What Is Mechanical Assembly? 7 Methods Compared

Mechanical assembly refers to joining manufactured parts to create a functional unit. The parts and components are assembled with mechanical fasteners, such as rivets, press fits, and threaded fasteners (along with adhesives and welding, in some cases).

This process takes place near the end of the production line, once all the parts have been manufactured. It is the step where the individual components form a finished working product.

Most of the production work goes into building the sub-assemblies. That’s where the parts are built, tested, and inspected. That extra care is a cost-saving measure, since defects that are caught before the final assembly are far easier and cheaper to fix.

Groover’s Fundamentals of Modern Manufacturing provides a stricter definition, where mechanical assembly is a joining process that uses fastening methods to attach parts. This distinguishes it from welding, soldering, and brazing, which join parts by fusing or bonding them (rather than using mechanical assembly).

While that is the textbook definition, we will be following the looser one that contract manufacturers tend to use. In practice, mechanical assembly generally refers to any assembly that does not involve electronics. So in other words, a gearbox, a hinge, or a sheet metal enclosure are all considered mechanical assemblies. But once you add a motor, a PCB, or wiring to the product, it falls under the electromechanical assembly category.

In this post, we’ll cover the various fastening methods, how mechanical assembly fits into the production line, and cost-saving design strategies.

---

What Fastening Methods Are Used in Mechanical Assembly?

Assemblers can use any mechanical fastener that does the job, but there are seven methods that are used most often.

Method	Is It Serviceable?	Typical Use	Main Failure Mode	Cost and Volume
Threaded fasteners (screws, bolts, nuts)	Yes	Anything that needs disassembly for service or adjustment	Self-loosening under vibration; stripped threads in soft material; under- or over-torque	Cheap per piece, but expensive per operation. Every screw is a handling, orientation, and torque step.
Self-clinching hardware (PEM nuts, studs, standoffs)	Yes (the mating screw)	Load-bearing threads in sheet metal that’s too thin to tap	Push-out or torque-out if sheet is too hard or hole is oversized	Installed at the fabricator with a press, before finishing. Removes tapping and loose nuts from the line.
Rivets (solid, blind, self-piercing)	No	Permanent joints in sheet metal, frames, brackets	Loosening in moving assemblies; cracking in very thin sheet	Fast, low skill, no torque spec. Blind rivets need access from one side only.
Press / interference fits	No (in practice)	Bearings, bushings, pins, dowels	Insufficient interference (slips) or excessive (cracks housing, galls)	Needs a controlled press and tight hole tolerances. No consumable fastener.
Snap-fits	Sometimes	Molded plastic housings, covers, clips	Fatigue of the cantilever; creep under sustained load; breakage on repeated opening	Zero fastener cost and the fastest assembly method. Requires a mold, so volume must justify tooling.
Adhesives	No	Dissimilar materials, thin sheet, sealing plus joining	Surface prep failures; temperature and chemical exposure; long cure holding fixtures	Adds cure time and process control (cleanliness, humidity). Difficult to inspect.
Welding / brazing	No	Structural steel frames, tube, pressure boundaries	Distortion, heat-affected zone, porosity	High strength, permanent. Not strictly “mechanical” assembly but often on the same BOM.

One requirement designers often miss: self-clinching nuts only work in sheets that are softer than the fastener. For example, PennEngineering rates its Type S and SS nuts for steel or aluminum sheet of HRB 80 or softer. For stainless CLS and CLSS, HRB 70 or softer is needed. And Type SP should only be used for stainless sheet up to HRB 90. Getting this wrong can cause the fastener to fail. If you specify a PEM nut in 304 stainless sheet that has work-hardened past HRB 90, the nut either won’t clinch or it will spin out on the first torque cycle.

The distance from hole to edge and from hole to bend matters, too. We covered those in our guide to designing sheet metal parts for hardware.

---

Why Do Bolted Joints Fail in Mechanical Assemblies?

Often, it’s because designs use bolted joints based on torque. But the joint is really held together by clamp load (or preload).

A torque wrench measures the bolt’s resistance to being turned. Most of that resistance is friction under the bolt head and in the threads, and that friction is affected by plating, lubrication, surface finish, and how many times the bolt has been run down. Only the remainder stretches the bolt.

