Machine elements on drawing: threads assemblies and standard parts
How to draw and specify the parts you don't construct yourself
En stor del af et maskinteknisk produkt består af standarddele, ingen konstruerer fra bunden: skruer, møtrikker, skiver, lejer, kiler, fjedre og gevind. De tegnes ikke realistisk hver gang — det ville tage lang tid og fylde tegningen med detaljer, alle alligevel kender. I stedet bruges en forenklet visning med faste symboler og en præcis betegnelse, der entydigt udpeger delen.
§Thread on drawing
A thread is not drawn as its actual spiral but simplified: the full line marks one thread diameter and a thin line marks the other so you can see the difference between an external thread (on a bolt) and an internal thread (in a hole). The important information is in the designation, not the drawing. A metric thread is indicated with M followed by the nominal diameter, possibly with pitch and length — the full designation tells production everything they need to know.
§Simplified view of assemblies
Bolted assemblies, bearings and similar are often shown simplified, especially at smaller scales or on assembly drawings. Only what is necessary to understand the assembly is drawn, and the standard part is referenced via the bill of materials. This keeps the drawing clean and means a change to a standard part only needs to be corrected in one place — in the bill of materials.
- 01Draw standard parts simplified — not with all their own details.
- 02Let the bill of materials carry the precise designation and quantity.
- 03Show only the cuts and views necessary to understand the assembly.
- 04Remember that axles, bolts, and nuts are not hatched along their length in a section.
§Welding symbols
Welds are indicated with standardized welding symbols instead of drawing the weld seam in detail. The symbol sits on a reference line and tells the seam type, location (which side), size and any post-treatment. It is a whole language of its own that both the designer and the welder must be able to read — so the seam is executed exactly as intended.
§Part list and item numbers
On an assembly drawing, each part gets a position number that points to the parts list. The parts list then collects the designation, material, quantity, and any standard reference for each position. The connection between position number and parts list is what makes a complex assembly manageable and purchasable.
| Element | How it is handled |
|---|---|
| Thread | Simplified view + full thread designation (e.g. M designation) |
| Bolt/nut/washer | Simplified view + standard designation in bill of materials |
| Hire | Simplified view + bearing designation in bill of materials |
| Welding | Standardized weld symbol on reference line |
| Standard part generally | Position number → bill of materials, not detail drawing |
§Why simplification is professional
Drawing a standard part in simplified form and referencing it correctly is not laziness — it is professionally correct. It reduces errors, makes the drawing faster to read, and ensures the part can be purchased exactly as specified. A drawing that redraws everything from scratch is both slower to make and easier to make mistakes in.
“Draw what you construct yourself. Refer to what the world has already standardized.”
— Principle of standard parts