The Design and Prototyping of Complex Compliant Mechanisms via Multi-Material Additive Manufacturing Techniques

The construction of compliant mechanisms is commonly performed in a single plane, due to the limitations on available manufacturing processes. This paper looks at the design and manufacturing of compliant mechanisms with members that cross over one another and thus need to be manufactured by alternative methods. The approach described here relies on the use of sheet-metal modeling software to create a 3D representation of a compliant linkage that can then be unfolded and 3D printed as a flat part. The modeling process begins with the development of a pseudo-rigid-body linkage to obtain a mechanism geometry to produce a specified motion. The PRBL is then converted to a lumped compliant mechanism by first laying out the locations of the flexural pivots in a modeling sketch, and then using the sheet metal feature of a solid-modeling program to create a model of the mechanism geometry. The sheet metal modeling process requires the user to separate the geometry into appropriate layers to provide clearance between links that cross over one another. The separation into proper layers includes the specification of a proper layer for each link, and the definition of the flexural pivots as belonging to either a single layer or to multiple layers. The sheet metal modeling must also pay attention to the need for the model to be able to unfold into a flat pattern. Once the sheet metal model is fully defined, the model is unfolded at the bends to obtain a flat pattern. The rigid portions of the mechanism are reinforced in the CAD model by thickening the regions of the model that lie between the sheet metal bends. The model is printed in its flat state, and then manually folded to its designed shape. This paper focuses on the design of relatively small models with the printing process being accomplished with the use of a desktop, dual material FFF 3D printer with soft PLA for the flexible material and ABS for the rigid reinforcement. The specific mechanism modeled in this paper is based on a Watt I six-bar linkage designed to imitate the motion of a finger. The size, space, and motion requirements of this design make the model an ideal candidate for conversion to a compliant linkage using the methods described in the paper. While the production process is simplified by the ability to use a 3D printer to produce the model, the design technique may also be applied to larger models where the parts are cut from flat stock using a variety of other manufacturing options.