Originally Published MDDI
Originally Published MDDI April 2003
PRODUCT DEVELOPMENT INSIGHTSolid Modeling and Medical Devices: Maintaining Compliance in a Virtual Design Environment
Advances in 3-D digital design have yielded huge gains in speed and flexibility, but going paperless also raises significant legal and regulatory hurdles.
By David Warburton
The design of complex parts has been revolutionized by the maturation of inexpensive, easy-to-use solid modeling CAD software. It is now routine for an engineer to create a virtual 3-D part model and e-mail that design file to a supplier. The supplier then uses and perhaps modifies that digital file to begin design of the tooling that will ultimately make the part. The engineer and vendor may rapidly communicate changes by exchanging revised files, resulting in rapid iterations of the part and tool design that shave weeks off the design cycle and, ultimately, time to market.
Except in the medical device industry, where things are never so simple.
The fictional engineer above is sending an uncontrolled document (a digital file) to a supplier, who is making changes to it without the benefit of formal change control. When the final part is received, the design of the part cannot be verified, because there is no controlled document to verify it against. The virtual 3-D part model does not contain crucial information about relationships between features and part tolerances, so there is no way for either the supplier or the development team to tell whether the part falls within the variation allowed by the design. In short, the requirements for design verification, product validation, and part traceability have not been met.
Furthermore, if, after all this fast-paced collaboration, the part doesn’t fit, it can be impossible to determine who is responsible, and ultimately, who will pay to correct the mistake.
The Role of Part Drawings
Thanks to years of incremental advances in both solid modeling CAD software on the front end and CAM software back in the machine shop, paperless design and manufacture of mechanical components is finally becoming commonplace. Now that the technical hurdles have been overcome, however, there are numerous legal and compliance implications that must be fully understood and managed by the engineering organization.
These issues with paperless design arise because the solid-model file does not contain many of the features of a traditional part drawing. A traditional drawing, with dimensions, tolerances, and the familiar title block in the lower right corner, is more than just a pictorial representation of the part to be fabricated. It is also a highly evolved legal contract between the supplier and the customer. Each aspect of an engineering drawing has gone through more than a century of refinement to eliminate ambiguity. For example, the dimensioning of the part drawings has gone from simple callouts of part dimensions to the complex and highly refined notation of geometric dimensioning and tolerancing.
Dimensions on a drawing perform two functions. First, of course, they define the size of features on the part. But more subtly, dimensions define relationships between these features. The way that a pattern of holes in a part is dimensioned, for example, tells the vendor how those holes relate to one another and to other part features. Is the spacing between holes in the part most critical, because another part must fit in the holes without binding? Or is the distance of the holes from the edge of the part most critical? The way the features are dimensioned will tell the fabricator.
|A drawing of a pulley section from 1906 shows that while drafting standards have evolved, the basic principles remain the same. The change from the 2-D drawing to the 3-D model has been much more revolutionary (Click to Enlarge).|
Tolerances on an engineering drawing, on the other hand, define the permissible variation in the part. Engineers recognize that although variation in physical parts is inevitable, this variation must be bounded. By explicitly defining how much variation will be permitted in the finished part, tolerances accomplish three things.
• From a legal standpoint, tolerances inform the vendor which parts meet the terms of the contract between buyer and seller, and ultimately which parts the customer will accept and pay for.
• From a functional standpoint, tolerances define the limits of variation in a single part that will still allow an assembly of many parts to perform within specification. Controlling “tolerance stack-up,” the sum of the natural variations of all the parts in a complex assembly, is the key to making parts interchangeable. Tolerance control is the reason you can walk into an auto parts store and buy an engine piston made in Brazil a few months ago that will fit perfectly into the engine of your Volkswagen Microbus, manufactured in Germany during the “summer of love.”
• From a regulatory standpoint, tolerances bound the limits of the validated device. Parts produced within tolerances on the released drawings are the first step in building assemblies that fall within
the space covered by the design verification and validation plans.
Ultimately, the successful manufacture of a medical device is based on a validation plan that does two things. First, it recognizes that variations do occur in piece parts within defined limits. Second, it validates both the product and the manufacturing process across that full range.
Another feature of a traditional drawing is revision control. In a controlled engineering environment, a drawing may not be changed without an engineering change order (ECO), which also forces a change to the revision of the drawing. The revision of the drawing will often appear on the purchase order sent to the vendor, making the vendor contractually obligated to produce parts only to that drawing revision.
Finally, a traditional engineering drawing usually contains legal language delineating who owns the information contained on the drawing, providing a company some legal recourse if the information is obtained by a third party.
However, for all the strengths of a traditional drawing, the problem with a paper drawing is the time required to go from design concept to finished part. Before digital collaboration was possible, an engineer started by creating either a 3-D CAD solid model or a 2-D CAD layout of the part. Then the engineer used the CAD software to create a dimensioned part drawing from that model. A complicated part might require hundreds of dimensions. The drawing was sent to the vendor, who had to interpret those hundreds of dimensions, often spread over multiple pages, and spend weeks entering the information into a CNC machine control application. Only then could the vendor begin to fabricate a part.
