Sample Student Work Mechanical Pencil Description

Introduction—General Description A mechanical pencil is a writing implement which uses refillable graphite (commonly and henceforth referred to as lead) which can be advanced by a simple button press. Mechanical pencils allow the user to write for extended periods of time without sharpening. A mechanical pencil consists of a lead reservoir, coil spring, chuck, chuck ring, lead retainer, and eraser, all of which are contained by the pencil case body and end cap. The pencil casing may have special features, such as a hand grip and pocket clip. For some pencils, the case can also be disassembled into the end cap and case body. Figure 1 below shows the components of a drafting pencil, which is a type of mechanical pencil. Figure 1. Drafting Pencil Components[1] Description of Parts and Their Function Lead Reservoir. The lead reservoir is a cylindrical, plastic component which holds thin rods of lead. Lead reservoirs tend to be similar in length to the pencil case body and can hold between 23 lead rods. Coil Spring. The coil spring is a small metal spring which connects to the lead reservoir. This spring provides resistance against the button mechanism and is what drives the return action of the pencil. Chuck and Chuck Ring. The chuck is the 3-jawed component which holds the lead in place, and is crucial to the mechanics of the pencil. The chuck ring is a collar which holds the jaws of the chuck closed. When the eraser is pressed in, the chuck moves past the chuck ring, allowing the chuck to open and release the lead. Both components are typically made from plastic, although higher quality pencils may use metal chucks and chuck rings instead. Figure 2 below displays the chuck and chuck ring operation. In the first stage, the pencil is in its relaxed state before the eraser has been pressed. Stage two shows how the chuck is pushed past the chuck ring and opens, releasing the lead. The return action is shown in stage three where the chuck retracts back into the chuck ring and clamps down on the lead. Figure 2. Chuck and Chuck Ring Mechanics[1] Lead Retainer. The lead retainer is a rubber component in the tip of the pencil which restricts movement of the lead by friction. This is what prevents the lead from being pulled back into the reservoir by the chuck during the return motion. Cheaper pencils may not have a rubber lead retainer. Instead, the end cap itself provides friction for the lead. End Cap. The end cap is a plastic component which forms the tip of the pencil and typically screws into the case body. An end cap is usually formed as a tapered cylinder about 1.5” long. This component houses the lead retainer, and provides a surface to stop the chuck ring during operation. Eraser. The eraser is a 0.375” long rubber cylinder which can be rubbed against the writing surface to remove graphite markings. It fits into a socket in the lead reservoir, and can be removed to refill the reservoir. Because the eraser is removable, it can easily be replaced when it becomes worn. Pencil Case Body. The pencil case body is a plastic, cylindrical component which contains the internal component. The case body is the second longest component of the pencil with a length of 4.5” and diameter of 0.25”. The end cap typically screws into the case body, and the case may have various modifications. For example, some pencils have a grip attached to the case for ease of writing. Others feature a pocket clip so that the pencil can be easily held in a pocket. Operating Description and Conclusion The operation of the mechanical pencil begins with the depression of the eraser/button. This action moves the reservoir, lead, chuck, and chuck ring towards the end cap. After traveling a certain distance, the chuck ring will hit the end cap and stop. After this, the chuck continues past the chuck ring, opening and allowing the lead to pass through it. Once the user disengages the button, the return action is begun by the spring. The chuck is moved towards the eraser, but the lead retainer prevents the lead rod from traveling with it. Once the chuck is forced back into the chuck ring, it clamps back down on the lead. The rest of the return action retracts the lead about half of its protracted distance. Once the return action is completed, the pencil is ready to write again. In a typical pencil, the lead reservoir, coil spring, chuck, and chuck ring cannot be removed from the pencil casing. Additionally, the user will rarely have need to separate the end cap from the pencil case body. The only frequently removed component is the eraser which needs to be removed to insert new lead.