Set screw

A set screw is a headless, fully threaded screw tightened with an internal drive (often a hex or Allen key) to lock one part to another—commonly securing a gear, pulley, collar, or knob to a shaft—by pressing its tip against the mating surface to stop rotation or movement without leaving a protruding fastener.

Set Screw

A set screw is a small but mighty fastener designed to lock two parts together without a protruding head. Unlike standard screws, set screws are fully threaded and usually sit flush or below the surface. You tighten them with an internal drive (often a hex/Allen), causing the tip to press against a shaft or component and hold it in place.

How a Set Screw Works

Set screws rely on friction and tip geometry. As you tighten the screw, the point pushes against the mating part—commonly a round shaft inside a gear, pulley, collar, or knob. The contact force resists rotation and axial (in-and-out) movement. Depending on the tip style, the screw may simply press and hold, slightly bite into the surface, or locate in a drilled dimple.

Because there is no head, the screw can be hidden in a counterbore or within a hub, leaving a clean look and preventing snags. This makes set screws common in hardware that needs a smooth exterior, like doorknobs, faucet handles, towel bar brackets, and appliance knobs.

Common DIY Uses and Applications

  • Securing knobs and levers on square or round shafts (doorknobs, faucet handles, appliance controls)
  • Locking collars on shafts for lawn equipment, garage door openers, and small machinery
  • Holding pulleys, gears, and sprockets on motor shafts in tools, fans, and HVAC blower wheels
  • Fixing loose towel bar brackets or robe hooks that mount to a hidden wall bracket
  • Electronics and hobby projects: 3D printer pulleys, RC models, and robotics assemblies
  • Electrical fittings: some ground lugs and connectors use set screws to clamp wire (different form factor, same principle)

Types and Variations

Drive styles

  • Hex socket (Allen): Most common; use the correct metric or SAE hex key.
  • Slotted: Simple, but easier to strip; often seen on small decorative hardware.
  • Torx socket: Better torque transfer, less cam-out.
  • Square or external hex head: Found on older or heavy-duty collars; not truly headless but used for similar locking duties.

Point styles (tip shapes)

  • Cup point: A small concave end bites slightly into the shaft. Good general-purpose hold for metal shafts.
  • Flat point: Flat end presses without digging in. Better for soft materials or where you want less surface damage.
  • Cone point: A sharp tip that penetrates for a very secure hold. Best for permanent or high-torque applications.
  • Knurled cup: A serrated version of cup point for improved grip under vibration.
  • Dog point: A cylindrical tip that fits into a mating hole or slot for precise location and repeatable assembly.
  • Half-dog point: Shorter dog for shallow holes.
  • Oval or nylon tip: Reduces marring on delicate surfaces; nylon tip adds vibration resistance.

Materials and finishes

  • Alloy steel (black oxide): Strong and economical; use indoors or in dry areas.
  • Stainless steel (18-8, 316): Corrosion resistant for bathrooms, kitchens, and outdoor use.
  • Brass: Softer, non-sparking, and kinder to mating parts; common in decorative hardware.
  • Nylon-tipped variants: Protect finishes and add friction.

Choosing the Right Set Screw

  • Match threads exactly: Identify metric vs inch (UNC/UNF). Bring the old screw to the hardware store or measure with a thread gauge. Common DIY sizes include M3–M6 and #4–#10.
  • Length matters: The screw should fully engage the threaded hole but not stick out. Too short can strip, too long can bottom out or block parts.
  • Pick the right point:
    • Cup point for most metal-to-metal shaft applications
    • Flat or nylon tip on finished or soft surfaces
    • Cone point for high load or where a dimple exists
    • Dog point if there is a matching hole or keyway feature
  • Choose material for the environment: Stainless for wet areas, alloy steel for dry interiors, brass for delicate trim. Use anti-seize on stainless to reduce galling.
  • Plan for vibration: Add medium-strength threadlocker (blue) or use a second set screw or a jam screw on top to lock it.
  • Tool fit: Use the correct hex key size (metric vs SAE) to avoid rounding the socket. A snug fit is critical.

Tips for Use and Maintenance

  • Prep the shaft: If possible, file or grind a small flat on the round shaft where the screw lands. This increases resistance to rotation and prevents slipping.
  • Use proper torque: Tighten firmly but do not overdo it. Refer to torque charts for the screw size and material. Over-torque strips sockets and threads.
  • Stage tightening: For hubs with two set screws, snug both, then alternate tightening to share load. For collars, position one at 90 degrees to the other if possible.
  • Threadlocker wisely: Clean threads with alcohol, apply a drop of medium-strength threadlocker, and allow cure time. Use low-strength (purple) on tiny screws you may adjust often.
  • Check after run-in: Recheck tightness after the first few hours of operation, especially on moving machinery.
  • Prevent corrosion: In bathrooms or outdoors, pick stainless or brass. Isolate dissimilar metals or use a protective paste to reduce galvanic corrosion.

Common Mistakes to Avoid

  • Wrong key size: Using an almost-fit hex key rounds the socket. Always match metric with metric and inch with inch.
  • Over-tightening: Stripped sockets or broken screws are hard to remove. Use a quality short-arm hex key for feel, not a long cheater bar.
  • No shaft flat: On smooth, hardened shafts, a cup point may skate. Add a flat or a small drilled dimple for reliable grip.
  • Using the wrong point: Cone points can gouge and make future adjustments difficult; flat points may slip on high-torque hubs. Choose based on the load and material.
  • Skipping threadlocker: Vibration can back a set screw out. A drop of threadlocker saves future headaches.
  • Mixing metals carelessly: Stainless screw in aluminum can seize. Use anti-seize and avoid moisture traps.

Related Terms

  • Grub screw: Another name for a headless set screw, common in the UK.
  • Shaft collar: A ring that clamps or locks onto a shaft; many use set screws.
  • Key and keyway: A square key in a slot that transmits torque; often used with or instead of set screws.
  • Allen wrench (hex key): The L-shaped tool used to drive most set screws.
  • Threadlocker: Adhesive that resists loosening from vibration.

Practical Examples

  • Fix a loose doorknob: Many knobs have a tiny set screw on the neck. Align the knob, insert the correct hex key, and snug the screw. If it loosens again, add a drop of threadlocker.
  • Secure a towel bar: The decorative post slides over a hidden bracket. A small slotted or hex set screw beneath the post clamps it tight. Tighten just enough to avoid denting the bracket.
  • Stop a fan pulley from slipping: Shut off power, loosen the pulley, file a flat on the motor shaft, align the pulley, and tighten the cup-point set screw with threadlocker. Recheck after a test run.
  • Restore a 3D printer drive gear: If the extruder gear is slipping, rotate the gear so the set screw lands on a flat, then tighten firmly with the correct hex key.
  • Add a locator feature: For repeatable position, drill a shallow dimple in the shaft and use a cone or dog-point set screw so the hub returns to the exact same spot after service.

By choosing the right point style and material, preparing the mating surface, and tightening with the correct tool and threadlocker, a set screw can provide a reliable, low-profile hold in countless household and workshop projects.

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