Moed Katan, Chapter 1, Mishnah 9. This mishnah deals with the utensils used for preparing food on Chol HaMoed, and with the question of which acts of setting up and repairing them are permitted during the festival days.
The words of the mishnah:
"Ma'amidin tanur v'chirayim v'reichayim ba'moed" - one may set up these three utensils on Chol HaMoed:
Tanur - an oven, for heating and baking.
Chirayim - a stove used for cooking.
Reichayim - a hand mill. This is not an especially large utensil, nor is setting it up a major labor; it is a relatively small item.
The ovens and stoves were movable to a certain degree: people would shift them from place to place and attach them to the ground when they wished to use them. It is this setting up that is permitted on Chol HaMoed, so that a person can fix the utensils in place and cook with them for the needs of the festival.
The words of Rabbi Yehudah:
"Ein mechabshin es hareichayim batechilah" - when one comes to use a millstone for the first time, the stones are too smooth to grind properly, and therefore they would first chisel them so that they would be rough and able to grind. This act is forbidden during the festival.
The distinction, then, is between two situations:
New millstones: the chiseling is done at the outset, for the very first time, and this may not be done during the festival.
Old millstones: these need nothing more than maintenance - additional cutting that adds grooves to them - and this act is permitted on Chol HaMoed.
In summary: the mishnah permits setting up an oven, a stove and a hand mill during the festival, since the needs of food preparation were permitted; Rabbi Yehudah, however, distinguishes between the initial chiseling of new millstones, which is forbidden, and the sharpening and maintenance of old millstones, which is permitted.