1. Overview
Joining refers to a group of manufacturing processes that connect two or more parts into a single structure or functional unit. If Primary Shaping creates the basic form from raw material, Forming deforms material, and Material Removal completes precision geometry by removing material, then Joining integrates individual parts into larger structures, assemblies, modules, and machines.
In mechanical design, Joining is not simply a process of “attaching parts together.” It is an important manufacturing decision that must consider joint strength, thermal distortion, residual stress, fatigue life, corrosion, disassembly, sealing, cleanability, inspectability, and maintainability.
Representative Joining processes include the following.
Welding
Brazing
Soldering
Adhesive bonding
Riveting
Bolted fastening
Pin joining
Clamp joining
Press fitting
Crimping
Snap-fit joining
Insert joining
Joining includes both permanent and detachable joints. Welding, brazing, adhesive bonding, and riveting are generally closer to permanent joining, while bolts, pins, and clamps are closer to detachable joining. Therefore, the designer must first decide whether the parts only need to be connected permanently, or whether they must be disassembled and replaced later.
2. Role of Joining
The core role of Joining is to integrate individual parts into one functional structure.
Most products are not made from a single part. Frames, covers, housings, shafts, brackets, piping, sensor brackets, motor mounts, guides, and sealing structures are connected to perform the final function. Joining determines how force, position, motion, sealing, electrical connection, and maintenance structure are established between parts.
The role of Joining can be summarized as follows.
Connects multiple parts into one structure.
Allows large structures to be manufactured in sections and then integrated.
Connects different materials.
Creates structures that transfer load.
Creates sealed or airtight structures.
Enables structures that can be disassembled and maintained.
Solves manufacturing, transportation, and assembly constraints.
Affects product strength, stiffness, precision, and service life.
For example, large machine frames are often fabricated by welding and then post-machined on reference surfaces. Covers are fixed with bolts or clamps so that maintenance is possible. In food machinery, welded and polished joints may be used in product-contact areas to reduce gaps, while clamp or hygienic fastening structures should be selected where disassembly for cleaning is required.
3. Basic Principle of Joining
Joining forms a mechanical, metallurgical, chemical, or physical bond between parts.
Basic joining principles can be classified as follows.
Joining Principle | Description | Representative Processes |
|---|---|---|
Metallurgical joining | Uses heat to melt or diffuse materials for joining | Welding, brazing, soldering |
Mechanical joining | Connects parts through geometry, friction, or fastening force | Bolts, rivets, pins, clamps, press fits |
Chemical joining | Uses chemical or physical bonding force of adhesives | Adhesive bonding, sealants |
Elastic joining | Uses elastic deformation of material for fastening | Snap fits, clips, clamps |
Hybrid joining | Uses multiple joining principles together | Adhesive + bolt, welding + post-machining, press fit + pin |
The basic Joining process flow is as follows.
Important common elements in Joining are as follows.
Joint surfaces must fit correctly.
Part position must be maintained during joining.
Thermal distortion or fastening deformation must be considered.
The joint must be able to transfer load.
Joint inspection and quality verification must be possible.
Disassembly and maintenance must be possible when required.
The joint must not become a source of corrosion, contamination, or leakage.
4. Classification of Representative Processes
Joining can be classified according to joining method and disassembly possibility.
Process | Joining Method | Disassembly | Main Characteristics |
|---|---|---|---|
Welding | Joins by melting base material or filler material | Difficult | Strong permanent joint, thermal distortion occurs |
Brazing | Joins using filler metal with lower melting point than base material | Difficult | Useful for joining dissimilar metals |
Soldering | Joins using low-temperature solder | Limited | Used for electrical and electronic joining |
Adhesive bonding | Surface bonding using adhesive | Difficult | Useful for dissimilar materials and large-area joints |
Riveting | Mechanical fastening by rivet deformation | Difficult | Suitable for sheet-metal structures |
Bolted fastening | Fixing by screw clamping force | Possible | Suitable for disassembly and maintenance |
Pin joining | Positioning or rotational connection using pins | Possible or limited | Used for positioning, hinges, and links |
Clamp joining | Fixing using clamping force | Possible | Useful for quick disassembly and cleaning |
Press fitting | Fixing by interference fit | Limited | Used for shafts, bearings, and bushings |
Snap fit | Joining through elastic deformation | Possible or limited | Suitable for plastic part assembly |
All of these processes connect parts, but their design criteria are different. Welding requires consideration of strength and thermal distortion, while bolted fastening requires consideration of clamping force and loosening prevention. Adhesive bonding requires attention to surface preparation and bonding area, while press fitting requires attention to interference amount and assembly load.
