How to Design Sheet Metal Parts for Manufacturability

To a layperson, the features, components, and design considerations of manufactured goods are often insignificant. A technologically literate person will analyze a product, and see it not only for its face value, but how and why it was manufactured.

Just as the infamous industrial designer, Charles Eames, once said “eventually everything connects.” The manufacturing industry is a great example of how crucial it is to be aware of every detail possible in regards to the goods that we produce.

Engineers, designers, and manufacturing technicians are consistently making decisions that solve the problems of our evolving technological world. How do the experts of manufactured goods make these decisions, and what are some of the reasons as to why their decisions are made? An area of manufacturing that may be of interest to examine is the subset of sheet metal forming and fabrication.

Sheet Metal Overview

To fully understand the design intent of sheet metal components in the manufacturing industry it is important to study the types of sheet metal, their characteristics, as well as where these components are used. Sheet metal is commonly defined as stock material between 0.006” to 0.250” in thickness. Although thickness may vary, material thinner than sheet is typically considered foil, and material thicker than sheet is called plate.

When referring to sheet metal thickness, it is common to use the gauge number that has been assigned to a certain sheet. Gauge of sheet metal is an assigned number based on the comparison of the thickness dimension to its weight per square foot. These gauges typically range from 3 to 38 depending on the type of material of the sheet stock. Ferrous and nonferrous metals will then logically have different gauges. A lower gauge number indicates a thicker piece of material, and a higher gauge number indicates a thinner piece of material.

The production process of sheet metal consists of the following
● Stages
● Melting
● Pouring
● Pickling
● Rolling
● Annealing
The end product is either delivered to the customer from the mill in rolls, or is processed further into dimensional sheets.

Sheet metal parts can be made from several types of metal including stainless steel, cold rolled steel, galvanized steel, aluminum, copper, and brass. A fundamental aspect in the design of sheet metal components is understanding the characteristics of the materials above. Common stainless steel selections for sheet metal parts include grades 316, 304, 410, and 430. While the 300 series of stainless steel offers high levels of corrosion resistance, the 400 series of stainless offers lower expense and higher levels of heat treatability.

When considering aluminum as a choice for sheet metal components, grades 1100, 3003, 5052, and 6061 are reliable choices due to their varying levels of strength formability, weldability, and corrosion resistance.

If the need for steel sheet metal exists, choosing a cold rolled 1008 or 1018 alloy is recommended. Cold rolled steel is of higher quality when compared to its counterpart hot rolled steel. Cold rolled steel will have a tighter thickness tolerance, and a descaled surface for better surface finishes.

As with most production materials, sheet metal components are utilized in industries that span from the medical and aerospace fields to kitchen appliances. The medical field requires sheet metal parts for stainless steel storage cabinets, medical imaging equipment, and pharmaceutical supplies. Perhaps the most well known industry that uses sheet metal parts is the aerospace industry. Components including aircraft wing ribs, aircraft bulkheads, and jet engine exhaust systems are all comprised of sheet metal assemblies.

Sheet Metal Manufacturing Processes

One of the challenges of designing sheet metal components is devising the optimal manufacturing process to yield the highest quality part in a competitive amount of time. Knowing the vast possibilities of how sheet metal parts can be manufactured, starts with understanding the following manufacturing processes. Separating, forming, and combining are three manufacturing processes that change the form and function of production materials. Each of these categories of manufacturing processes are accomplished by a wide variety of methods.

The separating process is divided into shearing and chip removal methods. Shearing methods cut material from the stock with no material loss. This method can be accomplished with tools such as
● Shears
● Punches
● Dies

Chip removal methods cut material away from the stock with a loss of material. This is due to a cutting tool being fed into, along, or through the material creating a chip and leaving what is known as a kerf. Chip removal methods can be accomplished by tools including
● Saws
● Mills
● Lasers
● Waterjet cutters
● Abrasives

The forming process is divided into bending, compressing, and stretching methods. Tools that perform these methods include
● Brakes
● Hammers
● Presses
● Rollers
● Drawing machines

Combining processes are carried out by bonding, coating, and mechanical fastening methods. The bonding method is accomplished by adhesive and cohesive bonds. Adhesive bonds join two components using structural acrylics, epoxies, and films whereas cohesive bonds are commonly achieved by welding in the sheet metal industry. Mechanical fastening methods are split into two categories of threaded and non-threaded fasteners. Threaded fasteners include bolts and screws while non-threaded fasteners are made up of rivets and pins.

As mentioned above, riveting is a staple process in the fastening of sheet metal parts. Contained in the list below are considerations for designing parts that will ultimately require rivets to be installed.

● Aerospace rivet terminology includes manufacturers head, shank, and shop head.
● Aerospace rivets have several types of heads including, flathead, brazier head, countersunk head, universal head.
● Use the rivet manufacturers recommended drill bit size for pre-drilling
● The resultant shop head should be 1-½ times larger in diameter than the shank of the rivet and ½ of the shank diameter in thickness.
● The distance from the edge of the sheet metal to the rivet is called edge distance and should be no less than two times larger than the rivet shank diameter.
● Rivet to rivet distance in rows is called pitch and should be no less than three times the rivet shank diameter.
● Rivet to rivet distance in columns is called gauge, and should be roughly 75% of the pitch.

Design for Manufacturability Considerations

All too often sheet metal components spend more time than necessary going through revisions and change orders. To avoid overlooking the manufacturability of sheet metal components in the early stages of product development there are several design considerations that may improve the outcome. The following is a list of some of these considerations to design sheet metal parts for manufacturability
● Including bend relief will decrease the chances of the metal tearing went bent.
● Flanges should be formed perpendicular to the grain structure of the metal to avoid tearing.
● Assigning tolerances tighter than necessary will increase the cost of the part and may cause premature wearing of punches and dies.
● Coining, collars, and chamfers added to corners and holes will increase the stability of the finished part.
● Simplifying multi piece assemblies by combining parts with similar roles will reduce product cost due to a reduction in production time.
● Using stock material close to the dimensions of the finished product will increase the manufacturability of the part.
● Using standard tooling will increase the manufacturability of the part.
● Ensuring that parts may be assembled from one side will increase the ease of assembly of a sheet metal product.
● Deburr all edges and sharp corners.
● Remove all heat affected zones following laser cutting.
● When at all possible leave the decision of welding method up to the manufacturer.

Designing Sheet Metal Parts Wrap-up

The end goal of any manufacturing effort is to provide the consumer with the best product possible at the most affordable price point. Technical knowledge and hands on experience with the materials, tools, and processes of sheet metal fabrication is essential to providing not only the consumer with the best product, but also the manufacturer as well. Keeping in mind the above considerations will result in a sheet metal component that is of high quality, and should provide assistance to the manufacturer of the end product.

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