Surgical Precision: How Advanced Technology Meets the Close Tolerances Needed for Medical Tools

Because the medical manufacturing market is more competitive than ever before, the search is on for new ways to perform more reliable, faster and more cost effective precision cutting of complex medical instruments made of exotic materials. Here is a closer look at how some of the latest developments in advanced technologies are meeting these needs.

There are 11,000 companies that currently compete in the $85 billion medical manufacturing market in the U.S.¹ Without exception, every one of them demands more reliable, faster and more cost effective manufacturing solutions to stay competitive in the tightening global marketplace, and they are all looking to advanced technology to meet their needs.

For example, there is heavy demand for the precision cutting of thin metal tubular structures used in instruments such as rigid endoscopic and arthroscopic devices, biopsy tools, needles and cannulas. The most common materials used in these and other surgical and implantable medical devices are stainless steel, cobalt chrome alloys, nickel alloys, titanium and Nitinol. In the dental sector, there is heavy demand for high-precision bridges, crowns and implants that also use titanium as well as prosthetics materials such as chromium cobalt, zirconia, ceramic, and plastic.

Several methods have been developed within the industry to cut these exotic materials, including laser, EDM, waterjet, chemical machining, milling and grinding. This series takes a closer look at the latest developments in two of these technologies: fine laser cutting and micro-milling.

FINE LASER CUTTING
New laser technology offers the surgical precision needed for medical tools.

Geoff Shannon, Miyachi Unitek Corporation

Over the past 20 years, laser cutting has been used in an increasing number of manufacturing applications in the medical, automotive, electronics, aerospace and other industries. Many of the pioneer laser cutting applications were performed with high power carbon dioxide (CO2) gas lasers, but CO2 was eventually found to be unsuitable for fine laser cutting due to lack of heat input control and focus spot size.

As a result, solid state lasers emitting a shorter wavelength – typically near one micron – emerged as the best choice for precision cutting applications and, over time, became the preferred choice for many thin wall metal tube cutting or machining applications, particularly when superior edge quality, tight dimensional tolerances and/or high volume production is required.

Now conventional solid state lasers are being replaced by fiber lasers that offer a reliable, stable energy source, have exceptional beam quality, high repetition rates and are easy to integrate into production manufacturing machines. In fact, fiber laser cutting is most effectively used in the “surgical precision” cutting of thin wall metal tube materials, such as those used in endoscopic and arthroscopic medical instruments.

The term “surgical precision” aptly applies to the need for clean sharp edges, contours, and patterns found in the tools and devices being introduced into this burgeoning field. From surgical instruments used in cutting and biopsy, to needles containing unusual tips and side wall openings, to puzzle chain linkages for flexible endoscopes, fine laser cutting provides higher precision, quality, and speed than traditional cutting techniques.

Integrated fine laser cutting technologies are advancing quickly into the market, including 5-axis motion packages that give the designer freedom to cut more challenging geometries in one pass. This advanced machine configuration integrates a more powerful fiber laser with increased speed and material thickness capability into a turnkey system with multi-axis motion capabilities that gives the medical device designer the freedom to create more challenging geometries with superior “as cut” edge quality.

Integrated fine laser cutting is ideal for the specialized cutting requirements found in medical tube tools and components. The key to successfully using this technology is properly integrating the system’s components into a process flow that works. The motion, laser, software, and tooling must all work together to get the desired end product.

The Small of It All
Fine laser cutting is ideal for working on small tubes that must be cut to high dimensional accuracy because the laser light used does not have any physical presence and makes no contact with the material. It does not push, drag, or impart force that might bend a part or cause flex that would have a negative impact on process control. Laser light also offers minimal thermal input, with fine control over how hot the work area gets. This is important since small parts heat up quickly and might otherwise overheat or deform.

Fine laser cutting is highly focusable to about 25 microns, or about one-quarter of the width of a strand of human hair. This makes it feasible to remove the minimum amount of material to make the cut, resulting in extremely high precision and high accuracy. This laser cutting technology uses an extremely fine control of pulse width, power, and focus spot size. Because the laser cutting tool does not rely on touching the part, it can be oriented to make any shape or form. Not limited by physical cutting geometry, laser cutting can be used to make unique shapes. Table 1 lists the technology’s key benefits.

How It Works
The technology most frequently used to make medical tubes and components is fiber laser cutting with gas assist, which means the laser is “assisted” with a coaxial gas, typically oxygen (O2). While O2 is usually the gas of choice, occasionally shop air can be used after oil filtering, usually when the tube thickness is less than 0.010 in or when the cut quality required is not very high.

This gas assist is used for stainless steels (300 and 400 series, 17-4, 17-7), MP35N (cobalt-chrome steel alloy) and Nitinol. The method can also be used for both on-axis (90 deg to surface) and off-axis (angled to surface) cutting.

