Riveting in Sheet Metal Fabrication(horizontal milling machine Victor)
- source:YESCOM CNC Machining
What is a Rivet?
A rivet is a mechanical fastener that consists of two main parts - the rivet body and rivet mandrel. The rivet body is a head on one end and a shank on the other. The shank is inserted through pre-drilled holes in the sheets being joined. The rivet mandrel (also called a rivet stem) is inserted into the hollow end of the rivet shank and driven to flare out the shank, creating a second head that permanently fastens the rivet in place.
The flared end holds the rivet tight against the reverse side of the workpiece, clamping the fastened parts together. The rivet heads on both ends also provide additional holding strength. Rivets come in a variety of head shapes, shank lengths and diameters to accommodate different assembly requirements. Common materials include aluminum, steel, copper, stainless steel, and Monel.
Benefits of Riveting
There are several reasons why riveting is a popular joining technique for sheet metal:
- Permanent - Unlike screws or bolts, rivets cannot come loose over time from vibration or thermal cycling. The flared shank permanently affixes the rivet in place for the life of the product.
- Strength - The rivet clamping force and headed ends provide excellent load distribution and resistance to shock and vibration forces. Properly installed rivets have high shear and tensile strength.
- Material versatility - Rivets are available in a wide range of materials to match the sheets being joined. This prevents galvanic corrosion that can occur when dissimilar metals are bolted together.
- No threads or rework - There are no threads to be damaged or wear out over time. No rework is required if rivets are driven inaccurately - they can just be drilled out and replaced.
- Sealing - The rivet heads and shank flare create a tight joint seal. This prevents leakage in fluid system components. Sealing washers can also be added if complete leak prevention is critical.
- Cost - Rivets are inexpensive relative to the total cost of the finished product. Both manual and pneumatic rivet installation is simple and fast for high production rates and lower labor time.
- Appearance - The smooth finished look of rivet heads is aesthetically appealing for exposed locations on finished products. Low profile head designs are available for applications where flush rivets are desired.
Riveted Joint Design
Properly designing riveted joints is crucial for creating high strength assemblies. Here are some key design considerations:
- Material and thickness - The rivet material should match the base material being joined to prevent galvanic corrosion. The rivet diameter must suit the combined thickness of the sheets.
- Hole Size - Holes must be correctly sized - too large reduces clamping force, too small makes insertion difficult and damages the rivet. Industry standard hole sizes relative to rivet diameters help ensure proper fit.
- Spacing and Pitch - Rivets must be spaced based on the load on the joint. The distance between rivet centerlines, known as pitch, impacts joint flexibility and potential buckling.
- Rows and Columns - For wide joints, rivets are arranged in rows and columns. This distribution of fasteners also affects joint strength and ability to resist failure.
- Edge Distance - Minimum spacing must be maintained between rivets and the open edge of sheet panels to prevent edge tearing. Industry specifications provide minimum edge distances for different materials.
There are two basic methods used to install rivets - manual and pneumatic.
The traditional tool used for hand riveting is the bucking bar. It is held against the back side sheet while the user hammers the rivet head down with a hammer until the shank flares. Manual riveting requires coordination and practice to produce a quality flare. It is still used for fieldwork in some industries but is not common in manufacturing.
Pneumatic rivet tools use compressed air to quickly and forcefully set rivets for high-volume production. There are two types - squeeze tools and impact tools.
Squeeze tools use hydraulic pressure to clamp jaws down around the rivet body, squeezing it to flare the mandrel. The flaring action is smooth and gradual. Squeeze tool riveting produces excellent results but is slower than impact tools.
Impact riveters use a piston driven by compressed air to strike and flare the mandrel in milliseconds. The impact force flares the shank very quickly but can work harden some materials. Impact tools allow one-handed operation and are ideal for high speed riveting lines.
There are also two methods for inserting rivets into holes for assembly - manual and automated.
Manual insertion involves an operator placing each rivet individually into the holes in the aligned workpieces. This allows flexibility for custom or low volume products but is relatively slow.
For mass production, automated riveting uses pick-and-place feeding systems to supply rivets directly into the pre-drilled holes. Vibratory bowl feeders or tape-and-reel feeds are common technologies used for automated high-speed rivet insertion.
Inspection and Testing
To ensure rivet joint integrity, inspection and testing should be performed during manufacturing:
- Dimensional checks of spacing, edge distance and pitch confirms rivet pattern accuracy.
- Confirming flushness of the rivet head verifies the mandrel was fully flared.
- Microscope inspection of the flared shank end can detect cracking or inadequate deformation that could lead to failure.
- Sample rivet pull testing measures the force required to shear the rivet shank. The results validate assembly process parameters are producing joints at or above the required strength.
- Leak testing of fluid systems verifies rivets have created a complete seal.
By following proven design principles and performing thorough quality inspections, manufacturers can produce riveted sheet metal assemblies that meet their durability, appearance and performance requirements. Riveting remains a trusted fabrication technique for connecting sheet metal across many industries. CNC Milling CNC Machining