The Shulhan Aruch presents the Halachot of Mukse in Siman 308.
A utensil whose primary function is for Melacha, such as a hammer, is called a "Kli She’melachto L’isur." Such a utensil may me moved for only two purposes. First, "L’sorech Gufo"-using it for a permissible function. For example, using a hammer to crack nuts. Second, "L’sorech M’komo"-for its place. The hammer is in the way and one needs its place.
The Poskim discuss whether one may use the hammer to crack nuts, even if he can use a "Kli She’melachto L’heter," a utensil meant for permitted actions, such as a nutcracker. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) is strict. He rules that it is only permitted to use a hammer L’sorech Gufo, if there is no other means available. He brings a proof to his opinion in his Sha’are Sion. The Kaf HaHaim concurs with the Mishna Berura.
On the other hand, Hacham Ovadia, in his Halachot Shabbat, clearly holds that once the Hachamim permitted using it, the leniency applies even if there is a better solution available. He marshals a long list of Poskim who are lenient. He also brings proof that Maran himself is of this opinion: If a garment is hung to dry on a rod, Maran permits either removing the garment from the rod or the rod from the garment. The rod is a "Kli She’melachto L’isur," and yet Maran permits moving it, even though there was another option of removing the garment from the stick.
The Menuhat Ahaba (R. Moshe Levi, Israel, 1961-2001) in Vol. 1 and in Tefila L’Moshe takes this one step further. He argues that even if the alternative is immediately available, it is still permitted to take the "Kli She’melachto L’isur" instead of the "Kli She’melachto L’heter." There is absolutely no difference between them. Accordingly, one may take the hammer to crack the nuts even if the nutcracker is on the table, in front of him.
Rav Moshe Feinstein (Russia-New York, 1895-1986) in his Iggerot Moshe (Vol. 5) makes a sort of compromise. He rules that if the nutcracker is immediately available, then of course he should use it. However, one does not have to trouble himself whatsoever to bring the "Kli She’melachto L’heter." For example, if he can bring a nutcracker from a neighbor’s house. He says that even the Mishna Berura would be lenient in that case.
The conclusion of the Sepharadic Poskim is that one may use the hammer in any event. However, if the nutcracker is immediately available, it is better to take the Mishna Berura’s position into account and use it, since there is no difference.
SUMMARY
One may use a hammer to crack nuts, even if he can bring a nutcracker. If the nutcracker is immediately available, it is better to use it, rather than the hammer.
