Way of emergency stopping the blade on tablesaw? EDIT 9-25-10 NOT A GOOD IDEA!
EDIT EDIT EDIT 9-25-10 After trying a couple of things with the tablesaw and listening to the replies, this is a BAD IDEA. It sounded good while I was in bed thinking about it. In practice tho, the momentum involved in trying to stop a 1 1/2 + motor and serveral pounds of spinning metal is tremendous and is best left to SawStop and their elegant but expensive gear. Thanks to John White for summing it up so well.
If I managed to jam the belt AND cut power to the motor, would that stop the blade on my Delta contractor’s saw? I have had the blade stall (too many times) so I know that this probably would not really hurt the equipment too much.
I was laying in bed when I had that thought about how to stop the tablesaw blade without the expense of a new “safety saw” with it’s $70 cartridge.
I figured that a touch light switch using the blade as a touch plate could be used to to jam something into the pulley while tripping a breaker for the motor. It does not stop the BLADE itself, but certainly would stop the blade from rotating! It also would cost nothing to reset. be cheap to implement, and at least would offer some protection.
I removed the pully guard from around the motor and tried sticking stuff into the pully as the saw was slowing down after I turned it off. Carefully! There is a hell of a lot of torque on that pulley, a LOT more than I thought. Crunched a wood stick pretty fast without effect. A piece of electrical cord went around the pully instead of jamming it.
I am looking around the shop now to see if there is something rubbery, round (3/4 inch should do it I figure), and hard enough to stop the belt.
Anyone have ideas about this? I know that I could get the electrical part to work (how to isolate the blade, have the blade wired to the switch touch plate, and trip the motor off), but have no idea of the mechanical aspects. The worst thing I could see happening is the belt breaking and the motor bucking up on its hinged, weighted “tensioner” Another possiblity is to just clamp down on the belt itself but I have a hard time picturing what could do that.
Geez, I could see this as a controversial post! If the moderators feel it is too dangerous for discussion, feel free to freeze it or remove it.
Replies
With two parachute malfunctions I don't scare easily but you are coming close.
Geez...
Have you ever considered taking another class on how to pack your 'chute?
Jammersix
Packing just doesn't get any better than this. Malfunction number one was a borrowed chute. Borrowed from a friend who was an area safety officer, a licensed rigger, and he had packed it for himself to use. In malfunction number two I was hit by a jumper still in freefall while I was under open canopy. He took out one full gore from apex to lower latteral bands as clean as a whistle. If he had hit me, instead of just my chute, we would have both died. He was max tracking and hit the canopy tangent to its curve and didn't tangle in any of my lines. I landed in a corn field and while field packing my canopy I included a few ears inside my pack. Boiled them up when I got home, took one bite...GAAAAAAGGGGG patooey... you guessed it... field corn. Things were just not going well that day..
Heh!
Yeah, the more I think about it, the less I like the idea. Gotta hurt the bearings of the motor if nothing else. If only there was some way of disengaging the pully from the motor in a hurry, THEN clamp things down. But I would like to see a one action response instead of a lot of cascading events all happening at once. Lots of actions means lots of things to go wrong. Just a bang and the breaker trips if the belt jerks up short.
If the pulley to the blade could move sideways, disengaging the motor and jamming the blade spindle to a stop , that would only affect the much lighter and easier to control blade spindle rather than the heavier motor armature.
I have to think about that one...
Contrary to the old song, in the case of a saw emergency, time is NOT on our side. Getting the blade to stop cold in a matter of milli or nano seconds is the objective, and that should be done without requiring user intervention. Once the user feels his or her hand being eaten by the blade, it's already too late.
There is always time, however, to use safe techniques, blade guards, push sticks, etc.
User modifications
This reminds me of the recent law suit over the day laborer who was mangled by an inexpensive saw that wasn't equiped with an electro/mechanical "saw stop", let alone the original OME safety guards and training. Unless you are both a licensed mechanical and electrical engineer, don't mess with the saw. Leave that to the Pros. God only knows what horrific results could happen by tampering.
Some ideas come to mind
1. Have the closed circuit detonate C4 explosive attached to the arbor. Hopefully someone with actual knowledge of explosives will chime in but my guess is you'll be somewhat protected by the cast iron top. As a backup you could also ignite thermite on bearings to instantly fuse the balls/rollers to the races and stop the blade. The bearings should cost less than the SawStop cartridges if you buy them in bulk.
2. OK, maybe you don't want to blow up your saw. How about making a special blade where the carbide teeth are held on by small electromagnets (There's only a small amount of cobalt in carbide teeth so they'd have to be very powerful). Closing the circuit energizes contacts in a magnetic switch which cuts the power to these electromagnets so the considerable centrifigual force will cause the teeth to fly off the blade. You'll still get cut, just not as badly (or cleanly). It will get you back in the habit of wearing your eye protection too.
3. Position the saw on a trap door. When the circuit is closed an electro-mechanical lock on the trap opens and the saw immediately falls into the SawPit (tm). You'll want to put an old mattress or something in the bottom to protect the saw from damage and probably rig some sort of lift system to get it out and reset the mechanism so you can get back to work.
Thanks for my morning laugh. Starting the day with a big smile on my face. Was going to say something about your idea of fuse the balls to the races, but decided I'd better not.
Spring-loaded table-top
How about instead of stopping the blade and dropping it under the table, as the Sawstop does, rig up an auxillary table that is spring loaded to tilt upwards. Hinge it at the front end and set it with, say, 30 lbs of upwards force and install a catch to hold the top down. Just trigger the switch and the table pops up and the workpiece comes clear of the blade. Just remember to duck or stand aside. (Oh, this won't work with a blade guard).
What you are basically describing is the SawStop mechanism, and to stop the blade as quickly as needed to prevent an injury, you have to absorb a tremendous amount of energy in just a few thousandths of a second, you aren't going to do this with a simple brake.
The SawStop cartridge when it hits the blade is destroyed, the large aluminum block that strikes the blade is ripped up and bent, the amount of damage done to the cartridge, but not the rest of the saw, is quite amazing.
John White is right about the energy load. NOT a good idea!
Until I started playing around with my tablesaw, I figured it would be easy to stop the blade. It is NOT easy, and as John White put it, the momentum is way too much for a simple fixture.
Much as I like the IDEA of this, I have to hand it (heh!) to SawStop for coming up with a way to stop the blade.
Well, It was a thought!
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