A medical device manufacturer recently received discouraging feedback from surgeons—the screw and small nut used in the hinge mechanism of the company’s surgical scissors, although secure and reliable, was either too loose or too tight to allow pinpoint operation accuracy making the scissor difficult to control.

Endoscopy surgical scissors need a consistent “feel” so that opening and closing of the blades is sensed the same way every time. It was evident to the surgeons during delicate procedures that although they visually positioned the scissors and cut by sight, of equal and critical importance was the minimal friction experienced when adjusting the blades.

In an effort to make medical surgical operations go smoother, the scissor manufacturing engineers sought a new method to fasten and assemble the blades together. With help from Jackson, MI-based Orbitform application lab engineers, these aspects of the scissor making processes were carefully analyzed:

• Screws -- Even with the most precise methods of process control, the screw and nuts weren’t holding with a consistent tension because of small thickness variations within the blade halves.

• Riveting -- Since the blades needed to be “firm yet flexible,” simply riveting them together to an exact stop point was not an option as this wouldn’t allow for needed flexibility.

• Orbital forming – It was determined that it was best to orbitally form the scissor parts to a clamp load that eliminated the part variations as a factor. Orbital forming is a cold-forming process that uses an orbiting tool to swage, head, crown, flare, or form a column or projection of malleable material for the purpose of fastening in assembly operations. It is also referred to as spinning, radial riveting, spin riveting, or peening.

Process intelligence

By using multiple load cells on a benchtop orbital forming machine, a form-to-force process control solution was created. This allowed the torque on the scissor joint to correspond to a pre-determined load, thus eliminating part size as a quality factor.

The Orbitform machine used a spindle-slide-servo motor arrangement. This provided accurate control of the .058-in. diam. pivot joints and permitted the forming machine to dwell at a position instead of a positive stop. Extensive testing by the scissor manufacturing engineers verified that each pair of medical scissors was able to repeat the opening and closing within the desired force range.

The work station featured process control digital readouts for orbital and anvil positioning and exact force monitoring. Also, a custom fixture for positioning the assembly helped increase the throughput and decrease the fastening time.