To crack the lock on a Sentry safe if you forgot the combination, you need to perform lock manipulation, which is when you work with the lock in order for it to reveal the combination to you. The first thing one must do when performing lock manipulation is to determine the contact points on the lock. Then, determine how many wheels are on the actual combination and graph the results. This will give you a variety of different combinations to try from. Many professional locksmiths use lock manipulation to open safes.