Medical Device Link.

Prototyping

Medical device manufacturers that need to test their designs require a quick turnaround. Advanced computer numerical control (CNC) programming software can help medical prototype manufacturers produce components with more-complex geometries in less time. The software makes it possible for such manufacturers to take on jobs that would be too time-consuming otherwise (i.e., if they had to program by hand). For example, advanced CNC programming software makes it possible to write programs for four-axis lathes in about an hour. Programs for wire electrical-discharge machining (EDM) can be written in 10–
15 minutes. Such program-writing times are considerably faster than what can be accomplished with conventional CNC programming software or by using manual programming methods. Advanced CNC programming software is particularly powerful when it is used to support advanced manufacturing technologies such as direct metal laser sintering (DMLS); the software can quickly write programs for DMLS parts that need subsequent machining.

CNC machining is used to produce a wide range of medical equipment due to its ability to produce components with complex geometry while holding high levels of accuracy. Examples of typical components produced by CNC machining include spinal implants, orthopedic devices, surgical instruments, surgical simulator systems, collimators for CT scanner systems, and PET scanner components. Programming is akin to cutting out the first piece by hand. Once it is completed, other pieces can be produced without the attention of a person except for loading raw materials and unloading finished pieces on the machine (although operators should occasionally check to ensure that the tool hasn’t worn out or broken). Powerful cutting-edge software such as that discussed in this article makes it possible to program even the most complicated parts in a surprisingly short amount of time.

DMLS is an additive technology that builds parts by sintering very fine layers of metal powders layer by layer from the bottom up, until the part is complete. With DMLS, 20-μm-diam metal powder is completely melted by the scanning of a high-power laser beam—free of a binder or fluxing agent—to build the part with properties of the original material. Eliminating the polymer binder avoids the burnoff and infiltration steps, and it produces a 95% dense steel part compared with roughly 70% density with selective laser sintering (SLS). DMLS is used primarily for small, complex parts (typically smaller than 10 × 10 × 10 in.) that would be time-consuming and expensive to make using traditional methodologies.

CNC software provides many options for manufacturing orthopedic instruments.

Applications for DMLS are wide-ranging and include inserts for plastic injection molding and die casting, as well as direct parts for a variety of applications. Typical DMLS medical applications include cranio-maxillofacial implants,
orthopedic instruments and saw guides, arthroscopy and key hole instruments, and custom devices. With the emergence of advanced materials such as the super-alloy cobalt chromium and 17-4 PH stainless steel, coupled with the design freedoms this technology offers, new applications are constantly being discovered. But many medical components require higher levels of accuracy than can be achieved with DMLS alone. In such cases, DMLS is used to produce a part that is larger than the net shape. Then machining is used to produce the part to its final dimensions.

Advent of Multifunction Machines

The latest multifunction systems machine with two live spindles, live tooling, and a y-axis for milling off-center. These machines are ideally suited for machining after DMLS. They provide the capability to condense what previously took three or four operations into a single operation, thereby reducing setup and cycle time and improving quality. But multifunction machines also substantially increase the complexity of the programming task. The operations are the same, but performing several of them simultaneously is much harder to choreograph.

Sophisticated CNC software can help manufacturers obtain unique shapes.

A customer’s solid model can be directly imported into advanced CNC software programs and then opened and oriented for manufacturing. Preliminary toolpaths are applied to the geometry to get an idea of how long the job will take. In the case of parts that will be machined on the lathe, a feature is selected that automatically creates a turning profile. The software examines the solid model and adjusts the turning so that it doesn’t violate the square, which can then be milled later. A machine definitions library includes a template that accurately reflects every component in the machine, including the spindle, chuck, and tooling. The library eliminates the need to manually define the machine geometry and also enables the machinist to identify potential interferences on the computer in order to avoid crashes on the shop floor.

Integrating the Solid Model

The CNC software is then used to automatically identify the part features of the solid model. In the majority of cases, the CNC software recognizes every feature in the part. When it misses a feature, an operator can go in manually and define the feature. This capability saves a considerable amount of time. The software also attempts to organize the features into a logical order for machining and usually performs this task well. To change the order of a feature, the user can drag and drop the feature into a different position in the sequence. The simple change options make it easy to reorganize machine operations to reduce cycle time, primarily by reducing the amount of time that the tool is cutting air.

