Rabeinu Tam holds that we never find a melacha that applies only in a partial fashion. It would seem to me that a partial fashion is possible as for instance in case of Ochel Nefesh and grinding on Yom Tov. I believe it is permissible to grind food in a small kli for use in a household but it is prohibited in a commercial or large grinder used for a large amount of food that would not be consumed on the festival. Can we say that the melacha is the melacha regardless of whether it is a small amount that is ground or a large amount and that the difference is simply that simchas Yom Tov (Ochel Nefesh) pushes the issur away until the needs of Yom Tov are met?
The Rishonim argue (in Maseches Beitzah) about the reason for the Isur of grinding with a large grinder on Yom Tov. One opinion maintains that the Isur is mid'Rabanan, because it appears as through one is preparing for the weekday (accordingly, mid'Oraisa it is not Asur). According to the opinion which prohibits it mid'Oraisa, the reason is because it is not an act of "food preparation" for consumption on Yom Tov, but rather it is only an act of Melachah (since it is preparing food for selling); even though the person himself plans to use some of it on Yom Tov, the Torah did not permit doing an act of Melachah in such a situation even for the needs of Yom Tov.