Medical Device Link.

Precision Machining Trends

Swiss machining is not a new concept, but recent advances have made it even more applicable to medical device manufacturing. The Swiss process originated in the 1800s when Swiss watch manufacturers developed a modification to lathe technology. The new process enabled users to make very small, precise parts for watches that required long, small-diameter machining turns. Swiss technology has continued to develop, addressing ever more intricate design needs. It now includes computer numerical control (CNC) and automated cell technology. Automated machining cells typically tie several machining stations together using robotic equipment to hand off parts from one station to the next, with no operator intervention.

This article describes the latest trends in automated Swiss machining and offers a manufacturing approach for the medical device industry that enhances quality, speeds shipments, and lowers costs.

Growing Demand

Cell configuration shows an overhead gantry robot that services the four adjacent Swiss machines on one side and the parts cleaning and inspection stations on the opposite side.

The demand for precision-machined medical devices continues to grow despite the current economic downturn because the number of patients that require surgery grows as the population ages. Precision-machined medical devices span an increasingly wide range of medical conditions and treatment. Some of these include the following: interventional cardiology devices (catheters and surgical tools), urological devices (surgical needles), orthopedic devices (bone screws, implants, and joint replacements), minimally invasive surgical devices (laparoscopic instruments), diagnostic devices (point-of-care testing instruments), wound-care devices (staples, suture anchors, and clips), and dental devices (implants and instruments).

The need for automated Swiss technology is broad-based and growing at an estimated 8% annual rate.1 In the United States alone, the medical device market is $100 billion, and applications for automated Swiss technology are contributing to that growth.

Today, Swiss machines are very precise and allow for a wide range of feature-generating techniques. Along with standard milling and turning, a CNC Swiss machine can perform broaching, polygon milling, honing, knurling, burnishing, hobbing (gear cutting), threading (including thread whirling and thread rolling), and other complex machining processes. In many cases, a CNC Swiss machine can produce parts in a single manufacturing operation that are complete and burr free, and don’t require additional finishing.

Four Swiss machines operate together with no operator intervention. The system has two robots, a parts cleaning station, and an inspection system to produce precision families of different parts.

The latest Swiss machines such as the Citizen Cincom L20 model are extremely fast and perform multiaxis machining operations simultaneously. Each machine has two spindles, a main spindle and a secondary spindle, whereby the machining is shared and handed off between the two spindles. Primary spindle speeds up to 10,000 rpm, secondary spindle speeds up to 8000 rpm, and gang rotary tool speeds up to 5000 rpm are the highest for this type of machine.

Automated Machining Cells

The output of these machines is multiplied when configured into an automated cell. Automated machining cells are typically made up of four Swiss machines and include the following components.

Gantry Robot. The cell’s most important part is a gantry robot on an elevated 30-ft-long track. The gantry robot transfers parts from all four Swiss machines to the secondary operation stations in the cell (i.e., robotic ultrasonic cleaning station and inspection station). Programming logic for the gantry robot ensures that parts are removed from the Swiss machine that finishes first (not necessarily in sequential order). Process design and programming minimizes delays and keeps parts moving efficiently through the cell.

The machining system’s cleaning station provides thorough cleaning of both finished parts and the robot’s jaws prior to part inspection.

Bar Feeder. Each Swiss machine is equipped with its own bar feeder. The bar feeder’s purpose is to supply the Swiss machine with raw material. Each bar is 12 ft in length, and the diameter of the bar is dictated by the required diameter of the finished part. Depending on the diameter of the raw material required for a specific project, anywhere from 13 to 50 bars of raw material can be placed into the bar feeder, ranging in diameter (respectively) from 0.75 to 0.06 in. The bar feeder gives the Swiss machine one bar at a time, and as it extracts the last remnant of the current bar it has been machining, it indexes a new bar into the feed channel. This reduces operator intervention.

Critical part features are verified automatically by the machining system’s inspection system. Any deviation in required tolerances triggers an alarm signal, stopping the specific machine from which the nonconforming part came.

Part Cleaning. Three-station cleaning is performed after each part is machined. This subcell includes its own small robot, which takes each part through the three separate ultrasonic baths to remove chips and cutting fluids. The cleaning station also contains a rinse tank. After the cleaning and rinse cycles are completed, parts are blown dry. The gantry robot’s jaws are also cleaned and blown dry prior to delivering finished parts into the gauging system for final inspection.

Part Inspection. The inspection station is the final stage of the cell. Every part can be inspected, or if the process capability warrants, a specified percentage of parts can be inspected (for example, one part in 20 from each of the four Swiss machines).

Parts can be assessed using a noncontact measuring system, such as a TESA Scan 50. The system measures lengths, diameters, angles, radii, and other features on cylindrically symmetric components. Runout and other dynamic measurements are also possible. In many cases, no other inspections are needed.

If an out-of-tolerance part is found, the system should shut down only the Swiss machine that created the nonconforming part. The rest of the cell can continue to operate uninterrupted. The operator is alerted and can identify the problem and attend to the affected machine.

The inspection gauge works well when used with software, such as Q-C Calc, which automatically downloads each measurement taken in order to generate statistical process control data and other process control information. This information is displayed at the cell so that process status can be viewed at a glance in real time.

Rolling pallets receive finished parts delivered by the robot. With parts organized on these pallets, lot integrity and part traceability is ensured.

Palletizing of Parts. Finished parts are palletized by the gantry robot in order to maintain lot integrity and part traceability. Each part can be traced not only to when it was made, but also to which specific machine each part came from, as needed. If there is a problem, nonconforming parts can be easily identified and segregated without significant operator sorting.

Benefits and Limitations

Not all medical device components are ideally suited to run in this type of cell, but when there is a match, customer benefits are substantial. The cell is designed to create specific manufacturing processes for longer-running jobs for the most cost-effective per unit cost. Designed as a low-cost process, the cell is best suited to medium to high volumes (20,000–500,000 pieces annually). The automated cell is a suitable choice for producing families of similar parts, and multiple components (see the sidebars, "Families of Parts" and "Multiple-Component Assemblies").

Here, 19 tools allow a wide variety of part configurations to be run with minimal changeover time.

Once a process is created, the cell is designed to minimize changeover between parts. Process standardization (preset cutting tools, custom-designed pick-off and ejection tooling, CAM program generation, and programming adaptations utilizing macros) enables seamless changeover from one part to the next. In addition, the ability to switch over quickly between different, longer-running jobs allows lower inventory levels while providing delivery schedule flexibility.

Conclusion

Swiss machining continues to be the gold standard for precision medical device manufacturing. For devices that require runs ranging from 20,000 to 500,000, device manufacturers may want to consider an automaton cell. As illustrated by the sidebars, part families and multiple-component assemblies, may be particularly suited to Swiss machining using automated cells. Additional processes, such as those illustrated by the sidebar, “Surgical Needle Devices,” can also be explored using automated cell processes. Although not covered in this article, factors such as material and design complexity must also be considered. For best results, it is advisable to discuss machining plans with a knowledgeable vendor.