Keilim, Chapter 13, Mishnah 3. This chapter continues to examine metal utensils that have been broken or damaged, and asks in each case whether enough of the utensil remains for it to still be considered a functioning kli, capable of receiving tumah. Our mishnah takes up two more such tools: a digging spade and a two-headed axe.
The chirchur
The first tool is the chirchur, a spade used for digging in the ground. Its edge is strengthened with a rim, and that rim is sharpened so that it can slice through roots which would otherwise obstruct the plow.
Here is the ruling itself: "chirchur shenifgam, tamei ad sheyinatel rubo." A spade from which a chip has fallen away, whether the loss came from the cutting edge or from the reinforcing rim along it, has not yet left the category of a kli, and tumah still takes hold of it. Susceptibility comes to an end only once the larger portion of that edge or rim is gone. The line is drawn at a majority: while fifty percent is still in place the spade will do its work of digging, but once more than that has broken away there is nothing left to dig with, and an implement incapable of performing any work does not become tamei.
The mishnah now turns to a different part of the same tool. The handle of the chirchur is fitted into a socket that is attached to the blade. What is the halachah if that socket itself was damaged, so that the handle can no longer be seated in it properly?
"Nishberah makufato, tahor." If the socket broke, the spade is tahor. Its blade may be perfectly whole, but without a socket there is no way to attach a proper handle, and a spade that cannot be held cannot dig. Since it can no longer serve its purpose at all, it is no longer a kli, and it does not receive tumah.
The kardom
The second tool is the kardom, an axe fitted with a blade at each end. One of the two is a broad blade, the sort carpenters use for chopping and hewing wood. The other is narrow, and its work is to split logs into pieces.
Because this tool has two working edges, the loss of one of them does not put an end to its usefulness. "Kardom shenital ushpo, tamei mipenei beit bikuo." If the wide chopping blade was removed, the axe is still susceptible to tumah, on account of the narrow blade that remains, which is still fit for splitting wood.
The mirror case yields the mirror ruling: "nital beit bikuo, tamei mipenei ushpo." When it is the slender splitting end that has been taken off, the axe still receives tumah because of the broad hewing edge, which remains entirely usable. A single surviving edge, whichever of the two it happens to be, suffices to keep the implement classified as a kli.
Here too, however, the mishnah closes with the socket: "nishberah makufato, tahor." If the socket broke, that is, the opening into which the axe's handle is inserted, the kardom is tahor. Neither of its two blades, however sharp and whole, can accomplish anything without a handle to swing it by. Having lost the ability to function altogether, it is no longer considered a utensil, and it is therefore tahor.
The two halves of the mishnah thus teach a single principle from two directions. A tool that retains any one of its working functions, whether half a blade on a spade or one of two edges on an axe, is still a kli and still receives tumah. But damage to the socket, the point at which the tool meets the hand that uses it, disqualifies the entire implement at once, no matter how sound the rest of it may be.