CNC Machining Free Forms
Getting an exact prototype is crucial in the product development environment, something often achievable only with CNC machining.
Reed Prototype and Model (RPM) (Austin, TX) concentrates exclusively on complex surface machining because it occupies a necessary space in the product development process - you achieve the accuracy and strength of a production part with quick turnaround.
The engineers who seek out RPM often start out using other RP processes to refine their design and move to machined plastic prototypes to do final testing and debugging. Others will go straight to machining because they need to know the exact behavior of their chosen material - not a review of the style and shape. That was the case for the Joey Clamp - an umbilical clamping and cutting device.
Putting CNC to the Free Form Test
The first thing any newborn will wear - the Joey Clamp - looks simple. It's a two part clamping and cutting device that is used to clamp and cut the umbilical cord of newborns in one motion. For the design team and RPM, machining was critical for all three rounds of prototypes and the device itself ended up being a greater challenge than anyone realized.Invented by an OB-GYN and designed by an Austin product development firm, the clamp replaces three separate instruments currently used for clamping the umbilical cord - two alligator-type Hollister clamps and one pair of scissors. In addition, it provides critical protection from blood spray - a growing concern given the increased frequency of blood-borne diseases.
Many engineers and designers think of CNC in terms of limitations - they believe that free form shapes and curves are beyond the capabilities of CNC. That is an outdated notion, according to Phil Reed, president of RPM. "We have helped to pioneer advanced fixturing techniques that allow us to machine virtually any shaped plastic part. We use the block of the material that the part is being machined from as the basis of the fixture and then have a process that allows for multiple setups," he says.
Carrie Bader, the project leader for the Joey, knew the free form design of the clamp and cutter wouldn't be a problem for RPM. Having seen previous free form designs from RPM, the team knew its initial prototype could be machined out of the exact material involved - polycarbonate. The bottom line was simple - they had to know the exact response of the material.
Prototypes and Setups
Bader and her team looked to the prototype to validate their intuition and experience with polycarbonate, especially given its application. The cutter involved a single-use living hinge - not a recommended use for polycarbonate - and there was a considerable internal debate whether or not the device would be too brittle. In addition to polycarbonate, the team considered K-Resin, acetal, Valox, Ultem and nylon. There was no "slam dunk material" for the project. Ultimately they settled on polycarbonate since the device was a single-use device.Once at RPM, the first Joey prototype required seven setups to create the free form curves on the cutter and four on the clamp. "We find that free forms like the Joey require special planning and many more setups - but they are well within the capabilities of CNC machining," notes Reed.
As expected, the first prototype taught designers a great deal. In the initial prototype the material twisted in an unexpected direction. The living hinge worked, but the flex hinge in the clamp moved so much that it pushed the face of the clamp off center. In the second and third prototype the design team made significant changes - they adopted a wave pattern on the clamp, added ridges on the "chin" of the clamp to increase clamping pressure and added guides to restrict the movement of the clamp while it was in the cutter. But one of the greatest challenges for RPM came with the translucent purple color of the clamp itself.
Surface Treatment
RPM developed a translucent surface treatment to capture the luminescent color now prevalent in plastics consumer electronics and personal computers.The process for surface coating CNC-machined prototypes involves chemical dyes. The approach is equal parts mixing and timing. Using chips of the same thickness as the part to be dyed, many test pieces are dipped. Once the range of color is reached, it's a matter of split-second timing - a couple of seconds' difference and the color will change dramatically.
In the end, the purple koala face has been a hit with doctors and nurses; the safety of the clamp/blood shield is widely praised; and Joey clamps are now being used on newborns.
When the prototype has to do more than function as an abstract, CNC machining gives designers and engineers the best tool - a model that will teach, enhance and capture the exact nature of the final product. RPM has repeatedly shown that with careful planning and exacting execution, virtually any form can be achieved through CNC machining.
For more information contact Reed Prototype and Model (Austin, TX) at its website at www.rpmaustin.com.