5. Welding
Welding is a process that joins metal parts using heat, pressure, or both. The base material may be melted directly, or filler material may be added to form the joint.
Representative welding methods include the following.
Arc welding
TIG welding
MIG welding
MAG welding
Spot welding
Laser welding
Plasma welding
Resistance welding
Friction welding
Ultrasonic welding
The advantages of welding are as follows.
Strong permanent joints can be created.
It is suitable for large structures.
It is widely used for frames, tanks, piping, and brackets.
The appearance can be simpler than bolts or rivets.
Sealed structures can be created.
The limitations of welding are as follows.
Thermal distortion occurs.
Residual stress remains.
Weld quality may vary.
Post-processing and inspection are required.
Disassembly is difficult.
Precision datum surfaces may require post-machining after welding.
Designers should consider the following items.
Welding method
Weld geometry
Weld length
Welding sequence
Thermal distortion
Residual stress
Welding accessibility
Welding jig
Post-machining datum surfaces
Non-destructive inspection feasibility
Grinding or surface treatment after welding
In welding design, it is necessary to understand how the weld transfers load. Rather than simply increasing weld length, it is more important to consider load direction, stress concentration, weld location, and deformation direction.
6. TIG, MIG, Spot, and Laser Welding
6.1 TIG Welding
TIG welding uses a tungsten electrode and shielding gas to perform precise welding. It is widely used for stainless steel, aluminum, thin sheet metal, and hygienic pipe welding.
The characteristics of TIG welding are as follows.
Weld quality is excellent.
It is suitable for thin materials and precision welding.
It is widely used for stainless steel welding.
It strongly depends on operator skill.
Welding speed may be slower than MIG welding.
In food machinery, TIG welding is widely used for stainless steel product-contact areas, chutes, hoppers, hygienic piping, and tanks.
6.2 MIG and MAG Welding
MIG/MAG welding is a method that welds by continuously feeding wire filler material while forming an arc. It has high productivity and is suitable for structural fabrication.
The main characteristics are as follows.
Welding speed is fast.
It is suitable for thick structures.
Automation is possible.
Spatter may occur.
Post-weld finishing may be required.
6.3 Spot Welding
Spot welding is a method in which two sheet-metal parts are placed between electrodes and locally joined by applying current and pressure.
The main characteristics are as follows.
It is suitable for mass production of sheet-metal parts.
It is widely used in automotive bodies and thin-sheet structures.
Fast repeated production is possible.
Joint location and electrode accessibility are important.
6.4 Laser Welding
Laser welding uses high-density laser energy to form a narrow and deep weld.
The main characteristics are as follows.
The heat-affected zone is small.
Precision welding is possible.
It is suitable for automation.
Equipment cost is high.
Joint gap control is important.
7. Welding Design Criteria
When designing welded structures, it is not enough to judge only whether welding is possible. Post-weld distortion, post-machining, inspection, and assembly must also be considered.
The main design criteria are as follows.
7.1 Welding Accessibility
The welding torch or electrode must be able to access the joint. If the joint is deep inside a structure or obstructed by interference, weld quality may deteriorate.
7.2 Welding Distortion
Welding applies high local heat, which causes shrinkage and distortion. Distortion can become especially large in long frames, thin plates, and asymmetric structures.
