety issue. I’ve always figured that if you’re in such a hurry to do something that you can’t wait for the blade to stop spinning, you’re working too fast and don’t have safety in mind anyway. Further, your hand shouldn’t be by the blade when it’s plugged in to begin with, no? It takes a few seconds to unplug, and that’s always enough for me to let the blade stop. On belt driven machinery (with the exception of the bandsaw) its only a few seconds after shutting down. That’s my opinion for what it’s worth!

Chop saw is a different story I guess. It would be a pain to have to wait for a chopsaw to spin down, since those cuts are repeated and you often use the blade to line up the next cut. But I haven’t seen a chopsaw w/out a brake.

-- Pete

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tvrgeek

621 posts in 2329 days


#11 posted 02-10-2020 09:06 PM

Double would be cheap. Look into actual production cost and how it relates to retail cost. A lot higher.
If they have it in the EU, then the engineering and production investment has already been done. I would not buy only to get it, but might be a tie breaker. As an example, between the 14 inch Laguna and Rikon band saws. Can’t go wrong with either. Only tie breaker is the sale this month may be a lunch better on the Rikon.


The reason is probably mostly cost. Unless they are required to add it like they are in Europe, they most likely won t add the cost and if ever are required, they will probably pass on double the cost to the consumer.

- Lazyman


View bigblockyeti's profile

bigblockyeti

6354 posts in 2400 days


#12 posted 02-21-2020 04:32 AM

My Unisaw has three drive belts and needs to run a good five minutes before getting any heat in them, as a result it slows pretty quickly even with a heavy, full kerf blade installed. My Delta Milwaukee RAS has an electronic three stage brake that starts slowing the blade ~0.3 seconds after power is removed from the motor, after the final stage it might still be spinning 300 rpm or so but with new bearings recently installed, it slows fast enough after that, BTW, it’s single phase.

-- "Lack of effort will result in failure with amazing predictability" - Me

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tvrgeek

621 posts in 2329 days


#13 posted 02-21-2020 02:28 PM

As I have a “new” drill press, I can experiment with the old. Basically, measure the DC resistance of the motor to determine the “normal” equivalent current to the AC RMS current, Then build a DC power supply with the voltage that would be the same current. Basically, just a single phase rectifier, a few resistors and a suitable cap. Looks like a fun little project.

Three phase motors can play games using one phase to generate the current to brake on another. Can’t do that on a single phase.

View ibewjon's profile

ibewjon

1381 posts in 3473 days


#14 posted 02-21-2020 03:22 PM

My grandfather’s 1950’s green DeWalt RAS had a braking system. It had a box on the saw containing vacuum tubes. Unfortunately the saw was lost in a flood and scrapped. Unfortunately Europe is far ahead of the US on safety. My DeWalt table saw doesn’t have a riving knife, but the European model does. DeWalt didn’t want to pay for the US testing from what I have read.

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Madmark2

1017 posts in 1268 days


#15 posted 02-21-2020 03:31 PM

I play a game when I turn off the saw. Can I get the blade cranked down before it stops spinning. You DO keep the blade retracted for safety, don’t you?

-- The hump with the stump and the pump!

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