Producing High Quality Rivets with CNC Machines(aircraft fasteners types Willie)
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- source:GAENOR CNC Machining
Material Selection
The material used for rivets depends on the application and strength required. Common materials include aluminium, steel, copper, brass and titanium. Aluminium and steel rivets are inexpensive, lightweight and corrosion resistant making them suitable for general applications. Brass and copper provide higher strength but are more expensive. Titanium offers very high strength-to-weight ratio but is costly.
The material must be rugged enough to withstand high pressure drilling, heading and driving into the workpiece without cracking or deforming. Ductility is important so the rivet head can form properly. Considerations like weldability, corrosion resistance and cost also influence material selection.
CNC Drilling
High quality rivet holes require precision drilling. On a CNC machine, the drilling process can be automated for consistency. Proper feeds, speeds and drill bit selection are key. Too slow of a feed rate leads to work hardening while too fast can overload the drill bit.
For most materials, a carbide twist drill bit is recommended. The drill point angle, web thickness, flute design and coating optimize chip ejection and hole accuracy. Through-tool coolant further improves drilling performance.
The CNC program should include peck drilling cycles for deeper holes. This periodically retracts the drill bit to clear chips. It prevents binding, reduces heat buildup and results in straighter, more accurate holes.
Rivet Shank Tolerances
The diameter tolerance on the rivet shank is critical for proper fit. Allowance must be provided to suit the hole diameter. For example, a 3mm diameter rivet would have a shank tolerance of 2.97-3.03mm for a 3.02-3.08mm hole. Too loose of a fit can cause vibration and joint failure while too tight leads to damage during rivet setting.
Shank surface finish is also important. A smoother surface reduces friction during setting. Typical finishes range from 1-4 micrometers Ra. Additionally, the shank should be straight, concentric and free of defects.
Heading Process
The rivet head is formed using a cold heading process. A punch presses the blank material into a die cavity at high pressure, plastically deforming it. This forms the rivet head. Done on a CNC header, the process can precisely control the force and position for consistency.
Several factors are vital for quality heading. The punch and die must be maintained in good condition and aligned. The accurate relief and curvature of the die is critical so the material flows properly to form the head. The punch-to-die clearance, typically 5-10% of rivet diameter, also affects how the material flows. Too much clearance causes an irregular head shape.
The pressure applied ensures the head fills out the die cavity completely. Harder materials require more pressure. Bottoming detection precisely controls the stroke depth during heading to consistently form the proper head height.
Proper lubrication prevents galling between the punch, die and material. It enables smooth material flow into the die cavity. Typical lubricants include oil, wax or grease-based formulations.
Driving Operation
The final step is driving the rivet into the workpiece. This expands the tail end, locking the rivet in place. On a CNC riveting machine, force, displacement and bottoming detection are programmable to control the precision and consistency.
Using the proper rivet set is crucial. It must fit the rivet head profile and diameter accurately. The cavity should be smooth to prevent marking or damaging the head. The set must be aligned squarely to the rivet and held rigidly during driving.
Appropriate force levels prevent damage while ensuring the rivet sufficiently fills the hole. Harder, thicker or grippier materials require more driving force. Bottoming detection helps avoid applying excessive force after the tail has expanded.
Quality Inspection
Post-production checks verify the rivet meets dimensional, visual and functional specifications. Dimensions like head height/diameter, shank length and tail diameter must fall within tolerance. Visual checks ensure no cracks, galling marks or other defects.
Functional tests like pull testing or mandrel retention evaluate rivet setting quality. Pull tests measure the force required to shear off or extract the rivet. Mandrel retention ensures it holds securely inside the set rivet. Random sampling and process monitoring identify any production issues requiring adjustment.
Proper CNC programming, tooling setup and machine maintenance enables fast, high volume rivet production. Attention to material selection, hole quality, heading and driving processes, and post-production quality control results in reliable, consistent and long lasting rivets suited to the application. Implementing these best practices for CNC rivet manufacturing delivers superior quality and performance. CNC Milling CNC Machining