How a Drafting Pencil Works

By: Guest Author John Hartzog

The modern automatic drafting pencil belongs to a sub set of the group of pencils called mechanical pencils. Mechanical pencils come in all shapes, sizes, colors and lead sizes. They also come in two major types: the twist or screw type, where the lead is advanced by turning some portion of the pencil’s body; and the ratchet type where the lead is advanced in stepped increments by the pressing of a button, either on top of the pencil or on the side or is some special cases, shaking the pencil up and down. The Modern automatic drafting pencil falls into the second category.

In a typical drafting pencil there will be a lead reservoir tube attached to a brass 3-jawed chuck, which is surrounded by a brass collar called a chuck ring. Generally there is a narrowing in the pencils interior between the chuck ring and the lead reservoir tube trapping them inside the pencil. Above the narrowing, surrounding the lead reservoir tube is a coil spring. The spring is trapped between the narrowing of the pencil’s interior and a widening in the lead reservoir tube. When the end cap is removed from the pencil the 3-jawed clutch and clutch ring can be seen. The interior of the end cap is designed so that there is space enough for the chuck to operate properly. There is also a stepped narrowing in the end cap that is designed to stop the movement of the chuck ring but allows the chuck to travel on for a bit. The end cap also houses the lead sleeve, the tube that extends beyond the end cap to protect and guide the lead. At the mouth of the lead sleeve is a tiny rubber device called a lead retainer. The lead passes through the lead retainer and the lead retainer holds the lead in place during lead advancement. Finally, at the opposite end of the pencil is the push button, which slides over the lead reservoir tube. In the mouth of the lead reservoir tube, under the push button, is the eraser. Stuck up under the eraser is the clean out rod. (See the illustration Parts of a Drafting Pencil).

To operate an automatic drafting pencil you must first put lead into the lead reservoir tube. To do this remove the push button and eraser and put in 1 or more pieces of lead of the appropriate diameter. After replacing the eraser and cap push down on the push button repeatedly. As this action is taking place a single piece of lead is aligning with the narrow tube inside the lead reservoir tube and funneling down into the 3-jawed chuck, which has been opening and closing with every “click” of the push button. When the lead is in the mouth of the chuck all the physics of the pencil come into play.

As the push button is pressed the spring is depressed and the 3-jawed chuck, chuck ring and lead begin to move downward in unison. When the lip of the chuck ring strikes the narrowing in the end cap it stops moving, releasing the chuck, which continues to travel for a short distance. As the chuck continues to move the jaws begin to open until fully opened. Just before the jaws fully open the lead falls through the jaws of the chuck and strikes the mouth of the lead retainer. Once the chuck reaches its maximum opening it is stopped by the fully depressed coil spring. As the push button is released the chuck begins to retract under spring tension. As it does the jaws begin to close around the lead. The lead, too, may move upward slightly but not enough to matter. The chuck also reengages the chuck ring as it travels upward. At one point the jaws are fully closed around the lead and the chuck is partially lodged in the chuck ring, which has fully returned to its resting place. The lead is now held in place by the pressure exerted upon it by the partially closed jaws. The jaws are held closed by the chuck ring and by the tension from the spring. (See figs. 1-3)

As the push button is pressed again the chuck, chuck ring and lead begin to travel downward from their new position. Before the jaws can open and release the lead the lead is pushed into and through the lead retainer. This process is continued until the lead protrudes from the opening in the lead sleeve enough for the user to write with. The lead is held in place during writing by the jaws of the chuck, not the lead retainer. The lead retainers job is simply to restrain the lead from moving in either direction during lead advancement. Without the lead retainer however, the lead would simply fall free of the pencil as soon as it was released from the jaws that very first time. As the lead is worn away from use a “click” or two produces more lead. See figs. 4-9).

Before putting away a drafting pencil, or before trying to carry it in your shirt pocket or pocket saver, retract the lead. This is done manually. Press and hold the push button then with a finger push the protruding lead back up into the lead sleeve then release the push button. The pencil is now ready for the next job.

John is a JetPens customer. You can read more about his passion for pencils on his blog


Leave a Reply

Your email address will not be published. Required fields are marked *