For example, a drill bit has two cutting edges (lips) (Figure 14). The drilling process requires the drill bit to rotate along its axis, then move the cutting edges downward. The rotating cutting edge creates a cone. Sweeping down the cone will remove a cylindrical volume of materials. Thus, the holes created always have a cone-shaped bottom. The turn tool has a single cutting edge. A layer of the material on a rotating workpiece is shaved off by the cutting edge. This layer is actually a tube- like volume. Therefore, the turn tool can reduce the diameter of a rotational workpiece.
|Figure 14 Drill Bit.|
A human process planner can use his or her experience and imagination to envision the shape a process / tool can create. However, in trying to automate process planning, it is essential to deﬁne the geometric capabilities of manufacturing processes explicitly. During process planning, for a given feature an inverse search is conducted to ﬁnd the candidate process(es) for the feature. The relation- ship between features and processes is deﬁned in a mapping between the two. In an automated process planning system, rules or algorithms are written based on this mapping.
1. Process for Features Mapping
The geometric capability of a process is summarized in Table 3. As can be seen, milling can create many different features (Volume Capabilities column). Drilling, reaming, and boring primarily create holes. Turning can create different axial symmetric parts. Other processes are also listed in the table.
TABLE 3 Geometric Capabilities
One can easily ﬁnd the entries for each process and determine the geometric capabilities of the process. Process planning is an inverse mapping. Given a feature, a process planner tries to ﬁnd all processes that can create that feature.
The table alone is not sufﬁcient. To select the correct process based on the geometry, one needs to look into the geometric constraints as well. Figure 15 provides a small sample of process con- straints based on geometry. For example, on the upper-right corner is the ‘‘large hole through a small slot’’ constraint. One should not try to drill such a hole after the slot has been cut. The drill center will be in the air and not cutting any material. It will tend to slip and thus produce an inaccurate hole.
|Figure 15 Process Constraints.|
2. Relative-Cost Table for Manufacturing Processes
When conducting a feature-to-process mapping, one may ﬁnd several candidate processes for the feature. Which process to choose also depends on the cost of the process. The process cost equation consists of a few terms: the tool and machine costs, the material removal rate, and the energy consumption. The relative cost of a process is the cost of removing a unit volume of material. Since the machining time is the inverse of the material removal rate (for a given machining volume), the cost is:
where tool and machine rates are overhead cost of using the tool and the machine and energy cost is the energy cost per unit time.
Processes such as drilling, milling, and turning have higher material-removal rates and thus can ﬁnish a job faster at a lower cost. Finishing processes such as grinding, boring, and polishing have very low material-removal rates, and also consume more energy for the same amount of material removed. The relative cost is higher. Table 4 gives the price of machine costs, which are one of the factors in the relative cost equation. The energy consumption, for example, for cutting cast iron is 0.5–1.2 hp min/in3. When grinding is used, the energy consumption is 4.5–22 hp min/in3. Non-traditional processes such as a laser process consume much more energy.