Riveting in Sheet Metal Fabrication(precision metal bending Bella)
- source:BREDA CNC Machining
What is a Rivet?
A rivet is a mechanical fastener that consists of two main parts - the rivet body and mandrel. The rivet body is a head on one end and a shank on the other. The mandrel fills the hollow shank portion during installation and is discarded afterward.
Rivets are installed by placing them in pre-drilled holes in the materials being joined. The rivet body shank is inserted through the holes and the tail end is upset, or splayed outward. This creates a second head that holds the rivet in place. The rivet mandrel is pulled out, leaving the hollow shank to fold. As the rivet cools, the metal hardens in the flared shape, creating a permanent joint.
Benefits of Riveting
Rivets offer unique benefits that make them a staple fastening method in sheet metal fabrication:
- Strong and Reliable Joints - The upsetting process work-hardens and expands the rivet shank, creating a tight fit in the hole. This provides excellent resistance to shock, vibration, and tension forces.
- Permanent Fastening - Unlike screws or bolts, rivets cannot come loose over time. The flaring process essentially welds the rivet in place for a lifelong connection.
- Lightweight - Rivets add very little weight versus other mechanical fasteners. This helps minimize the mass of sheet metal assemblies, especially in industries like aerospace.
- Quick Installation - Riveting is generally faster than welding or using nuts and bolts. Automated tools allow for rapid mass riveting.
- Variety - Rivets come in many head shapes, shank diameters, and materials to accommodate different design needs.
The most common rivet materials in sheet metal fabrication are:
- Aluminum - Offers good corrosion resistance at a low cost. Different alloys allow riveting of most any thickness.
- Steel - Available in a range of grades and strengths. Carbon steel provides high shear and tensile strength. Stainless steel adds corrosion resistance.
- Copper - Provides excellent conductivity and corrosion resistance. Mostly used in electrical applications.
- Monel - A nickel-copper alloy resistant to saltwater corrosion. Common in marine environments.
- Titanium - Extremely strong but lightweight. Used in aerospace applications despite the higher cost.
There are several specialized rivet types designed for different sheet metal joining needs:
- Solid Rivets - Most common type with a solid shank. Available in round, flat, or countersunk head profiles.
- Blind Rivets - For accessing only one side of a joint. Mandrel is expelled after installation. Used where backside access is limited.
- Split Rivets - Shank splits during flaring. Allows pre-insertion into holes. Used where loose rivet retention could cause issues.
- Drive Rivets - Installation forces the domed head into the rivet shank rather than flaring the tail. Creates a low-profile finish.
- Large Flange Rivets - Extra-wide domed head distributes load over more area. Withstands vibration in aircraft and trucks.
- Structural Rivets - Extra-strong for critical structural joints. Usually have larger diameters than standard rivets.
- Self-Plugging Rivets - Sealant fills the hollow shaft during installation. Prevents leak paths through rivet holes.
Rivet Hole Preparation
Proper hole sizing and preparation are critical for rivets to function as intended. Guidelines include:
- Hole Diameter - For most rivets, drill sheet metal stack holes 0.05mm larger than rivet shank diameter. This provides optimal clearance.
- Hole Alignment - Align holes precisely when drilling multiple sheets to prevent rivet joint offset. Clamp pieces together when drilling if needed.
- Hole Deburring - Use a deburring tool to remove any sharp edges or burrs around holes that could cut rivet shanks during installation.
- Surface Cleaning - Clean sheet surfaces around holes to maximize rivet-to-metal contact area for optimum strength. Remove any oils, rust, or paint.
- Rivet Gripping Range - Rivet length should be selected to match the thickness of the sheet stack. The rivet must be long enough to flare properly but not overly long to prevent bottoming out in the hole.
There are manual and automated riveting techniques available:
- Hand Riveting - Uses simple tools like a hammer, dolly, and bucking bar for one-off jobs. Operator skill affects quality. Slowest option but requires minimal equipment.
- Pneumatic Riveting - Rivet gun pulls the mandrel and upsets the rivet via compressed air. Faster than hand but still relatively slow. Portability allows riveting at various locations.
- Impact Riveting - Uses electromagnetic force, air, or hydraulics to quickly upset rivets. High speed makes it ideal for mass production. Requires large equipment not easily moved.
- Orbital Riveting - Automatic CNC machine precisely presses a rotating rivet head into the workpiece. Computer control ensures consistent high quality and speed. Limited to workshop use.
Rivet Strength Testing
Testing finished rivet joints helps ensure sufficient strength and quality:
- Tension Testing - Measures rivet strength against forces trying to separate joined materials by pulling them apart. Verifies rivets meet minimum strength specs.
- Shear Testing - Determines rivet strength against shear forces, which act parallel to the rivet axis. Confirms adequate resistance to shear loads.
- Micrographic Analysis - Cross-sections cut from riveted joints are inspected under a microscope. Checks for cracks, voids, flare geometry, and other defects.
- Process Monitoring - Production parameters like installed rivet height and pulling force are tracked to identify any trends indicating issues.
- Tap Testing - Rivets are lightly tapped with a hammer and the sound is assessed. Solid tight rivets give a clear "ring" versus a dull thud for loose rivets.
Proper application of rivets creates robust sheet metal assemblies capable of withstanding years of abuse. Understanding available rivet types and installation methods allows fabricators to select the optimal riveting solution for their specific application needs. Thorough strength testing provides quality assurance and confidence in the finished product. With the right knowledge, rivets can be an extremely effective and reliable fastening method in sheet metal fabrication. CNC Milling CNC Machining