Sharing solid-model files has reduced this time from weeks to hours. Now, an engineer models a part and e-mails it to a vendor. The vendor then takes that solid-model file and downloads it directly to a stereolithography machine, producing a physical model of the part in just 48 hours. Using downloaded models, full-steel injection molds can now be made in weeks instead of months.
Mark Vaughan is director of engineering at the Diagnostics Division of Bayer Healthcare (Medfield, MA). He says that “solid modeling and rapid prototyping technologies such as stereolithography have allowed us to quickly create several models of a product design.” Multiple models are useful, he points out, because “sometimes you don’t know how something is going to work until you can hold it.”
Moreover, with each advance in the state of the art in injection molding, injection-molded parts have become increasingly more complex and organically shaped. The problem of describing and fully dimensioning a part on a traditional drawing has become increasingly difficult. According to Vaughan, Bayer “has some parts that could not have been produced without solid modeling. In the caseworks of a recent product, the combination of exterior curved surfaces, for industrial design purposes, and support of interior functional parts could not have been done practically with 2-D CAD.” Vaughan also notes that with these advances come challenges. “The ability to design and fabricate more-complex parts increases file sizes. E-mail limitations and maintaining associativity of drawings and solid models in zip files has been an issue for us in transferring information to vendors.”
The move from the traditional paper drawing to paperless fabrication is inevitable, given the revolutionary gains in efficiency paperless fabrication affords and the increased design sophistication it allows. To be competitive, a medical device engineering team must embrace this technology. To be compliant, however, it must develop ways to address the technology’s shortcomings and meet FDA requirements for design control.
Strategies for Compliance
An organization’s strategy for addressing the issues inherent in paperless design should address four areas:
• Engineering change control of the content of the model files.
• Definition of the contractual obligations of the vendor regarding the model files.
• Declaration of the ownership and confidentiality of the model files.
• Control of critical dimensions and tolerances.
|This exploded view of the MediSense Precision Ultra shows the complex shapes that solid models can create. Before solid modeling, a complete drawing for this complex part would run to several densely dimensioned drawings showing multiple views.|
Controlling the model files is the first step. Charles Deschenes is an engineering manager at Abbott Laboratories, MediSense Products (Bedford, MA). He explains that at MediSense, the files must be under some level of ECO control and must be archived in documentation before they can be sent to a vendor. This provides both formal revision control and a history of the evolution of the design for the design history file.
Control of the files can be improved further by limiting communication with the vendor to a single point of contact. This role is typically filled by the purchasing staff. They are ultimately responsible for releasing the purchase orders that authorize any changes.
The next two areas of concern can be addressed simply by a policy of including a traditional drawing with the model file whenever a model file is sent to the vendor. (The drawing may be sent in digital format such as a PDF file.) Doing so can provide all the legal and contractual protections contained in that drawing, especially if the contractual language on the drawing explicitly covers the accompanying model file. Given these legal protections, it is useful to send the drawing with the model file even if the actual pictorial part drawing is in a preliminary form. “Bayer engineers,” says Vaughan, “have found it useful to maintain consistent revision levels between the solid model and the accompanying drawing to avoid confusion with suppliers.” To explicitly extend the legal protections of the drawing, the standard confidentiality disclaimer on the drawing can be expanded to read as follows:
Digital representations of the information contained in this document may be included with this document for reference. These digital files are the sole confidential property of Medical Device Corp. Copying, reproducing, modifying, or transmitting this file to a third party is forbidden without the express written permission of Medical Device Corp.
Because the drawing represents part of the legal contract between customer and supplier, it is advisable that the purchase order or the drawing itself contain the following language:
Digital files may be included with this drawing as a REFERENCE ONLY. The supplier is responsible for meeting the specifications on the drawing and for ensuring that the number and revision of the drawing match those on the purchase order.
The last of the four areas of concern can also be addressed by including a drawing with a model file when it is sent to an external supplier. “The drawing identifies critical dimensions and relationships between features,” says Vaughan. This is probably the most important reason that a drawing should accompany any model file sent to a vendor, even during the earliest phases of engineering development. As the team reaches the later phases of product development, the drawing evolves to contain increasing amounts of detail. In addition to identifying key features and dimensions, it provides a revision trail to demonstrate design control for the device history record and a working document for design verification activities, including the validation of the vendor’s tooling.
Once the part is released to manufacturing, the drawing must also do the following:
• Meet the needs of the supplier’s in-house inspection.
• Meet the needs of the customer’s incoming inspection.
• Demonstrate manufacturing control for the device master record.
The combination of solid modeling and rapid prototyping is slashing new product development time, while allowing the creation of highly integrated parts, lowering product part count, and shortening assembly time. These gains are so dramatic that some companies have begun using these technologies without fully understanding either the contractual or regulatory implications. Before you do the same, remember that a drawing is more than a pictorial representation of a part. It is a complex, and powerful, legal document.
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