Producing High Quality Rivets with CNC Machines(champhered Ernest)
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- source:YESCOM CNC Machining
Defining Quality Rivets
A quality rivet has certain essential attributes that differentiate it from inferior fasteners:
- Strength - The rivet must securely hold materials together under expected load conditions without failing. Quality rivets are typically forged from steel alloys optimized for hardness, ductility and fatigue resistance.
- Dimensional accuracy - All dimensions of the rivet shank, head and point must conform precisely to specifications so the rivet fits into drilled holes and deforms properly during installation.
- Consistent head formation - The shop head of the rivet is precisely formed to sit flush against the top material without burrs or irregularities. The buck tail is symmetrically deformed.
- Surface finish - The external surface of a quality rivet is free of cracks, seams, scale or other defects that could weaken it or hinder fit and function.
- Materials compatibility - The rivet composition and plating is suitable for the service environment and mating materials to prevent issues like galvanic corrosion.
CNC Machining Basics
CNC machines use programmable computer controls to automate machining tasks like milling, drilling, turning and riveting formerly done manually. Here’s a quick overview:
- Instructions are coded via CAM software to command the tooling, axes of motion and other parameters.
- The programs are then loaded into the CNC machine’s controller which interprets the instructions.
- Machine components like spindles, cutters, axes and drill heads are precision-actuated by servomotors and ball screws according to the program coordinates and commands.
- Workpieces are fixtured onto the machine bed and tooling is changed out as needed to perform the sequential operations.
- Continuous feedback control loop adjusts operating parameters in real-time for accuracy.
CNC technology boosts rivet manufacturing productivity while ensuring each rivet meets specifications. Now let’s look closer at how it achieves this for each production step.
Blanking Raw Rivet Shapes
The first stage in rivet making involves cutting the rivet blank from wire stock or sheet metal. This establishes the basic cylindrical or tubular shape at a prescribed length. Blanking can be done via:
- Punching - A hydraulic punch press forces a hardened die through sheet metal tightly clamped in a die set. A simple stroke produces finished blanks.
- Roll forming - Rotating rollers progressively form metal sheet or strip into a tubular shape which is then cutoff.
- Wire cutting - Spinning circular cutter blades shear wire fed from a coil to slice off blanks.
For high production rates, CNC turret punches offer the most efficient rivet blanking. Programmable auto-indexing of large turret-mounted tool stations allows cutting different blank sizes and shapes without changeover. Precise stroke control and die alignment ensures blanks meet dimensional tolerances.
Forming the Rivet Head
The next key process step is forming the shop head on one end of the rivet blank. This involves using pressure to flare out the rivet end into a symmetrical dome shape. Shop heads secure the rivet in place during assembly before the tail is deformed. Common CNC forming methods include:
- Impact heading - The rivet end is struck by a shaped punch to flare it outward into a head within a cavity die. CNC enables high-speed sequencing of blows with fine force regulation.
- Orbital forming - The blank end is pressed against a spinning convex domed roller which gradually shapes it into a head. CNC controls the forming pressure and roller speed for smooth heads.
- Rotary forging - The rivet is gripped and rotated at high speed as a shaping die pounds the end into a precise head profile. Programmable CNC hammers deliver controlled blows for head uniformity.
- Compression heading - The blank is axially pressed against a motorized forming die by a programmed ram stroke. The confined material flows outward into a smooth shop head.
The CNC system ensures correct axial alignment, force and dwell during heading to achieve flawless high-volume production. In-process sensors confirm head dimensions stay within tolerance.
Machining the Rivet Shank
In some cases, the rivet shank requires additional machining before final finishing. Common CNC operations include:
- Turning - Rotating the clamped blank against a cutting tool forms the smooth, precise shank diameter and chamfers the tip.
- Drilling - Accurately drilling a concentric cross-hole through the rivet shank enables expanded tail deformation to improve strength.
- Threading - Tapping or rolling threads onto the shank end can provide stronger engagement and pull-out resistance when paired with a nut.
- Grooving - Cutting circumferential grooves into the shank provides reservoirs for adhesive or sealants when used in riveted joints.
- Knurling - Roller patterned teeth added to the shank produces an interlocking mechanical bond when flared in place.
Programmable feed rates and spindle speeds optimize cutting accuracy, surface finish and tool life throughout these CNC processes. The capability to machine complex rivet geometries is unlimited.
Automated Rivet Pointing
The pointed tip of the rivet serves to locate it precisely into aligned holes for installation. Blunt tips can cause problems. CNC enables advanced techniques to form optimized points:
- Percussion pointing - The rotating rivet is impacted against a contoured cavitating die which indents the tip into a point. CNC controls force and timing.
- Rolling - Passing the rivet between opposed roller dies produces the desired point profile with minimal material distortion.
- Grinding - High speed abrasive grinding wheels accurately shape and sharpen the rivet tip. Programmable feed rates prevent thermal damage.
- EDM pointing - Electric discharge machining vaporizes material using precisely timed sparks to cut a sharp rivet point. CNC allows unlimited point angle and shape.
Monitoring systems ensure the rivet points meet stringent specifications for symmetry, sharpness and chamfer angle. Blunt or irregular points are automatically rejected.
Finishing Processes
The final critical production step is applying the required finish to the CNC machined rivet. Common finishing methods include:
- Plating - Electroplating the rivet with zinc, cadmium or other metals applies a protective corrosion resistant coating just a few microns thick.
- Passivation - Chemically treating the rivet surface forms an impervious oxide layer to resist corrosion while allowing visual inspection.
- Coatings - Spray or powder coating provides thicker polymer coatings like epoxy or polyurethane for extreme environments.
- Heat treating - Heating and quenching hardens and strengthens the rivet material for improved performance.
CNC enables very precise control of rivet finishing processes. Parameters like bath chemistry, dwell times, cure temperatures and coating thickness are programmed, monitored and maintained within tight tolerances. This ensures consistently high-quality rivets.
Automated Inspection and Sorting
The last step is 100% inspection and sorting of the finished rivets. Vision systems and sensors integrated with the CNC machines verify critical dimensions, surface defects, hardness and other key parameters on a pass/fail basis. Rejected rivets are automatically sorted from acceptable parts. This closed-loop quality control within a CNC workcell guarantees only rivets within specifications are packaged for shipment.
Conclusion
From blanking to finishing, computer numerical control systems enable fast, flexible and cost-effective production of high quality rivets with repeatable tolerances. The programmability of CNC machines allows manufacturers to quickly optimize the entire riveting process while minimizing labor through automation. This results in maximizing productivity and profitability while delivering consistent rivet quality customers can rely on. With continued advances in smart CNC technology, the future looks bright for innovating the manufacturing of this vital industrial fastener. CNC Milling CNC Machining