Smoothing the Ups and Downs of Surface Machining

Sophisticated CAM technology smooths the transition to steep/shallow milling
Freeform surface machining is widely used to produce sculptured surfaces, generally using multi-axis CNC machining centers. Parts manufacturers supplying industries like aerospace, automotive, die-and-mold, optics, and energy use advanced computer-aided manufacturing (CAM) systems to compute toolpaths that flow across multiple surfaces.

The technology in a CAM system must use both surface shape and special algorithms to recognize unique surface characteristics like curvature, normal vectors, and convex and concave regions. Depending upon the level of complexity, toolpaths for surface finishing might be applied to the entire surface of the part or by sequentially selecting limiting contours.

To meet modern demands for productivity and surface quality, ESPRIT® CAM provides sophisticated machining strategies to calculate advanced surface toolpaths, simulate the process, and ultimately generate NC code to drive CNC machine tools via their control units.

A few key machining strategies form the basis for most steep/shallow milling. First, toolpath planning is critical to achieving a final part that meets engineering requirements in terms of geometrical shape and surface quality. A machining process may involve a single machining operation for the whole surface or a series of machining operations subdivided by the contours of the surfaces. However, a machining strategy that produces an excellent finish on shallow or flat areas of a model often produces poor results on steep or vertical areas. For this reason, most CAM systems offer a range of choices for finishing strategies.

Freeform surfaces are most often machined with three different toolpath generation methods. These methods traditionally include planar, Z-level, and offset.

In planar processing, toolpath generation is done by intersecting surfaces with evenly spaced planes in Cartesian space. This method of machining is like a 2D zig-zag pocket toolpath, except that the toolpath is projected down along the tool axis onto a 3D part. (See Figure 1.)

While the planar method is both simple and robust, a drawback becomes immediately apparent when the normal vector of a surface is close to that of the parallel intersecting planes. As the slope of a surface increases, the constant distance between passes can produce large scallops of material on near-vertical walls. (See Figure 2.)

Decreasing the distance between the intersection planes can reduce the scallops on walls but leads to redundant machining of flatter zones. Therefore, applying a single planar strategy to an entire model almost always leads to either excessive machining time or a substandard finish quality.

To address this problem, CAM systems often offer an adaptive planar method that partitions surfaces into different regions according to their slope.

The Parallel Planes Finishing cycle in ESPRIT includes a strategy that lets the programmer partition and exclude zones above a specified slope angle. These “lateral” walls are avoided in the initial machining, and then only those walls can be machined with planar toolpath that is kept perpendicular, rather than parallel. (See Figure 3.)

A Z-level toolpath is also planar, except the model is sliced with evenly spaced horizontal planes. In this process, a trace of the model is generated at each level. As with planar toolpath, when a near-horizontal surface has a normal vector that is close to that of the intersection plane, surface quality suffers (See Figure 4 and Figure 5.)

An offset toolpath is another technique that works well on shallow or flat regions; it uses the shape that bounds the region to define the shape of the toolpath. The CAM system uses a slope threshold to identify and partition steep and shallow regions, and then the toolpath is generated by offsetting the boundary of each region by a constant distance (See Figure 6 and Figure 7.)

Combination Finishing — When a complex model has many areas of steep walls and shallow floors, applying a single machining strategy to the entire model often leads to excessive machining time that results only in a substandard finish. Producing an acceptable finish on the model involves the extra work of identifying the boundaries between steep/shallow zones and applying separate strategies that can lead to “waterline” marks along those boundaries as the tool must transition to machine each zone individually.

ESPRIT controls the machining of steep/shallow surface topology without the need for steep/shallow boundaries. This means that the CAM system can analyze surface angles of a model at runtime to identify and partition machining zones based on slope angle (See Figure 8.)

There are some typical problems when a CNC programmer chooses a combination of toolpaths to cut a 3D model:
— Multiple toolpaths require multiple retracting movements, with the tool constantly being repositioned from one area to another. The more surfaces on the part, the more retract moves will result as the tool moves from one style of toolpath to another. No matter how accurate the machine may be, these moves will leave a mark due to tool wear, deflection, and the locations of previous toolpaths.
— Multiple toolpaths take more time to plan, test, and machine, and may result in an inconsistent surface finish across the entire part. Some toolpaths may produce a better finish on certain areas than toolpaths in other areas, requiring the programmer to continuously adjust the machining parameters for each toolpath.
— Depending on the machining criteria specified for the toolpath and the CAM system, some areas of the model might be machined multiple times when multiple toolpaths overlap.

CAM systems that have "intelligent" machining capabilities solve these problems with a sophisticated type of toolpath that intelligently applies the appropriate combination of Z-level and projection cutting passes based on the shape of the 3D model.

Comprehensive Solution — ESPRIT features a three-axis "global finishing" method that combines the best of Z-level finishing and offset finishing in one comprehensive solution for steep/shallow milling. Global Finishing optimizes machining not only by calculating the most appropriate toolpath for steep and shallow zones, but also by applying fluid transitions between zones to keep the cutter in continuous motion, thus eliminating visible marks between zones and improving machine utilization. (See Figure 9.)

The advantages of using a single, global solution include:
— A single operation with a single threshold angle requires less time to program and less guesswork. ESPRIT uses the threshold angle to identify and partition regions by surface angle and then automatically apply Z-level cutting passes to steep areas and offset passes to shallow areas.
— The Global Finishing interface is streamlined to present only the main parameters, such as tolerance and step-over, for faster programming. Climb milling and spiral connections are enabled by default for smoother finishing.
— Sophisticated logic gives priority to continuity of toolpath as surfaces transition from flat to vertical and attempts to preserve uninterrupted Z-level passes as much as possible. The system first calculates Z-level toolpath on the entire part. All toolpaths that flow entirely on areas above the slope threshold are maintained. Toolpaths in areas below the slope threshold are replaced with offset passes. (See Figure 10, Figure 11, and Figure 12.)

The first implementation of Global Finishing keeps options to a minimum so as to optimize programming time and minimize cycle time. Faster, smarter programming is accomplished through a simplified user interface and an emphasis on automated application of Z-level and offset cutting passes, with continuity of toolpath taking priority.

Future development plans for Global Finishing will offer CNC programmers a choice of patterns for horizontal areas, parallel or offset. Extra options are planned that will let the programmer choose a priority for time versus surface quality. For example, optimizing cycle time could be prioritized for foam samples or parts with less complexity, while a higher priority could be placed on optimal surface quality for parts with high complexity.

Global Finishing will continue to evolve as a comprehensive solution for sculptured surface machining through the continuous development of sophisticated CAM logic based on customer feedback and lab testing.

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