A machine is not one part. Dozens of components sit together, locate each other and work as a unit. Technical drawing therefore uses two different kinds of drawing: the assembly drawing, which shows how the parts sit together, and the detail drawing, which shows how each part is made on its own.
They are not interchangeable. You cannot make a part from an assembly drawing, and you cannot tell where a part fits from a detail drawing.
The assembly drawing
An assembly drawing shows all the parts of a machine or sub-assembly in their assembled position. Its purpose is not manufacture but relationship — what goes where, and how the parts work together.
- It is usually drawn in section, so that the internal arrangement is visible.
- Every part is given an item number.
- Only assembly dimensions are given: overall size, mounting centres, seating diameters and fits.
- Manufacturing dimensions, tolerances and surface symbols are not put on an assembly drawing.
- A parts list always accompanies it.
Item numbers (balloons)
- The number is connected to the part by a thin leader line with a heavy dot where it touches the part.
- It is written inside a circle (balloon) outside the outline, in figures larger than the dimension figures.
- Leader lines must not cross each other, and are drawn sloping — never horizontal or vertical.
- Balloons are arranged in a tidy row or column around the drawing.
- Identical parts share one number; the quantity is stated in the list.
- Numbering usually starts at the main body and follows the assembly sequence.
Parts never hatched in a longitudinal section. When the cutting plane runs along their axis these are shown uncut: shafts, bolts and nuts, washers, pins, rivets, keys, couplings, bearing balls and rollers, and solid arms or spokes. Sectioning them adds no information and makes the drawing harder to read. Cut across the same parts and they are hatched normally.
Hatching adjacent parts
- Two touching parts are hatched in opposite directions.
- If a third part joins them, change the spacing, or hatch at 30° / 60°.
- All cut areas of the same part are hatched identically — in every view.
- Parts thinner than about 2 mm (sheet, gaskets) are filled solid, with a small gap between adjacent ones.
The parts list
The parts list translates the item numbers. It sits directly above the title block in the bottom right corner of the sheet and is filled in from the bottom upwards — item 1 at the bottom, the last item at the top. The header row is at the bottom of the list, immediately above the title block.
| Column | Contents |
|---|---|
| Item No | Same as the balloon number on the assembly drawing |
| Part name | Housing, shaft, bush, cover… (standard name for bought parts) |
| Quantity | How many of that part are used |
| Material | C45, S235JR, GG25, CuSn8, Al… |
| Size / Standard | Raw size, or the standard number (e.g. ISO 4762) |
| Remarks | Heat treatment, coating, drawing number |
Standard parts get no detail drawing. Bolts, nuts, washers, circlips, pins, keys and bearings are bought in. They are drawn and numbered on the assembly, but no separate detail drawing is made. The parts list entry — size plus standard number, e.g. “Bolt M8×30 — ISO 4762” — is all that is needed.
The detail drawing
A detail drawing carries everything needed to manufacture one part. The person at the machine must be able to make it without reference to any other document. That means nothing may be missing.
- A separate sheet for each part (small parts may share a sheet, each with its own field).
- The minimum number of views that describe the part; sections where needed.
- Every dimension written out — nothing left to be calculated.
- Dimensional tolerances (Lesson 8) and geometrical tolerances (Lesson 9) where required.
- Surface texture symbols (Lesson 7) on the relevant faces.
- A complete title block: material, quantity, scale, projection method, drawing number, drawn by, date.
- General notes: heat treatment, coating, edge break, general tolerance standard.
Assembly drawing vs detail drawing
| Assembly drawing | Detail drawing | |
|---|---|---|
| Purpose | Show how parts fit together | Make one part |
| Scope | All parts | One part |
| Dimensions | Assembly dimensions only | All manufacturing dimensions |
| Tolerances | Fits only | Size + form + position |
| Surface symbols | No | Yes |
| Item numbers | Yes | No (item number is in the title block) |
| Parts list | Yes | No |
| Section | Usually | If needed |
Drawing numbers
Assembly and detail drawings are tied together by their numbers. A common scheme: the assembly is 10-00 and its detail drawings are 10-01, 10-02, 10-03… From any detail drawing you can find the assembly it belongs to, and from the assembly you know which details to look for.
Test yourself
- Why are not all dimensions shown on an assembly drawing?
Its purpose is not manufacture. Manufacturing dimensions live on the detail drawings; putting them all on the assembly would make it unreadable. - Why is the parts list filled in from the bottom upwards?
It sits directly above the title block and grows upwards, so new items can be added without moving the title block. - Why is the shaft not hatched in the section?
Solid parts cut lengthwise — shafts, bolts, pins, keys, rivets — are never hatched; sectioning them adds nothing. - What happens if two adjacent parts are hatched the same way?
Their boundary disappears and they read as a single part. - Why is there no detail drawing of a bolt?
It is a standard part, bought in. The parts list entry with its size and standard number is enough. - Can a detail drawing say “this dimension follows from the others”?
No. The machinist must not be made to calculate; every required dimension is written out. - You have detail drawing 12-03. Which assembly does it belong to?
Assembly drawing 12-00.
Next lesson. All the fundamentals are now covered. Lesson 12 brings them together into a single skill: reading a drawing step by step, and turning what it says into manufacturing decisions.
Technical Drawing Training — all lessons
- Lesson 1: Introduction, Paper Sizes, Title Block and Scale
- Lesson 2: Line Types, Line Widths and Lettering
- Lesson 3: Geometric Constructions, Tangency, Polygons and the Ellipse
- Lesson 4: Orthographic Projection, Views and First vs Third Angle
- Lesson 5: Section Views, Hatching Rules and Parts Never Hatched
- Lesson 6: Dimensioning Rules, Systems and Tolerance Accumulation
- Lesson 7: Surface Texture, Ra and Surface Symbols
- Lesson 8: Dimensional Tolerances and the ISO System of Fits
- Lesson 9: Geometrical Tolerances — The 14 Symbols, Frame and Datums
- Lesson 10: Pictorial Projection, Isometric Drawing and Sketching
- Lesson 11: Assembly Drawings, Detail Drawings and the Parts List ← you are here
- Lesson 12: Reading a Drawing for the CNC Operator
- Technical Drawing Symbols and Abbreviations Glossary