Design for Manufacturing & Assembly

Design For Manufacturing Methods

Today it takes a lot to make a product successful. Not only does your product have to meet all performance requirements but it has to do so while being affordable. This requires adding design for manufacturing and assembly (DFMA) to your product development process.


Methods

Manufacturing should be a critical variable during every design decision. There are several ways to incorporate design for manufacturing into your product development process.

Minimize Part Counts

Minimizing part counts for a single product or family can do wonders for manufacturing cost reduction and is therefore a critical part of design for manufacturing. When you use fewer unique parts in a product you dramatically increase individual part volumes.

Manufacturing Costs Linked To Part Volume

Individual part costs are closely linked to volumes. A part that costs as much as $20.00/unit for 100 units can go down to $2.00/unit for 5,000 units. Manufacturing techniques that are dependent on volume significantly drive part costs. Many parts have threshold quantities at which point it makes sense to use a much higher volume machine to produce. Use of high volume machines such as stamping presses instead of laser cutting can lead to dramatic reductions in unit cost.

Get Quotes for Multiple Volumes

I have seen large custom screws that cost as much as $10.00/unit at 5,000 units go down to $2.00/unit at 10,000 units due to meeting volume requirements for using a higher volume machine. While it didn’t make sense for us to order 10,000 units of every part, it did make sense for the screws. If we never quotes for multiple volumes then we may have missed out on massive savings. Examples of typical volumes to get quotes for an initial product launch are 100, 500, 2,000, 5,000, and 10,000 units. These provide a nice trend line so that you can identify if there is a volume in which unit cost declines dramatically.

Minimize Manufacturing Part Setup Time

A significant portion of manufacturing cost is allocated to setup time for each unique part. This makes reusing the same part at multiple locations throughout a product far more affordable compared to using unique parts.

Examples of Minimizing Part Counts

  • Use standard fasteners across products
  • Use standard structural reinforcement components
  • Create a product family primarily relying on standard parts
  • Use standard printed circuit boards (PCBs), and sensors across a product family

Use Off-The-Shelf Parts

You can use many readily available off-the-shelf parts for many components. Some readily available components include: fasteners, springs, brackets, bushings, bearings, structural components, motors, sensors, batteries, adhesives, and more. By using off-the-shelf parts you get to take advantage of high volume production processes without needing to pay for high volumes of parts.

Stocked Part Sources

This is just a small example of all the different distributors that can help supply components for your product.

Niche Product Suppliers

There are many other sources for stocked parts. Components such as straps, detent pins, linear actuators, batteries all have manufacturers which supply these parts to anyone in need of an affordable solution for their product. Prior to designing a custom component do a search to see if there is already a supplier making a part which will work in your design.


Design With Affordable Materials

It’s no secret that materials such as carbon fiber, titanium, chromoly steel, and gold offer extraordinary properties. However, despite these material’s properties you rarely see them in successful products. Instead materials that offer adequate performance at low cost such as plastics, aluminum, medium carbon steel, and copper are used far more frequently in successful products. This is because these materials offer far better manufacturability at a low cost.

Measuring Machinability

Material machinability ratings have been quantifies by the American Iron and Steel Institute (AISI). These ratings were determined by measuring the weighed averages of the normal cutting speed, surface finish, and tool life of a material. AISI then arbitrarily assigned 160 Brinell B1112 Steel a machinability rating of 100%. Note that the higher the percentage the more machinable a material is.

How Material Properties Affect Machinability

Different materials have varying degrees of compliance during manufacturing. Materials that have properties which offer substantial compliance are easier to form into a finished product. Machinability allows for increased feed rates, better finishing, reduced power consumption, and low tool wear. All of this reduces combines to reduce manufacturing costs.

Hardness
  • Hardness is a measure of the resistance to localized plastic deformation induced by mechanical indentation or abrasion.
  • This greatly affects the types of tools you need to use during manufacturing. Once hardness gets high enough you risk damaging tooling.
  • Materials with high hardness also make it difficult to maintain tight tolerances as tools may deform while cutting.
  • Typically if a material such as steel needs significant hardness it is heat treated after machining.
  • Materials such as glass and ceramics which have high hardness level and low fracture toughness which makes them require specialized cutting tools. In order to handle the high friction without shattering or cracking the material, tungsten carbide spear-tipped and diamond-tipped drill bits are used.
Ductility
  • Ductility is a measure of a material’s ability to undergo significant plastic deformation before rupture.
  • This reduces compliance because the materials tend to deform plastically during machining. This plastic deformation can make maintaining tolerances difficult.
  • Increased ductility also tends to produce continuous chips that don’t cut away from the material easily because they deform well beyond the yielding point.
  • The plastic deformation that also occurs at the surface of a cut leaves residual stresses and causes strain hardening to occur.
  • While you don’t need to worry as much about fracturing with high ductility materials there are other significant trade offs.
  • Materials with moderate ductility typically offer the best machining capability as they are resistance to fracture but also don’t yield excessively.
Thermal Properties
  • Material removal can apply significant thermal loads due to friction.
  • Oil is used to help cool the tooling and part while also lubricating the cutting surface.
  • Materials with high thermal conductivity can heat up significantly if the feed rate is set too high. This heat tends to reduce the materials yield strength tend to decrease leading to unintended deformation.
  • The decrease in strength that occurs due to thermal loads can also increase the wear on a tool.
  • Thermal expansion can also cause a cut to remove the wrong amount of material.

