Reading a dimensioned shop drawing

Orthographic views and projection, hidden lines and sections, how dimensions are chained or measured from a datum, fractions and rounding, and a checklist before cutting.

A shop drawing is a set of instructions written in lines. Reading one well means knowing which view you are looking at, which dimension governs, and what the drawing is deliberately not telling you. Most drawing-related mistakes in the shop come from reading a dimension from the wrong view or measuring from the wrong edge.

The views

Furniture drawings usually show the piece in several orthographic views, each looking straight at one side:

Front view: the most informative face, usually the one you see in use.
Top view: looking down.
Side view: looking from the right or left.

In North America, drawings normally use third-angle projection: the top view sits above the front view, and the right-side view sits to the right of it. Many European drawings use first-angle projection, where those positions are swapped. If a drawing looks mirrored, check which projection it uses; there is often a small symbol in the title block.

An isometric or perspective view shows the piece in three dimensions for context. It is excellent for understanding how parts relate and poor for measuring. Take dimensions from the orthographic views.

Lines that mean something

Solid lines are visible edges.
Dashed (hidden) lines are edges behind a surface: a mortise inside a leg, a groove inside a panel.
Centre lines (long and short dashes) mark the centre of holes, curves and symmetrical parts.
Section views cut the piece along a line, usually marked with arrows and a letter such as A–A, and show what is inside. They are the clearest way to see a joint.
Detail views enlarge a small area, such as a joint or a moulding, at a larger scale.

Dimensions

Overall dimensions give the size of the whole piece: width, depth, height.
Part dimensions give each part’s finished size.
Location dimensions say where things are: the height of a shelf, the position of a mortise.

How location dimensions are arranged matters:

Chained dimensions run one after another: 4 inches, then 6, then 6. Each measurement adds its own small error, and errors accumulate.
Baseline (datum) dimensions are all measured from one reference edge or face: 4, 10, 16 inches from the bottom. Errors do not accumulate.

When you mark out, measure from the same reference face the drawing uses, even if the drawing is chained. Add the chain up and mark each position from the datum with one tape or a story stick.

Fractions and rounding

Woodworking drawings are usually dimensioned to the nearest 1/16 or 1/32 inch. A value that has been rounded for display may not be exact; the underlying model or calculation might be 0.719 inch while the drawing says 23/32. Where a rounded value matters, such as a joint fit, check the actual stock. Some tools, GrainDraft included, mark rounded values with an approximation sign so you know when a displayed fraction is not the exact size.

What a drawing does not tell you

Never scale a print. Measure only written dimensions. Printers and PDF viewers rescale drawings.
Finished, not rough. Dimensions are finished sizes. Milling and rough-cut allowances are yours to add; see milling rough lumber.
Stock variation. A drawing assumes stock is the thickness it was drawn at. If yours is not, dependent dimensions change; see nominal and actual sizes.
Fastening and finishing, unless stated. A top drawn resting on its aprons still needs a fastening method that allows movement.

Before you cut: a checklist

Find the overall dimensions and confirm they suit the space.
Match each part in the cut list to the drawing by its name or reference number.
For each joint, find the view or section that shows it and note the governing dimensions.
Identify the datum for each part: which edge or face measurements come from.
Check the grain direction marked for each part.
Compare stock thickness against the drawing and adjust dependent dimensions.

In GrainDraft

GrainDraft’s shop packet and drawings come from the model, so a change to the model updates the drawings rather than leaving an old sheet behind. Part sheets carry part references that match the cut list, and datum markers show which corner each part’s dimensions are measured from. The sample packet linked from the homepage shows them on the starter side table, and in the app, section views expose joints that are hidden in the assembled piece.

Numbers here are starting points from common shop practice and published references. Check your own stock, tools and hardware instructions, and cut a test piece before committing project wood. Spotted an error? The contact address is on the About page.