Bossard’s guide to preload and tightening torque quantifies this. Based on VDI 2230, it states that an M12 property class 8.8 zinc-plated bolt with a friction coefficient of 0.14 has a maximum tightening torque of 93 Nm and reaches 41.9 kN of preload at that torque. Assuming a commercial torque wrench and an estimated friction coefficient, VDI 2230 applies a tightening factor of 1.6 to 2.0 for scatter. In Bossard’s worked example, that gives a minimum preload of 23.3 kN. In other words, you’ve got the same bolt and the same torque setting, but the clamp load can still land anywhere between 23 and 42 kN. Now suppose your design requires a preload of 35 kN to keep the joint from slipping, this means that some percentage of the units you ship will have joints that can’t hold as intended. And even if you did a torque audit, you still wouldn’t catch them before they reach the end user, because the torque was fine even though the preload wasn’t.

This is especially important when the assembly will be subjected to transverse vibration. The vibrations can cause the clamped parts to slide, which drops the thread friction that holds the nut in place down to almost zero. This can cause the bolt to loosen even if it had been tightened properly during assembly. This phenomenon has been well understood since Gerhard Junker measured it back in 1969. His testing methods have since been standardized as DIN 65151. Following those methods, a bolted joint would need to be verified under a controlled transverse load with preload recorded over time. That would enable plain nuts, wedge-lock washers, prevailing torque nuts, and lock washers to be compared directly. As Bossard had observed, in some cases the preload is lost completely. This is why you need to specify the locking method for fasteners if your assembly will be sitting on anything motorized, because a torque value doesn’t guarantee it won’t fail under vibration.

There’s another reason bolted joints fail, and this one doesn’t have to do with the fastener itself. It’s because the tolerances for different components can stack up and introduce a higher risk of failure. Any interface that’s added between two parts will throw its own variation into the mix. So if you have three separate parts that each have a tolerance of ±0.25 mm, combining them could result in a total variation of ±0.75 mm in the worst case, which can create problems with alignment. When you’ve got a screw that won’t thread properly, it might really be that you have two hole patterns that are 0.6 mm apart. And a snap-fit that’s too fragile and breaks easily could be due to a mating wall that is on the thicker end of its tolerance. Basically, the more interfaces you have, the more tolerances will stack up together, which increases the risk of defects or parts that can’t be assembled cleanly.

---

How Does Design for Assembly Reduce Cost?

Undergoing a design for assembly (DFA) review helps manage cost by minimizing the number of parts needed to assemble the product. It essentially involves asking three questions about every item on the bill of materials (BOM). Following the method outlined by Boothroyd Dewhurst, for every component on your design, ask if it 1) needs to move relative to its neighbors, 2) has to be made from a different material, or 3) must be kept separate for service or assembly. If the answer to all three is “no,” then that part can be consolidated. Most fasteners, brackets, stiffeners, and split housings fail this test, and consolidating them can result in assemblies with 30 to 60% fewer parts.

A DFA should be done early in the design process. According to Boothroyd Dewhurst, approximately 80% of the assembly cost is already committed by the time the design is frozen. Once the tooling has been cut, removing parts from the assembly means you’ll have to reopen the molds and revalidate the fits. To avoid this, Komaspec runs a free DFMA review before giving a production quote.

Choosing the right type of fastener for your production volume can also help keep costs down. If you’re only making a couple hundred units per year, screws are probably a decent option. Investing in the tooling required to replace them is a cost you’re unlikely to recuperate at such low runs. But if you’re producing a few thousand units each year, that changes things. Unless there’s a compelling reason to stick with screws, tab-and-slot, snap-fits, or PEM studs installed at the fabricator are often more cost-effective.

---

What Does the Mechanical Assembly Process Look Like in Production?

There are four major things that change when mechanical assembly moves from the prototype bench to the production line.

Production line assembly uses fixtures to prevent parts from going in at the wrong angle or direction. These fixtures also hold parts in place so they don’t shift out of position when other components are screwed into them. Poka-yoke features like part-present sensors and go/no-go gauges ensure that the operator can’t proceed unless the parts have been assembled properly.

Production level assembly also uses controlled tools. Assembling a joint with a spec should be done using torque tools that have been calibrated. Following Bossard’s torque tables, you’ll want to keep the tool’s inaccuracy to 5% or less. But if you’re using a click wrench that hasn’t been calibrated in nearly a year, you can assume that it deviates more significantly than that. And in cases where traceability is required, the operator will log the torque and angle and pair them to the part’s serial number.

