Rebuah Regel tefillin battim are made square by operating a milling machine powered by foot pedals. Some battim are made ‘Yad Yad’ meaning that the squaring of the battim is made only with hand power. The question is: is ‘yad yad’ really better than rebuah regel? I asked Reb Avraham Tzvi Wosner and he said there is no difference (except the higher price for yad yad). There are also battim that are made by a milling machine that is powered by electricity. These are called ‘mechonah’. They are less expensive than rebuah regel, and are the most common type of gassos battim.