Bolts, screws, rivets, and nuts all join parts, but they do it in different ways. Choosing the wrong fastener can create assembly delays, weak clamping, damaged threads, difficult maintenance, or unnecessary machining cost.
For CNC machined parts, fastener selection should be reviewed with material, wall thickness, access direction, thread depth, tolerance, finish, and service load. These details are best included early in CNC machining drawings rather than solved after parts are cut.
Quick Comparison
| Fastener | How it works | Typical connection | CNC design note |
|---|---|---|---|
| Bolt | External thread clamps parts, usually with a nut | Removable | Needs clearance holes, access, and suitable bearing area. |
| Screw | Threads into a tapped hole or directly into material | Removable | Needs thread engagement and enough wall thickness. |
| Rivet | Deforms during installation to lock parts together | Permanent | Useful when future disassembly is not expected. |
| Nut | Internal thread pairs with a bolt or threaded rod | Removable threaded joint | Needs wrench access and anti-loosening plan if vibration exists. |
When to Use Bolts
Bolts are useful when a strong, serviceable joint is needed. They are common in machine frames, fixtures, housings, structural brackets, and assemblies that need maintenance access. A bolt usually works with a nut, although a tapped component can sometimes replace the nut.
In CNC parts, designers should confirm head clearance, tool access, washer or bearing surface, grip length, and whether the bolt will load the part in tension, shear, or both.
When to Use Screws
Screws are often chosen when the assembly is compact or when the fastener must thread directly into the machined part. Machine screws in tapped holes are common in covers, enclosures, instrument housings, panels, and precision fixtures.
The key design issue is thread engagement. Soft materials such as aluminum or plastic may need more thread depth, inserts, or a larger diameter. MTSPi can review material choice through CNC material options before the threaded features are finalized.
When to Use Rivets
Rivets create a permanent mechanical joint. They can be useful for sheet metal, light structures, covers, and assemblies where loosening must be avoided and future disassembly is not required. Some rivets can be installed from one side, which helps when rear access is limited.
The tradeoff is repairability. Removing a rivet usually damages or changes the joint, so rivets should not be used where routine service, adjustment, or repeated inspection access is expected.
What Nuts Add to an Assembly
Nuts are not usually used alone. They complete a threaded joint with a bolt or threaded rod. Hex nuts, lock nuts, flange nuts, and prevailing-torque nuts solve different access and vibration problems.
For CNC machined assemblies, nut access should be checked in the model. A strong joint on paper can become expensive if there is no room for a socket, wrench, washer, or locking feature.
Design Checks for CNC Machined Fastener Features
| Design check | Why it matters | Practical action |
|---|---|---|
| Thread depth | Too little engagement can strip threads | Match thread length to material and load. |
| Edge distance | Holes too close to edges can crack or deform | Leave enough material around holes. |
| Counterbore and countersink | Head style affects surface fit and tool access | Call out dimensions clearly on the drawing. |
| Vibration | Threaded joints can loosen | Consider lock nuts, thread lockers, washers, or rivets. |
| Finish buildup | Coatings can affect threads and holes | Plan masking or post-finish thread checks. |
Fastener choices also affect machining cost. Extra tapped holes, deep threads, tight position tolerances, and difficult counterbores can increase cycle time. Review these details with reduce CNC machining cost if the assembly has many fasteners.
Conclusion
Use bolts for strong removable joints, screws for compact threaded fastening, rivets for permanent connections, and nuts to complete bolted assemblies. The best choice depends on load, access, service needs, material, and machining cost. Define the fastener strategy early so the CNC parts are easier to make, inspect, assemble, and maintain.
