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Tough piece of wood - saved by the riving kife

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Forum topic by bbasiaga posted 565 days ago 820 views 0 times favorited 8 replies Add to Favorites Watch
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bbasiaga

112 posts in 621 days


565 days ago

I was milling down a 2×4 today for part of a jig for my table saw. The first thing I noticed was that one end of it kept ‘crooking’ even after jointing. The board was flat, but developed about a 5 degree bend at one end. I’d cut it long and didn’t think much of it. I would just cut that part off as I went to final length.

After doing that, it came to rip to final width. So i set the fence and started going. About 11” in, it started to get hard to push though the blade, as if it was dull. Its a new blade so that wasn’t it. There was a knot in the area so I thought I was just getting in to that, slowed down the feed speed and the next thing I know – the circuit breaker tripped. I hit the power switch on the saw and went to clear the piece of wood, and had to literally pry it off the blade and riving knife. It was ON THERE! Once free, the 1/8” gap left by the saw blade snapped closed with such force it activated the electronic cut off on my hearing protection. I couldn’t pry that back open if you paid me.

Obviously, there was some kind of hidden stress in the wood, and had it not been for the riving knife who knows what would have happened. I’m pretty new to this hobby, so I can’t really explain why it happened. Maybe someone more knowledgeable about wood can provide some theories. All I have to say is – Thank God for safety devices.

-Brian

-- Part of engineering is to know when to put your calculator down and pick up your tools.


8 replies so far

View Rick M.'s profile

Rick M.

3872 posts in 1006 days


#1 posted 565 days ago

Weird. I’ve had wood with stress in it but never to that extent.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

View oldnovice's profile

oldnovice

3688 posts in 1994 days


#2 posted 565 days ago

I had that happen to me a long time ago when I still lived in Illinois and my shop was in the basement. There was no apparent reason why the blade stopped rotating!

I had no riving knife (but I did have my blade guard with a splitter) the breaker didn’t trip but the blade refused to turn. The blade was a fairly new 36 tooth carbide tipped Craftsman blade and I was ripping a new 2”X4”!

After I turned off the power to the TS I had a heck of the getting the wood off of the blade. To this day I do not know what caused and it never happened again.

-- "I never met a board I didn't like!"

View Tony_S's profile

Tony_S

420 posts in 1709 days


#3 posted 565 days ago

Knots in lumber can have an extreme amount of stress in and around them, caused simply enough by the lack of definite direction of grain in that area.
Find a board with nice tight straight grain and follow the direction of the grain as it approaches the knot. It will start to travel off and around the knot, also getting much more hard and dense(some knots can have a rockwell hardness that is greater than steel, which is why they dull carbide quickly and can destroy high speed steel knives)Because of the directional change of the grain and the change in density, there is quite often a large amount off stress created in that area as the lumber dries.(one of the main reasons why woods like crotch walnut can be a bitch to work with)
The amount of stress is usually fairly proportionate to the size of the knot vs. both width and thickness of the board your ripping.
I tell my guys in the shop, basically….don’t cut directly through large knots on a table saw. If you need to cut fairly close to a large knot, rough cut (close to finish) on the bandsaw, rejoint if need be and then rip.

Oldnovice,
If there where no knots or swirled grain in the 2×4 you were ripping, it could have been an issue of ‘case hardened’ lumber, which is another dangerous(and frustrating at the very least)beast all together.

Both knots and case hardened lumber can be quite dangerous to cut on a table saw depending on the situation(case hardened lumber is immediately sent back to the supplier). Ive actually seen lumber split halfway through a cut and more often the last 8 to 12 inches of the rip because of internal stress in the board.
I tell the new guys coming into the shop, this is how it happens…..your ripping a board, knots or otherwise, pressure increases as the cut progresses. Natural instinct is to push harder…big mistake. Your hands are traveling in the direction of a meat grinder. If the push stick(which you BETTER be using) slips off the board or the board splits(which isn’t common, but can happen) your hands go rocketing into the blade.
Ive ripped, or been involved in ripping literally thousands of miles of lumber over the years…seen some scarey stuff happen.
Most recently, A young girl ripping a piece of 6/4 white oak about 8”x24” long on a five horse cabinet saw(who couldn’t be bothered to put the splitter back on the saw) got half way through the center of a golf ball size knot….started to pinch the blade and then she panicked, let go of the board and as she jumped back, took the board right across the boobs…ambulance ride.
Another that happened years ago that I’ll never forget, because it was both dangerous AND funny as hell! Similar situation as above but a piece of 8/4 walnut.
I hear a guy screaming HELP from across the (large) shop, look over my shoulder to see this guy in front of the saw holding onto the board with all his might…smoke billowing off the blade…and a look of sheer terror on his face! Another fella that was close buy did a quick sprint and a dive for the kill switch.
All was well, no injuries except to the guys pride. He asked me if he could go home….because he actually shit his pants!

End of ramble LOL!

-- "The trouble with people idiot-proofing things, is the resulting evolution of the idiot."

View rrww's profile

rrww

263 posts in 740 days


#4 posted 565 days ago

Good advice Tony_s on the case hardened lumber – its almost impossible to get that stuff straight. When they crack at the end it will scare the heck out of you.

View Rick M.'s profile

Rick M.

3872 posts in 1006 days


#5 posted 565 days ago

Coincidentally there was a bit on Tommy Mac’s show about this and how good sawyers check for it. The sawyer cuts a foot long piece from a board and cuts a long U notch in it. If the fork closes there is too much moisture in the center of the wood; if the notch spreads apart there is too much moisture on the outside.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

View Cole Tallerman's profile

Cole Tallerman

391 posts in 811 days


#6 posted 565 days ago

That exact same thing happened to me first time I ever used a saw with a riving knife. Haven’t been without one since

View Holbs's profile

Holbs

515 posts in 656 days


#7 posted 564 days ago

thanks for sharing this. i never knew a knot in wood could be so dangerous.
i assume, this would apply to all species, hard and soft?

View bbasiaga's profile

bbasiaga

112 posts in 621 days


#8 posted 563 days ago

The piece I was working on was a softwood species.

-Brian

-- Part of engineering is to know when to put your calculator down and pick up your tools.

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