3. Process planning for HMP

#### 3.1 Process planning

Process engineering or process planning is the activity that determines how a product will be produced. That is, process engineering determines which manufacturing methods will be used in order to transform a product from one state (typically a part number) into another more valuable state (again, typically a new part number). In other words, it is the selection of the manufacturing method(s) to be used to convert a raw (or semi-finished) material into a final part requirement.

It is desirable to perform all processes at a single manufacturing station because material handling is eliminated (a non-value added process), but the use of multiple resources requires the scheduling/coordination of these resources. Unfortunately, most high-performance mechanical components are produced on a number of manufacturing resources, such as: casting processes, machining processes, heat treating processes, grinding, and other high-finishing processes. Determining which of these processes will be used, along with specifying what tooling and operating parameters will be executed, is the function of process planning. Process planning may also include defining what intermediate geometries, tolerances and material allowances are required between these processing steps. Process planning is a critical part of the engineering process because it determines the primary manufacturing cost for a product.

To illustrate this, we will use an engine block as an example. See Figure 5. Engine blocks are normally cast from gray iron. In order to plan a part like this Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing DOI: http://dx.doi.org/10.5772/intechopen.88560

Figure 5.

machining fixturing, part location in the machine due to variability in the AM processes, support structure removal, required tolerances, and required surface finish. Although there are additional considerations that must be made to accommodate the use of additive manufacturing in hybrid processes, the buy-to-fly ratio and costs can be lower than machining alone due to the material waste associated with subtractive only manufacturing [7]. Figure 3 shows the flow chart and key

Sample part which could replace multiple components and become part of an assembly after finishing

The component shown in Figure 4 is an excellent example of an additive manufacturing component that could be used in a functional assembly. However, the tolerances of the functional surfaces would not meet the requirements as is and

Process engineering or process planning is the activity that determines how a

manufacturing methods will be used in order to transform a product from one state (typically a part number) into another more valuable state (again, typically a new part number). In other words, it is the selection of the manufacturing method(s) to be used to convert a raw (or semi-finished) material into a final part requirement. It is desirable to perform all processes at a single manufacturing station because material handling is eliminated (a non-value added process), but the use of multiple resources requires the scheduling/coordination of these resources. Unfortunately, most high-performance mechanical components are produced on a number of manufacturing resources, such as: casting processes, machining processes, heat treating processes, grinding, and other high-finishing processes. Determining which of these processes will be used, along with specifying what tooling and operating parameters will be executed, is the function of process planning. Process planning may also include defining what intermediate geometries, tolerances and material allowances are required between these processing steps. Process planning is a critical part of the engineering process because it determines the primary manufactur-

To illustrate this, we will use an engine block as an example. See Figure 5. Engine blocks are normally cast from gray iron. In order to plan a part like this

product will be produced. That is, process engineering determines which

considerations for additive-subtractive HMP processes.

would need to be finished before assembly.

3. Process planning for HMP

3.1 Process planning

Figure 4.

(reproduced from [8]).

Mass Production Processes

ing cost for a product.

122

Engine block with some assembled components. (reproduced from [9]. Photo by Garett Mizunaka on Unsplashed).

using traditional casting, machining, and then finishing, the process engineer would first determine how much additional material would be necessary to use in the near net-shape casting. Once this is done, the process engineer would create a "new pattern" that would be used in the green sand casting process. This pattern would allow for enough material of the critical features (faces and cylinders) for subsequent steps; this is the machining allowance. Next, the machining processes would be planned, where drilling, boring and milling operations would typically be used to create the next step in the production. Finally, finishing operations of the highly toleranced surfaces would be conducted. Planning each of these activities requires experience and a detailed understanding of the precision of each process. Tolerance stacks must be identified and used to properly sequence the operations that will be used.

The planning of each of these processes can be both time consuming and expensive. For each of the three production activities illustrated in this example (casting, machining and finishing), these activities represent "fixed costs" associated with each of these activities. Planning time for each of these activities would typically be on the order of 3–10 days depending on the complexity, tolerances and experience with similar products. For very small quantities of parts, process engineering can be the dominant cost component.

