The First Snap-Fit Handbook

Prefaces to Previous Editions

Preface to First Edition

This book is a reference and design handbook for the attachment technology called snap-fits or sometimes, integral attachments. Its purpose is to help the reader apply snap-fit technology effectively to plastic applications. To do this, it arranges and explains snap-fit technology according to an Attachment LevelTM knowledge construct. The book is intended to be a user-friendly guide and practical reference for anyone involved with plastic part development and snap-fits. It is called The First Snap-Fit Handbook for two reasons: I believe it is the first book written that is devoted exclusively to snap-fits. I also hope it leads to increased interest and more books on the subject.

The reader should consider this book to be a “good start” in the ongoing process of understanding and organizing snap-fit technology. There is much more to be done, but one must begin somewhere. Although the original “attachment level” construct (created in 1990 and 1991) has proven to be fairly robust and complete, many details have evolved over the years as I learned more about the topic. The construct will continue to evolve and I encourage and welcome reader’s comments on the subject; they will certainly help in the process.

My interest in the subject of snap-fits grew out of a very real need at General Motors. As a long-time fastening expert, I had typically been involved with threaded fasteners and traditional mechanical attachments. In the late 1980s and early 90s, as GM embraced design for manufacturing and assembly, the philosophies of Dr. Geoffrey Boothroyd and Dr. Peter Dewhurst [Product Design for Manufacture and Assembly, 1988, G. Boothroyd and P. Dewhurst, Department of Industrial and Manufacturing Engineering, University of Rhode Island, Kingston, RI] were formally adopted as the corporate direction, and were rolled out in a series of intensive training/workshop sessions. As a result, product designers and engineers began looking for alternatives to traditional loose fasteners, including threaded fasteners. Snap-fit attachments immediately became popular but we soon discovered that there was little design information available in the subject. Calculations for cantilever hook performance could be found in many supplier design guides or as software, but beyond that, no general snap-fit attachment expertise was captured in design or reference books. GM needed to bootstrap itself to a level of snap-fit expertise that was not written down anywhere. An intensive study of snap-fit applications resulted and eventually patterns of good design practices began to emerge. A “systems level” understanding of snap-fit attachments began to grow.

I called this systems level organization of snap-fits attachment level to emphasize its focus on the interface as a whole and to distinguish it from the traditional feature level approach. I have been teaching about snap-fits according to this attachment level model since 1991. The reaction after each class has been that attendees had indeed reached a new or better understanding of snap-fits. I trust and hope this book will have the same results for the reader.

The Attachment Level Construct (ALC) was only a personal vision in 1990. I believed it had potential and that it represented a unique approach to understanding snap-fit applications but I needed much more than that to make it reality. I needed verification that I was not just reinventing or paraphrasing some existing but obscure snap-fit design practices; an extensive literature search verified that systems-level snap-fit practices were not documented anywhere. I also needed impartial validation that the model was indeed useful and worth pursuing. A colleague, Mr. Dennis Wiese, who was Manager of the Advanced Product Engineering Body Components Group at that time, provided that initial validation. He also gave moral support and generously provided resources including his own engineers and significant amounts of his own time for debate and discussion of the fledgling snap-fit design methodology. Those discussions, sometimes lively and always useful, drove the insights that helped shape the original attachment level model. Dennis was certainly the midwife of the attachment level approach and I cannot thank him enough for his help. Other GM people involved with the infant methodology included Florian Dutke, Tom Froling, Daphne Joachim, Colette Kuhl, Chris Nelander, Tom Nistor, Tim Rossiter, and Teresa Shirley.

Finally, Mike Carter, of GM University, deserves special thanks because in the early 1990s he asked me, what are you fastening guys going to do about too many loose fasteners in our products? That question was the beginning of my involvement with design for assembly. Mike, this book is your answer.

As pressure of other work grew, the development team dwindled back to one (me). In 1992, Tony Luscher, the project manager of a planned snap-fit program at Rensselaer Polytechnic Institute (RPI), and I learned of each other’s work and made contact (once again, thanks to Mike Carter). The RPI program was originally designed around feature level research but Tony enthusiastically embraced the concept of attachment level thinking. Tony, with the concurrence of Dr. Gary Gabrielle, the project leader, modified the RPI program to include some aspects of the attachment level method. Tony’s technical insights, contributed during many hours of personal discussion and through exchange of correspondence, helped drive more refinements to the method. Under his guidance, some work to apply and extend the methodology occurred under the RPI program. Tony is now a professor at the Ohio State University and he has carried his interest and enthusiasm for the subject to his new position. Tony and I shared a longterm vision for snap-fit technology: that attachment level thinking will lead to evolution of the snap-fit design and development process from an art to an engineering science.

The original motivation for the attachment level work was to provide support for Design for Manufacturing and Design for Assembly initiatives at General Motors. Joe Joseph, then the Director of the GM DFM Knowledge Center, supported my early efforts by providing a site for snap-fit training classes. This also gave the kind of validation needed to justify continued efforts to develop the methodology. Joe is now Dean of the Engineering College of the GM University and he continues to provide valued moral support. The patience and support of Jim Rutledge, Dave Bubolz, and Roger Heimbuch is also greatly appreciated. They provided an environment in which ongoing development work could flourish and gave me much encouragement. Tony Wojcik also deserves thanks because he first sent a publisher my way. That marked the beginning of the snap-fit book project.

I must also acknowledge the creative people who designed and developed the numerous snap-fit applications that I have studied. In products from around the world, the level of cleverness and creativity evident in many snap-fits is truly impressive. My admiration for and fascination with these designs helped to drive the original ideas behind the Attachment Level Construct in the following manner:

• Observation: There are many clever, well-designed, and complex snap-fit applications in existence; there are also many poor snap-fits.

• Hypothesis: Many snap-fit designers must possess tacit knowledge that allows them to develop good snap-fits; others do not.

• Problem: Snap-fit application design information could not be found as documented knowledge. Principles of good snap-fit application design were not written down anywhere.

• Solution: Discover the information and define it. Study successful snap-fit applications and look for patterns of good design practices. Capture and organize the concepts behind good snap-fit design.

• Result: A deep understanding of snap-fit concepts and principles organized in a knowledge construct.

I cannot claim credit for the clever snap-fit applications I describe here. Most were found on existing products or inspired by products. I simply interpreted them, inferred a logical process by which they could have been developed, and organized everything I found into a knowledge structure. The only new “invention” here is the construct itself. Hopefully, it will inspire readers to create their own product inventions.

My wife and son have provided endless encouragement and understanding through the long process of writing this book, putting up with my long hours at the computer, and tolerating (barely) my monopolization of same.

With thanks and appreciation to all.

 
Rochester, Michigan 1999
Paul Bonenberger

Preface to Second Edition

The first edition of this book introduced a systematic way of thinking about snap-fit attachments. By intent, it did not spend a lot of time or space on calculations of feature behavior because this information was available elsewhere. That information is still available in various resources, including online sources; therefore, no new calculations have been added. However, equations for locking feature analysis are available online. The reader should check Appendix A for resources providing snap-fit feature calculations.

This second edition is an opportunity to add clarification and more detail in some areas. Most significantly, a new chapter, “Creating a Snap-Fit Capable Organization ‒ Beyond Individual Expertise” has been added. This chapter is targeted primarily toward engineering executives and managers. It explains how engineering...