When you are ripping a pc. of wood on the table saw your blade makes an 1/8" (or so) kerf (wood turned in to sawdust where the blade cuts). What happens, many times, and especially with pine from big box stores that is improperly dried, not dried completely or dried too quickly, is that the grain is under tension. When you
cut through some of this, sometimes, it causes the grain to move and when it does, it closes the kerf. Basically, like taking the palm of your hands and putting them on both sides of the blade and trying to make it stop- something has to give when the kerf closes. Either the blade stops spinning, or it thows the wood back at you. Very dangerous and it happens in the blink of the eye.
When I first started teaching wood shop about 6 years ago, I used the machines we had in the shop. A good ole' Delta Unisaw with no guards whatsoever. Well, it got me one day. I was making a
cut for a student- one that I didn't want him to make and before I understood very much about kickbacks, etc. That pc. of wood came out of there like a rocket and tagged me a few inches away from my youhaha. It dropped me to the ground and freaked out most of the students. It didn't break skin but like you, I could see the imprint of the corner of the board. I was black and blue for 3 months, literally. I joked that, "Well, I already had kids........"
One of my better students was making a
cut and doing everything he was supposed to (guard, splitter, push sticks, standing to the side of the blade, etc.) A board kicked out and flew clear across the room and hit another kid in the back. The kid it hit was just getting ready to make a
cut on the radial arm saw (that could have been bad!). He let out a few choice words but was ok.
Right next to the radial arm saw, in my old shop, was a large garage door that had many pcs. of plywood screwed over broken windows. I had always thought it was from vandals but after those two situations, I knew it was from years of kickback on the table saw.......the door was about 30ft away but in direct line of the table saw.
Be careful, use a splitter/anti-kickback device (they work WELL and worth EVERY penny as you now know). Stand to the side of the work, if you can. AND MAKE YOURSELF SOME PUSH STICKS AND USE THEM!!!
I took a pc. of pine and made a
cut about half way through it. I held it down and shut the machine off. I had to use a screwdriver to pry the board off of the blade. It can have an unbelievable grip on the blade. Anyway, you can see where it closed up the kerf you and can't open it back up with your hands. I keep the pc. of wood as a demonstration to my students as to what can happen. If I had a camera, here, I'd take a pic of it for you and post it.
edit: check this out, it's not me....
http://www.metacafe.com/watch/910584...demonstration/
**I've seen more kickbacks or possible kickbacks from the kerf closing up on the blade......so there are several ways....
http://www.youtube.com/watch?v=sVpGi85HfnY
This is an inexpensive splitter.......see the kerf/
cut in the board........imagine if the little plastic splitter wasn't there and the kerf began to close up and grab the blade with a lot of force...
http://www.rockler.com/product.cfm?page=10889
sorry i keep adding to this but the thoughts keep coming..
when you are cutting and it gets hard to push for some reason, that is the kerf closing up.....sometimes you
cut past the tension and all is fine......sometimes the tension increases to a point where you can actually feel it pushing back against you......at that moment, you either need to press the board down with your hands and turn the maching off with your knee or use one of you hands (keep holding lots of down pressure) OR in a worst case scenerio, step to the right side, let go (if you can safely) and let her fly....i've had to do both of these, many times...