The final cost of any manufactured component will be the sum of the costs at each step of the production plus the materials, holding and overhead costs. At each step, the production cost must be determined. In general, we can define the cost of a product as:

$$\text{Product Cost} = (\text{One} - \text{time Cost}) + (\text{Batch Setup Cost}) + (\text{Processing Cost}) \tag{1}$$

In order to put cost as a function of volume, we can express this as cost per part or:

$$\begin{array}{l} \text{Product Cost/Part} = (\text{One} - \text{time Cost}) / (\text{Total Parts Produced}) \\ \quad + (\text{Batch Set} - \text{up Cost}) / (\text{Batch Size}) + (\text{Processing Cost}) \end{array} \tag{2}$$

Or in terms of variables:

$$\mathbf{C}\_{p} = \frac{\mathbf{C}\_{1-time}}{n\_t} + \frac{\mathbf{C}\_{mo}t\_{set-up}}{n\_b} + \frac{\mathbf{C}\_{mo}}{t\_p} \tag{3}$$

Where C1�time, Total one-time costs; nt, Total parts produced; Cmo, Cost of the machine resource and operator per unit time; tset�up, Time required to set-up for a new batch; nb, Parts per batch; tp, Total time to process a part.

or are recognized automatically using a series of rules, topology maps, or the

For instances of hybrid finishing of castings, where the near-net-shape component is produced using casting and later finished with machining, several factors must be taken into account during the planning stages. The numerous factors to be considered can be grouped into three categories which include those regarding: (1) which features need to be finished, (2) how cast parts are prepared for finishing, and (3) how the cast parts are finished. A general list of factors are

Several scholars have attempted to address these areas primarily focusing on how to identify features for finishing stages and plan for the casting stage [12, 13]. Some have also assessed the economic costs of finishing castings, which were briefly mentioned in previous sections [14]. Few scholars however, actually attempted to address the full complexity of the entire hybrid process [15]. Kim and Wang addressed this through an algorithm that has stages for feature recognition, casting allowance recognition, and machining volume selection [15]. From the author's understanding, a complete planning system is still required to span from feature identification in a computer-aided design (CAD) model and generation of intermediate models and process selections to the output of tool paths for the finishing

Traditionally the process planning for injection molds has relied heavily on the experience of past mold designers and fabricators. There have been significant strides to develop computer aided process plans for traditional mold making but not with the integration of multiple processes; these models are becoming more complex and time consuming [16]. As the molds are increasingly complex so are the

Considerations for feature based process planning are crucial in not only designing and manufacturing a mold from scratch but also repairing or refitting an injection mold. This is an iterative process in which molds are cycled through machining and testing. Many mold making facilities have an onsite injection molding machine for testing. However, some require shipment between the end customer and tool shop during this iterative process. In industry today, the most common method for process planning of injection molds is to allow experts to complete the task. However, there is a decreasing trend in qualified personnel to manufacture custom molds since the process is highly variable and requires strong

The considerations specific to injection molding are similar to those mentioned previously for cast components, however there are some differences. Special factors include the identification of mold components, the development of the injection mold (including its material and tolerance specifications), and the finishing required for the mold and subsequent parts. This is further defined in Figure 7.

Similar to the other hybrid process planning methods, process planning for additive-subtractive HMPs can either be feature-based or feature-less, and many methods utilize computer aided process planning. Utilizing CAPP methods is especially important when dealing with Additive-Subtractive HMPs due to the variability between parts manufactured both within a build and between builds. As with the other processes, the parts that are built in the first stage, must have the ability to

problem solving skills and a high level of self-confidence [17].

decomposition of volumes within the part [10, 11].

DOI: http://dx.doi.org/10.5772/intechopen.88560

Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing

3.2.2 Process planning for injection molding-subtractive

need for better process planning techniques.

3.2.3 Process planning for additive-subtractive

125

shown in Figure 6.

of the final product.

One can quickly see that to determine the production cost to plan a new product is a complex activity at each step. To make this even more difficult, the geometries and allowances at intermediate steps are also planned, and these specifics affect all downstream costs. This makes this a difficult engineering problem.

Multiple processes have been used to successfully produce mechanical parts for decades. The difference between traditional serial process planning and hybrid process planning and manufacturing is illustrated in Figure 6. This figure shows that in order to plan for hybrid processing, the process engineer must examine the effect of decisions made at each stage of manufacturing in order to develop the most efficient combination of processes and intermediate components.

#### 3.2 Process planning for HMP

Process planning for hybrid manufacturing processes, is similar to that for single manufacturing method processes. Many of the key considerations are the same and include: how to minimize machining time, how to maximize tool life, how to minimize the number of tool changes, and how to minimize the number of times a part must be setup in a machine or machines. However, hybrid manufacturing processes require careful planning in design and development phases to ensure that parts and tooling are optimized for the full manufacturing flow that spans multiple manufacturing technologies.

