Riveting in Sheet Metal Fabrication(what is a cmm Shirley)
- source:ZIEG CNC Machining
Sheet metal parts are ubiquitous in machinery, vehicles, aircraft, ducting, enclosures and more. Riveting is a quick and inexpensive way to assemble sheet metal components into more complex parts and structures. This article will provide an overview of riveting technology and its role in modern sheet metal fabrication.
What is a Rivet?
A rivet is a mechanical fastener that consists of two main parts - the rivet body and mandrel. The body is a head on one end and a shank on the other. The mandrel is inserted into the shank and helps form the rivet when set.
Rivet bodies are commonly aluminum, steel, stainless steel or titanium. They are tubular in shape with the shank end flared outwards. Mandrels are hardened steel bars that fit into the rivet shank.
To set a rivet, the mandrel and rivet body are inserted through aligned holes in the sheets to be joined. The rivet body head rests against one sheet while the shank end sticks out through the other sheet. A rivet gun provides a strong force which upsets and spreads the rivet body shank to form a second head. The expanded rivet shank permanently joins the sheets together.
Benefits of Riveting
There are several key reasons riveting sees continued widespread use in sheet metal fabrication:
- Speed - Automated riveting is extremely fast. Robotic riveters can install thousands of rivets per hour. Even manual riveting moves quickly.
- Strength - The cold forming process work hardens and strengthens the riveted joint. Rivets reliably withstand shearing and tensile stresses.
- Permanence - Unlike screws or bolts, rivets form a permanent connection that does not loosen over time.
- Vibration resistance - Riveted joints maintain integrity in high vibration environments.
- Simplicity - Basic riveting requires only simple tools and training. Mechanically fastened joints are easier to inspect than welds.
- Appearance - The smooth exterior surface of flush rivet heads provide a pleasing aesthetic. They avoid the splatter marks of welding.
These benefits make rivets ideal for many sheet metal uses like aircraft fuselages, automobile chassis, electronic enclosures, ducting, and more. The speed and strength of riveted joints outperform welding and other mechanical fastening methods.
Rivet Materials and Finishes
The most common rivet materials are:
- Aluminum - Offers good corrosion resistance at low cost. Used for non-structural applications.
- Steel - General purpose structural riveting. Carbon steel provides high strength while stainless resists corrosion.
- Titanium -Highest strength-to-weight ratio. Ideal for aircraft and aerospace applications where weight savings are critical.
- Copper - Used for its electrical conductivity when riveting bus bars and other current carrying components.
- Monel - Corrosion resistant nickel alloy used in marine and other wet environments.
Rivets come in a variety of head styles and finishes:
- Countersunk - Flush heads for smooth aerodynamic or cosmetic surfaces.
- Round head - Domed shape provides thickness for load distribution.
- Brazier head - Low profile rounded head.
- Large flange head - Wide flattened head for greater bearing surface.
- Flat head - Cylindrical flat top head.
- Colored and lacquered - Decorative and identification purposes.
- Grooved and serrated - Reduce spinning and vibration loosening.
Rivet Type Classifications
There are two major classifications of rivets defined by how they are set:
These one-piece rivets do not require access to the backside of the workpiece. Blind rivets come with a built-in mandrel that is pulled to expand the rivet. Installation tools range from simple handheld rivet guns to pneumatic squeezers for high volume production:
- Pop rivets - Most common type of blind rivet. The mandrel snaps off after setting.
- Split rivets - Similar to pop rivets but mandrel is removed by screwing out the set tool.
- Blind bolts - Installed like rivets and threaded internally to receive a bolt.
- Self-piercing rivets - Cut through softer materials and upset in place without drilling.
- Expanding sleeve rivets - Hollow rivet body expands into an outer sleeve.
Solid rivet installation is fast, easy, and requires minimal training. Structural integrity depends on material thickness. Best suited for light duty applications.
Two-Piece Structural Rivets
These rivets utilize a separate mandrel tool. They allow for solid riveting of thick stacked materials and are primarily used in heavy structural fabrication. Types include:
- Drive rivets - Require hammering the mandrel head to flare rivet body.
- Compression rivets - Rivet body is pressed onto mandrel stem using dies.
- Impact rivets - Pneumatic rivet hammer pounds mandrel to set.
- Countersunk head rivets - For flat, smooth surface finish.
- Universal head rivets -Accommodate variable material thickness.
- Flush head rivets - Provide smooth exterior surface.
- Bulbed Cherry rivets - Modified for improved fatigue performance.
Two-piece rivets produce very strong, permanent structural joints. However, installation is slower, more labor intensive, and requires access to both sides of assembly.
Rivet Hole Preparation
Proper rivet hole sizing, spacing, and alignment are critical to achieving maximum strength. Recommended practices include:
- Drilling holes 1/64” larger than rivet diameter. Ensures minimal force is needed for insertion.
- Adding chamfer to hole edges. Reduces potential for cracking at hole edges.
- Maintaining 2-3 times rivet diameter edge distance. Prevents rivet imposing shear along edge.
- Spacing at 4-6 times rivet diameter. Avoids excessive material deformation between rivets.
- Aligning holes to within 1/32” accuracy. Eliminates excessive shear stress from hole offset.
Proper hole preparation optimizes rivet performance and dramatically improves joint strength.
While specific procedures vary, the general riveting process involves:
1. Fixturing and clamping workpieces
2. Drilling, deburring and inspecting holes
3. Selecting appropriate rivet type and size
4. Inserting rivets into aligned holes
5. Setting and upsetting rivet with rivet gun or hammer
6. Forming and compressing rivet shank
7. Trimming excess shank material
8. Inspecting set rivet button for defects
Automated riveting systems greatly increase production rates by reducing labor and improving quality and consistency.
Riveted Joint Design Considerations
Well-designed riveted connections optimize joint efficiency. Factors to consider include:
- Materials - Match or exceed base material properties. Select rivets for shear, tensile and bearing strength.
- Direction of applied load - Determine if joint will undergo primarily shear or tensile stress.
- Rivet pattern and spacing - Distribute load appropriately and avoid distortion.
- Interlocking joints - Overlap multiple sheet edges to improve resistance to peel forces.
- Waste allowance - Add excess material to joint regions to be trimmed after riveting.
- Sealants - Seal rivets to prevent leaks in pressurized systems.
Proper rivet joint design maximizes strength while allowing for economic fabrication.
In summary, riveting remains a highly useful joining method for sheet metal fabrication. The speed, reliability, and relative simplicity of riveting make it a cost-effective choice for many applications. With an understanding of rivet materials, types, hole preparation and design principles, strong and durable riveted sheet metal assemblies can be efficiently produced. Riveting will continue enabling high-volume manufacturing across diverse industries far into the future. CNC Milling CNC Machining