Riveting in Sheet Metal Fabrication(cnc machining basics Cornell)

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Rivets are a fundamental fastening method in sheet metal fabrication. They create strong, permanent joints between two or more pieces of sheet metal. Compared to other mechanical fasteners like screws or bolts, rivets offer unique advantages that make them ideal for joining thin sheets of metal.
What is a Rivet?
A rivet is a mechanical fastener that consists of two main parts - a smooth cylindrical shaft and a head. The shaft is inserted through holes drilled or punched in the sheets to be joined. The tail end of the shaft is then mechanically deformed to form a second head. This process is called riveting.
The two heads grip the sheet metal on both sides, creating a solid riveted joint. The rivet shank fills the hole and prevents lateral movement or separation of the joined pieces. Rivets are available in a wide range of materials, sizes and styles to accommodate different sheet metal fabrication applications.
Benefits of Riveting
There are several key benefits that make riveting a preferred joining technique for sheet metal:
Permanent Joints - Once installed, rivets form permanent joints that do not loosen over time. This prevents fatigue failures or separations caused by vibration in assemblies.
High Strength - The riveting process work-hardens and expands the rivet shank, creating very strong joints. Pound for pound, rivets have higher shear and tensile strength compared to screws or bolts.
Vibration/Impact Resistance - Riveted joints reliably withstand high levels of vibration, shock or impact loads better than threaded fasteners. The rivet shank dampens and absorbs energy rather than transferring it to the joined sheets.
Stress Distribution - Rivets apply clamping force over a relatively large area, distributing stresses much more evenly. This reduces risk of tear-outs or damage to thin sheet metal.
No Thread Damage - There are no threads to be damaged or worn over time. Rivet joints maintain consistent strength over the life of an assembly.
Air and Water Tight - Properly installed rivets create air and water tight joints along the rivet shank. This allows riveting of enclosures, ducts, water tanks and more.
Quick Installation - Riveting is generally faster than bolting or screwing. No nuts or washers are required. Multiple rivets can be quickly gang set.
Riveting Applications in Sheet Metal Fabrication
Rivets are commonly used to assemble sheet metal parts in a wide range of industries and products, including:
- Aircraft and aerospace vehicles - fuselage skins, wings, empennage, engine nacelles, etc. Rivets are extensively used because of their vibration resistance.
- Heavy trucks and automobiles - cabs, trailers, body panels, wheels, etc. Rivets provide durable fastening for sheet metal vehicle parts.
- Electronics enclosures - computer cases, appliances, instruments, business machines. Rivets allow quick joining of chassis and cover panels.
- HVAC ducts and equipment - rivets create strong, airtight joints in duct assemblies and air handlers.
- Architecture - decorative metal panels, railings, curtain walls and skylights often involve riveted connections.
- Transportation - rivets are ideal for assembling railway cars, ships, storage tanks and cargo containers.
- Furniture - chairs, cabinets, shelving and office furniture items use rivets to fasten sheet metal components.

Common Rivet Types
There are various types and styles of rivets tailored to different fabrication requirements:
Solid/Blind Rivets - These have a pre-formed head on one end and are driven by a rivet gun on the opposite end. Blind rivets are useful where access is only available from one side.
Self-Plugging/Friction Lock Rivets - These have locking features that mechanically grip the sheet when driven for reduced hardware. Often used in softer materials like aluminum.
Drive Rivets - Hardened steel rivets that are driven using hammer blows rather than a rivet gun. Used for heavy duty boiler and structural applications.
Split Rivets - These have a split shaft that flares outward when driven to lock into the sheet hole. Allows removal and installation using only hand tools.
Structural Rivets - Extra large rivets with higher shear/tensile strength for critical structural connections. Materials include steel, aluminum, titanium and copper.
Tubular Rivets - Hollow and lightweight while still providing good grip strength. Used extensively in aircraft assembly.
Flow Drill Rivets - Installed by flow drill screws that use rotary friction and pressure to form the rivet head.
Riveted Joint Design Considerations
Proper design of riveted connections is important for joint strength and durability:
- Spacing - Rivets should be spaced 3-4 diameters apart in rows and columns based on the rivet diameter. This prevents local deformation around holes.
- Edge Distance - Minimum 1.5 x rivet diameter edge distance maintained from sheet edges and other holes. Prevents tear-outs under load.
- Hole Fit - Rivet hole diameter should match the rivet shank with a tolerance of +0.13mm/-0.00mm for snug fit. Oversized holes will reduce joint strength.
- Material Thickness - Joint members should not exceed 3x sheet thickness to avoid bending vs shear failure.
- Load Direction - Rivets have maximum strength in shear rather than tension. Joints should be oriented so loads apply parallel to the rivet axis.
Riveting Process
Installing rivets involves a simple three step process:
1. Drilling/Punching - Matching holes are formed in the materials to be joined using CNC drills or punch presses. Hole dimensions match the specific rivet size being used.
2. Insertion - Rivets are manually or machine fed into the aligned holes through all sheet layers. Automatic riveting systems use vibratory bowl feeds.
3. Rivet Setting - A rivet gun, squeeze type tool or hammer is used to deform the rivet tail and form the second retaining head. This upsets and expands the rivet shank to clamp the sheets.
Setting applies tremendous force - up to 15,000 psi for large structural rivets. This work hardens and prestresses the rivet for maximum strength.
Properly driven rivets completely fill the joints holes while forming neat button-head profiles. Improperly set rivets can result in loose joints, cracks or rivet failure.
Automated Riveting
Many modern manufacturing plants use automated riveting technology to improve quality and throughput. This includes:
- CNC Machines - Multi-axis computer numerically controlled (CNC) machines that drill holes, insert rivets and set them in programmed sequences. Allows precise positioning and riveting of complex aircraft and vehicle components.
- Robotic Riveting Arms - Articulated robots that can be programmed to follow riveting patterns optimized for speed and efficiency. Robots never get tired or deviate from quality standards.
- Autofeed Riveting - Systems that automatically supply and sort rivet sizes/types coupled with fast setting pneumatic or hydraulic tools. Enables rapid mass production riveting.
- Vision Inspection - Cameras and sensors to check inserted rivets for proper type, orientation, grip length and flushness with the joined surface.
Automated systems can install over 1,000 rivets per hour consistently with perfect quality. They are ideal for high volume manufacturing.
In summary, rivets offer unique benefits that make them a preferred joining method for sheet metal fabrication across many industries. Their strength, permanence and vibration resistance allow durable and reliable fabrication of aircraft, vehicles, enclosures and structures using both manual and automated riveting processes. Continued innovation and technology improvements will further expand the applications for versatile rivet fasteners. CNC Milling