Courtesy of All photos courtesy of UNC Charlotte

Figure 1. Deformation machining bending mode: thin-wall machining (top), followed by single-point incremental forming (middle) to create an inclined wall (bottom).

Deformation machining is a new hybrid process that combines two manufacturing techniques: thin-part machining and single-point incremental forming (SPIF). Researchers at the University of North Carolina at Charlotte (including the author), Northwestern University and Clemson University developed the process, which allows use of a 3-axis CNC machine to create part geometries that usually require a 5-axis machine. This technique could possibly reduce the machining time and cost of producing impellers, which are typically milled on a 5-axis machine.

Thin-part machining uses a special milling tool and machining strategy to create monolithic components with walls and floors that can be as thin as sheet metal (see the September 2009 "Machine Technology" column). Monolithic machined parts have replaced sheet metal assemblies in numerous aerospace applications.

SPIF is a forming operation originally developed to manufacture sheet metal components without the expense of creating a die. A tool that looks like a ballnose endmill without teeth contacts sheet metal held in a frame at a single point. The tool follows a path that incrementally deforms the workpiece into the desired geometry.

Figure 2. This array of thin fins produced via deformation machining weighs less than a similar array made with 5-axis machining because the acute angle at the root of the wall does not have to allow for tool clearance.

Figure 1 summarizes deformation machining. The thin-wall machining progresses in layers from top to bottom and a wall is machined to its finished dimension at each step. In this way, wall stiffness is maintained during machining.

Then, the user replaces the milling tool with a solid-carbide ballnose rod for the SPIF operation. The tool moves along a path that applies force on the wall perpendicular to the tool axis, and the wall is plastically deformed. The deformation occurs as the tool moves back and forth across the workpiece in layers from top to bottom. The wall is about 50mm tall and 1mm thick.

Figure 3. An array of thin pins with 0.5mm-square cross sections, approximately 75mm tall, made on a conventional 3-axis machine tool using deformation machining.

The toolpath is not straight because spring-back near the edges of the thin wall is greater than near the middle. The toolpath needed to make a straight wall is curved, and selection of a toolpath to create the desired geometry is not trivial. Accurate prediction of sheet metal spring-back is required to produce the desired finished part geometry without iteration. Almost all of the available data, however, is based on rolled sheet metal. In deformation machining, the thin sections are made by cutting—not rolling—them, and spring-back prediction is an ongoing research topic.

Measurements have shown that the forces required for deformation machining are lower than typical machining forces and well within the capabilities of conventional machine tools.

For thin floors, a pocket is machined on both sides of a workpiece, leaving a thin, flat floor in between. Then the deformation tool pushes the floor into a dome shape, which would be difficult to create with a conventional machining process. Such geometry could be used, for example, as a heat sink.

Commercial applications for formed thin walls include T stiffeners, return flanges and mold line geometries in aerospace components. An application for formed thin floors is in pressurized bulkheads. The dome shape would allow the floor area to be larger while carrying the same pressure, similar to the bottom of an aluminum beverage can.

The process is still under development, but it is already clear that complex part geometries are feasible in a range of materials. Ongoing work is directed toward operational sequencing, process parameter selection, toolpath computation, achievable tolerances, fatigue life and machine and tooling requirements. CTE

About the Author: Dr. Scott Smith is a professor and chair of the Department of Mechanical Engineering at the William States Lee College of Engineering, University of North Carolina at Charlotte, specializing in machine tool structural dynamics. Contact him via e-mail at [email protected]. The work described in this article was supported in part by the National Science Foundation CMMI-0800433 and 0758607.