By Pete Marut and Dale Pereira, Connecticut Spring and Stamping
Engineering expertise is taking on an ever-more important role in designing for manufacturability in the world of metal stamped parts and springs. Particularly in today's new medical devices engineering expertise is invaluable in driving down manufacturing costs. Expertise in prototyping parts to test and prove design concepts, suggesting ways to reduce secondary operations which reduces cost, and providing value engineering consulting expertise, are key engineering skills that ensure the success of projects. Each is built on a foundation of communications and two-way dialogue that opens up the lines of communication.
One fact of life in the metal stamping and springs industry is that metal components are often the last to be sourced. Since plastic parts cannot be changed without significant mold costs and very long lead times, stamped metal parts and springs frequently need to adjust to other parts' restrictions. This makes prototyping a very important part of the design development process.
No matter what the industry, each project should begin by establishing lines of communication to get an in-depth understanding of a customer's design, objectives, and material requirements. A discussion of part dimension tolerance is essential. Some tight tolerances may add significant cost, and may not be critical, others may be achievable at no additional cost. Understanding the key dimensions and most the critical tolerances of the part is extremely valuable to both parties when developing the final part print. Is raw material cost a main concern, or is tooling cost their biggest issue? Getting this information up front is essential to develop a prototype or series of prototypes to meet their needs. With a brand new product, enough detail is needed to work out the best material to make a prototype that can be manufactured in a production scenario.
The key is to use the prototyping process to identify ways to reduce costs and make the part manufacturable at the production phase in as short a time frame as possible. Prototypes vary tremendously depending upon the project, and a company may be called upon to produce everything from a one-piece prototype up to 10,000 pieces, for those parts where repeatability is a must.
Prototyping should be used to initiate a dialogue on how to steer the project in ways that assure manufacturing consistency & reduce cost. Engineers may call out areas where they have concerns, point out exceptions, or look at dimension and features that can be made without exorbitant tooling or added secondary operations.
In those instances where the designed part does not meet a particular need, engineers would use the prototyping process to request a material change or redesign the part to increase its strength. Or, if a material is called for that is not available without making an inordinately high material purchase, engineers may suggest a material change.
"Soft tooling" is utilized for most prototype orders. "Soft tooling" refers to simple form tools and standard items likely to be found in a tool makers tool box drawer. Mounted in small die sets, the soft tooling can be used to produce prototypes in smaller numbers, before making the more sizeable investment in production tooling. This is frequently the best method for getting to the production phase. In some instances, a customer may commission low volume production tooling to help develop the prototype. If a significant number of parts are needed to bridge the lead time of full production tooling, "bridgetooling" may be used to produce parts for a short run. Bridge tooling refers to using low cost but accurate tooling to make low volume production.
One example is a large medical device company, for whom CSS produces a stamped flat spring located underneath the water tank of a continuous positive airway pressure (CPAP) system used to treat sleep apnea. Working with the company, CSS developed an initial prototype, and worked through several design iterations before settling on a good design that could meet the requirements. A short run of 2500 proved the design, and more than 750,000 of the machines have since been manufactured.
Wound spring prototyping is a somewhat simpler engineering exercise because tooling is not involved; prototypes are typically wound by hand. After receiving the drawing, spring engineers review the drawing to see if the design works, performs spring calculations using specialized software, and note exceptions. Again, this begins the dialogue on any material changes that may save on costs, or about the most cost-efficient machine to use.
In many instances, about five to ten springs will be made for use in these early prototypes. They are inserted into an application and then engineers work from there, testing greater or lesser loads, diameter, or material, working through the process and many iterations, until the spring works in the customer's device or application with their restrictions. In one case, for a compression spring for a medical device, CSS made 70,000 prototype parts for testing, since high volume parts need to run qualification testing to get the repeatability they need for FDA approval.
Rather than commission prototype springs and test them at their manufacturing facility, customers can purchase machine time at CSS, where they can run tests on a dozen different spring samples in a day, sitting down at the spring machines and making their changes directly. CSS will make a spring in accordance with the drawing. Customers can bring the particular application with them, inserts the prototype spring into the application, and tests it. If it doesn't perform in accordance with the specification, they can adjust the diameter, sometimes going through twelve or more iterations in a day.
The process saves weeks, if not months over the typical one, in which samples are developed and sent to the customer for testing at their facility. The key here is that the company gains access to expert engineering assistance during the spring prototyping stage of the development, which allows them to experiment with numerous choices until they find the one that works best for their application. They leave with the iteration that worked best in their hands, to take back for further testing. Then, they may make a larger prototype run of about 2000 and try those during the next testing phase. Their development process may generate one or two variations, which continue to undergo testing before a decision is made on which spring to take through to commercial production.
Engineers are frequently called on to reduce costs by creatively designing tools with features to eliminate secondary operations. Lean approaches are being adopted and used to cut out secondary operations wherever possible. More automated machinery and computer numerical controlled (CNC) equipment allows for repeatable set ups that aid in this goal. In the past, manufacturers had to run extra material or do a secondary trim on a torsion spring, but this can now all be done completely by an automated CNC machine.
One example of an innovative way to reduce secondary operations is a spring that CSS manufactures in its automated machinery for a particular customer. The finished spring had been sent to an outside vendor for color-coding, but to save on costs, engineers suggested a system in which the part was marked as it was being wound by incorporating a marking solution to the winding machine. The new system is estimated to save the customer about 10 percent, and also saves on lead time because the customer no longer has to go to an outside vendor.
Stamped metal and springs are used in millions of products across the gamut of industries. Experience on how metals move and where they are likely to fail goes a long way to reducing costs and developing solutions. In addition, experienced engineers can increase value and reduce costs by designing tooling with flexible options for change. This is frequently done by adding skip stations in a die for a nominal up front tool cost where additional cutting or forming can be added if needed.
In the recent past, as manufacturers moved from machining to stamping metal parts to achieve cost savings, there was a concern that the stamped parts would lose features compared to traditionally machined parts. In today's metal stamping world, with enhanced engineering design capabilities, very fine details can be incorporated using CNC milling equipment to finish the parts,. Working on this upfront adds value and reduces manufacturing costs. This is also true with regard to springs, where engineers running spring calculations software can go back to the customer to work out cost saving details right from the beginning of each project.
Value engineering helps customer design parts that are within a tight tolerance and yet very manufacturable and consistent in the long run. For example, CSS is currently working with a significant medical device company that is currently purchasing one of its parts as a completely machined tube, but wanted to explore the possibility of moving to a stamped part. The project is tricky, since the tube has important features that had to be duplicated in a stamped part. A pin has to smoothly ride on a surface to create the torque necessary to grab flesh. Also, there could be no bumps or ridges that might cause the tube to skip or the surgeon to feel tension. Finally, a coined feature was needed where wires can be attached that made the instrument head articulate.
CSS engineers have successfully moved towards changing to a stamped part, finishing the critical features using a CNC machining process. The result is a savings of about $6 per part, a true slam dunk for the customer, which manufactures about 100,000 of the parts per year.
This happy result stems from the fact that CSS design engineers had made a similar part for a completely different industry, so were already familiar with the concept. After adjusting the 3-dimensional CAD model and marking up the original drawing with their initial ideas, they had numerous discussions before finalizing it into something that could be manufactured. The tooling costs were significant, about $44,000, but the high per part savings made the investment worth it. When the part was made as part of a tube, it was held to a tolerance of plus or minus 1/1000th of an inch. The stamped part is capable of plus or minus 2/1000th of an inch. Even though the tolerance is 1/1000 more, the part is fully functional in the design at a significant savings.