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Lesson 3.6DeformChapter 3 · Bulk forming

Thread rolling and tapping

Press a thread onto a blank, or cut or press one in a hole

Thread rolling presses the thread onto a screw blank between grooved dies, without cutting away any metal. Tapping makes the matching thread inside a hole, with a tool called a tap. Together they finish the screw story: wire (3.4), head (3.5), and now the thread.

Thread rolling and tapping: a real photo
Photo: R. Henrik Nilsson, CC BY 4.0, via Wikimedia Commons
Threads are pressed in by grooved dies, or cut or pressed into a hole by a tapLeft: outside threads on a screw blank (thread rolling). Right: inside threads in a hole (tapping).Thread rolling(end view of the blank)moving diefixed die1blank rollsbetween dies2die slides; its ridges press the thread3Before and aftersmooth blankthreaded screwno metal is removed:it flows up into crests,so the outside diameter growsTapping(cut-away of a hole)parttapturns andfeeds in4drilled hole,a bit smaller5thread cutor pressedin the wall6reverse to back the tap out; the thread staysKey:metal being shapeddies and tap
Thread rolling between flat dies, and tapping a holeLeft: thread rolling (steps 1 to 3). Right: tapping a hole (steps 4 to 6). Circled numbers match the steps below.

How it works

  1. Feed the blank. A headed blank from cold heading (lesson 3.5) drops between two hardened steel dies. Their faces carry slanted ridges shaped like the thread.

  2. Roll it. One die stays still while the other slides past it. The blank rolls between them, and the ridges press into its surface; the metal flows up between the ridges to form the crests.

  3. Out comes a screw. In a single pass, typically a fraction of a second, the thread is complete. No metal is removed, so the outside of the thread ends up larger than the original blank.

  4. Drill the hole. For an inside thread, you first drill a hole slightly smaller than the thread size (the tap drill size).

  5. Turn the tap in. The tap (a hardened screw-shaped tool) is turned and fed into the hole. A cutting tap cuts the thread and makes chips; a form tap has no cutting edges and presses the thread into the wall instead.

  6. Back it out. The tap is reversed and unscrewed, leaving the inside thread ready for a bolt.

Key facts

Materials
Rolling: ductile metals such as steels, stainless steel, aluminum, brass, titanium. Tapping: almost any machinable metal or plastic
Typical parts
Screws, bolts, studs, threaded rods (rolled); threaded holes in engine blocks, housings, nuts (tapped)
Part size
Rolled threads from about 1 mm to tens of millimeters; taps from under 1 mm to about 50 mm, bigger ones cut by other means
Precision
Thread tolerance classes typically hold sizes to a few hundredths of a millimeter
Surface finish
Rolled threads: smooth and shiny. Cut threads: slightly rough, with tool marks
Batch size it suits
Rolling: thousands to millions. Tapping: from one hole by hand to millions on automatic machines
Tooling and machine cost
Rolling dies: hundreds to thousands of dollars a pair. Taps: a few to tens of dollars each
Speed
Rolling: often hundreds of parts per minute. Tapping: seconds per hole on a machine

Strengths and limits

Strengths

  • Rolled threads are stronger: the grain follows the thread and the surface is work-hardened
  • No chips and no wasted metal in rolling and form tapping
  • Very fast and cheap per part
  • Taps are cheap and work on almost any machine or by hand

Limits

  • Rolling needs ductile metal and a precisely sized blank
  • Rolling suits outside threads; holes need tapping instead
  • Taps can break in the hole, especially small ones, and are hard to remove
  • Cutting taps leave chips that must be cleared, especially in blind holes

Where it's used

  • FastenersScrews, bolts, studs, threaded rods
  • AutomotiveWheel studs, engine bolts, tapped holes in engine blocks and gearboxes
  • AerospaceHigh-strength bolts with rolled threads for fatigue resistance
  • MachineryTapped mounting holes in frames, housings and brackets
  • PlumbingThreaded fittings and pipe ends

How to recognize it

  • Rolled threads: crests that are smooth and shiny, sometimes with a faint seam at the very top.
  • Rolled threads: the thread's outside is wider than the plain shank next to it.
  • Cut threads (taps or lathes): fine tool marks, and sometimes small burrs at the start of the thread.
  • Form-tapped holes: a smooth thread with no chip marks, often with a tiny split line at each crest.

Compare with

Cold heading3.5makes the headed blank that thread rolling turns into a screw.Turning5.1a lathe can cut threads, which is slower but suits one-offs and very large sizes.Knurling5.6another rolling process that presses a pattern into the surface instead of cutting it.

Watch it

Haas Automation, Inc. · Tap types, tap drill sizes and tapping on a CNC machine

Mr. G-Code Machining · Cutting and form tapping side by side

TeskerMFG · A thread rolling machine at work

TR Fastenings · A fastener factory rolling threads at speed

See real photos

Remember

  • Thread rolling presses a thread onto a blank between grooved dies, with no chips.
  • Tapping makes the matching thread inside a drilled hole, by cutting or by pressing.
  • Rolled threads are stronger than cut ones because the metal's grain follows the thread.

More in Bulk forming

All 100 processes
3.1ShortForging (open-die and closed-die)Squeeze hot metal between flat or shaped diesOpen the lesson3.2RollingPull metal through the gap between turning rollsOpen the lesson3.3Extrusion (aluminum profiles and plastic pipes)Push metal or plastic through a shaped dieOpen the lesson3.4Wire drawingPull a rod through ever-smaller diesOpen the lesson3.5Cold headingCut wire into blanks and punch a head, coldOpen the lesson

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