Riveting in Sheet Metal Fabrication(circular spring Alexander)
- source:GAENOR CNC Machining
What is a Rivet?
A rivet is a mechanical fastener consisting of a cylindrical shaft and head. During installation, the rivet is placed in a pre-drilled hole through the materials being joined. The tail end is then upset, or deformed, to create a second head. This anchors the rivet in place. The rivet transfers load between the joined parts through bearing stress as it tries to straighten back out.
There are many types of rivets available for sheet metal fabrication. Common options include:
- Solid rivets: Made of a single material like steel, aluminum, copper or stainless steel. Used for general sheet metalwork.
- Blind rivets: Tubular rivets designed to be installed from one side when access to the back is limited. Common in aircraft and appliance production.
- Self-piercing rivets: Similar to blind rivets but pierce the sheet while upsetting instead of requiring pre-drilled holes. Often used to join dissimilar or coated metals.
- Drive rivets: Set by hammering a drive pin through the hollow body. Used when an air hammer is not available.
- Structural rivets: Extra-large rivets designed for joining structural steel components.
Rivet Strength Ratings
The strength rating of a rivet depends on the material and amount of load it can support before failing. Important measures include:
- Shear strength: Maximum load the rivet can withstand perpendicular to the axis of the shaft. Typically 500-1,000 MPa for steel rivets.
- Tensile strength: Maximum axial load it can handle before being pulled apart. Around 400-650 MPa for most steel rivets.
- Fatigue strength: Maximum cyclic stress it can endure through repeated loads. Varies based on application.
Higher strength ratings allow the use of fewer rivets or wider spacing between fasteners. Aircraft-grade rivets offer some of the best combinations of high shear, tensile and fatigue strength.
Rivet Material Selection
Choosing the right rivet material for sheet metalwork depends on several factors:
- Base material strength: Match rivet strength to the sheets being joined. Using a stronger rivet than needed is wasteful.
- Corrosion resistance: Select rivets that won’t corrode prematurely if exposed to weather or chemicals.
- Cost: More exotic rivet materials like Monel or titanium cost significantly more than steel or aluminum.
- Appearance: Paintable rivets are available for visible applications where looks are important.
- Temperature resistance: Some rivets maintain strength and integrity better at high temperatures.
For general sheet metal fabrication, aluminum and mild steel rivets cover most situations effectively and economically.
Common rivet diameters range from 1/16” to 1/4” (1.6mm-6.4mm). Length depends on the combined thickness of the materials being joined, needing to span the entire grip length. Allowance should be made for any gap between the sheets. Standard length increments are often 1/16” (1.6mm).
The hole size drilled to accept the rivet should be matched to the rivet diameter based on standard clearance allowance:
- 1/16” rivets: 0.046”-0.051” holes
- 3/32” rivets: 0.093”-0.099” holes
- 1/8” rivets: 0.126”-0.134” holes
Larger holes will reduce rivet strength while smaller holes can damage materials and make installation difficult.
Rivet Strength Calculations
Engineers perform rivet strength calculations during design to properly size fasteners for an application. This ensures rivets selected can withstand expected stresses and loads without failing. Key factors in rivet strength calculations include:
- Shear loads - forces perpendicular to the rivet axis
- Tensile loads - forces pulling along the rivet axis
- Number of rivets sharing/transferring a given load
- Spacing between rivets
- Rivet material properties like shear and tensile strength
- Yield margin - avoiding stresses near rivet yield strength
Software tools are available to model and simulate riveted joint strength in various configurations. However, testing of prototypes is still needed to validate real-world performance.
Drilling Rivet Holes
Accurately drilling rivet holes is crucial for proper fit-up and joint strength. Hole misalignment is a leading cause of rivet failure. Several best practices include:
- Using a drill press for perpendicularity
- Indexing off part edges or datums for consistency
- Employing a backing plate to minimize burrs and distortion
- Proper drill bit sharpening to avoid wandering
- Spotting holes with a center punch for drill bit stability
For production runs, CNC drilling machines excel at delivering hole accuracy and repeatability. Fixturing ensures parts remain aligned hole-to-hole.
Hand-installed rivets rely on a rivet gun, bucking bar, and hammer to upset the tail. Pneumatic rivet guns provide the force needed to deform the rivet for a tight fit. Proper riveting technique involves:
- Inserting the rivet into the hole through all layers
- Bucking the rivet tail flush against a bar on the backside
- Pulling the rivet gun trigger to upset the tail into a second head
- Avoiding damage to the surrounding material surface
Self-piercing and blind rivets utilize specialty tooling to control their installation in limited access applications.
For large rivet volumes, auto-fed riveting machines take indexing trays of pre-drilled parts and install rivets hands-free. Operators simply load and unload parts from the machines. Precision clamping aligns holes prior to automated riveting.
Inspecting Riveted Joints
Post-installation inspection ensures rivets meet quality standards. Checks include:
- Confirming flush rivet seating without gaps
- No cracks or splits in rivet heads
- Proper rivet bulge and tail deformation
- Tight joints between parts without looseness
- Verifying edge distances around rivets
Failed rivets must be drilled out and replaced. Periodic maintenance sustain programs test riveted aircraft components for continued airworthiness.
Some helpful tips for effective riveting in sheet metal fabrication:
- Use alloy rivets for joining dissimilar metals to avoid galvanic corrosion
- Apply sealant to the faying surface for watertight or airtight joints
- Avoid using excessive force which can crack rivet heads or distort parts
- Install rivets in holes at the center of parts first, working outwards
- Drill undersized holes or ream for proper fit with close-tolerance rivets
Properly incorporating rivets during product design and manufacturing ensures robust, reliable joints in sheet metal assemblies across countless industries. CNC Milling CNC Machining