Like various smaller sized rapid prototyping equipment and 3D printers, the CNC is following suit and shrinking to a “personal” size. The design concessions necessary to achieve that size are fewer than you might think.
The term ‘Personal CNC’ describes the new Tormach mill and correlates to the first personal computers when they shrank in size and price to become practical tools for the individual. Greg Jackson, CEO of the Waunakee, Wis. based company and his development team have spent three years working in conjunction with machine tool manufacturers to achieve their goal: create a small precision CNC machine that can meet the needs of short-run production at low cost.
A personal CNC enabled medical device manufacturer KFx Medical to rapidly prototype bone screws of various metals for its line of medical fasteners that eliminate the need for sutures.
The Tormach PCNC has a standard 34 in. x 9.5 in. table and 17 in. vertical clearance, as well as comparable precision of larger mills. It can cut iron, steel, titanium, and chromium alloys such as 300 or 400 series stainless.
The design concessions that make it affordable are that of spindle power and the speed of rapids–it takes a few minutes longer to cut a precise part than on a larger industrial mill. “A 1.5 hp CNC mill with 65 IPM rapids is ludicrous in a large volume production environment where minutes per piece are crucial,” noted Jackson. “However, in a prototype environment, where run time is a tiny fraction of setup time, those extra minutes per part are simply not relevant.”
The design of the mill also carefully leverages mass against surface cutting speed, allowing for exceptional precision in a machine that weighs little more than half a ton. The design uses maximum dynamic stiffness to maintain stability and precision. “Mass and strength are important, but they must be in the right place,” Jackson said. “If you only have a thousand pounds of iron to work with, it does not make sense to put a lot of it in a supporting base.”
The PC and Windows control computer, g-codes and m-code commands, drawing and image file support, spindle taper, machine dimensions, and the electrical interferences are all industry standard.
The new medical fastener system also required a special tool for doctors. The plastic handle had to have the proper ‘feel’ in a surgeon’s hands. The ability to quickly prototype various handle variations kept development time short. These photos show the initial idea, (A) turning it into a machine ready program, (B) , the rough cut version, (C), and one of the finished prototypes (D).
This PCNC came in handy when a customer needed to prototype a new medical device. The San Diego-based firm, KFx Medical, is a venture capital start-up with a new idea for orthopedic surgeons who repair rotator cuffs. The company’s new technique, called “SutureCross Knotless Anatomic Fixation,” involves simpler and stronger fasteners to hold tissues, eliminating suture knots.
To move the ideas for the fixtures and the application instruments from the virtual realm of CAD to real testable prototypes, Mike Ko, senior product development engineer, and his development team did the precision cutting with the Tormach PCNC 1100.
The Tormach PCNC cutting medical screw samples.
The team cut prototype and pre-production components out of plastic, aluminum, and stainless steel. Even 420 stainless steel injection mold cavities were cut in house in a few days. “We were even able to use the 4th axis rotary capability of the Tormach to fabricate prototype 316SS bone screws with a variety of tapering spiral thread designs–all in house,” said Ko.
One of the most challenging R&D tasks for the suture system was the ergonomics of the plastic handle of the tool that the doctor holds during surgery. “Like all hand-held instruments, the device had to have the proper ‘feel’ in the surgeon’s hands. To find this elusive feel, numerous full size prototypes of handle variations were made and put into the hands of the surgical advisory board members as quickly as possible,” explained Ko. “The current pallet of high-tech rapid prototyping methods (SLS, SLT, and RP) could accurately reproduce the required shapes of the handles, but not in materials that would stand-up to the rigors required of an orthopedic device.”
Various medical screw prototypes made on the PCNC.
”With the Tormach and the current suite of 3D solid modeling and CAM software, we were able to go from a 3D CAD solid model to a machined handle component in one day and to a completed, fully functional handle assembly in one week,” Ko said.
CAM converts CAD files to g-code. The software for g-code generation is FlashCut, Visual Mill, and DeskProto. Other inexpensive programs automatically convert the g- and m-codes into the actual commands that move the machine. KFx Medical uses CNC controller programs like MACH2 or FlashcutCNC.
The Tormach machine, working with standard g- and m-codes, is compatible with all brands of CAM software. The company recommends TurboCAD/CAM and SprutCAM as effective programs for low cost. Similarly, it offers inexpensive CAD and 3D modeling programs, such as TurboCAD and Alibre Design.
Now that hardware and software have come together to create a personal package, Jackson believes the accessibility to this size of CNC technology will have a deep impact on many aspects of industry (not just in rapid prototyping fields) and education. And as a side benefit, companies find that the designs sent to production are improved because the design engineer’s direct involvement with building prototypes leads to a better understanding of regular production.
KFx Medical Corporation
Filed Under: Software • 3D CAD, Software • CAM, Machine tools + subtractive manufacturing, Screws • nuts • bolts, Medical-device manufacture, FASTENING • JOINING • locks • latches • pins, MECHANICAL POWER TRANSMISSION