A set screw is a headless threaded fastener that passes through a tapped hole in a hub, collar, or pulley and clamps against a shaft to prevent rotation or axial movement. The tip geometry — not the thread size — dominates whether the joint holds, marks the shaft, or slips under load.

The five standard point styles:

• Cup point — a hollow concave tip with a sharp annular edge. The default choice. The edge bites into the shaft and creates high localized contact stress. Holds well on hardened or unhardened shafts; leaves a permanent mark.
• Flat point — a flat face. Used when you need to reposition frequently without gouging the shaft, or when clamping against a hardened shaft with a machined flat.
• Cone point — a 90° or 118° pointed tip. Requires a matching drilled dimple in the shaft. Highest holding power of any style — essentially a permanent alignment feature.
• Oval point — a rounded tip. Lower holding force than cup, but designed for shafts that must slide or index axially against the screw (e.g., adjustment mechanisms).
• Dog point — an unthreaded cylindrical protrusion that seats into a hole or slot in the shaft. Used as an anti-rotation key rather than a friction clamp.

Real-world example: A timing pulley on a stepper motor shaft. A cup-point M4 set screw torqued to spec holds fine at low load — until micro-vibration lets the pulley creep, and now your printer skips layers. The fix: file a shallow flat on the shaft where the cup point lands. The screw now bears against a flat surface with a wider contact patch and, more importantly, cannot rotate the pulley around the shaft even if preload drops. For high-torque applications, drill a dimple and use a cone point, or add a second set screw at 90°.

Rule of thumb for holding torque: For a cup-point socket set screw on a mild steel shaft, holding torque is roughly T_hold ≈ 0.2 × T_seat × (D_shaft / D_screw), where T_seat is the tightening torque. For an M6 cup point at 9 N·m of seating torque on a 20 mm shaft, you get roughly 6 N·m of slip resistance — often insufficient for anything but a light pulley. Two screws at 90° roughly double this. A machined flat adds another 2–3× depending on flat depth.

Practical rules: Never use a single set screw for high torque; use two at 90° with the primary screw over a flat. Match the hex drive size to the screw — stripping a 1.5mm hex on an M3 is nearly guaranteed if you don't use a fresh key. Use thread locker (blue Loctite 243) — set screws in vibrating machines back out constantly.