Food preparation is permitted on Yom Tov, and certainly on Hol HaMoed. We are therefore not all that surprised to learn in the Mishna that setting up a stove or an oven is permitted on Hol HaMoed. Similarly, the establishment of a grindstone in a mill is permitted, since producing flour for the holiday is permitted. Rabbi Yehuda limits this, in that he says ein mekhabshin et ha-reihayyim ba-tehilla – forbidding one le-khabesh (to chisel) the millstone.
Two suggestions are raised by the Gemara in an attempt to explain what mekhabshin means. According to Rav Yehuda it means cutting grooves into the stone that has become smooth so that it will grind better. According to Rav Yehiel it means to make the hole in the center of the stone, in which the grain is placed for grinding.
The Ra’avad points out that it is not clear which explanation is the more surprising one. On the one hand, making the eyehole in the stone is the work of a trained craftsman, and we usually forbid such work on Hol HaMoed; on the other hand, even without the grooves, the millstone will work and successfully grind flour, albeit with some trouble, so fixing it on Hol HaMoed may be seen as unnecessary work on the holiday.
Millstones come in pairs. The base, or “bedstone,” is stationary. Above the bedstone is the turning “runner stone,” which actually does the grinding. Although there are many types of runner stones, depending on the type of mill, it is essential that this stone have a central hole into which the grain is poured so that it can be distributed between the two stones and crushed into flour.
In order for the stones to successfully grind the grain, grooves must be made in the stones so that the kernels of grain will get caught and crushed between the stones. Since these grooves can become worn down with time, it becomes necessary to chisel out new grooves in the stone.
Although most of us no longer work with millstones on a regular basis, the Shulhan Aruk (Orah Hayyim 540:8) rules in this case like the Tanna Kamma and permits both making a hole in the millstone and gouging out grooves on Hol HaMoed.