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Production of screw threads

Short answer

Threads are machined by single-point cutting on a lathe, taps and dies, or milling; rolled by pressing between hardened dies causing plastic flow; ground with a formed wheel for accuracy on hardened parts. Rolled threads are stronger because grain fibres follow the contour, cold working raises strength 10–20%, and surface compressive stress improves fatigue life.

Describe the methods of producing screw threads by (a) machining, (b) rolling and (c) grinding. Why are rolled threads stronger?

(a) Machining methods

  • Single-point cutting on a lathe: a tool ground to the thread form is moved by the lead screw; flexible but slow, used for single parts.
  • Taps and dies: hand or machine cutting of internal and external threads.
  • Thread milling: a single-form cutter traverses along the thread, or a multiple-form cutter on a thread milling or CNC machine cuts the full thread length in about one revolution of the work.
  • Self-opening die heads, thread chasing and thread whirling for production work.

(b) Thread rolling

The blank is pressed between hardened dies carrying the thread form, and the metal is made to flow plastically (cold working) to form the roots and crests, with no chips.

  • Dies: flat dies (one stationary, one reciprocating), or two or three cylindrical rolls fed radially.
  • Very fast (hundreds of parts per minute); used for bolts, screws and studs.

(c) Thread grinding

A grinding wheel dressed to the thread form (single-rib or multi-rib) is traversed along the rotating job. It gives high accuracy and a fine finish and can produce threads on hardened parts: lead screws, taps, gauges, ball screws. Centreless thread grinding is also used.

Why rolled threads are stronger

  • The grain fibres follow the thread contour instead of being cut across.
  • Cold working increases the hardness and surface strength (tensile strength rises by about 10 to 20%), and the smooth surface with residual compressive stress greatly improves fatigue life.
  • There is no material waste.

✓ Answer checked by Claude Opus 5.5 (high effort)

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