### 3.2.1 Process planning for casting-subtractive

As mentioned previously, hybrid process planning is used to define how a product will be most efficiently produced by accounting for the effects of each manufacturing stage. While computer-aided process planning (CAPP) systems can be grouped into various subcategories of variants or generative approaches, such as feature-based technologies, knowledge-based systems, Petri nets (PN), agent-based technologies, internet-based technologies, neural networks, genetic algorithms (GA), or fuzzy set theory/logic, more recent interest has been shown in the development of feature-based planning approaches. Feature-based approaches are favored in many instances because large varieties of parts can be represented by individual features. Features used for plan generation are either specified manually

Figure 6. Considerations for casting-subtractive HMP process planning.

#### Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing DOI: http://dx.doi.org/10.5772/intechopen.88560

or are recognized automatically using a series of rules, topology maps, or the decomposition of volumes within the part [10, 11].

For instances of hybrid finishing of castings, where the near-net-shape component is produced using casting and later finished with machining, several factors must be taken into account during the planning stages. The numerous factors to be considered can be grouped into three categories which include those regarding: (1) which features need to be finished, (2) how cast parts are prepared for finishing, and (3) how the cast parts are finished. A general list of factors are shown in Figure 6.

Several scholars have attempted to address these areas primarily focusing on how to identify features for finishing stages and plan for the casting stage [12, 13]. Some have also assessed the economic costs of finishing castings, which were briefly mentioned in previous sections [14]. Few scholars however, actually attempted to address the full complexity of the entire hybrid process [15]. Kim and Wang addressed this through an algorithm that has stages for feature recognition, casting allowance recognition, and machining volume selection [15]. From the author's understanding, a complete planning system is still required to span from feature identification in a computer-aided design (CAD) model and generation of intermediate models and process selections to the output of tool paths for the finishing of the final product.

#### 3.2.2 Process planning for injection molding-subtractive

Traditionally the process planning for injection molds has relied heavily on the experience of past mold designers and fabricators. There have been significant strides to develop computer aided process plans for traditional mold making but not with the integration of multiple processes; these models are becoming more complex and time consuming [16]. As the molds are increasingly complex so are the need for better process planning techniques.

Considerations for feature based process planning are crucial in not only designing and manufacturing a mold from scratch but also repairing or refitting an injection mold. This is an iterative process in which molds are cycled through machining and testing. Many mold making facilities have an onsite injection molding machine for testing. However, some require shipment between the end customer and tool shop during this iterative process. In industry today, the most common method for process planning of injection molds is to allow experts to complete the task. However, there is a decreasing trend in qualified personnel to manufacture custom molds since the process is highly variable and requires strong problem solving skills and a high level of self-confidence [17].

The considerations specific to injection molding are similar to those mentioned previously for cast components, however there are some differences. Special factors include the identification of mold components, the development of the injection mold (including its material and tolerance specifications), and the finishing required for the mold and subsequent parts. This is further defined in Figure 7.

#### 3.2.3 Process planning for additive-subtractive

Similar to the other hybrid process planning methods, process planning for additive-subtractive HMPs can either be feature-based or feature-less, and many methods utilize computer aided process planning. Utilizing CAPP methods is especially important when dealing with Additive-Subtractive HMPs due to the variability between parts manufactured both within a build and between builds. As with the other processes, the parts that are built in the first stage, must have the ability to

Where C1�time, Total one-time costs; nt, Total parts produced; Cmo, Cost of the machine resource and operator per unit time; tset�up, Time required to set-up for a

One can quickly see that to determine the production cost to plan a new product is a complex activity at each step. To make this even more difficult, the geometries and allowances at intermediate steps are also planned, and these specifics affect all

Multiple processes have been used to successfully produce mechanical parts for decades. The difference between traditional serial process planning and hybrid process planning and manufacturing is illustrated in Figure 6. This figure shows that in order to plan for hybrid processing, the process engineer must examine the effect of decisions made at each stage of manufacturing in order to develop the most

Process planning for hybrid manufacturing processes, is similar to that for single manufacturing method processes. Many of the key considerations are the same and include: how to minimize machining time, how to maximize tool life, how to minimize the number of tool changes, and how to minimize the number of times a part must be setup in a machine or machines. However, hybrid manufacturing processes require careful planning in design and development phases to ensure that parts and tooling are optimized for the full manufacturing flow that spans multiple

