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Sawstop table saw braking system works by putting a low voltage on the blade, and when it detects that voltage shorted to ground, the blade is retracted and slammed into an aluminum block. The claim is that it can do this in the time it takes 3 tooth to pass, and their video corroborates this. Looks like a 60 tooth blade.

The big downside to this: You have to replace the brake cartridge at around $60 US. and often replace or at least service the blade. No one likes it when they lose a 200 dollar saw blade.

At 3600 rpm, that's 60 rev per second, or 3600 teeth per sec. To stop in 3 teeth is 3/3600 or 1/1200 of a second or not quite 1 ms. This is 1/20 of a single voltage cycle.

The second downside is getting false trips from either wet wood or from nails in the wood.

So what alternatives are technically possible?

Ran into a guy on another forum who claimed that a VFD driven 3 phase motor could stop a table saw blade fast enough that the arbor nut would spin off.

Can a VFD do this in 1 ms without destroying the motor? (I'm thinking in terms of transient voltages) Presumably you have a big fat power resistor to soak up the energy.

The other method I was considering as a sawstop alternative:

A pair of spring loaded (think high performance engine valve spring) brake pads on a set of callipers round the bottom of the blade.

A formula 1 engine is limited to a max 18,000 rpm. 3.3 ms/rev. Valve closings must happen in a small fraction of that time. So the time to close is ball park correct. Can it grip hard enough to stop the blade in 15 degrees of the blade's rotation?

The forces on the blade are considerable. You have to stop the motor too. Typically you're talking a 3-5 HP. These are not pick up in one hand motors.

Options for this: My snowblower (84" wide, 48 hp PTO) has a linkage with two 4" diameter 1/8" steel plates. They are linked with a common 3/4" machine bolt. Hit a chunk of firewood with the snowblower, the bolt sheers. Usually takes me longer to find a replacement bolt than to be up and running again.

The second way would be like my bushhog mower, which has a clutch between the gearbox and the blade. Some industrial sewing machines do this too.

What ways can you stop a moving device fast enough to save injury, but not damaging the machine?

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  • $\begingroup$ Seen a 24" diameter brake disc with 4 calipers stop a wind turbine set of blades full speed to stop... the turbine tower vibrates a lot!! 2 of us were in the tower - brown trouser moment... $\endgroup$ – Solar Mike Nov 4 '19 at 21:29
  • $\begingroup$ Destin in Smarter Every Day made a video about doing something similar with machine learning and a magnetic field: youtube.com/watch?v=OdW7vhYYSdM $\endgroup$ – Wasabi Nov 8 '19 at 1:40
  • $\begingroup$ Very cool. Not directly applicable to table saw. See my attempt at an answer. $\endgroup$ – Sherwood Botsford Nov 11 '19 at 18:46
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First alternative I've run into was suggested by Wasabi in a comment to my question.
The video is here: https://www.youtube.com/watch?v=OdW7vhYYSdM

This example uses a circular saw -- hand tool. In essence they put an accelerometer chip to control the power and braking inputs to the saw. While it doesn't stop as fast as sawstop, it triggers the halt at the onset of kickback, where sawstop triggers on blade contact.

A somewhat different approach could be made on a table saw:

Mount strain gauges on the arbor mount. When kickback starts there is going to be an abrupt torque on the arbor, increasing the pressure on the mounts on the leading edge of the blade, and decreasing pressure on the trailing edge. On top of that the blade speed is going to drop abruptly, and the current rise.

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