Before using a drill turn on the power and check it for trueness and vibration. Do not use a drill bit that wobbles or is slightly bent. Trueness may be visibly checked by running the motor.

The most common error made by the inexperienced person is to hold a portable drill at an incorrect angle to the work. Make sure the drill is held at right angles to the work. When you are drilling in a horizontal position, you can see if the drill is too far to the right or left, but it is difficult to tell if the rear of the drill is too high or too low. Until you

Figure 13-12.-Pneumatic drills.

learn how to hold a drill at the correct angle, another person should sight the angle before starting the drill. Another common mistake is to put too much pressure on the drill. Pushing or crowding a drill may break the drill point. It could cause the drill to plunge through the opposite side of the sheet and leave rough edges around the hole. It could also cause the drill to side slip on the metal, causing hole elongation. The drill should not be stopped immediately upon breaking through. It should continue to be inserted for approximately half its length while still running, and then withdrawn. This operation requires judgment and skill because it is very easy to ream the hole. If this is done properly, cleaner holes will result.

Pneumatic drills are available in various sizes and shapes. The drills are designed to provide a rotary shaft that is equipped with a chuck capable of holding a drill bit. Most are powered by a vane air motor, and the speed is adjustable by using the variable restrictor built into the motor body. Normal maintenance of the unit requires only a clean, dry air supply and periodic lubrication of the vane assembly. Lubrication can be accomplished by inducing a small amount of light oil into the air supply. The two most used types are the straight and the pistol grip. See figure 13-12.