In this process, a highly focused laser is used to melt a thin sliver of material. While the material is still molten, a 0.020 in diameter gas jet nozzle that is coaxial with the laser blows away the molten material. The desired features are produced using this continual cycle of melt, then melt ejection. The distance between the laser and the material must be maintained precisely.

The O2 serves two purposes: (1) it blows away the molten material and (2) serves as a heating element, because the heated material reacts with it and heats up. The heat reaction caused by the presence of O2 adds about 30 percent to 50 percent more heating energy to the cutting area. The gas assist is a key factor in increasing cut speed and cut quality. Figure 1 illustrates a basic fiber laser cutter with gas assist. Figure 2 shows a close-up of an actual laser cutter at the work piece.

Fine laser cutting with gas assist produces the highest cut quality and high resolution cut paths, a key objective of the makers of medical tube tools and components. Dimensional accuracy is paramount to measuring cut quality – does the part match the print? Other considerations include surface roughness (better than 12 micro inches) and the absence of thermal damage.

Cut width can be extremely small with laser cutting, less than 0.001 in, and dimensional accuracy is extremely precise, at about ±0.0005 in. This accuracy is very useful for producing the jagged teeth used in some cutting tools. Dross or burr left on the underside of the cut (which can become attached and re-solidified) is minimized or eliminated to significantly reduce the amount of post-processing needed.

The recast level layer (a small amount of material that doesn’t get blown away during the process) is less than 0.0005 in. Figure 3 illustrates the typical cut quality with no post processing of features and edges when cutting 0.010 in thick stainless steel (304SS) tubing, showing the excellent quality of laser cut faces.

How It Compares
Fine laser cutting speed and precision compares favorably with that of its chief competitive technology, electro discharge machining (EDM). To obtain the same high quality cut as a laser cutting machine, EDM requires up to four passes, which slows down processes considerably. But on the plus side, EDM allows multiple parts to be processed all at once.

The width of cuts produced by a fine laser cutter is as small as 0.001 in, while that of EDM is around 0.004 in. Feature sizes are limited with EDM and sharpness is not as good, compromising cut resolution. The EDM process is also limited by the fact that it works best with certain geometries, such as tubes with a symmetrical profile. Problems arise if there is a hole in a tube that does not go through both sides. In other words, EDM is like the ubiquitous cheese cutter that uses a wire to slice through a block of cheese. All is well with a symmetrical, solid shape, but the wire cannot handle any complex geometries.

A final issue is floor space, especially for factories where space is at a premium. A typical EDM machine can be as large as 10 sq ft to 12 sq ft, while a fine laser cutting system is only five sq ft to six sq ft.

Another competing technology is electro chemical grinding (ECM), which removes electrically conductive material by grinding with a negatively charged abrasive grinding wheel, an electrolyte fluid, and a positively charged work piece. ECM is a fast cutting method that gets quality similar to EDM. With ECM, the electrolyte being used must be disposed in accordance with OSHA as hazardous waste, and some electrolytes produce hexavalent chrome when cutting steels. Finally, this sort of use of hard tooling makes it much less flexible than laser cutting.

A third competitor is waterjet cutting, which slices into the metal using a jet of water at high velocity and pressure, or a mixture of water and an abrasive substance. Waterjet cutters offer restricted cut geometry. Only symmetrical through-features or end cuts are possible using this technology. Figure 4 compares the cut quality of EDM and laser. Note the smooth sharp edges in the cut made by the fiber laser equipment.

Whole Integration
The explosion of new non-invasive surgery tools has introduced some unique and innovative shapes. This, in turn, requires motion packages that offer a new level of cutting geometry. The ability to keep a part in a machine and make intricate cuts gives designers the objective of cutting more challenging geometries in one pass.

This objective is accomplished in 5-axis laser cutting systems that use control software to command the laser and the motion together. The integration of these two functions provides a rigid structure, free from vibration. The 5-axis motion consists of 3 linear axes and 2 rotary axes in a unique set-up that allows system engineers great flexibility to choose the best axis configuration for a particular cut because they are no longer limited to where they are on the part.

Engineers can mix and match and set combinations to create a more efficient process. For example, designers might place four axes on the part and one on the focus head, or switch them around according to the best solution for the application. This is significant, because even though fine laser cutting for medical tube tools and components has many benefits, actually achieving them depends in no small part on successful system integration. The entire system must integrate the motion, laser, software, and tooling into a whole that works properly and supports the desired process flow. Putting these pieces together can be a challenge.

