Achieving High Quality Rivets with CNC Machining(online sheet metal fabrication Ingemar)
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Rivets are a crucial fastening component in many manufacturing applications, from aircraft construction to industrial machinery. While riveting was traditionally a manual process, computer numerical control (CNC) machining has enabled automated, precision riveting for mass production. However, achieving consistent, high quality rivets requires careful programming, fixturing, tool selection and inspection methods. This article will examine key considerations for optimizing CNC riveting processes.
Rivet Material Selection
The material used for rivets significantly impacts performance and machinability. Common rivet materials include aluminum, steel, titanium, copper and various alloys. Aluminum and steel are most economical, while titanium and MONEL offer high strength. Copper rivets provide excellent conductivity. The material must be selected based on strength, corrosion resistance, fatigue life, and cost requirements. Ductile, shear-resistant materials like 1117 steel or 2024 aluminum alloy are ideal for rivets in high vibration applications.
Hardness is another important factor, as too soft of material will deform during riveting and too hard will be brittle. 1018 steel and 6061 aluminum strike a good balance. Pre-headed rivets are recommended for CNC machining, as the head is precisely formed under high pressure during manufacturing. This avoids potential mushrooming of the head during installation.
CNC Programming
Five-axis CNC machines enable drilling the rivet hole and installing the rivet in one continuous operation. This requires programming of the tool motion along the riveting path, calculating drill depth based on rivet length, and coordinating the spindle and rivet setter movements. Programs are coded in G-code, utilizing canned cycles for drilling and rivet setting operations.
Precision fixturing is critical to hold parts in the correct position and prevent warpage under riveting force. Programmers must also implement peck drilling cycles and chip breaking operations to clear debris from deep rivet holes. Rivet setter force and dwell time parameters must be dialed in to develop sufficient head tension without damage.
Tooling Selection
Carbide or high speed steel drills matched to material hardness are used for rivet holes. Straight flute drills prevent chip wrapping at depth. Precision step drills may drill the initial pilot hole and gradually increase to full size for greater accuracy. Automated rivet setting tools use servo-pneumatic or hydraulic force to compress and buck the rivet tail. Nosepieces must be selected to fit the rivet head diameter.
For blind rivets, specialized tools cut and pull the stem to deform the tail. Tool alignment is verified with test holes prior to production runs. A hollow mill can also cut counterbores around holes to recess the rivet head. Lubricants containing extreme pressure additives help tools survive aggressive riveting operations.
Inspection and Quality Control
Several inspection methods verify rivet quality on CNC machinery:
- Head height gauges check that the formed head meets minimum height specifications
- Bucking bars or ring gauges confirm sufficient tail compression and hype expansion
- Pull tests measure rivet tensile strength using hydraulic jacks
- Micrometers, calipers and optical comparators verify hole diameter and rivet shank/head dimensions
- Dye penetrant testing detects surface cracks in the rivet head
- Coordinate measuring machines (CMM) provide 3D measurements of hole and rivet geometry
- Eddy current and ultrasonic testing identify subsurface defects
Sample rivet coupons should be subjected to shear, tensile and fatigue testing per industry standards like NASM 1312 and ASME B18.2.6. Statistical process control monitors production to identify any deviation from standards.
By selecting optimized tooling, materials and processes, manufacturers can leverage CNC automation to produce large volumes of quality rivets quickly, consistently and cost-effectively. Attention to programming, inspection and quality control will ensure rivets withstand the demands of critical assemblies. CNC Milling CNC Machining