Threads look like a detail on a drawing and behave like a design decision in service.
Cut threads
A tap or single-point tool removes material to form the thread. Standard, predictable, works in any machinable material.
Best for: most applications in steel and thicker aluminium sections.
Watch for: thread depth. Specifying a thread deeper than 1.5 times the diameter rarely adds strength — the first few engaged threads carry most of the load — but it does add cycle time and tap breakage risk.
Formed (rolled) threads
The material is displaced rather than cut. No chips, and the grain structure follows the thread form, which makes a stronger thread in ductile materials.
Best for: aluminium and softer alloys where thread strength matters.
Watch for: the hole size before forming is different from a cut thread, and not every material tolerates it. Brittle materials will crack.
Threaded inserts
A separate insert — helical coil or solid bushing — installed into a plain hole.
Best for: soft materials, parts that will be assembled and disassembled repeatedly, and any thread that must survive more cycles than the base material can support. Also the standard repair route when a thread is damaged in service.
Watch for: added part cost and an assembly step. And the insert has to be specified: type, material, and whether it is locking.
The decision in practice
Ask how many times the joint will be undone. A thread in aluminium that is assembled once at the factory and never touched again can be cut. The same thread on a service panel that a technician opens twice a year should have an insert — this is the single most common cause of stripped threads we see on returned parts.
Ask what the joint is carrying. If a calculation shows the thread near its limit in the base material, a formed thread or an insert changes the answer without changing the part size.
What belongs on the drawing
- Thread designation and class of fit
- Thread depth, and whether it is full-form depth or including the incomplete leading threads
- For inserts: the insert specification, not just "insert"
- Whether the thread must be free of burrs on the far side, which is a real machining step and not automatic
The short version
Cut threads are the default. Use formed threads for strength in ductile materials, and inserts wherever the joint will be opened repeatedly. Do not over-specify depth — it costs time and buys almost nothing.
