Sound investment

Linear slides sit at the heart of machine that makes products with a musical heritage. Julie Bieles reports


Sevenoaks-based music string manufacturer Rotosound has added 12 semi-automated winding machines to its manufacturing capacity.
The company’s string range includes guitar strings, bass strings, double bass and classical strings, and, since it was set up in the 1960s, customers have included Jimi Hendrix, The Who’s John Entwistle and Lou Reed. Rotosound strings are known for their distinctive sound and sound quality, which comes from a careful choice of raw materials and a bespoke winding process, which is fundamentally unchanged since the sixties. During this process a wrap wire is wrapped over core wires and then, in many cases, a layer of silk is glued on.
The new semi-automated machines are an evolution of the hand winding machines originally built by company founder James How. The original machines, with a throughput of 20-30 strings per hour, are labour-intensive and require the wire to be fed manually by an operative.
Some of the strings are still made on the hand-winding machines but many of the more popular ones, including the Swing Bass, are manufactured on the new semi-automated machines.
Details of the machinery and the manufacturing process are a closely guarded secret. But Rotosound chairman – and the founder’s son – Jason How, told Eureka that the strings’ distinctive sound was due to a “combination of the type of material, the ratio of the wrap wire to the diameter of the core wire, and the tensions applied to the core when it’s being wound. Also the tensions of the wrap wire, and the angle of the wire feeding onto the core.”
He says: “We have certain specifications for each string which go back to the 1960s.”
The 12 new semi-automated winding machines were made in-house by How with only a few components, including the electronics, the pneumatics, and the cradle and linear slide, sourced externally. The software controlling the machines was bespoke, written for Rotosound by a company that specialises in designing and writing machine programmes.
Each machine can be adjusted and programmed to make many different individual strings. “Normally we run for one or two days on a certain size, and then we change to another size, depending on what the requirements are,” says How.
How used HepcoMotion linear slides to guide the cradle that carries the wire spool set – Generation 1 Hepco slides were specified on the original winding machines.
“In truth I have never considered any systems other than Hepco,” says How. “I know of designers who have used recirculating technology in the way of shaft and ball bushings but I didn’t feel this could give me the rigidity I needed. There was danger of deflection in the end supported shafts, any load could cause the shaft to wobble.”
The rigidity allows the carriage to run true and steady, and maintains the constant tension of the core and wrap wires and their respective feed angle. The only specification change on the latest machine is the introduction of a dual slide system, mounted in parallel with a set of bearings on each slide track which carries the wire feeder mechanism. Because the system runs dry, this keeps the process as clean and friction-free as possible.
The payback on each of the new machines should be two to three years.
“And crucially the quality of our manufactured product is much more consistent, which is of vital importance in the light of demand,” says How.
Hepco, Astrosyn

Mellow sounds
Streetly Electronics has developed a new Mellotron, an electronic keyboard instrument in which each key controls the playback of a single pre-recorded musical sound – essentially it’s a sample playback machine
The new model M4000 Mellotron – the first production of a cycling tape replay machine in nearly 40 years – contains 35 separate lengths of magnetic tape containing recordings. It uses a stepper motor and driver from motion control solution provider Astrosyn to ensure the pre-recorded tapes are precisely positioned for playback.
Inside the instrument a sprocket, fitted to the stepper motor, drives a Reynolds chain. This loops around front and back drums holding opposite ends of the tapes. When a key is pressed a digital interface relays commands to the Astrosyn driver and the stepper motor transports the appropriate tape to the correct position under the playback heads.

Tom Shelley

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