The old saw about knowing when to "hold 'em" and when to "fold 'em" takes on new meaning when applied to fuse, or shear, pins. These pins hold a jet engine securely on an aircraft wing but still break away when necessary. By allowing the engine to separate from the wing in a crash or other emergency, the pins prevent catastrophic structural failure and fires.

Sonic Industries, Torrance, Calif., manufactures fuse pins and other aerospace components. The company makes fuse pins out of steel and stainless steel alloys, including 318 and 15-5, in various diameters and lengths to 23 ". The pins start as a bar forged to specific geometry and are gundrilled and bored to a rough, preliminary hole size. The parts are heat treated and samples are tested for shear strength before the entire lot is sent for final machining and OD grinding.

Courtesy of Sunnen Products

An example of a fuse pin (top) and a look inside the automated honing machine purchased by Sonic, with a superabrasive tool in place.

The heads are finish machined with slots or hexes before the pins are bored to size and honed to establish required ID size, geometry and surface finish. The ID tolerance is 5µm to 7µm. ID surface finish must be from 8µin. to 16µin. rms. ID geometry can vary from a through-hole to a blind-hole with an angle and a radius or just a bottom radius.

Sonic had been honing the pins with manual machines from Sunnen Products Co., St. Louis, but needed higher throughput to meet increased demand. "We needed to increase productivity and decided automating the honing process was the best way to accomplish it," said Facility Manager Roy Franks. "Before purchasing a Sunnen SV-1000 automated machine, we conducted time studies with them. The machine has since exceeded the time study estimates and the finish is superior to the previous manual honing method."

Upgrading to automated honing also enabled Sonic to use Sunnen's MMT superabrasive tools. Designed to work with SV-1000 series machines and engineered to the application based on width, length, expansion angle and number and placement of stones, the tools are accurate to 0.6µm for diameter, roundness, straightness and taper, according to Sunnen. Machined with a body and feed wedge made from hardened tool steel, the superabrasive tools typically last five times longer than conventional designs and reduce per-part cost by 30 percent, according to Sunnen.

Another advantage of the automated machine is its longer stroke length. "Our old machines were labor intensive and had a maximum stroke length of 9 "," Franks explained. "That limited the parts we could produce, but now we're able to make more sizes, and we're doing it faster." The SV-1000 handles diameters up to 3 " and has a 31 " stroke length.

Automated honing also freed up operators to tend more than one machine. "With the manual hones, our process was hone a little, check the part, hone a little more, then check the part again," Franks said. "With the CNC hone, we dial in the settings and 99 percent of the time the part comes out to spec." After honing, parts are measured using a scanning air gage.

Sonic has already more than quadrupled output of fuse pins, from nine parts to 40 parts per day, but Franks thinks that number can climb even higher. "We're looking at possibly purchasing another SV-1000 and that one may have a rotary table, which would allow us to continue honing parts while others are being checked," he said.