BY MIKE JONES
About the Author: Mike is a Principal Mechanical Engineer at SIGMADESIGN with a background in controls design and system integration. He is a certified “Black Belt” in “Design for Lean Six Sigma”. He enjoys the creative phase of the design process and working cross-functionally with the teams to achieve customer goals.
Part cost, assembly cost, and reliability are some of the factors that drive the differences between engineering a product for a low volume versus a high volume production. How are production volumes used to determine manufacturing processes? What are some of the differences between low and high volume product designs? What are some of the greater implications and considerations when planning to produce a high volume product? Let’s explore these questions.
Below, we present Part 1 of a 3-part series on the subject of product manufacturing. In this first segment, we’ll dive into defining production volumes, and explore examples of selecting a manufacturing process for a product such as sheet metal, or CNC machining vs. die casting. Finally, we’ll look at DFX to understand more design considerations for higher volume manufacturing. DFX is a Lean Six Sigma term adopted by many to describe focus on specific design attributes.
In Parts 2 and 3 of this series, we’ll discuss more about Improving Reliability and Implications of High Volume Manufacturing. Check out the next Satellite Newsletter editions to continue to learn more about these topics in detail. Now, let’s get into Part 1!
DEFINING PRODUCTION VOLUMES
What is low volume? Volume refers to the annual build quantity, which is actually a rate. There is no hard cut-off to low volume manufacturing. It can range from one or even less than one to as many as 10,000. 10,000 may seem like a lot for low volume. Context is important. Certainly that would be high volume for a complex assembly like the Space Shuttle. But, this is low volume for a soft tooled injection molded part. The transition to high volume manufacturing is often considered to be 50,000 or more. At this rate there can be advantages to adopting particular manufacturing processes. Some examples of very high volume products are: Playstation4 – averaged 16 million units per year, HP Deskjet printer – 18 million units per year, household dishwashers – 24 million per year. Within these products there can be components such as bearings and connectors that are used in multiple locations. It easy to imagine the demand for tens if not hundreds of millions of some high volume parts.
Why does this matter? The manufacturing process predominately drives the cost of tooling, setup, material, and labor. There are also overhead costs. These are expenses to provide a work environment, power, and indirect material costs. Below are three production volume groups and some commonly associated manufacturing processes.
Low volume: 1-10,000 — Characterized by short lead times and low tooling and setup costs
- CNC machining metal
- CNC machining plastic
- 3D printed plastic
- Laser and press break sheet metal
Mid volume: < 10k < 50k — Characterized by low to moderate lead times and tooling costs
- Powder metal
- Thread rolling
- Die cast metal
- Turret punch and press break sheet metal
- Modular tooling sheet metal
- Injection molded plastic
High volume: > 50k — Characterized by moderate to high lead times and tooling costs
- Die cast metal
- Modular tooling sheet metal
- Progressive Die sheet metal
- Injection molded plastic
These groups may guide our expectations as we consider what manufacturing processes will be best suited to support a given production rate. The goal, however, is to determine the most economical manufacturing process. To do that we need to estimate part and tooling costs for each manufacturing process being considered. Next, let’s look at a couple of examples.
EXAMPLES OF SELECTING A MANUFACTURING PROCESS
CNC SHEET METAL
In this instance, we’ll consider a sheet metal part and four manufacturing processes. Our model includes a one- time tooling and setup charge, material costs, and a combined labor and machine run time cost.
In this case the part has a $200 tooling and setup charge for laser and press brake. It is $35,000 for the progressive die. Material cost is considered equal in all cases. As the run rate increases, the tooling and setup costs become a smaller proportion of the total part cost. Each process reaches its asymptote near the combined total cost of material, labor, and run time. For laser and press break this is $3.73. For Progressive die this is %0.50.
(Note both axes of the graph are logarithmic)
From the graph we can see volumes at which the part costs for these processes cross over. One becoming more economical than another. All other factors being equal, the lowest cost process may be selected. The highlighted green cells in the table below identify the lowest cost option on an annual basis for various production volumes.
However, cost should not be the only consideration. Here are a couple more factors to consider when selecting a manufacturing process.
- Setup time – press brake parts may only take a couple weeks, progressive tooling may take 16 to 20 weeks
- Flexibility to changes – Laser cut part provide the most flexibility while changes to a progressive die set can be expensive and have long lead times
- Precision and repeatability of the part (within a feature and feature to feature) – these metrics depend on several factors including feature size and material thickness.