That challenge is exacerbated by the fact that many integrators do not have a good understanding of the laser cutting tool, which may tempt them to rely on the manufacturer of the laser cutter to integrate the system. Then, when or if there is a problem with the laser or if changes are needed to adjust to a new product, the integrator is in no position to fix the system.

To prevent this problem, users should assess the in-house laser cutting capabilities and knowledge of the cutting process of the integrator that is being considered. Though the integrator may not be a cutting laser manufacturer, they do purchase the laser from an OEM and should integrate it into a complete system with motion, software and tooling.

Any integrator that can add fine laser cutting to other related capabilities, such as laser welding and marking/engraving, should be able to provide designers with a one-stop shop for system integration, including running samples of the entire process in-house to ensure that it does the entire job as specified, and answering in-depth application questions.

References
1. Society of Manufacturing Engineers market research data, Dearborn, MI, January 2012.

Geoff Shannon is the laser technology manager for Miyachi Unitek Corporation, 1820 South Myrtle Avenue, Monrovia, CA 91016, 626-303-5676, [email protected], www.miyachiunitek.com.

THE NECESSARY “BITE” FOR DENTAL MILLING TOOLS
Micro-milling takes center stage in the surgical machining of dental parts.

Bridges, crowns, implants and other dental parts are complex high-precision components with rigid quality demands in medical manufacturing. To meet these tight specifications, successful dental laboratories are turning to micro-milling technology from machine tool distributors such as ARO-tec GmbH (Rheda-Wiedenbrück, Germany) that combines a high-speed machining process with advanced interior tool clamping techniques.

ARO-tec, a German distribution and service partner of Haas Automation (Oxnard, CA), has combined Haas machinery with an innovative Swiss toolholder from Rego-Fix (Tenniken, Switzerland) to develop customized “dental packages” based around the micro-milling capabilities of the combined system.

For example, OM2A Dental micro-milling machines from Haas Automation perform extremely precise and economical machining of dental components made from ceramic, plastic and metal. They accomplish complex chipping tasks used in mass production or the quick turnaround of prototypes of small, high-precision 2D or 3D medical parts by expanding from a standard three-axis structure up to five axes using a rotary table or rotary/swiveling table, thereby allowing five-axis simultaneous machining.

ARO-tec combines this 5-axis multitasking capability with a special ISO 20 ERM toolholder from Rego-Fix that ensures a required precision run-out with 100 percent dimensional accuracy, even with small tool diameters, a function that is crucial in elevating the productivity of the multitasking equipment. “A tool can provide absolute machining precision, with top surface quality in high-speed machining, but only if it is optimally clamped and can positively impact the entire system with its precise run-out,” explains Oliver Stabenow, a sales engineer at ARO-tec.

Rego-Fix developed the ISO 20 ERM toolholder system specifically for Haas Office Mill machining centers such as those used in medical manufacturing. “The system is 100 percent precision balanced at 40,000 min^-1 in 2.5G. Combined with a high rigidity and very good run-out with figures of under 3 µm, this innovative tool clamping system ensures the best processing results,” explains Martin Brönnimann, the head of product development at Rego-Fix.

This system is perfectly demonstrated when machining dental implants with the filigree milling of chrome cobalt. A 1 mm ball cutter is used at 30,000 rpm, with feed rates of up to 2,000 mm/min. Despite the hard load, the mini-milling tool must show a high level of run-out precision in view of the extremely close tolerances specified. This is the only way to provide the necessary surface quality.

Quiet and vibration-free running also significantly increases the service life of the tools. The same holds true of the process when machining other difficult materials that are commonly used in dental technology, such as titanium and zirconium. In all cases, the dental implants have a perfect precision surface quality, which means expensive and time-consuming manual reworking is not necessary.

“Using the OM2A Dental with the ISO 20 ERM toolholder, dental technicians can overcome these special challenges quickly and economically,” notes Stabenow. “Productivity increases significantly through the precise run-out. The ease of use and the quality of the milling results, even with materials that are difficult to machine, such as plastic, zircon and metal alloys, speak for themselves. Manual reworking is no longer necessary. On balance, it is no surprise that customer satisfaction is very high and that demand for our dental packages is rising steadily.”

The original source of this article was Meditec INTERNATIONAL, www.meditec-international.com.

ARO-tec GmbH, Siemensstr. 12, 33342 Rheda-Wiedenbrück, Germany, +49 (5242) 9649-0, Fax: +49 (5242) 9649-19, www.aro-tec.org.

Haas Automation, Inc., 2800 Sturgis Road, Oxnard, CA 93030, 805-278-1800, Fax: 805-278-2255, www.haascnc.com.

Rego-Fix AG, Obermattweg 60, CH-4456 Tenniken, Switzerland, +41 61/976-1466, Fax: +41 61/976-1414, [email protected], www.rego-fix.ch.