The Compass

The compass is a fundamental tool in technical drawing, a field that focuses on the precise graphic representation of objects, components and designs. The compass is used to draw circles, arcs, curves and other circular shapes with extreme precision. With the use of CAD drawing it has lost much of its importance but there are disciplines in which it is still indispensable.

Compass Components: A compass consists of two arms connected by an adjustable joint. One arm has a pointed tip, while the other has a wheel or pen for drawing. The joint allows you to adjust the radius of the circle or arc you are drawing.

Precision: The compass is known for its precision. It can be adjusted with extreme accuracy to obtain circles of specific dimensions or to create precise arcs and curves.

Using the compass: The compass is used to draw concentric circles, create curves and arcs, determine points equidistant from a center, and much more. It is an important tool for constructing geometric figures and for representing circular components in technical drawings.

Detail drawing: In technical drawing, it is often necessary to represent complex details or components that have a circular or curved shape. The compass allows these representations to be made accurately, making it easier for the reader to understand the project.

Materials and design: Compasses can be made of a variety of materials, including metal, plastic, or wood. Some compasses have locking mechanisms to keep the set size stable. Some advanced models may also feature a wheel guide to improve accuracy.

Based on the characteristics above, compasses are divided into:

Balustrades (like the one in the picture), if they have a needle as a holding tool, a lead or a nib as a writing system and a small wheel in the centre for regulating the opening;

Bowstrings, when they have the same characteristics as balusters but lack a regulating wheel (the compass is maintained at the chosen angle only thanks to the hardness of the connecting gears between the rods, which must therefore be tightened regularly);

Blackboard Compasses, if they have a suction cup as a fixing tool and a piece of chalk or a blackboard marker as a writing system; typically, they are made of wood or aluminum-based metal alloys (to make them lighter given their large size).


The perfect compass
(The so-called compass perfect).
TAll the compasses we have talked about above are useful, but they allow you to directly draw only circles or arcs of these; on the contrary, the so-called perfect compass It also allows you to create images of hyperbolas, parabolas and ellipses. It is composed, like all other compasses, of two rods hinged by a gear;
the main difference, which then determines the different functioning, consists in the fact that one of the two rods can be lengthened, increasing the eccentricity of the figure. A diagram of the functioning of the perfect compass is illustrated in the figure on the side: in it one can easily notice how the drawing instrument in question allows one to generate the image of an ellipse by intersecting a virtual cone on the α plane (which can be made up of a sheet of paper or any other graphic support). Precisely on the basis of the principle of intersection of the virtual cone one can easily understand how this instrument is able to represent all the conic sections.

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