7.3 Welding Sequence
Welding sequence greatly affects the amount of distortion. Symmetric welding, distributed welding, jig fixing, and tack welding can reduce distortion.
7.4 Post-Machining Datum
Welded structures requiring precision datum surfaces must be designed with post-machining in mind. LM guide mounting surfaces, bearing mounting surfaces, and assembly datum surfaces may require milling or grinding after welding.
7.5 Weld Inspection
Welds should be verifiable through visual inspection, penetrant testing, ultrasonic testing, radiographic testing, leak testing, or other appropriate methods. Welds that cannot be inspected are difficult to quality-assure.
8. Brazing and Soldering
8.1 Brazing
Brazing is a process that joins parts by melting a filler metal with a lower melting point than the base materials, without melting the base materials themselves.
The advantages of brazing are as follows.
It is useful for joining dissimilar metals.
Complex assemblies can be joined at once.
Damage to the base material is small.
It is suitable for thin parts.
Relatively uniform joints can be produced.
The limitations of brazing are as follows.
Joint strength may be lower than welding.
Joint gap control is important.
It may be limited in high-temperature service environments.
Flux residue may need to be removed.
Brazing is used for heat exchangers, piping, carbide tool tips, dissimilar metal joints, and precision assemblies.
8.2 Soldering
Soldering is a process that joins parts using low-melting-point solder. It is mainly used for electrical and electronic components.
The advantages of soldering are as follows.
Joining is possible at low temperatures.
It is suitable for electrical connection.
It is useful for small parts.
Repair and rework may be possible.
The limitations of soldering are as follows.
Mechanical strength is limited.
It may be weak in high-temperature environments.
Oxidation and surface cleanliness affect quality.
Reinforcement may be required in vibration environments.
In mechanical design, soldering is important mainly in relation to electrical components, sensors, connectors, and PCB assembly.
9. Adhesive Bonding
Adhesive bonding is a process that joins the surfaces of two parts using an adhesive. It can join various materials such as metals, plastics, rubber, glass, and composites.
Representative adhesives include the following.
Epoxy
Acrylic
Urethane
Silicone
Instant adhesive
Structural adhesive
Anaerobic adhesive
Sealant
The advantages of adhesive bonding are as follows.
It is useful for joining dissimilar materials.
Load can be distributed over a wide area.
Stress concentration can be reduced because holes are not required.
Sealing function can be provided at the same time.
Appearance is clean.
The limitations of adhesive bonding are as follows.
Surface preparation is important.
Curing time is required.
Temperature, humidity, and chemicals affect performance.
It may be weak against peel loads.
Long-term durability must be reviewed.
Disassembly is difficult.
Designers should consider the following items.
Bonding area
Adhesive layer thickness
Surface roughness
Surface cleaning
Curing conditions
Shear load
Peel load
Temperature condition
Chemical resistance
Food-contact suitability
Inspectability
In adhesive joint design, load direction is important. Adhesives are generally strong against shear loads, but may be weak against peel loads or progressive peeling.
10. Mechanical Fastening
Mechanical Fastening is a joining method that connects parts using mechanical elements such as bolts, nuts, screws, pins, rivets, and clamps.
Representative mechanical fastening methods include the following.
Bolted fastening
Nut fastening
Tapped-hole fastening
Stud fastening
Riveting
Pin joining
Clamp fastening
Snap rings
Set screws
Keys and keyways
The advantages of mechanical fastening are as follows.
Disassembly and reassembly are possible.
It is suitable for maintenance.
Joint quality is relatively easy to verify.
No thermal distortion occurs.
It can be used for joining dissimilar materials.
It is easy to apply at the assembly site.
The limitations of mechanical fastening are as follows.
Fastening space is required.
The number of parts increases.
Loosening may occur.
Stress concentration may occur.
Assembly time is required.
Additional structures are required when sealing is needed.
Designers should consider that fastening methods do not merely fix parts. They determine assembly sequence, maintainability, safety, vibration behavior, and tool accessibility.
