What is a machine drawing?
A machine drawing is a technical drawing of a machine part. It is a set of instructions in pictures. There are two main kinds:
- Detail (working) drawing: one part only, with everything needed to make it.
- Assembly drawing: many parts put together, showing how they fit (see the next lesson).
Drawings follow standards (rules everyone agrees on), mainly ISO standards, so a drawing made in India can be read in Japan or Brazil. Sizes are in millimetres, and the unit is usually not written.
Views and section views
We use orthographic views: front view, top view and side view. Each view looks straight at one face. Choose the view that shows the most shape as the front view. Use only as many views as you need: a simple shaft often needs just one.
Line types
- Thick continuous line: visible edges.
- Thin dashed line: hidden edges.
- Thin chain line (long dash, dot): centre lines and axes.
- Thin continuous line: dimension lines and hatching.
Section views
When the inside is complex, imagine cutting the part with a cutting plane. Remove the front half and draw what is left. Cut solid material is filled with hatching: thin parallel lines, usually at 45°. Types: full section, half section (one quarter removed), and local (broken-out) section. Shafts, bolts, keys and ribs are normally not hatched when cut along their length.
Dimensions, tolerances and surface finish
Every size must appear once, in the view that shows it best. Symbols help: Ø = diameter, R = radius, M10 = metric thread 10 mm, 1 × 45° = chamfer.
No machine makes a perfect size, so we give a tolerance: the allowed range. Ø40 ±0.05 means 39.95 to 40.05 mm. Tolerance = upper limit − lower limit = 0.10 mm. ISO fits like H7/h6 tell how a hole and shaft go together: clearance (loose), transition, or interference (tight, pressed in).
Surface finish (roughness, for example Ra 1.6 µm) says how smooth a surface must be. Smaller number = smoother = costs more.
Title block, scale and project documents
The title block sits in the bottom-right corner. It holds the part name, drawing number, material, scale, projection symbol (first or third angle), who drew and checked it, date and revisions.
Scale: 1:1 is full size, 1:2 is half size, 2:1 is twice the size. Standard paper sizes are A4 to A0. A full project keeps all drawings, their numbers and revisions in an organised set, often made with CAD software.
How to read a detail drawing: 1) read the title block, 2) find the front view, 3) build the 3D shape in your head, 4) read sections, 5) read sizes and tolerances, 6) read notes.
Key formulas and definitions
- Tolerance = upper limit − lower limit
- Upper limit = basic size + upper deviation; lower limit = basic size + lower deviation
- Drawing size = real size × scale (scale 1:2 → drawing is half size)
- Ø = diameter, R = radius, M = metric thread, □ = square
- Hatching: thin lines at 45°, same direction for one part
Worked examples
1. A shaft is marked Ø25 ±0.02. What are its limits and its tolerance?
Upper limit = 25.02 mm, lower limit = 24.98 mm. Tolerance = 25.02 − 24.98 = 0.04 mm.
2. A part is 180 mm long. It is drawn at scale 1:2. How long is it on paper?
180 × 1/2 = 90 mm on the paper. The dimension written is still 180.
3. A hole is Ø20 +0.021/0 and the shaft is Ø20 −0.007/−0.020. What kind of fit is it?
Smallest hole = 20.000, largest shaft = 19.993. The shaft is always smaller, so there is always a gap: a clearance fit. Smallest gap = 0.007 mm.
Common mistakes
- Writing the same dimension in two views. Each size appears once only.
- Hatching a solid shaft or bolt cut along its length. These are left unhatched.
- Using the drawn length instead of the real length when the scale is not 1:1. Always write the real size.
- Forgetting the tolerance. A size with no tolerance uses the general tolerance in the title block, not 'exact'.