As mentioned previously, hybrid process planning is used to define how a product will be most efficiently produced by accounting for the effects of each manufacturing stage. While computer-aided process planning (CAPP) systems can be grouped into various subcategories of variants or generative approaches, such as feature-based technologies, knowledge-based systems, Petri nets (PN), agent-based technologies, internet-based technologies, neural networks, genetic algorithms (GA), or fuzzy set theory/logic, more recent interest has been shown in the development of feature-based planning approaches. Feature-based approaches are favored in many instances because large varieties of parts can be represented by individual features. Features used for plan generation are either specified manually

new batch; nb, Parts per batch; tp, Total time to process a part.

downstream costs. This makes this a difficult engineering problem.

efficient combination of processes and intermediate components.

3.2 Process planning for HMP

Mass Production Processes

manufacturing technologies.

Figure 6.

124

3.2.1 Process planning for casting-subtractive

Considerations for casting-subtractive HMP process planning.

be fixtured in the second and other subsequent stages of finishing. One such strategy is to create sacrificial supports that can be removed from the part once the components have been finished using CNC machining. Figure 8 shows an as-built component at the back left, a finished component with the sacrificial support still in-tact in the back right, and a finished component in the front center.

described above, the production costs and product costs can be calculated to determine if additive-subtractive HMP is the appropriate manufacturing solution

Advanced Manufacturing Using Linked Processes: Hybrid Manufacturing

When developing a process plan for any manufacturing method there are many avenues in which the plan can be developed. The most intuitive process plan can be developed from the perspective of the features themselves; this is a precedence based approach where higher precedence is given to more critical features for final part function. In this situation each feature is completely manufactured before the next feature is considered. This is the most logical method for creating

However an optimized process plan might consider is the minimization of the number of tool changes. A tool change can occur multiple times in manufacturing a single feature. This can take a significant amount of time, especially if the tool change process is manual. In this situation the process plan is developed such that each tool is used on as many features as possible before changing tools. The drawback to this method is that multiple features may be in process at any given time. If features have critical tolerances based on each other this process plan can result in a

Another optimized process plan may consider manufacturing parts one that reduces the number of orientation changes required. In an automated 5 axis CNC machine, orientation changes are often not a problem, however in a more manual process, changing the orientation of a part can take hours to re-fixture and re-center the part. In this scenario every feature in each orientation is machined before reorienting the part. Again, multiple features are in-process at the same time. Even further optimized process plans can be developed combining any of the three techniques: precedence, minimizing tool change, or minimizing orientation changes. Each of these methods are important especially to HMP parts since often complex or unusual features are the driving force for choosing such complex and time consuming manufacturing methods. If the process plans are then developed manually this can take days, weeks, even months to develop an initial plan delaying a project entirely. If the plan needs to be optimized for precedence, tool changes, orientation changes, or a combination of the three the process planning phase can take an extremely lengthy amount of time delaying the project even further. Therefore, there is a considerable need for computer aided process planning software which can account for the complex geometries of such HMP parts. An Excel based prototype has been developed and is described further in the next section.

4. Feature-based advanced hybrid manufacturing process planning

(FAH-PS) presented by [19] may be applied to multiple types of hybrid manufacturing processes such as casting-, injection molding-, and additivesubtractive. FAH-PS utilizes a modular and extensible software framework, which was intended to address: (1) the determination of operations final order in a process plan, (2) the types of processes supported in a hybrid process plan for holes, flats and slot features, and (3) the general extensibility of process planning systems for future program advancements [19]. The decision structure of FAH-PS uses feature specific geometric, tolerance, and material data inputs to generate automated

The Feature-based Advanced Hybrid Manufacturing Process Planning System

for a particular part.

a process plan.

3.3 Optimizing process planning for HMP

DOI: http://dx.doi.org/10.5772/intechopen.88560

part that does not meet standards.

system (FAH-PS)

127

Additionally, hybrid finishing of additive manufacturing requires considerations that can be grouped into three key areas: feature considerations, additive manufacturing process considerations, and finishing process considerations. Figure 9 lists key example considerations for each area. Using the strategies

Figure 7. Considerations for injection Mold-subtractive HMP process planning.

#### Figure 8.

Sacrificial support strategy example part (reproduced with permission from [18]).

Figure 9. Considerations for AM-subtractive HMP process planning.

described above, the production costs and product costs can be calculated to determine if additive-subtractive HMP is the appropriate manufacturing solution for a particular part.