The next step is applying machining operations to each feature. Advanced CNC software enables users to create a knowledge base of preoptimized machining operations that include a particular tool, cutting speed, feed rate, depth of cut, etc. The knowledge base can be used to define carefully optimized machine operations for features that are common to a company’s products. Then the software automatically applies these operations when it encounters similar features. This ensures that the program takes full advantage of the capabilities of the machine and cutting tools. It also saves programming time and cycle time for future parts that utilize a similar feature. The use of standardized operations optimizes productivity and reduces machining time.

Programmers can manually define toolpaths for parts with complex geometries.

Advanced CNC software provides a wide range of options for harnessing the special capabilities of multifunction machines. For example, there are eight different options for clearance planes used for entering or exiting the cut. The tool rapidly advances to the clearance plane to avoid wasting time cutting air. The tool can also enter in the z-axis by recognizing the x-axis position of the cut and feeding in from a perpendicular direction. This is also done to avoid wasting time cutting air. The CNC software saves additional time by automatically recognizing where holes start and stop, even if they are on an angle or counter-bored. The tool is automatically rapid traversed to the beginning of the cut without cutting air.

Optimizing Multifunction Machining Operations

A key strength of advanced CNC programming software is the collection of tools that it provides to optimize the operation of a multifunction machine. After the operations have been created, the CNC software makes it easy to assign them to different turrets, change their sequence, and synchronize operations in different turrets. The programmer can then view a simulation that shows the machine, turrets, spindles, tools, and workpiece in real-time operation. The realistic graphical depiction of the machining operation often helps engineers think of ways to improve the CNC program. They might go back and change the order of a few operations or change the sync points and run the simulation again. The comparison function highlights any variation between the part machined by the program and the design intent, such as excess or overremoved material. By a process of continuous improvement, programmers can often use multifunction machines to reduce cycle times for some common components by as much as 80% compared with single-function machines.

Programmers also perform interference checking to fine-tune the program during the simulation. Multitasking machines have more turrets and spindles that move simultaneously, so avoiding crashes such as a tool hitting the machine can be challenging. The ability to visualize the machine, spindle, tooling, fixture, and workpiece makes it possible to do all of the prove-out (checking the program to make sure it works as expected) and debugging at the computer-aided manufacturing station. This station houses PCs used for CNC programming, as opposed to having the PCs on the machine where the parts are produced. Operators can then post directly to the machine tool without any editing.

Wire EDM Programming Process

The programming process is a bit different for wire EDM, although the programmer still imports the customer’s model and orients it for manufacturing. For two-axis wire EDM, the programmer manually applies the chain feature to the part, which identifies what will actually be cut by the wire. The software has an add-in utility that integrates it with the EDM system. The utility works with the EDM system’s technology files to provide the burn settings and offset the toolpath for the overburn of the wire.

Shown here is an example of a component created with the aid of the ESPRIT computer-aided manufacturing system.

With four-axis wire EDM, upper and lower heads move independently of each other. In many cases, it takes only a few mouse clicks to begin cutting parts on these machines because the CNC software can recognize the solid model and apply toolpaths automatically. In the more difficult cases, in which the geometry is very complex or there are problems with the solid models, the programmer can manually define the upper and lower toolpaths and link them together.

Conclusion

Conventional CNC programming software is designed around the requirements of single-function machines, making it a difficult task to write working programs for multifunction machines and much more difficult to achieve their full potential. Advanced CNC programming software provides a range of tools that make it possible to achieve the full productive potential from multifunction machines. The simulation and postprocessing capabilities make it possible to achieve editless posting, which saves a substantial amount of time spent on these machines.

The result is that medical manufacturers can explore new methods of prototyping, such as four-axis wire EDM and four-axis lathes, to manufacture parts that would be difficult or impossible to manufacture by conventional methods. With the ability to create CNC programs for all but the most exotic shapes in less than an hour, advanced CNC software enables the prototype manufacturer to respond rapidly to its customers’ requests. Use of this new technology might enable the typical medical prototype manufacturer to double its machining throughput.

Chuck Mathews is vice president of DP Technology (Camarillo, CA).