eli5: How do some tablesaw know when to autamatically stop when it is about to cut you?
In: Engineering
Sawstops have a low voltage current running through them and the blade. When you complete the circuit by touching the blade with your finger, (or running through a nail in old wood, or accidentally touching the blade with anything conductive), a piston fires, instantly dropping the blade below the table and stopping it.
It ruins the blade and the cartridge, but a $75 cartridge and a $100 blade are a lot cheaper than a finger.
It applies a 500-kHz sine-wave current to the blade and measures the voltage response every 6 msec. A sudden drop in voltage indicates a conductive foreign object, like a finger, is drawing current off the blade.
Once the touch is detected, analysis software acts to stop the blade as quickly as possible. When signaled, capacitor sends a surge of current into a fuse wire that holds back a mechanical spring. The current vaporizes the wire within 15 millionths of a second and releases the spring to force an aluminum “brick” into the teeth of the blade.
https://www.machinedesign.com/community/guest-commentary/article/21818329/capacitance-system-stops-table-saws-from-amputating-fingers
Answer:
It’s not the saw that knows, it’s a seprate part called a SawStop that is installed into the saw housing. The SawStop gives the blade a small electric charge and when your finger (which also has a small electric charge) touches the blade, it completes the circuit and sends a signal to the SawStop to activate. Activation uses a spring loaded mechanism to bring the blade to a stop.
Here’s an explanation and some slo mo footage: https://www.youtube.com/watch?v=SYLAi4jwXcs
Lumber with a high moisture content can also trigger the locking mechanism. Hardwood lumber is typically kiln dried to a 7-8% moisture content. Always best to keep a spare break on hand…
Note: SawStop is a brand name. I believe other companies have product lines with the same capabilities.
The blade and your hand connect to create an electrical circuit which fires a brake into the blade almost instantaneously stopping it. The brake is a big piece of aluminium and is fired with compressed gas.
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