I was participating in a discussion recently in which someone had been attempting to raise a table saw blade through a new zero clearance insert (ZCI). The saw had a riving knife installed and the instructions that were provided with the machine did not address the fact that it is not possible to raise a running saw blade up through a ZCI without first removing the riving knife.
In fact, the instructions that came with the ZCI implied that it was indeed possible to do this. At any rate the guy did try to crank the blade up with the riving knife in place. The ZCI had a locking feature so when the knife came in contact with the underside of the ZCI, it was deflected downward and came in contact with the saw blade. There was some question about how much force the user was applying by continuing to crank the blade up once he felt resistance. Obviously, it was considerable since it was sufficient to actually bend the riving knife down into the blade. The problem was exacerbated by the fact that this particular machine was a SawStop and the instant the riving knife contacted the blade, the braking mechanism was triggered.
(It is interesting to note that the SawStop people became aware of this discussion and subsequently stated that all new ZCIs would have a warning not to attempt to cut through it without first removing the riving knife. Furthermore, they provided the customer with not only a new brake cartridge free of charge but also replaced his damaged carbide tipped saw blade.)
Most of the ensuing discussion leaned heavily toward the apparent lack of common sense the guy displayed. If you look at a saw with a blade and riving knife installed, it is very obvious that the knife is going to hit the ZCI and prevent the blade from cutting through it. There was some consensus that the idea that it should be incumbent on a manufacturer to warn about the danger in attempting something obviously impossible. The fact that this particular manufacturer was not only willing to acknowledge the event as preventable via the addition of a warning sticker placed on all ZCIs sold from that date forward, but go so far as to treat the whole thing as a “warranty” issue and take care of the guy at no cost, is commendable.
This whole thing reinforced my theory that most of the ways in which an accident can happen in a woodworking shop have yet to be discovered.