Joining Technologies

Introduction

Fastening is certainly one area of appliance assembly that requires manufacturers to carefully weigh solution benefits and drawbacks. Options range from clinching and spot welding to self-piercing fasteners, standard rivets and even adhesive bonding. The best applicable technology is determined by the demands on quality of the final product, available production time and the investment budget. There's a strong interaction between these three factors:

Naturally, manufacturing/assembly considerations also have to be taken into account. The choice for the optimal joining technology also depends on:

• the production a day
• the ease of automation
• the materials to join and the thickness of parts being joined
• the construction and the accessibility to the joint
• the dimensions and the tolerances of the joints
• the joint length

Joining methods

To narrow down the fastening options, the joining technologies can be subdivided into three categories:

• Adhesive Bonding (Chemical and Physical): applied in Aerospace and Automotive industries where high performance fastening is required. However, automation of adhesive bonding demands high investments and the joint result depends strongly on the surface quality.
• Welding: Applied in almost every industry, easy applicable but low quality results. The choice of materials is limited, material is weakened by change of structure and a follow-up treatment is mostly required.
• Mechanical joining: Highly versatile and low investment costs. Material structure remains intact, no internal stresses, no follow-up treatment and no surface quality requirements. However, mechanical joining requires forming tools and additional fasteners.

Riveting

Verborg Engineering specializes in automating mechanical fastening processes in medium to high production runs. Riveting processes can be automated easily by CNC-controlled riveting heads. Semi-automatic riveting brings efficiency improvements within reach of almost every mounting or assembling process. The advantages are, among others:

• Fast ROI (Return on investment); even in small to medium runs, see calculation.
• Universal purpose machine: Quick retooling, can be placed in different production runs with different products, materials...
• High and constant mounting quality (Koni shock absorbers, AXA locks); joints are identical.
• Low TCO (total cost of ownership); Riveting machines are reliable and durable.

Comparison of Joining Technologies

The table below shows the benefits and drawbacks of six different joining technologies; four mechanical methods, one welding method and one adhesive bonding method. Most of the results are gathered from a research project of Böllhoff ('Böllhoff is one of the leading suppliers of joining and assembly technologies in Europe and America').

	Radial/Orbital Riveting	Clinching	Self Pierce riveting	Screw fastening	Spot Welding	Bonding
Corrosion of coated material	Little	Little	Little	Little	High	None
Joining and strength alteration at joining point	None	None	None	None	Yes	None
Dynamic load-resistance	Good	Very good	Very good	Less good	Less good	Very good
Crush load-resistance	Less good	Less good	Very good	Less good	Less good	Good
Static load-resistance 1. Shearing stress 2. Head Stress	Very good Very good	Good Good	Very good Very good	Very good Very good	Very good Very good	Good Good
Edges - Burring - Splinters	None	None	None	Edges	None	None
Joining consumable required	Rivet	None	Punch rivet	Nuts, bolts, washers, tap	None	Glue
Energy consumption	Little	Little	Little	Little	Very high	Very high
Materials	All solid materials	Only metals	Only metals	All solid materials	Only some metals	All solid materials
Environmentally friendly	Good	Good	Good	Good	Poor	Very poor
Handling	Simple	Simple	Simple	Simple	Simple	Time consuming
Reproducibility	Very good	Very good	Very good	Good	Satisfactory	Good
Dependence of joint result on surface quality	None	Little	None	Little	High	Very High
Pre-processing	Drilling	None	None	Drilling	Washing, etching	Washing, etching
Possibility of joining different materials	Very Good	Poor	Poor	Very Good	None	Very good
Possibility of joining fragile parts	Very good	None	None	Good	None	Very good
Investments	Medium	High	High	Low	Medium	Very high

Conclusions

• Radial/Orbital Riveting is highly versatile with controlled investments, it ensures high quality joints between different materials, non-metals and even fragile parts.
• Clinching and Self Pierce Riveting both don't require pre-processes like drilling but demands high investments and are of limited applicability; Limited choice of materials, non-fragile parts and low tolerances.
• Screw Fastening is labour intensive and difficult to automate and therefore expensive in medium to high production runs.
• Spot Welding is even less versatile than Clinching and Self Pierce Riveting. The changes it brings to material structures and internal stresses makes it even worse.
• Adhesive bonding is a new and promising joining technology for high-tech applications or mass production of high-performance products. Only in these production processes the enormous investments are profitable.