Comment by Spivak
8 months ago
I'm actually kind of surprised that any implementation destroyed the blade. Like I don't actually care that the blade is moving, I care where the blade is moving. It seems like a trigger to yank the blade under the table would be the easier and more obvious way to do it.
A few-milliseconds yank covering up to a couple inches of blade height feels like a harder engineering problem than "trigger brakes already right near the blade to grab the shit out of the blade"
So we're somewhat lucky from an engineering standpoint. Because the blade is circular the only interval of time that really matters is from detection to first movement away. Because it triggers on touch the difference between getting sawed and not is millimeters. The time from first movement to full retraction only needs to be fast on human scale time in case the person's hand is still moving into the blade. Name brand SawStop is actually fairly slow on the retraction because it uses the blade's momentum to drive it and that's plenty of speed.
However, the blade-preserving system puts the explosive between the table and the pivot that's already there for retracting the blade. The full explosion force is there to force the blade down and it ends up being faster than the SawStop. Which while cool the SawStop was already fast enough so it's all the same.
So I don't know, I guess to me I'm surprised that the solution we jumped to first was a brake when the action of moving it out of the way takes far far less energy. It's only the energy to move the weight of the blade and bar down at the requisite speed, instead of needing to absorb the full energy of the spinning blade.
> Because the blade is circular the only interval of time that really matters is from detection to first movement away. Because it triggers on touch the difference between getting sawed and not is millimeters. The time from first movement to full retraction only needs to be fast on human scale time in case the person's hand is still moving into the blade. Name brand SawStop is actually fairly slow on the retraction because it uses the blade's momentum to drive it and that's plenty of speed.
Do you mean in a system with both moving it away and a break?
The only time my fingers hit the blade of a table saw they were moving with a fair amount of momentum and hit first low on the blade - dropping the blade at the speed of gravity wouldn't have been enough.
I haven't seen an explosive system like you mention - is that what Bosch had for a bit? - so I don't know just how fast that is, though dropping a spinning-towards-you blade also seems to have some other potential risks of grabbing shit with it, too. If it's fast enough I wouldn't be concerned as much, but at relatively slow speed it seems maybe nasty.