Technical Sketching and Shape Description

Technical Sketching Technical Sketching: Freehand sketches are of great value to designers and engineers in organizing their thoughts and recording their idea. The term “freehand sketch” does not mean a crude or sloppy freehand drawing. A freehand sketch should be made with care and with attention to proportion, clarity, and correct line technique. A freehand sketch only requires a pencil, paper, and eraser.

Technical Sketching Soft pencils, such as HB or F, should be used for freehand sketching. A sharp point is used to produce thin lines for drawing center lines, hidden lines, and dimension and extension lines. These lines should be thin and dark. A rounded point is used to produce visible object lines that are thick and dark. A sharp point is also used to draw construction lines. Construction lines are drawn thin and light.

Technical Sketching Sketching Straight Lines: A good freehand line is not expected to be as rigidly straight or exactly uniform as a line drawn with instruments.

Technical Sketching Sketching Circles and Arcs: One method of sketching circles is to lightly sketch the enclosing square, mark the midpoints of the sides, draw arcs tangent to the sides of the square, then heavy in the final circle. Another method is to sketch the two center lines, add light radial lines, sketch light arcs across the lines a the estimated radius distance from the center, then heavy in the final circle.

Sketching Three Views Steps in Making a Sketch: Step 1: block in the enclosing rectangle for the three views using construction lines. Step 2: Block in all details using construction lines. Step 3: Sketch all arcs and circles using construction lines. Step 4: Lighten all construction lines. Step 5: Darken in all final lines.

Sketching Three Views Hidden lines: Hidden lines are used to show hidden features. They are made thin and dark (dense black). A hidden line is a dashed line consisting of 1/8” dashes with 1/32” spaces. Correct and incorrect practices in drawing hidden lines.

Sketching Three Views Center Lines: Center lines are used to indicate axes of symmetry, bolt circles, paths of motion and in dimensioning. They are made thin and dark (dense black). A center line consists of a long line, short dash, and a long line. Center lines extend 1/4” past the feature for which they were drawn. Examples of center line applications.

Sketching Three Views Precedence of lines: Visible object lines, hidden lines, and center lines often coincide on a drawing. The drafter must determine which lines to show and which ones to eliminate. A visible object line always takes precedence over hidden lines and center lines (A) & (B). A hidden line always takes precedence over a center line (C).