CNC MACHINING VERSUS DIE CASTING
There are many types of die casting processes: High pressure hot and cold chamber; High integrity vacuum and squeeze; Low pressure; and gravity sand casting to name a few. For this example we are comparing die casting an aluminum part using the high pressure cold chamber process with an equivalent CNC part. Die cast tooling cost is $20,000. CNC setup cost is $200.
The cross over near 1000 indicates the volume at which die casting begins to have an economic advantage. The table below shows an annual savings of $320,200 at a production volume of 10,000.
Note there will be additional design costs converting a part from CNC manufacturing to die casting. Here we add $15,000 for 3 weeks of engineering. In this case, with the additional engineering expense, the financial advantage and choice is still clear.
Further considerations may include the following:
- Tool life
- Design constraints for cast parts (there are many details, here are some points)
- Draft requirements
- Minimum and maximum thickness
- Thick to thin transition ratio
- Material strength
- Secondary machining operations
- Plating and coating
In these examples we demonstrate how to select a manufacturing process based on part cost. Furthermore, we also recognize that manufacturing processes drive different engineering costs as well as other manufacturing considerations. Next, we will look at further design considerations with higher production volumes. Examples are designing for manufacturing, assembly, and reliability.
DFX – MORE DESIGN CONSIDERATIONS FOR HIGHER VOLUME MANUFACTURING
With high volume manufacturing, efficiencies are important. The repetition of even small tasks quickly adds up to significant costs. For a production volume of 50,000 per year, saving 10 seconds on a part adds up to 138 hours per year. Reducing time reduces expenses. DFX is a Lean Six Sigma term adopted by many to describe focus on specific design attributes. Let’s examine a few of these attributes and see how the techniques can improve efficiencies.
DFM, DFA and DFS
Designing for Manufacturing, Assembly, and Service addresses part requirements with goals to improve yields, reduce assembly time, and reduce assembly mistakes. Some of the following design characteristics have benefits across all three of these DFX’s.
- Tight tolerances equate to increased cost. Tolerances should be equal to or larger than the capability of the process used to create the feature.
- Within a feature: For example a standard punched hole in sheet metal may have a process capability of ±0.05mm. If the drawing tolerance is ± 0.025 it may be achieved by under punch and shave or by reaming. These secondary operations add cost to the part. Verify the need for tight tolerances and increase allowances whenever possible.
- Within a system: Tolerances should be driven by alignment requirements. When designing assemblies, the alignment requirements should drive the creation of datum structures, a tolerance study, a tolerance allocation (a budget for each feature allowing for the capability of the process to create the feature), and the use of GD&T to document the requirements. In this way the manufacturing expense to achieve the alignment goals is applied only where it is needed.
- Avoid the temptation to use calibrations to fix uncertainties in alignments. Calibrations take time and are prone to errors. Can the system be designed to achieve the alignment requirements by a tolerance study and statistical analysis of the system, by changing materials or the manufacturing processes, or the use of fixtures?
- Create designs that are easy to orient and if possible are self-fixturing. In many cases the addition of self-fixturing features to a part can improve alignments, reduce assembly time, and make assembly easier. Here are some examples: adding a tab to hold sheet metal parts together prior to joining by riveting, screws or welding; using half shears, holes and slots in sheet metal parts for alignment.
- Design assemblies that minimize re-orientations during assembly and provide sufficient accessibility for tools.
- When using fasteners such as screws and rivets, minimize the quantity, the number of types and sizes, and avoid those that are difficult to handle. Consider snap fits, staking, and sonic welding instead of screws. Minimize the number of tools needed. For example, use common fastener head types.
- Minimize the number of parts. Look for opportunities to combine parts or features into a given part.
- Eliminate or reduce the use of adhesives to reduce fixturing, assembly, and curing time as well as the need to manage potential toxic out gassing.
- Apply mistake proofing such as “poka-yoke”. An example is using unique connectors to avoid miss-plugging.
We hope Part 1 of this 3-part series gives you a better understanding of production volumes, and how to decide on various production methods depending on your product and needs. Additionally, with a better understanding of DFX you are able to adjust your design considerations for higher volume manufacturing.
Stay tuned for Parts 2 and 3 of this series, coming soon. We’ll discuss more about Improving Reliability and Implications of High Volume Manufacturing. Feel free to contact us if you have questions.