Achieving High Quality Rivets from Your CNC Machine(zinc casting Eileen)
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Rivets are a crucial component in many manufacturing and construction applications. While rivets may seem simple, getting consistently high quality rivets from your CNC machine requires careful consideration of several factors. In this article, we'll explore the key elements that contribute to rivet quality and how you can optimize your CNC machine setup and processes to achieve the best results.
The Role and Properties of Quality Rivets
A rivet is a mechanical fastener that consists of two main parts - a threaded shank and a head. Rivets are inserted through holes in two or more layers of material and the shank is mechanically deformed to create a mushroom-shaped head. This joins the rivet securely to the materials.
Quality rivets have heads that are uniformly round and without defects. The shank is straight, smooth, and has consistent mechanical properties and geometry. This ensures the rivet can be inserted cleanly, makes maximal contact with the joined materials, and has uniform strength to withstand shear and tensile stresses during use.
For weight-critical applications like aerospace manufacturing, rivets are often made from lightweight aluminum or titanium alloys. In other cases like construction, carbon steel rivets are common. The material must be malleable enough to flow and form the rivet head during installation, while also being strong enough to maintain integrity as a fastener.
CNC Programming Considerations for Riveting
To produce high quality rivet shanks suitable for further forming, the CNC program must ensure:
- Tight tolerances on the shank diameter based on the hole size it will fit into. +/- 0.005 inches is a common tolerance.
- Precise control of shank length to match the total material thickness. Length can be adjusted by controlling cut-off position.
- Excellent surface finish to facilitate insertion into the rivet holes. Surface roughness value (Ra) of 16 to 32 microinches is typical.
- Straightness of within 0.005 inches per inch of shank length prevents binding during insertion.
- Consistent material properties through workpiece selection, like 6021 aluminum alloy.
This requires a rigid CNC machine, sharp tooling, moderate cutting feeds/speeds, and a dynamic CNC control system to closely track position for tight tolerances. Coolant is also helpful to evacuate chips and prevent built-up edge.
Forming High Quality Rivet Heads
The rivet head is formed by mechanically deforming the tip to flare it outward into a mushroom shape. This is done with a tool called an upsetting die that surrounds the tip and exerts pressure.
To achieve a nicely domed head, the CNC program must:
- Align the rivet shank precisely perpendicular to the die surface as the head is formed.
- Use a die radius approximately 1.5 times the shank diameter to form a smooth rounded head.
- Control the amount of deformation to tightly fill the die geometry without excessive material overflow.
- Retract the die at the optimal moment to avoid over-compressing the formed head.
This requires advanced programming techniques like custom macros or parametric subprograms to manage the motion, alignment, force application, and retraction. CNC machines with sufficient rigidity are also critical.
Secondary Rivet Head Processing
After the initial rivet head formation by the CNC machine, additional finishing steps may be used to ensure quality:
- The head surface can be planed, shaved, or machined to achieve an optimal smoothness and geometry if needed.
- Rolling processes can further refine the dome shape and improve thickness consistency.
- Heat treatments might also be applied to enhance material properties.
- Coatings like anodizing can protect the rivet head from corrosion.
- Automated quality inspection can ensure all dimensions, geometry, and material specs are within requirements.
These secondary processes allow the riveting system to be fine-tuned for the highest quality standards.
Optimizing CNC Riveting Workholding
Effective workholding is crucial during CNC riveting to locate each fastener hole accurately and keep parts under firm control as the rivet is inserted and headed. Typical options include:
- Precision jigs and fixtures that orient all rivet hole locations in 3D space and resist deformation as force is applied.
- Vacuum tables that pull sheets flat but avoid introducing stress that could distort holes.
- Dedicated mandrels and anvils that fit cleanly into each rivet hole and support from within.
- Customized robotic grippers that hold complex aerospace geometries.
- Programmable tailstocks, backups, and ball locks that apply targeted support forces.
The workholding strategy must align each rivet hole precisely to the machine axes while keeping surrounding material fully supported. This enables the CNC machine to execute its motions accurately and form quality heads.
Conclusion
From a programming perspective, achieving consistently high quality rivets requires control of tolerances, finish, geometry, alignment, and material properties. Physically, the CNC machine must have high precision and rigidity. Effective workholding is also essential to present each rivet hole in an optimal position. Paying attention to each of these areas will allow manufacturers to dial in their CNC riveting process for the best possible results in this critical application. The increased reliability and durability from quality rivets ultimately supports lighter, stronger, and longer-lasting end products across industries. CNC Milling CNC Machining