Surrogate Parts


apps_surrogate_red_aero_parts.ashxThere are times when a component’s only function is to occupy the space that it will take in the final product. Claiming this space provides assessment and verification of critical installation issues such as assembly, serviceability, routing and interfaces. It also allows assessment of product performance aspects that are adversely affected by clearance from a neighboring sub-assembly.

One evaluation option is to manufacturer or purchase the component. However, for high-value parts with complex designs, and possibly long lead times, the investment may be unwarranted and ill advised. In the time span between installation assessment and final product assembly, design modifications may occur and parts may be damaged during repeated installation cycles. If this occurs, the component must be either repaired or replaced. Another downside to use of production components is that work-in-progress (WIP) expense increases, and schedules may be delayed by late deliveries of components. This is especially critical when working on new equipment designs.

Application Outline

Mock-ups may be substituted for production components during the assembly and interface evaluation phase of a project. For simple configurations, all necessary detail can be incorporated in a machined or fabricated part that is inserted in the product assembly. However, with complex, intricate subassemblies, the mock-up may be oversimplified, which can result in an oversight of installation and interface problems.

Surrogate components preserve all of the critical details for an installation while minimizing expense and lead time when they are manufactured with Fused Deposition Modeling (FDM). Produced as needed, with up-to-date configuration changes, the FDM surrogates will confirm clearances and interfaces for installation assessment; highlight serviceability issues; and validate routing interfaces for wiring harnesses and fluid conduits. As the product nears completion, the FDM surrogates may also be used as a training aid for assembly technicians or field service personnel.

FDM works by extruding small beads of thermoplastic material through a nozzle that is moved by a numerically controlled mechanism in layers that harden immediately. The additive process constructs surrogates in hours or days at a fraction of the cost of production components. This lowers acquisition cost and defers WIP expenses until final assembly while shortening lead times to validate installations.

When neighboring subassemblies are modified or the surrogate reveals clearance issues, design revisions are easily incorporated in subsequent iterations since FDM requires no tooling. New surrogates are conveniently produced to the latest design revision. The thermoplastic surrogates also offer the advantages of being lightweight and non-marring. This makes installation easier and reduces the possibility of damaging nearby components or structures.

As efforts shift to final product assembly or functional testing, FDM surrogates can highlight the need for removal. Manufactured from colored material— for example red—the surrogates are visible to assembly technicians. Optionally, embedded RFID sensors communicate the presence of non-production components. Either option will assure that all surrogates have been replaced with the production parts they represent.

Process Overview

The configuration of FDM surrogates can be adjusted to match the immediate needs. For space claim and assembly assessment, it is usually best to produce the subassembly to the final design. However, with minor CAD adjustments, the FDM part can be simplified to include only areas of interest. If evaluating interfaces of items such as fluid fittings, electrical connectors, air ducting or mounting surfaces, either include them in the CAD model or attach production fittings to the FDM surrogate. For smart surrogates, incorporate pockets in the model to accept RFID tags or sensors that will be embedded in the part. If weight or balance is important when training installation and service technicians, add strategically placed pockets in the CAD model that will be filled with ballast material.

Surrogate Configuration Options:

Envelope – Verify fit (space claim) and access for installation and service.

– Basic: simplified representation that eliminates non-functional features.

– Advanced: Complete representation for assessment of functional clearances (e.g. sway and cooling zones).

Interface – Validate routings and connections (e.g., fluid fittings and electrical connectors).

– Integrated: interfaces constructed in surrogate.

– Hybrid: production hardware mounted to surrogate.

Ergonomic – Represent weight and balance in training aids.

Ballast: add sheet, bar or shot material to surrogate.

Smart – Integrate feedback devices for surrogate detection and data capture.

– RFID: encapsulate or attach tags for surrogate identification.

– Sensor: embed or attach measurement devices.

Customer Story

Bell Helicopter manufactures the heavy-lift, tilt-rotor Osprey, the    hybrid aircraft that combines features of both airplane and helicopter.

To assess Osprey’s tail-wiring configurations, Bell Xworx used an FDM    system to build polycarbonate wiring conduits. Technicians installed    the branching conduit’s six mating sections inside the Osprey’s twin    vertical stabilizers for on-the-ground confirmation of the wiring path.

“It takes a long time to design an aircraft. Starting from scratch it    can take five years, and it’s a rigorous development process to go    through,” says RP lab technician Mike Storp. “When using FDM    over the course of development for a new aircraft, there is great    potential to reduce costs and development time.”

Using FDM surrogates, conduits were ready for installation in two    and a half days. This is nearly a six-week reduction from Bell’s alternative, using cast aluminum parts. And    according to Storp, “We obviously saved money as well.”

As is often the case, the surrogates revealed needed design modification. According to Storp, “The    efficient process allows us to do more iterations than we could with other processes. That results in    better-designed components.”

How Did FDM Compare to Traditional Methods for Bell Helicopter?



Cast metal 6 weeks
FDM 2.5 days
SAVINGS 5.5 weeks (92%)