11. Bolted Joint
Bolted fastening is the most widely used detachable joining method. It fixes parts using bolt axial force and friction between contact surfaces.
The core elements of a bolted joint are as follows.
Bolt diameter
Bolt strength grade
Tightening torque
Axial force
Coefficient of friction
Washer use
Thread engagement length
Fastening direction
Tool accessibility
Loosening prevention
Repeated disassembly
In bolted joint design, it is important that the bolt does not simply act like a pin directly resisting shear force. In a typical friction joint, bolt axial force presses the contact surfaces together, and contact surface friction transfers the load.
Designers should consider the following items.
Securing sufficient thread engagement length
Securing tool access space
Ensuring washer or bearing surface quality
Preventing loosening under vibration
Preventing thread damage during repeated disassembly
Managing fastening sequence and torque
Preventing galvanic corrosion between dissimilar metals
Applying gaskets when sealing is required
In equipment subject to vibration, spring washers alone may not be sufficient to prevent loosening. In such cases, thread-locking compounds, double nuts, prevailing torque nuts, Nord-Lock washers, pin locking, or structural anti-loosening designs can be considered.
12. Rivet, Pin, and Clamp Joining
12.1 Riveting
Riveting fixes parts by deforming a rivet. It is used for sheet-metal structures, thin metal plates, aircraft structures, and covers.
The characteristics of riveting are as follows.
It is close to a permanent joint.
It is suitable for joining thin sheet materials.
It can be used for materials that are difficult to weld.
No thermal distortion occurs.
Disassembly is difficult.
12.2 Pin Joining
Pin joining uses a pin to locate parts or create a rotational connection.
Pin joining is used in the following areas.
Hinges
Links
Locating structures
Rotational joints
Detachable fixing structures
Shear load transfer areas
In pin joining, pin diameter, hole tolerance, shear load, wear, lubrication, and pin-retention structure must be considered.
12.3 Clamp Joining
Clamp joining fixes parts using clamping force. It is suitable for structures that require quick disassembly and assembly.
The advantages of clamp joining are as follows.
Quick disassembly is possible.
Tool-less operation may be possible.
It is useful for cleaning and maintenance.
It is suitable for fixing pipes, covers, and guides.
In food machinery, clamp structures are widely used for hygienic piping, covers, guides, and parts that require cleaning.
13. Press Fit and Interference Fit
Press fitting is a method that fixes parts by creating interference between a shaft and a hole, then forcing them together so that friction and contact pressure hold the parts.
Representative applications include the following.
Bearing press fits
Bushing press fits
Pin press fits
Gear and shaft joints
Hub and shaft joints
Insert installation
The advantages of press fitting are as follows.
Separate fastening parts are not required.
The structure is simple.
It can be useful for securing concentricity.
It is suitable for repeated production.
It saves space.
The limitations of press fitting are as follows.
Assembly load is required.
Disassembly is difficult.
Excessive interference can damage parts.
Thin housings may deform.
Quality may degrade during reassembly.
Designers should consider the following items.
Interference amount
Elastic modulus of materials
Surface roughness
Assembly load
Difference in thermal expansion
Housing thickness
Disassembly possibility
Lubrication
Press-fit direction
Assembly jig
In press-fit design, the interference amount is very important. If interference is insufficient, slipping may occur. If interference is excessive, cracking, deformation, or bearing damage may occur during assembly.
14. Joining Dissimilar Materials
Joining dissimilar materials means connecting materials with different properties. Examples include joining metal and plastic, aluminum and stainless steel, rubber and metal, or composites and metals.
Important issues in dissimilar material joining include the following.
Difference in coefficient of thermal expansion
Electrochemical corrosion
Difference in stiffness
Difference in surface energy
Difference in adhesion
Stress caused by temperature changes
Recyclability and disassembly
Food-contact suitability
Representative methods for joining dissimilar materials include the following.
