‘Near Net Shape’ components (NNS) are products that are almost in their final or finished form, and just need some finishing work in order to come within the required specifications, such as dimensional precision and surface roughness. Removal technologies are still the most common processes used for making the final finish that leads to the functional product. Therefore tool path generation for controlling CNC machines is essential.
As the focus of research is generally on the finishing strategies, university graduates in Mexico decided to shift their focus towards a more comprehensive method. They carried out research on new algorithms for both rough milling and finishing 3D NNS models. No matter whether the basic shape involves NNS or a primary form (e.g. block, cylinder), the new method is applied whether or not it is used for finishing. Based on four steps, the new algorithms are briefly explained below.
1. Using points analysis on 3D models
The 3D model is first sliced. Specifically this means an intersection of the 3D model with vertical surfaces (fig. 1). This results in a set of profiles (fig. 2).
Fig. 1: (a) model (b) intersection with surface (c) sliced profile
Fig. 2: (a) multi-profiles (b) single profile
To reduce the number of points on the model approximation, a visibility analysis (applying three methods) is carried out to eliminate the invisible edges from the field of vision of the cutting machine. Figure 3 shows an example of method 1, which involves cutting edge discretization.
Fig. 3: (a) original profile (b) disintegrated profile (c) resulting edge after visibility analysis with edge discretization
2. Accessibility analysis of the cutting tools
This comes down to finding feasible tool positions for the cutting tools where each type of cutter (key-way, radial and toric cutters) is defined with a reference point (fig. 4).
Fig. 4: reference point definitions for three types of cutters
3. Evaluation of production tolerances versus processing tolerances
The minimum number of points according to the feed movement of the cutter are calculated so that the calculated tool path can cut the work-piece to within the defined tolerances. This tolerance is called the ‘cut step’.
4. Collision analysis
An evaluation of whether the tool path points generated avoid collision is done in the final phase. The analysis takes account of both the tool and the tool-holder.
The proposed method for generating tool paths was implemented in an open source C++ application using the ACIS® kernel for the necessary geometrical operations. The algorithms have not yet been implemented in commercial software and are only used within an academic environment.