Sketching by hand

Why it matters

Even in a world of CAD, the fastest way to explain a part, a bracket you need, or a conduit route is a quick sketch. Engineers sketch in design meetings, electricians sketch as-built changes, and mechanics sketch a worn part before it goes to the machine shop. A clear sketch with the right views and a few key dimensions can save hours of back-and-forth.

What you need

• Pencil (a mechanical 0.5 mm works well) and an eraser.
• Grid paper (5 mm or 1/4 in squares) for orthographic sketches, and isometric grid paper if you have it (free printable sheets exist online).
• A ruler for checking, though good technical sketches are freehand.
• The part itself, or a picture of it, and a tape or caliper to measure it.

Freehand line technique

• Draw lines with your arm, not just your wrist. Mark the start and end points first, then sketch light strokes between them.
• Pull lines toward you or across, rotating the paper if needed.
• Sketch light construction lines first; darken the final outline at the end.
• Use line weight to communicate: dark, thick visible lines; medium dashed hidden lines; thin centre and dimension lines.

Proportion over precision

A sketch does not need to be to scale, but proportions should be right. Use the grid: if the part is 60 mm by 40 mm, make it 12 squares by 8 squares on 5 mm paper. Estimate features by halves and quarters of a known length.

Sketching three views: step by step

Use third-angle projection (US convention) unless told otherwise.

1. Choose the front view: the face that shows the most shape.
2. Block in the front view as a light rectangle using the overall width and height.
3. Block in the top view directly above, the same width, using the depth for its height. Leave a gap of a few squares.
4. Block in the right side view to the right of the front, the same height, using the depth for its width.
5. Project features between views with light horizontal and vertical lines. A step in the front view appears as a line in the top and side views.
6. Add circles and arcs: sketch the centre lines first, mark the radius on each line, then join the marks.
7. Add hidden lines for features you cannot see.
8. Darken visible lines, then add centre lines.
9. Add key dimensions with extension and dimension lines, once each, on the view that shows the feature best.
10. Add a title block: part name, your name, date, units, "NOT TO SCALE" if it is not.

Sketching an isometric view

1. Draw three light axes from one corner: one vertical, two at 30 degrees from horizontal (follow the isometric grid).
2. Mark the overall width, depth and height along the axes and box in the part.
3. Cut away notches and steps from the box.
4. For holes, sketch ellipses on the faces (a circle in isometric is an ellipse). Box the ellipse first in a rhombus.
5. Darken the visible edges only. Isometric sketches usually leave out hidden lines.

Worked example: a small angle bracket

A bracket is an L shape seen from the front: 50 mm wide base, 40 mm tall upright, 5 mm thick, 30 mm deep. Two Ø6 holes go through the base, 12 mm in from each end.

• Front view (12 × 8 squares at 5 mm, close enough): the L outline. The base holes appear as hidden lines.
• Top view: a 50 × 30 rectangle with a line 5 mm from the left end where the upright rises, and two circles with centre lines on the base.
• Side view: a 30 × 40 rectangle with a line 5 mm above the bottom for the base thickness.
• Dimensions: overall 50, 40 and 30; thickness 5; hole locations 12 from each end, centred in depth; 2X Ø6 THRU.

Common mistakes

• Starting with details before blocking in the overall size.
• Views that do not line up, so features cannot be projected.
• All lines the same weight.
• Dimensioning the same feature in two views.
• Forgetting units.

Recap

• Sketch light, then darken; use line weights to separate visible, hidden and centre lines.
• Block in all three views first, aligned, then project features across.
• Use grid paper for proportion; circles in isometric become ellipses.
• Add key dimensions once each and a simple title block.

Try it

The Do tab walks you through sketching a real object from your home in three views and an isometric view.