Micro-coil winders from Engineering by Design wrap coils from wire to 0.001-in. diameter about a spinning mandrel (to 3,600 rpm) of the same diameter at wind angles to 0.5°. Wire tension is controllable to 60 gm or less. Shown is a micro-coil (0.001-in.-diameter wire, 0.003-in. coil) compared to a human hair (about 0.004 -in. diameter) and President Lincoln's nose on a penny.

A recently developed micro-coil winder capable of tensions lower than 3 gm helps a medical device company produce miniature, multi-pitch coils faster and more accurately than manually controlled winders. Micro-coil winders from Engineering by Design, San Jose, precisely control wire tension and slack by use of a proprietary "dancer" mechanism combined with dual, independent servoloops, a coil-angle sensor, and an electronically geared mandrel drive. The result of all this is finer control of tension at levels lower than what is possible with conventional drag brakes.

"Drag brakes tend to apply jerky, inconsistent tension and don't work well at tensions below about 20 grams," says Dale Henson, president of Engineering by Design. Brakes that rely on fluid-suspended magnetic particles to apply force further complicate matters. This is because the fluid introduces a drag force that decreases linearly with temperature (due to viscosity) and increases exponentially with rotational speed (due to fluid shear). The dancer arrangement eliminates drag brakes altogether.

The medical-device maker, Conceptus Inc., San Carlos, Calif., uses the machine to build tiny, implantable contraceptive coils. The coils go inside fallopian tubes via a nonsurgical procedure. "These variable-pitch coils (0.005-in.-diameter wire over a 0.010-in.-diameter mandrel) proved difficult to produce on machines with manual controls," says Conceptus' Roberto Silva-Torres. Pitch is a measure of the distance and angle between adjacent coils. "The new machine can be programmed to accurately repeat up to five different pitch angles."

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