Bolted fastening
Insert fastening
Adhesive bonding
Overmolding
Brazing
Clamp structures
Rubber-to-metal bonding
Plastic snap fits
For example, if aluminum and stainless steel are in direct contact, galvanic corrosion may occur depending on the environment. In such cases, insulating washers, coatings, sealants, or changes in material combination should be considered.
When joining plastic and metal, thermal expansion difference and creep at fastening areas must be considered. In structures subject to repeated loads or temperature changes, insert nuts, metal bushings, and wide fastening surfaces are preferable.
15. Design Criteria for Joining
The core criteria that designers should review in Joining are as follows.
15.1 Load Transfer Path
The joint is located in the path through which load passes. Load direction, shear, tension, bending, torsion, and fatigue loads must be considered.
15.2 Joint Accessibility
Welding torches, fastening tools, adhesive application equipment, and inspection equipment must be able to access the joint.
15.3 Deformation Control
Welding, press fitting, and fastening can create deformation in parts. In precision assemblies, changes in datum surfaces and positions after joining must be considered.
15.4 Disassembly
Parts that require maintenance, cleaning, or replacement are better joined with bolts, clamps, or pins rather than welding or permanent adhesive bonding.
15.5 Sealing
When leakage or entry of fluids, air, dust, or food residue must be prevented, gaskets, O-rings, sealants, or welded sealing structures should be considered.
15.6 Corrosion and Contamination
Joints can become sources of crevice corrosion, galvanic corrosion, contamination accumulation, and poor cleanability. In food machinery, joint geometry is directly connected to hygiene.
15.7 Inspectability
Joint quality must be verifiable. Welded joints, adhesive joints, press-fit joints, and bolted joints each require different inspection methods.
16. Defects and Quality Issues
In Joining processes, various defects can occur depending on the joining method.
Defect | Main Process | Cause |
|---|---|---|
Weld cracking | Welding | Material, heat input, cooling rate, residual stress |
Porosity | Welding, brazing | Gas entrapment, contamination, poor shielding gas |
Lack of penetration | Welding | Insufficient heat input, incorrect welding conditions |
Distortion | Welding, press fitting, fastening | Thermal shrinkage, excessive fastening force, assembly load |
Loosening | Bolted fastening | Vibration, insufficient axial force, bearing surface settlement |
Adhesive peeling | Adhesive bonding | Poor surface preparation, peel load, poor curing |
Leakage | Welding, fastening, gasket | Poor joint surface, insufficient compression, surface defect |
Galvanic corrosion | Dissimilar metal joining | Potential difference and electrolyte environment |
Wear | Pins, hinges, clamps | Repeated motion, insufficient lubrication, poor tolerance |
These defects are not caused only by production conditions. They are directly related to joint location, load direction, material combination, accessibility, tolerance, post-processing, and inspection method. These factors must be considered from the early design stage.
17. Joining and Post-Processing
After Joining, post-processing may be required to secure joint function and quality.
Representative post-processing operations include the following.
Weld bead removal
Polishing
Stress-relief heat treatment
Surface treatment
Rust prevention
Passivation
Cleaning
Leak testing
Torque inspection
Non-destructive inspection
Assembly inspection
Welded structures may require correction of distortion or post-machining of datum surfaces after welding. Stainless steel welds may require oxide color removal, polishing, and passivation. Bolted joints may require torque verification, marking, and loosening prevention checks.
Joining design that does not consider post-processing creates major problems in manufacturing. In particular, narrow spaces where post-weld polishing is impossible, or joint gaps that are difficult to clean, can lead to quality and hygiene problems.
18. Production Volume and Economic Feasibility
The economic feasibility of Joining depends on production volume, joining method, automation possibility, and post-processing requirements.
The relationship between production volume and Joining can be summarized as follows.
When selecting Joining, the following cost elements should be reviewed.
Joining equipment cost
Work time
Operator skill
Jig manufacturing cost
Joining material cost
Fastener cost
Post-processing cost
Inspection cost
Defect rate
Rework possibility
Maintenance cost
Disassembly and replacement cost
Welding can reduce the number of parts, but post-processing and distortion correction costs may occur. Bolted fastening allows disassembly, but increases part count and assembly time. Adhesive bonding is useful for appearance and dissimilar material joining, but requires curing time and surface preparation cost.
