Joint Makeover

Recently, a major medical device manufacturer was getting some discouraging feedback from its main customer group: endoscopy surgeons. They said the company’s surgical scissors were difficult to control. It seemed the screw and small nut used in the hinge mechanism, although secure and reliable, was either too loose or too tight to allow pinpoint accuracy.
By David Parham, Orbitform Group December 1, 2009
LINKS
Pre-assembly of components at Behr leads to flexible, accurate assembly. http://budurl.com/kmhu

 

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Recently, a major medical device manufacturer was getting some discouraging feedback from its main customer group: endoscopy surgeons. They said the company’s surgical scissors were difficult to control. It seemed the screw and small nut used in the hinge mechanism, although secure and reliable, was either too loose or too tight to allow pinpoint accuracy.

Endoscopy surgical scissors, whose blades are remotely actuated by a cable attached to the handle of an instrument, need a consistent “feel” so that the surgeon senses the opening and closing of the blades the same way every time when performing delicate procedures in the operating room. To make the medical operations go smoother, the medical device manufacturer sought a new method of fastening scissor blades together. With help from Orbitform Group, specialists in forming, fastening, joining, and assembly equipment, all aspects of the scissor-making processes were carefully analyzed.

Using multiple load cells on the machine, a form-to-force process control solution was created that allowed the torque on the scissor joint to correspond to a pre-determined load.

Using multiple load cells on the machine, a form-to-force process control solution was created that allowed the torque on the scissor joint to correspond to a pre-determined load.

Joining options

In its analysis, the team considered three main joining approaches:

  • Screws: Even with the most precise methods of machine control, the scissors’ existing screws 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.

  • Orbital forming: Orbital riveting is a method of joining parts together by cold forming the end of a rivet. The resulting joint may or may not allow rotation of the mated parts, depending on the product design. Orbital forming is a method of deforming material to a desired shape: to flare, swage, and crown material from 0.02 in. to 6 in. diameter. This process allows precise control and consistency with minimal or no effect on non-contact areas of the workpiece.

 

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Orbitform engineers determined the best way to assemble the blades was to form the scissor parts orbitally to a clamp load. This approach eliminated part variations as a factor, since each forming operation was able to compensate for blade thickness differences.

By utilizing multiple load cells on an Orbitform Model 125 benchtop orbital forming machine, a form-to-force process control solution was created that allowed the torque on the scissor joint to correspond to a pre-determined load. This eliminated part size as a quality factor.

This orbital forming machine uses a specially designed spindle, slide, and servo motor arrangement to provide accurate control of the .058 in. diameter pivot joints. It features Watchdawg process control digital readouts for orbital and anvil positioning, and exact force monitoring. This capability, combined with a custom fixture for positioning the assembly, helped increase the throughput and decreased the fastening time.

Watchdawg is a monitoring and control system accessed by a touch screen menu that is easy to navigate and logical in its design. The control system uses standard, commercially available parts which, allows customers to use a single PLC control system.

 

Orbitform engineers determined the best way to assemble the blades was to form the scissor parts orbitally to a clamp load, which eliminated part variations as a factor.

Orbitform engineers determined the best way to assemble the blades was to form the scissor parts orbitally to a clamp load, which eliminated part variations as a factor.

Orbitform’s application lab and product team brought this project from analysis to solution within budget and ahead of schedule. Most importantly, extensive testing by the medical device manufacturing engineers verified that each pair of medical scissors was able to repeat the opening and closing well within the desired force range. The resulting application-specific machine is now producing these surgical scissors around the clock. Thankfully for surgeons and their patients, the cuts have never been better.

Author Information
David Parham is marketing manager for Orbitform Group. Reach him at dparham@orbitform.com.