From: Rabbi Moshe Sofer
To: Rabbi Avraham Auerbach
The question is whether a mohel may prepare and sharpen his fingernail on Shabbos to use as an instrument for priah during circumcision. The Chasam Sofer rules this constitutes a full Torah prohibition, classifying it not as mere tikkun but as boneh kli gamur — creating a complete instrument — rendering the mohel liable under makeh b'patish.
Source: Shu"t Chasam Sofer OC §71