CyberCut: A Coordinated Pipeline of Design, Process Planning and Manufacture
4.4 Architecture
4.4.5 Microplanner
After setup planning, each setup is sent to the Microplanner, which selects the optimal set of tools out of the available tools in the database for the given feature geometry. The microplanner first obtains a list of feasible tools (i.e. tools that can machine the area without collisions) and computes the accessible regions for each of these tools. It is clear that the larger tool can be used to remove material in a shorter amount of time, but will not be able to reach all regions of the contour geometry. A
(a) (b)
(c) (d)
(e) (f)
Figure 4.6. Fixture plans for various geometries of the part: (a) two configurations for vise – all features machined from one setup; (b) two setups required due to insufficient area and deep features; (c) non-planar faces allow vise clamps only on shaded region; (d) two setups required for toe-clamps; (e) previously machined faces used for toe-clamps; and (f) single setup is sufficient for toe-clamps
smaller tool will take longer to machine, but will be able to reach all regions of the feature. The problem is to find an optimal combination of the larger and smaller tools that will minimise the time taken to machine the entire feature geometry.
Figures 4.7(b)–(d) show the regions of a pocket (Figure 4.7(a)) that are machined using three different tools. Each of these regions is called a decomposed feature for the corresponding tool. For a single feature, the problem of selecting an optimal sequence of tools can be posed as the problem of finding the shortest path in a single-source, single-sink directed acyclic graph [4.19]. The edges represent the cost of machining, while the nodes represent the state of the stock after the particular tool has completed machining. In order to generate the machining costs, this module interacts with the tool-path planning module, which calculates the actual tool paths and machining cost. The machining costs are stored in the edges of the graph and the minimum cost path is computed. At this stage, tool holder collision checks are also performed to ensure collision-free tool paths.
Freeform surface microplanning involves two major operations – rough machining and finish machining. For rough machining, the freeform surface is decomposed into slices of specified thickness [4.20]. The thickness of the slice controls the amount of finishing that needs to be performed later and the time taken for rough (and finish) machining. The freeform pocket is first decomposed into slices of equal thickness. The tools and their accessible areas for each of the slices are computed. Some of these slices can be merged depending upon the maximum
allowable depth of cut of the tools that are to be used. Thus, the tool is allowed to plunge to its full depth capacity to machine the accessible area of the deepest slice.
The unmachined areas remaining in the “higher” slices are then removed in turn to complete the rough machining. In addition, a similar graph-based strategy to the 2.5D microplanner is adopted to compute the optimum set of tools that will machine the slices. The final machining is done on a tool-by-tool basis rather on a per slice basis,i.e. all the regions that can be removed by a particular tool are machined first before proceeding to the next tool. Thus, the tool paths for a particular tool have to be connected optimally to minimise air-travel time. This will be discussed in the next section.
(a) (b) (c) (d)
(e) (f) (g)
Figure 4.7. Microplanning and tool-path planning of example feature: (a) original feature; (b) decomposed feature for 0.5” tool; (c) decomposed feature for 0.25” tool with areas left over by the 0.5” tool; (d) decomposed feature for 0.125” tool; (e) tool path for 0.5” tool; (f) tool path for 0.25” tool; and (g) tool path for 0.125” tool
Different strategies for finish machining have been explored in the CyberCut system. One approach [4.21] is similar to that used for microplanning where multiple tools are used to machine the surface. The surface is decomposed into sub- surfaces based on four independent criteria (this leads to four possible decompositions): (1) Based on the feature contour similar to the approach for microplanning; (2) Based on bottom surface – large tools cannot access all regions of the surface due to self-intersections and gouging. The surface can be decomposed into different sub-surfaces that would correspond to intersection-free and gouge-free regions for the various tools; (3) Based on flat and steep regions – flat regions can be easily machined by designing the tool paths for the 2D contour of the feature and then projecting onto the offset surface of the design surface, whereas steep regions can be machined by using a slicing approach; and (4) Based on directionally flat and steep regions – given a direction and a critical slope angle, the surface can be decomposed into sub-surfaces that are directionally flat or steep. The first two criteria are concerned with tool selection, whereas the last two are concerned with
the tool-path strategy. The different decompositions are combined in order to give minimum cost and machining time.
Another approach that CyberCut uses is the inverse tool offset and zigzag tool- path strategy [4.22]. This method does not use multiple tools as it is observed that switching tools during finish machining leaves marks on the surface due to tool- offset measurements at the machine tool and other alignment problems. The surface is first discretised by sampling the design surface at a resolution that depends upon the curvature and slope of the surface with respect to the access direction. Next, the offset surface is computed using the inverse tool-offset method, i.e. by taking the inverse of the tool in the access direction with the centre of the inverted tool moving on the design surface. For each discretised point, the inverse tool-offset algorithm gives a tool centre location so that the tool is tangent to the surface. These points of the offset surface, however, do not ensure that the tolerances or scallop requirements are met. Points are interpolated to the points of the offset surface both in the feed- forward and the side-step direction to meet these requirements.