Fastened joints on a budget

It's important to view fasteners not just as "nuts and bolts" but as integrated parts of a sophisticated assembly.

Bruno Marbacher
Engineering Manager
Bossard U.S.
Portsmith, N.H.

Low production costs are a common mantra for engineering departments. To reach this goal, however, the emphasis is generally on hard material costs. But it's often the soft costs - assembly, assembly preparation, and logistics (i.e., consolidation of parts) - that are underestimated.

Assembly is key to streamlining production. The actual design of a product represents only about 10% of the overall cost of production. But, designers set the stage for an assembly process that is either economical or costly. In fact, they influence the cost by as much as 70%.

Choosing a method

Though designs influence cost, the price of a fastener affects the total cost very little. For a given design it accounts for only about 15% of the fastened joint. In contrast, soft costs have a much bigger impact representing about 85% of the fastened joint. These associated soft costs include the overall logistics of procuring parts, all steps necessary to prepare parts for assembly, and final assembly.

The greatest potential for savings lies with final assembly costs. For example, a 10% reduction in fastener price reduces overall costs of the fastened joint by only 1.5%. In contrast, reducing associated assembly costs by 10% drops the fastened joint's overall cost by 8.5%.

The choice of an optimum fastening method largely depends on the subsequent disposal of the product. However, equally important is the cost savings gained through both efficient assembly and sometimes through ease of maintenance and repair.

Threaded fasteners account for about 50% of all fastened joint designs. Frequently, washers, inserts, and locking elements serve in place of multifunctional fasteners. But multifunctional fasteners can be a means of reducing part counts and the number of assembly steps.

Early consultations with fastener and assembly experts can find parts to eliminate and identify how multifunctional fasteners might consolidate operations. Reducing the number of times a part rotates and/or moves during assembly, for example, makes the process more efficient.

Assembly facilitators

Assembly facilitators make it easier to mount specific parts and prevent improper assembly. A few tips may help ease assembly headaches down the road:

• Whether the parts are assembled manually or automatically, allow enough clearance for both assembly tools and the individual parts being assembled.
• Fasteners, such as those outfitted with cylindrical heads and a favorable length-to-diameter ratio, are assembly friendly and easier to feed via automated methods. Flat-head screws tend to "shingle," clogging the feeding channels. Top-heavy screws tend to fall over, also clogging the feeding channels. When screwbotics is used, fasteners with bulky heads or noncylindrical heads (e.g., flat-head screws) are more likely to jam up lines.
• Principally, fasteners with an internal drive are better suited for automated assembly. For cases, drives with excellent stick-fit capabilities greatly ease assembly. Well-known examples include socket screws with six-lobe drives (torx).
• Each additional process needed to install fasteners increases component production costs. A fastened joint is most economical, when it doesn't require predrilled holes. But when a hole is unavoidable, it is best to use a shape that allows the use of a multifunctional fastener.
• Multifunctional fasteners minimize operations and combine functions of different elements. For example, either a sems screw or screws with an integrated dished flange can often replace a screw with a washer and a split lockwasher. These fastening systems speed assembly or let it be automated. This eliminates the possibility of locking elements being accidentally left out.
• Another factor designers must consider is fastener size. Too many different sizes boost the cost of procurement and inventory. Assembly is easier when there are only a few sizes and the process of servicing is simpler as well.
• Drive style is another important metric. As with fastener size, it's best to consolidate drive types when possible. This streamlines product range, makes for a more economical assembly, and keeps the number of tools on the line to a minimum

All screws are assembled from one side making this assembly efficient.

10% price reduction brings a 1.5% total in-place savings. A 10% reduction in the associated costs results in an 8.5% total in-place cost savings.

Screws placed too close to the housing wall impede the use of power tools.

Multipurpose heads eliminate the need for washers and lock washers.

Multifunctional screws eliminate assembly steps and thereby cut costs.

Standardized features on fasteners reduce tooling costs and keep manufacturing costs low.

Cost-saving hints

Before designing a special fastener, consider using something standard and off the shelf, particularly for small quantities. Special fasteners are tougher to procure and have long lead times.

Fasteners come in standard lengths. It is important that engineers stick to these "off-the-shelf" dimensions. It is often more economical to change a design so it accommodates fasteners of a standard length. (For metric fasteners: Lengths such as 8, 10, 12, and so forth are readily available.)

If special features such as "assembly facilitators" are necessary, the design should follow recognized standards. This will let a fastener manufacturer use the same tooling repeatedly. Also, it lets manufacturers pass on savings to their customers.