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Designing outside the part catalog

Apr 1, 2007 12:00 PM, Tom Solon Application Engineer Kerk Motion Products Hollis, N.H.

One of my more memorable college classes, Machine Design, was taught by the Director of Engineering from Brown and Sharp. A rule for class projects was that any component used must have an identified source and cost. So for three years we studied design theory but now came face to face with the axiom, “Don't reinvent the wheel,” meaning don't design a part someone has already built. Even though we learned to design a perfect gear or the optimum linkage, we then had to find a company that made these products, or worse, find a way to use approximate sizes from a catalog. There was no CAD or Internet yet, just those big green directories.

Today, it is quite common to literally search the world for components. It does not take long to learn that the best price, best delivery, and lowest cost design comes from using “catalog standard” products. Readily available software helps put standard components directly into designs. But this approach may not yield a lowest cost. This is not to suggest reinventing the wheel, but instead of choosing only standard fare, have it your way.

Product versus process

As an application engineer for a motion-products manufacturer, I work hard getting customers thinking of the company catalog as a design guide rather than a parts list. Many other companies work with a similar philosophy. When an engineer can view our firm as a specialized manufacturing company rather than a supplier of lead screws, we are half way there.

Despite many standard configurations, every item shipped is manufactured to a customer specification. Many companies operate similarly and we often find ourselves partnered with them on our customers' behalf.

When a company has the equipment in place to make parts similar to those you want, the company might be the one to make the exact parts you want. A first step to finding out is to understand the capabilities of various manufacturing operations. The more you know about manufacturing, the greater the chance you already know what equipment will be used to produce the components that make up your design. Machining, molding, stamping, extruding, forging, drawing, and so on, all have their own strengths and benefits. Each company has set itself up to be efficient and effective in specific areas and for certain volumes.

With this knowledge comes awareness of what is possible. When reviewing a customer specification, we often request details of the next level of assembly. We are looking for ways to combine or consolidate parts, eliminate assemblies, reduce part counts, or eliminate secondary machining. Once freed from the constraint of a standard part, we get to see a design at its best. As a designer, your responsibility should be to learn the capabilities of your suppliers. They in turn should make every effort to educate prospective customers.

The difficult part comes next: Finding a best source for a required process, not a product. Most companies spend a great deal of energy promoting product features and benefits. You are most likely to find a company's process information when researching online sections called “About the company”.

It is not necessary to carry the research to an extreme. If a standard component meets your needs, use it. But understand which parameters are easy to vary and which are difficult.

For example, a motor casting is much harder to vary than its windings. A polymer lead-screw nut is more easily customized than a ball-screw nut. Even better, a polymer nut can often be combined with the next level assembly.

In many mechanisms, it is common practice to use one component for guidance and another to generate motion. Linear rails and lead screws are an example. Load is typically transported by a carriage that is connected to the bushings on the guide rail and the nut on the lead screw. At a minimum, this requires five separate components: carriage, lead screw, lead nut, guide rail, and bushing. But combining the carriage, bushing, and nut drops the component count to three. Further simplification is possible by including sensor flags, mounting brackets, and other elements. Such an approach was successfully used in pick-and-place mechanisms and syringe drives.

Benefits of the approach include a lower part count, simplified assembly, and reduced space requirements. Some products from our company have rolled these attributes into a family of products that are custom-built for each application.

Another example comes from Smiths Medical when it was designing its award-winning Cozmo insulin pump. Its limiting packaging requirements called for a lot of new thinking.

The final design included a drive nut that comprised a concentric combination of guiding and sealing surface, syringe plunger, anti-rotation and mounting features for a tool-free assembly, in addition to a precise-fitting, self-lubricating lead nut. If that was not enough, Smiths engineers specified a custom high-accuracy lead for the screw. This would match conventional encoder counts to an existing syringe bore size to dispense the required increments. The medical company also specified unusual features such as a knurl and contoured bearing journal that satisfied the requirements of their pump-assembly process and housing tolerances. This innovative product was the result of Smiths and Kerk engineers literally working side-by-side on design details.

Helical Products Company Inc. makes mechanical couplings. Sales and Marketing Manager Tom Puerling says only 50% of its coupling sales are catalog products or standards. “Clients often start with a standard and during their testing decide to add or change the coupling in some way, shape, or form,” he says. Because the product begins as bar stock, many features can be incorporated easily without tooling or NRE charges. Thousands of variations to date have been made, some adding, some removing features found on the standard catalog product.

Price versus value

The message is that using a standard product may offer a lowest price, but getting a custom product may offer the greatest value. Choosing suppliers based on their manufacturing methods and capabilities, rather than only catalog offerings, lets creative designers optimize each component.

Value comes from maximizing performance while controlling cost, not just minimizing the cost of each component. Maximum value means the lowest cost of ownership which includes purchase price, maintenance costs, reliability, service life, and other intangibles.

It is the designer's responsibility to weigh options and make difficult choices. Standard parts provides a path of least resistance. Going to the next level requires extra creativity and greater knowledge of what is possible. The successful designer will have learned how each element can be manufactured, the capabilities of each manufacturing process, and will be able to exploit that knowledge to maximize the value of their design.

Putting a part catalog to work

An example where standard or off-the-shelf products might not work well is in the application of step motors. These products come in many shapes and sizes. For some users, choosing the right motor might simply involve browsing through a catalog and making a purchase. For most users, however, this is not the case.

For example, a medical company approached step-motor manufacturer Lin Engineering with an application for a peristaltic pump. “Because of size constraints for the overall system, the pump was already designed and not easily altered,” says Chief Technical Engineer Richard Badgerow. “The medical company had already chosen a step motor for its application by going through a number of catalogs and selecting what they thought would be the right one.” Unfortunately, that was not the case.

“For starters, the selected motor was too big and the sides extended too far from the pump. Also, the motor's mounting holes did not easily align with the mounting surface of the pump. Lastly, the motor's standard shaft was too short,” he says.

Badgerow says his team evaluated the customer's pump, and then designed a motor that solved their problems and ended up being more cost effective as well.

This company also treats its catalog as an engineering guide and so designed its 3609 series step motors with a modular frame rather than the typical square design. The company modified the mounting plate on the motor to adhere to the customer's design. Finally, Lin extended the 3609 motor's shaft to accommodate the customer's requirements.

The motor delivered the needed performance without having to alter the pump. This saved the client a great deal of engineering time and expense.