19. Application of Joining in Food Machinery
In food machinery and meat-processing machinery, Joining directly affects hygiene, cleanability, corrosion resistance, and disassembly. Therefore, joint design is more important than in general industrial machinery.
Representative application areas include the following.
Stainless steel frames
Food-contact chutes
Hoppers
Tanks
Hygienic piping
Covers
Guides
Parts for disassembly and cleaning
Sealing structures
Motor brackets
Sensor brackets
When applying Joining to food machinery, the following should be considered.
Joints must not have gaps.
Weld beads must not become contamination accumulation points.
Welded areas must be polishable.
Water pooling and cleaning-liquid residue must be prevented.
Areas requiring disassembly and cleaning should use clamps or bolted structures.
Adhesives or sealants unsuitable for food contact must not be used in product-contact areas.
Corrosion caused by dissimilar metal contact must be prevented.
Foreign-material generation from fastening areas should be reduced.
Reassembly after cleaning should be easy.
For example, food-contact chutes should preferably be welded and polished to remove gaps and beads. In contrast, guides or covers that must be frequently removed for cleaning are better designed with hygienic bolts, clamps, or quick-release structures.
In food machinery, Joining is not merely structural connection. It is a core element of hygienic design and maintainability design.
20. Connection with Other Knowledge Tracks
Joining is closely connected with other Knowledge Tracks.
20.1 Connection with Material
Material affects weldability, adhesion, corrosion, thermal expansion, and fastening strength.
20.2 Connection with Mechanism & Motion
Joints in moving parts are affected by repeated loads, vibration, wear, and alignment precision. Joining design is important in links, hinges, pins, and shaft joints.
20.3 Connection with Assembly
Joining is a core element of Assembly. The joining method directly affects assembly sequence, tool accessibility, disassembly, and tolerance stack-up.
20.4 Connection with Manufacturing Quality
Joint quality is directly connected to product safety and service life. Weld inspection, torque inspection, leak testing, and adhesive strength testing may be required.
21. Core Thinking
The core thinking in Joining is as follows.
The method of connecting parts determines the structure and maintenance method of the product.
A stronger joint is not always a better joint.
Permanent joining should not be applied where disassembly is required.
Welding is strong, but it creates thermal distortion and residual stress.
Bolted fastening allows disassembly, but loosening and fastening space must be considered.
Adhesive bonding is useful for dissimilar materials, but surface preparation and load direction are important.
Press fitting is simple, but interference amount and assembly load are important.
Joints exist on the load transfer path.
Joints must be inspectable.
In food machinery, joints must not become contamination accumulation points.
Good Joining design considers strength, distortion, disassembly, hygiene, inspectability, and maintainability at the same time.
22. Conclusion
Joining is a group of manufacturing processes that connect two or more parts into one structure or functional unit. Processes such as welding, brazing, soldering, adhesive bonding, riveting, bolted fastening, pins, clamps, and press fitting directly affect the structure, strength, assembly, and maintainability of mechanical products.
However, Joining is not simply a process of attaching or fixing parts. Designers must consider joint strength, load direction, thermal distortion, residual stress, disassembly, sealing, corrosion, tool accessibility, post-processing, and inspectability together.
From the mechanical design perspective, the key points of Joining are as follows.
What load does this joint transfer?
Is permanent joining required, or is detachable joining required?
Is distortion after welding or fastening acceptable?
Can tools and inspection equipment access the joint?
Could the joint become a source of corrosion, leakage, or contamination?
Are post-processing and cleaning possible?
Are maintenance and replacement possible?
Is the joining method suitable for production volume and cost?
Therefore, Joining is a core section in the Manufacturing Track for integrating individual parts into actual product structures. It is closely connected with Material, Mechanism & Motion, Assembly, and Manufacturing Quality.