Safe-cracking is the process of opening a safe without either the combination or the key. Physical methods Different procedures may be used to crack a safe, depending on its construction. Different procedures are required to open different safes, so safe-crackers need to be aware of the differences. Lock manipulation Lock manipulation is a damage-free combination recovery method and a well known surreptitious bypass technique. Manipulation only requires fingers, eyes, and proper technique, though it is a skill that takes years to develop. While manipulation of combination locks is usually performed on Group 2 locks, many Group 1 locks are susceptible to manipulation. It involves the manipulation of the lock in order to obtain the combination one number at a time. Manipulation procedures can vary, but they all rely on exploiting the presence of mechanical imperfections in the lock, unlocking the safe and recovering its combination, which can then be reused to open the safe lock. Similar damage-free bypass can also be completed by a brute-force attack from a computerized auto-dialer or manipulation robot. These auto-dialer machines may take 24 hours or more to reach the correct combination, although modern devices with enhanced advanced software may successfully do this more quickly. Mechanical safe locks are manipulated primarily by feel and vision, with sound helping the process occasionally. To find the combination the operator uses the lock against itself by measuring internal movements with the dial numbers. More sophisticated locks use advanced mechanics to reduce any feedback a technician could use to identify a combination. These group 1 locks were developed in response to group 2 lock manipulation. Wheels made from lightweight materials will reduce valuable sensory feedback, but are mainly used for improved resistance against radiographic attacks. Manipulation is often the preferred choice in lost-combination lockouts, since it requires no repairs or damage, but can be time consuming for an operator, the specific difficulty depends on the unique wheel shapes and where the gates rest in relation to them. A novice's opening time will be governed by these random inconsistencies, while some leading champions of this art show consistency. There are also a number of tools on the market to assist safe engineers in manipulating a combination lock open in the field. Nearly all combination locks allow some "slop" while entering a combination on the dial. On average 1% radial rotation in either direction from the center of the true combination