Q. Can I use a metal or stone knife sharpener for fleshig and milchig knifes? Do they require tvila?

A. Poskim permit using the same sharpening stone or steel for meat and milk knifes and utensils even when used that day (Ben Yomo) and even if the sharpening creates enough heat that reaches the Yad Soledes temperature (about 45°C). However they should be clean, but could be slightly wet. (Shevet Hak’hosi Y.D. 192 – Moriah, year 16: Av: p. 88, quoting Horav Y. N. Karelitz Shlit”a – Ohel Yaakov 92: n. 72.)

They do not require immersion in a Mikva since they don’t come in contact directly with food.

Horav Shlomo Miller’s Shlit”a opinion is similar

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a