Tuesday, December 28, 2010

What’s the big deal about a riving knife?

A couple folks have written to ask why I’m sort of fixated on having a riving knife for the next table saw I own. A splitter would be better than nothing, but a riving knife would be even better.

So what’s the difference between a splitter and a riving knife? A splitter is a piece of metal the same thickness as a saw blade mounted in a fixed location behind the blade. It prevents the two parts of the board that have been cut from coming back together because of internal pressure and pinching the blade, which leads to kickback. A riving knife moves up and down relative to the blade. One advantage of a riving knife over a splitter is that it can be used in groove cuts. It doesn’t so much help to prevent kickback in groove cuts, though it probably does to some degree, as it makes it unnecessary to remove it. That makes it more likely that it will remain installed for future cuts. The idea is that if you remove the splitter to do a groove cut, you might not put it back on immediately. Any barrier to installing the splitter, no matter how trivial, makes a saw less safe.

When ripping the entire length of a board, a splitter performs almost identically to a riving knife, though the riving knife is closer to the blade and follows the arch of the blade so that there is a bit more protection. A riving knife also prevents cut off pieces from drifting back into the blade. Check out this photo of a riving knife on the Ridgid R4511 saw taken by PurpLev for his review on Lumberjocks. This is a great view showing how close the riving knife stays to the blade at all times. Also, check out this excellent video from Finewoodworking.com that shows how riving knives prevent kickback.

I had an experience once where a riving knife would have been nice to have. I was taking a woodworking class and was ripping a 5’ long piece of cherry. For some reason this piece had a lot of crazy, internal spring in it. During the cut – about one foot in – the two pieces on either side of the just-cut kerf clamped together, pinching the blade and bogging down the saw. I was already out of the way, but I ducked a little bit more just to be sure. In the same motion I was able to turn off the saw. The grip was so tight that I had to drill a little hole on the far end of the blade to release some of the pressure and remove the board from the saw. Needless to say, that was a cursed piece of wood and I chopped it up and got rid of it. I’m lucky that the pinched blade didn’t throw the board back at me. Though I was in the correct position and probably wouldn’t have been hit, I didn’t even want it to come close. A good splitter or riving knife would have prevented the wood from pinching the blade. A riving knife would have been more likely to have been installed in the first place.