Mechanical assembly in production follows a defined sequence. For instance, all rivets and PEM hardware will be installed before the part is powder coated; otherwise, adding them could compromise the coating and the protection it is meant to provide.

Parts are also inspected in-process so defects are identified before it would be difficult or expensive to replace the part. The final product will be inspected to ensure it functions properly (e.g., seals hold pressure, latches engage smoothly, hinges swing through the defined range). But certain components or features will get checked before the assembly is complete (e.g., verifying hole alignment while the enclosure is still open, testing torque before the cover goes on).

Komaspec maintains eight assembly lines that are designed to handle high-mix, medium-to-high-volume production. Units from the same product category can share fixtures and operators, even if they have distinct SKUs. And we use real-time production monitoring to keep yield visible.

---

What Should You Give a Contract Manufacturer for a Mechanical Assembly Quote?

To get a more accurate quote, make sure to include the following six items in your request.

Assembly drawings and exploded view with balloon numbers that match the BOM you supply.

BOM with fastener specifications detailing everything from the size, thread, and length to the material, torque value, and finish. Too many designs simply list something like “M5 screw,” which is not precise enough to pick out the exact fastener that will be needed.

Which parts are consigned and which are sourced. In a pure contract assembly arrangement, you would be the one supplying the parts. But if you’re working with a turnkey manufacturer, they will be the one responsible for sourcing or fabricating them. Most productions are some mix of the two, so you need to specify exactly who is responsible for what.

Annual volume, batch size, and SKU count. This affects things like the production line layout, whether automation is a viable option, and how many fixtures will be needed.

If you provide all this information to a vertically integrated manufacturer, you’ll be able to get DFM feedback before pricing. That’s because a supplier who has the capability to fabricate the sheet metal, mold the plastic, install the hardware, and assemble the finished product will have a better idea of whether any parts in your design could be combined or which fasteners could be eliminated. This is how Komaspec’s product assembly service works, with facilities located in China, Vietnam, and Mexico.

---

Frequently Asked Questions

What does a mechanical assembler do?

They build finished products (or sub-assemblies) by joining together fabricated parts. It involves creating and using fixtures for certain parts, driving fasteners to specified torque, pressing bearings and inserts, and performing in-process checks to catch any defects before the assembly is complete.

How much does an assembler make?

The median annual wage for assemblers and fabricators was $45,450 in May 2025 (according to the U.S. Bureau of Labor Statistics).

What is an example of a mechanical assembly?

Any product that is assembled with mechanical fasteners and does not have electrical parts. A gearbox, for instance, which consists of a housing, shafts, gears, bearings, seals, along with the fasteners that hold everything together. Larger assemblies can involve hundreds of components (for example, a bike rack built by Komaspec involves more than 300 BOM lines, covering extrusion, sheet metal brackets, molded parts, and fasteners). Valve bodies, hardware-fitted enclosures, and furniture frames also fall under this category.

What is the difference between mechanical and electromechanical assembly?

Simply put: mechanical assemblies don’t have electrical components, while electromechanical assemblies do. But to be more precise, a mechanical assembly joins metal and plastic parts using fasteners, press fits, rivets, or adhesives. Electromechanical assembly is used to create products that use electricity to perform a mechanical function, so it also involves components like motors, sensors, switches, PCBs, and wiring. Electromechanical assemblies also need additional test steps (continuity, function under power) and ESD controls that a mechanical assembly does not.

How do you keep fasteners from loosening in a mechanical assembly?

The fasteners need a locking method. Torque isn’t sufficient to keep the fasteners tight, especially when they’re subjected to transverse vibration. Instead, you’ll need something like prevailing-torque nuts, wedge-lock washers, thread-locking adhesive, or even a design that removes the fastener altogether. If the joint is safety-relevant, validate the fastener with a Junker test to DIN 65151 instead of relying on a torque audit.

Is mechanical assembly automated?

Parts of it can be automated, but whether that’s economically viable will depend on production volume and consistency in the parts used. In high-volume, low-variety production lines, it’s common to see automated screwdriving, robotic insertion, and vision inspection. But for high-mix production lines that handle several distinct assemblies, most of the work tends to be done manually, usually with fixtures and other methods for ensuring consistent quality.