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The Machine

The Machine · Elements

Whirling striker drum

Design stage

Updated 2026-10-18

Spinners are motors whirling a wooden ball on a short tether. They run at full speed all the time and strike only when moved so the ball's path crosses the drum head.

Main version. One drum head facing the audience, with four spinners just in front of it on extendable arms. Retracted, the ball spins clear of the head; extended, it strikes the head.
Fig. 1Main version. One drum head facing the audience, with four spinners just in front of it on extendable arms. Retracted, the ball spins clear of the head; extended, it strikes the head.
Head up variant. Head up, spinners dip on pivot arms. Raised, the ball spins clear; dipped, it strikes the head.
Fig. 2Head up variant. Head up, spinners dip on pivot arms. Raised, the ball spins clear; dipped, it strikes the head.
Sideways variant. Heads facing the sides.
Fig. 3Sideways variant. Heads facing the sides.
Spinner arm options. Lead-screw linear actuator, telescoping tube, and scissor mechanism compared. A build uses one type.
Fig. 4Spinner arm options. Lead-screw linear actuator, telescoping tube, and scissor mechanism compared. A build uses one type.

How it works

  • Spinners are motors whirling a wooden ball on a short tether. They run at full speed all the time and strike only when moved so the ball’s path crosses the drum head.
  • Main version: one drum fixed in the center of an extrusion rectangle, head facing the audience. Four spinners sit just in front of the head: two lowered from the top bar and two raised from the bottom bar, each on a motorized extendable arm.
  • Each ball whirls in a horizontal circle just in front of the head. Retracted, the circle clears the drum; extended, it crosses the head and strikes once per revolution.
  • Keep each hub close to the head’s plane so the ball meets the head nearly square.
  • How far an arm extends sets where the ball lands: near the rim is brighter, nearer the center fuller. Spin speed sets loudness.
  • Space the two top spinners, and the two bottom ones, so their ball circles can never meet.
  • At full speed the tether runs nearly flat. While a spinner recovers speed after a hit, its ball droops lower; the arm can compensate.
  • Each hit slows only the light ball; it is pulled back up to speed within a revolution or two. The four take turns.

Extendable arms

  • Options: lead-screw linear actuator (simplest), telescoping tube, or scissor mechanism (most visible). A build uses one type.
  • Only lead-screw slides are truly off the shelf (CNC and 3D printer parts that mount to extrusion). A scissor arm is a build, not a purchase; drive it fast near its folded end (crank or cam) rather than with a slow screw, since off-the-shelf scissor lifts are geared for force and are slow. A belt-driven slide is the fast alternative to a screw.

Timing

A single hit means moving in after the ball passes and out before it returns, within one revolution. Make that window workable:

  • Spin slower with a longer tether: about 30 cm at 120 rpm keeps the tether within roughly 12 degrees of flat, hits at about 3.8 m/s, and gives half a second per revolution.
  • Park each spinner just clear of the head with the slow positioning move, so the strike move is only 1 to 2 cm (about 80 mm/s, within a plain lead screw’s reach).
  • A sensor on each hub tracks the ball; the move in starts just after the ball passes the strike point. Staying in for n revolutions gives n hits.
  • Set rpm to the tempo (120 rpm = 120 BPM, one revolution per beat) so every hit lands on the grid. Spinners at different multiples play different subdivisions. Briefly changing a spinner’s rpm shifts its hits off the grid.

Tether

  • Use stiff trimmer line rather than soft string, like a string trimmer, so it straightens itself after each hit. A felt- or leather-covered ball reduces wear on the head.

Risks

  • Tangling is the main risk: the ball rebounds, the tether goes slack, and it can wrap the hub. Tie the tether to the end of a short rigid arm on the hub, use a fishing swivel, and switch to a short chain or a hinged rigid link (flail) if tethers keep fouling.
  • Clear panels across the front keep a broken tether contained while the audience watches the balls whirl.

First test

  • One spinner on a cordless drill, 30 cm of trimmer line with a wooden bead, held so the path just reaches a drum head, filmed in slow motion. It shows whether the tether recovers cleanly and how it sounds.