The Redi-Edge® Challenge Coin Knife Sharpener is unmatched in its design and effectiveness. The convenient “challenge coin” design enhances the portability of the sharpener. The Air Force version of the Redi-Edge® Challenge Coin is engraved with the USAF logo on the front and the US Air Force Wings on the back. The Redi-Edge® Challenge Coin’s Duromite™inserts are harder than any knife blade; and factory set at a 40° inclusive angle, maintaining a consistent 20° double edge on your knives. Produced from military-grade aluminum for extreme durability, long life, and lightweight.