QUESTION: In the Mishnah, Beis Hillel and Beis Shamai agree that one may put beams on top of olives or grapes in the olive press or winepress at any time before Shabbos.
RASHI (DH she'To'anin) writes that the act of squeezing grapes or olives itself is an Av Melachah. Rashi's words are difficult to understand for two reasons.
First, squeezing juice out of a fruit is not an Av Melachah, but it is a Toldah of the Melachah of "Dash," threshing.
Second, Rashi himself later in the same comment states that squeezing grapes or olives on is a Toldah of the Melachah of threshing!
Why does Rashi first write that squeezing grapes or olives on Shabbos is an Av Melachah?
ANSWER: RAV DOVID COHEN (in GEVUL YA'AVETZ) explains that Rashi uses the phrase "Av Melachah" to refer to any Melachah d'Oraisa of Shabbos (as opposed to an Isur d'Rabanan of Shabbos, or a Melachah of another Halachic category, such as working one's field during the Shemitah year). Rashi's source for calling a category of work with regard to Shabbos (whether it is an Av or a Toldah) an "Av Melachah" is the Gemara in Pesachim (5a) which says that from the fact that Rebbi Akiva said "kindling is an Av Melachah" we derive that the prohibition of kindling on Shabbos is a category of Melachah with regard to Shabbos and is punishable with Kares, and it is not just a Lav punishable with Malkos.
We see this usage of the phrase "Av Melachah" again in Rashi later (68a, DH Av Melachah). Rashi there says that the words "Av Melachah" in the Mishnah are not meant to exclude Toldos, but rather they refer to any category of Melachah.
OPINIONS: The Gemara cites a Beraisa that says that one is permitted to open his sluice gate, before Shabbos, to allow water to flow into his garden or field on Shabbos. Similarly, mid'Oraisa one is permitted to put wheat into a watermill before Shabbos and have the mill grind the wheat on Shabbos. The Rabanan, however, prohibited letting a mill operate on Shabbos because it makes excessive noise (according to Rabah).
Does this Gemara have any implications with regard to setting a timer or "Shabbos clock" prior to Shabbos in order to cause a Melachah to be performed on Shabbos?
(a) RAV MOSHE FEINSTEIN (in IGROS MOSHE OC 4:60) rules that one may not set a timer before Shabbos in order to cause a Melachah to be performed on Shabbos. He writes that although the Gemara permits certain actions to be done prior to Shabbos when the results of those actions will occur on Shabbos, that allowance applies only when the process of the Melachah began prior to Shabbos. In the case of Shabbos clocks, however, the Melachah begins on Shabbos.
Rav Moshe Feinsten writes two reasons why the use of a Shabbos clock should be prohibited. First, the Halachah prohibits one from telling a Nochri to perform Melachah for him on Shabbos. Just as one may not tell a Nochri to perform Melachah for him on Shabbos, one may not "tell," or program, a mechanical device to perform Melachah for him on Shabbos. Second, Rashi (DH she'Yitchanu) explains that one may not have his millstone operate on Shabbos because the noise that it makes is a disgrace to Shabbos, and if people were to have their mills running on Shabbos they would be transgressing the Mitzvah of Kavod Shabbos, honoring the Shabbos. Similarly, setting a Shabbos clock to do Melachah on Shabbos is a disgrace to Shabbos and a violation of the Mitzvah of Kavod Shabbos.
Nevertheless, Rav Moshe Feinstein does permit the use of a Shabbos timer for setting lights to go on and off. Even though he prefers to disallow the use of a Shabbos timer altogether, he permits using it for lights, because it was the accepted common practice in Europe to have a Nochri extinguish and re-kindle the lights in the homes of Jews at given hours. We do not have to be more stringent with regard to a Shabbos timer.
(b) Other authorities differ with Rav Moshe Feinstein's ruling. The CHAZON ISH (OC 38:2-3) permits setting a Shabbos timer to cause a Melachah to be performed on Shabbos, and this is the Halachah as recorded in SHEMIRAS SHABBOS K'HILCHASAH.
RAV SHLOMO ZALMAN AUERBACH (in MINCHAS SHLOMO #11) even permits changing on Shabbos the time that a Shabbos timer is set to perform a Melachah by altering the controls in such a way that one delays the action that the Shabbos clock is set to cause, because doing so is not considered an act of performing a Melachah. (For specific details about the proper way to change the time of a Shabbos timer on Shabbos, see SHEMIRAS SHABBOS K'HILCHASAH 13:25-27).