Other Affordable Materials

Today plastics are everywhere especially in electronics. Fiber reinforced plastics in combination with rubber now offer adequate damping and strength to survive most standard falls. Plastics provide huge cost savings when compared to most metals thanks to injection molding and low material costs. For components that don’t have significant structural or other requirements then designers should look into using plastic to reduce cost.


Use Loose Tolerances

One way to drive manufacturing costs through the roof quickly is to require overly tight tolerances. This is why tolerancing is one of the most critical components of design for manufacturing.

Figure 1 – Logarithmic Tolerances vs Costs
Source: CNC Cookbook

As shown in figure 1, as tolerances are tightened beyond +/- 0.005 inches, costs increase exponentially. Today most CNC machines can hit +/- 0.005 inches. However, if you want to be able to use more affordable manufacturing techniques such as water jet cutting, plasma cutting, or casting then you’ll need looser tolerances.

Tolerances Background

Tolerances are the allowable amount of variation from the defined dimension on a drawing. One of the goals of an engineer should be to define the loosest tolerances possible that will allow for a working machine. Tolerances also help show an engineers intent to a manufacturer so that they understand which dimensions are critical to a design and which are not. As a component of your design for manufacturability process you should check every tolerance and determine whether you can adequately justify them.


Optimize Design for User Requirements

At first it may seem exciting to be able to get more performance out of a product, however if your customer won’t ever use that performance its far better to reduce the products cost. Having a strong understanding of how your product will be used allows you to eliminate excess materials and costs. Design for manufacturing involves optimizing designs in order to reduce material and part counts required for your product.

Learn User Requirements

In order to optimize a product’s design you need to first gain a strong understanding of how it will be used. Some things to learn while observing and interviewing users are the following.

  • How frequently will it be used?
  • Is the product likely to be dropped?
  • How much load will it experience?
  • What environment will the product be used in?
  • How critical will it be the user?
  • How does the user currently solve the problem?
  • Does the user work with a partner?

Structural Optimization

There is a saying among structural engineers “Anyone can design a building that stands, but it takes an a structural engineer to design one that barely stands.” While that saying may not be entirely accurate, the point is that by having a strong understanding of engineering principles and user requirements you should be able to design a product that meets those requirements cost effectively.

Design by Analysis

Using FEA you can model the performance of your product under loading. With the FEA results you can then optimize your design to remove excess material or reinforce areas that are in need.

Mechanical Optimization

Every mechanical component such as gears, bearings, motors, and springs add cost to your product. By solving a users problem with the fewest possible mechanical components or most affordable components you can significantly drive down costs.


Employ Low Cost Manufacturing Techniques

Design for manufacturing involves thinking about which manufacturing technique will be used for each component. Can you use a casting or do you need the component to be machined? Would injection molded plastic work or do you need superior performance?

Metal Casting

Source: Image by Brennan Emerson from Pixabay

Metal castings involve creating a mold and pouring in liquid metal to form a component. These castings then sometimes have some features machined into them such as surface finishes, threads, and more. There are several types of castings including sand, plaster mold, shell, permanent mold, die, and more. Each casting method comes with its own unique cost and tolerance capabilities. As there is little material waste in a casting, it is typically far more affordable when compared to machining especially for larger components.

Plastic Injection Molding

Plastic injection molding is similar to casting, except it uses plastics and higher speed automatic feed systems. While it allows for low cost and precise plastic components, there are still limitations. Parts must be sized to fit within the plastic injection mold system. Plastics also don’t offer the same strength and performance properties as other materials. Fiber reinforced plastics have however been working to dramatically improve the structural properties of plastics making it a far more useful material for more applications.

Stamping

Stamping takes rolled sheet metal and forms it with a series of high energy controlled hits to add in features. Some features include cuts, threads, folds, and more. This is commonly used for automobile bodies as well as many other industries. Once stamping is setup it allows for a high rate of production without significant oversight leading to low cost components. Since most of the features are formed instead of cut (with the exception of holes), there is little excess material involved which helps to further drive down costs.

Production Volume

While the above manufacturing methods offer low costs at high volumes, their upfront costs associated with manufacturing dies and molds make them uneconomical for low volume and prototyping manufacturing. For low volume production machined parts or processes such as laser jet cutting are preferred. Prototyping methods depends on the type of prototype needed. 3D Printing can perfectly satisfy the requirements for some prototypes while others need machined components.


ASR is a mechanical engineering firm that specializes in engineering design and analysis. If you are in need of design for manufacturing & assembly services then contact us today to speak to one of our experienced engineers or for a free quote.

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