# Motor Reversing

Page 8 of 8
• posted on October 14, 2010, 5:36 pm

A car wheel spinning at 5000RPM's isn't stopped within a single rotation by a caliper disc brake. Why would you expect a saw blade to be?
A car brake is designed to avoid locking up, yet that's exactly the behavior you would want with a sawblade, and you'd want it to lock in much less than a single revolution.
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<%-name%>
• posted on October 14, 2010, 7:39 pm

I don't think you'd want to stop that tire in one rotation. <G>
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<%-name%>
• posted on October 14, 2010, 10:13 pm

If a car wheel has a rolling diameter of, say, 17", the car will travel about 4.45 feet per revolution. 5000rpm means the car will be travelling at 22,251 ft/min or 252mph. I'm not surprised given the car probably weighs upwards of a ton.
However, as anyone who has done much driving before the advent of ABS will tell you, it is quite easy to lock up the wheels of a car if you stamp on the brake hard enough.

Because the energy in a rotating saw blade is bugger all compared with a moving car.
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<%-name%>
• posted on October 14, 2010, 11:42 pm
Nobody wants to stop a saw blade in one rev. Convenience stop only.
This was barked at when the SawStop came into comparison.
A car wheel spinning at 5000RPM's isn't stopped within a single rotation by a caliper disc brake. Why would you expect a saw blade to be?
A car brake is designed to avoid locking up, yet that's exactly the behavior you would want with a sawblade, and you'd want it to lock in much less than a single revolution.
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<%-name%>
• posted on October 15, 2010, 1:43 pm

Because the blade masses a tiny fraction of what the car does.

If a car brake doesn't ever lock then the car doesn't stop. In any case, absent ABS it's possible to lock the brakes on any car on which the brakes are in good condition. Don't believe me, put both feet on the pedal and shove hard. If you have ABS then pull the fuse on it first.
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<%-name%>
• posted on October 14, 2010, 3:25 am
snipped-for-privacy@snyder.on.ca wrote:

You want to put that goober under a car where it has to contend with dirt, water, oil, ice, salt, road-kill entrails, and have it work reliably???
Common sense says it won't work and can't be made to work. Don't believe your lyin' eyes.
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<%-name%>
• posted on October 14, 2010, 4:11 am
wrote:

and sawdust. A purpose built caliper could be used on the opposite end of the motor on a direct drive saw - but a drum would be just as effective and smaller for the same stopping power. You don't need the fade resistance of a disk brake for the application, nor the thermal mass of a Lincoln car caliper.
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<%-name%>
• posted on October 13, 2010, 2:12 am
"electromagnet/solenoid would do the trick"
My God man, you've invented the Saw Stop!
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<%-name%>
• posted on October 13, 2010, 2:30 am
On 10/12/10 9:12 PM, Hoosierpopi wrote:

Hardly. There's a lot more more going on with that thing than a simple current running through a magnet.
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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<%-name%>
• posted on October 17, 2010, 6:28 am

I have a Delta radial arm that uses an electromagnet and puck against a disk of the opposite side of the motor from the blade. Fair function- stops the blade in about 6 seconds.
--
Jim in NC