John Gill Commentary 1 Chronicles 20:3

"And he brought forth the people that were therein, and cut [them] with saws, and with harrows of iron, and with axes. And thus did David unto all the cities of the children of Ammon. And David and all the people returned to Jerusalem." — 1 Chronicles 20:3 (ASV)

(See Gill on 1 Chronicles 20:1).