Introduction(machine finish Mark)
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- source:WEINBERG CNC Machining
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Mechanical Fastening
The primary use of rivets in CNC is mechanical fastening. Rivets create permanent joints between two or more workpieces when installed correctly. The head and shank of the rivet when deformed create a solid connection that can withstand vibration, weathering, and extreme forces. Rivets are commonly used to fasten metal sheets, plates, brackets, frames, and other components where a reliable mechanical bond is required. Proper rivet selection ensures the joint has sufficient shear and tensile strength for the application.
Blind Riveting
Blind rivets have become a popular choice where only one side of a workpiece is accessible for fastening. The rivet is inserted into pre-drilled holes in the materials being joined. A pulling tool draws the stem of the rivet through the body, causing the blind side to flare out and clamp the workpieces together. Blind rivets enable CNC machinists to make attachments where welding, adhesive bonding, or standard rivets are impractical. Aerospace, appliance, electronics, and automotive industries rely heavily on blind rivets.
Joining Dissimilar Materials
Rivets present a simple solution for joining different or incompatible materials in CNC workholding applications. They can bring together metals, plastics, composites, wood products, and more regardless of surface chemistry issues. Mechanical fastening circumvents unfavorable fusion bonding, deformation, galvanic corrosion, and other incompatibilities faced when welding or adhering dissimilar materials. Rivet strength relies on mechanical interlock rather than metallurgical fusion.
Permanent vs Removable Fastening
For nonpermanent attachments, several rivet types allow for fastening and removal for maintenance or reconfiguration purposes. One common example is the Huck bolt, used for quickly fastening components within CNC machining fixtures and workholding jigs. Huck bolts provide a vibration-resistant alternative to clamps, fixtures pins, and screws. Other removable rivet options include drive rivets, pan head rivets, and tri-fold rivets. Permanent rivet options, such as round head, flat head, and countersunk rivets, once set, can only be drilled out for removal.
Rivet Materials
Steel remains the most prevalent material for CNC riveting applications, providing high shear strength and durability. Stainless steel and aluminum are also regularly used for enhanced corrosion resistance or weight savings. Monel and copper rivets offer specialized corrosion protection, while brass provides decorative value. Exotic alloys like nickel, Inconel, and titanium give added performance in extreme environments. The rivet material chosen should match or exceed the strength and longevity requirements of the working environment.
Standard vs Custom Rivets
While standard off-the-shelf rivets work for most CNC uses, custom rivets provide engineers an option for specialized strength, precision, or design demands. Custom diameter, length, head type, tolerance, material, and coating specifications can be obtained. This allows rivets to be optimized for unique joint requirements or operating conditions. However, custom rivets come with higher lead times and per piece costs.
Installation Methods
Hand-set rivets continue to be commonly used in CNC when pieces quantities are low or access is limited. However, for higher volume production, pneumatic, hydraulic, and impact rivet tools are employed. Robotic riveting integrates well with CNC automation, allowing for precise placement and clinching force. Self-piercing rivets are also growing in popularity, eliminating the need for pre-drilled holes. Each approach has its advantages depending on factors like: joint design, materials, rivet type, assembly time, and cost targets.
Quality Assurance
Proper rivet installation is critical in CNC workholding and assembly. Underdriven rivets result in loose joints while overdriving leads to material failures. Robust quality control during the riveting process reduces scrap and prevents catastrophic in-service failures. Common quality checks include: rivet length measurement, pull testing removal force, push testing for minimum button size, sonic testing for hidden defects, and visual inspection of driven rivets. Automated inspection eases the burden for higher volume production.
Design Considerations
Several factors need to be weighed when designing CNC riveted connections: shear strength, hole size, spacing, edge distance, grip range, joint thickness, access for tools, secondary processing issues, inspection, and repairs. Thorough engineering analysis via hand calculations or Finite Element Analysis ensures rivet joints have sufficient strength and fatigue life. Proven riveting best practices should be utilized when designing new CNC components and machinery.
Conclusion
In closing, rivets offer CNC operators and engineers an invaluable fastening solution thanks to their simplicity, cost-effectiveness, and reliability. When properly selected and installed, rivets create robust, permanent joints between an array of materials and components. While often considered basic shop essentials, rivets enable successful function and fabrication of critical CNC machined parts and assemblies across every manufacturing sector. Continued innovation and clever application of rivet technology provides the opportunity to improve manufacturing efficiency, quality, and safety well into the future. CNC Milling
