A medical device manufacturer recently received discouraging feedback from surgeons. It appears its surgical scissors were difficult to control. The screw and nut in the hinge, although secure and reliable, were either too loose or too tight to allow sufficient accuracy. Endoscopic scissors need a consistent “feel” so users sense opening and closing the blades the same every time. Surgeons said that although they visually positioned the scissors and cut by sight, of equal importance is minimal friction.

To make surgeries go smoother, engineers at the scissor company sought a new way to fasten and assemble the blades. With help from the Application Lab at Orbitform Group, Jackson, Mich.,(orbitform.com) the engineers analyzed several aspects of their manufacturing processes.

They found that small thickness variations in the blades kept the screw and nut from holding consistent tension. The engineers considered several fastening methods that also generate a pivot. For instance, because the blades had to be “firm yet flexible,” simply riveting them together to an exact stop point was not an option because it wouldn't allow the needed flexibility. They determined that orbitally forming the scissor parts to a clamp load would reduce thickness variations. The team built a forming machine to shape the scissor blades.

Several load cells on the bench-top orbital forming machine created a form-to-force process control which let the torque on the scissor joint correspond to a predetermined load, thus eliminating part size as a quality factor.

The forming machine used a custom spindle, slide, and servo motor. For accurate control of the 0.058-in. diameter pivot joint, the controls let the forming machine dwell at a position while it cold works the rivet, instead of a positive stop.

Extensive testing at the medical-device company verified that each pair of scissors opened and closed within the required force range. The new orbital work station features digital readouts for orbital and anvil positioning and exact force monitoring. Also, a custom fixture for positioning the assembly increased throughput and decreased fastening time.