Introduction to Process Planning
This article is focused on processes within manufacturing industry. That is, you put raw material into a manufacturing system and the output from the system is a product or part of a product. A Manufacturing System includes many things, basically it may include parts of, or perhaps also the entire company, depending on what perspective you choose to look at it. To be able to utilize the manufacturing system in its best way it is important to plan which processes that are going to be used, i.e Process Planning.
What is Process Planning?
Well, one can say it is the process of planning for manufacturing and the realisation of a product according to a product specification. The product specification (it may also be referred to as Product Design Specification) is a document describing the product; it’s features, design, tolerances and what kind of specific behaviour certain components should have etc. Basically it should describe the product in great detail. This document is usually the final thing when having designed the product and perhaps also produced a prototype of some kind.
This product specification is needed because you may want to send it to manufacturing companies that should produce parts of the product. However it is equally important if you want to produce parts or the entire product within you own company.
The process plan is basically a plan of how your parts will be produced, what machines to use and in what order, to achieve the correct tolerances etc. It involves strategic decisions and careful analysis with production engineers and expertise in order to plan and adapt the production of every single component that you wish to build yourself. Process planning is often seen as the interface between design and manufacturing stages.
Process planning becomes more and more important depending on the number of components and also the total number of products that are going to be produced. If you only produce a small quantity of the final product you perhaps can put down very much detail and perhaps create very fine tolerances on the components since you have time to do it, basically you can check and recheck the important components until they are perfect. However of you have large quantity’s of your product you can not check every component but perhaps every 100 or so, then it becomes important that you have very high repeatability within you manufacturing processes so that you know that the products/parts are produced within tolerances.
Your company’s Manufacturing system may be of different types such as, Project, Job Shops, Batch, Mass/Flow or Continuous/process. Continuous processes means essentially steel, glass or plastic industry, project type are larger things such as ships, aircrafts, special made machines etc, Job shops involves small production rates of made to order machines/products, Mass/Flow type means producing large quantity of products such as cars, domestic appliances etc. The Batch type of manufacturing system is the most common one which involve a product range from a few number of products up until a few thousands.
The product variety change between for example project driven production (high customization) until the lowest when in continuous processes (basically only trimming of some parameters in the process). Your company may use some of the following strategies regarding processes, Make to Stock, Assemble to order, Make to order and Engineer to order. The connection with the customer or end user is often distant when using Make to stock while Engineer to order involves much more customer interaction and perhaps also an entry into the customers design process to adapt the product to fit the customers need or processes in the best way.
To be able to plan the different processes that are needed to produce the parts or product in a cost efficient and suitable way a number of activities have to be conducted. These activities may include:
- Engineering Drawing interpretation
- Many engineering drawings are drawn or created with the purpose of showing dimensions and tolerances that are important from a design and function point of view. These dimensions may not be the same that are important from a production perspective. So sometimes you have to interpret the drawings to understand what dimensions are important from a production point of view.
- Chosen materials is important both from a design perspective but also from a production perspective. To understand or have knowledge about what characteristics and behaviour selected materials possess will affect the results from the manufacturing processes. It directly affects the chosen machining processes and tools selection. It may also be that the designer have chosen a material sometimes referred to as “unobtainium” which from a design perspective is the very best for the product however from a total cost perspective it may not be most suitable.
- Manufacturing processes
- You need to have a good understanding of what kind of machining or manufacturing processes that are needed and what the benefits and pitfalls are within them.
- Fixtures, Jigs and fastening of work piece
- Also the selected manufacturing process will affect what kind of fixtures that are needed. Often, to be able to obtain the tolerances set in the drawings various types of jigs and fixtures may be produced in order to have repeatability and stability the manufacturing processes. It is not always the case that the drawings for the jigs are given at the same time as the product drawings since it depends on what type of machinery you will choose.
- Reference data, tooling, material, machining
- You have to be able to understand and reference different types of catalogues of manufacturing tools and their machining capabilities. If very exclusive materials are used special tooling have to be used and perhaps ordered from the tooling supplier. There are various types of manufacturers and suppliers in the business, each claiming to be the best. So it is up to you to be able to judge which one is best for your certain case. Often it can be good to have some kind of standardization of what types of materials and tools that you are used to work with and occasionally have some custom jobs that forces you to take a step forward. However it is important to always keep up to date since new tools are developed continuously that may enhance your production rate and quality and perhaps also lowers the total cost.
- Costs within the company
- Here we focus on the costs for machining processes, energy costs, materials costs and tooling. What cost coverage do you have? Remember that giving stuff away for free rarely gives you more money only a demand for the same discount that your customers had the last time.
- Can you calculate the cost for your tooling and perhaps also what lifetime the tools have. Prolonging the lifetime of tooling keeping the same quality that is needed affects the profit.
- Quality assurance and methods for standards etc.
- It is important that the processes that you put in to process the raw material into the product or component wanted also produce the results you expect. Thus it is important that you have some kind of quality and inspection routines to always keep you machinery and measuring tools at it’s best. This may include routines for cleaning, upgrading, repairing, changing for more environmental friendly chemicals if those are needed.
- Some customers also demands that the company are certified according to some kind of quality system such as ISO 9001 or ISO 14001.
Process Planning Documentation
This phase includes usually two different types of documentation first the routing sheet, which includes what data about what machines and processes that are going to be used in the machine shop. The document usually follows the component or part around so that operators can follow what has been done before the component reaches the operator. The second document is the operations list, in the operations list the different operations are carefully detailed in what order, what clamps, tooling feeds and speeds that are necessary and should be held during manufacturing.
Process Planning methods
There exist some basic classification of different methods of process planning
On the first level you have the manual approach and Computer-Aided Process Planning. Within the manual approach you have to different types first the traditional approach which basically means that you manually have to check each drawing and determine, processes used, machines, fixtures etc, also select materials and tooling that are suitable if not specified already, create an routing sheet using manuals and other literature. After that you create the detailed operations list for each of the processes used.
The other approach of Manual Process Planning is called Workbook approach which basically means that you have standardized certain operations and sequences of operations for certain types of work pieces. So when you have identified what manufacturing processes that are needed for a certain part you can then choose an appropriate operations sequence from the workbook.
Manual process planning may be very time consuming and it is easy to make mistakes. Therefore numerous variants of Computer Aided Process Planning (CAPP) software have been developed. IT usually are two types of approaches when using these systems, the Variant approach and the generative approach. The variant approach is similar to manual process planning as it retrieves a standard plan and modifies according to the specifics for a certain part. The generative approach means creating a unique new plan for each and every part. It includes having access to Computer Aided geometry (3D) and other subsystems that are needed for specifiying process parameters, operation sequences, production equipment available. They are very useful but may be a bit complex to set up the first time. However software tend to become more and more userfriendly and development is contionously improving, thus it can be a good idea to examine what kind of CAPP-software that may suit your needs.
Computer Aided Process Planning (CAPP)
Project Manufacturing System
Jobbing Shop Manufacturing System
Batch Manufacturing System
Mass/Flow Manufacturing System
Continuous/process Manufacturing System
Make to Stock (MTS)
Assemble to Order (ATO)
Make to Order (MTO)
Engineer to Order (ETO)
Halevi, G., Weill, R., D., “Principles of Process Planning - A logical Approach”, 1995, Chapman & Hall
Stoll, H., W., “Design for Manufacture: an overview”, In Design for Manufacture – Strategies, Principles and Techniques pp. 107-129, Addison-Wesley
Fox, M., J., “Quality Assurance Management”, 2nd ed., Chapman & Hall
Dieter, G., E., “Engineering Design”, 2000, 3rd ed., McGraw-Hill