A set screw secures one object inside or against another by pressing directly into a surface, using friction and compression to prevent movement. Unlike regular screws that clamp two pieces together through a hole, set screws work by tightening into a threaded hole until their tip pushes against a mating part, locking it in place. You’ll find them anywhere a component needs to stay fixed on a shaft, adjusted to a precise position, or held without a visible bolt head.
How a Set Screw Works
A set screw is essentially a fully threaded bar with no traditional head. It threads completely into a tapped hole so that nothing protrudes above the surface. When tightened, the tip of the screw presses against a contact surface, creating enough friction to resist movement. This makes set screws ideal for situations where a protruding bolt head would interfere with nearby parts or where a clean, flush appearance matters.
The most common example is locking a pulley, gear, or knob onto a rotating shaft. The set screw threads through the hub of the component and presses against the shaft, holding everything in place. Shaft collars, sprockets, and handles all use this same principle. In electronics and furniture, small set screws (sometimes called grub screws) secure knobs, decorative hardware, or connectors without any visible fastener on the outside.
Point Types and When to Use Each
The tip of a set screw determines how well it grips and whether it marks the contact surface. Choosing the right point type matters more than most people expect.
- Cup point: The most common type. A thin-edged cup digs slightly into the contact surface for a strong hold across a wide range of materials. Good general-purpose choice.
- Flat point: Makes smooth, even contact on flat surfaces without leaving a mark. Useful when you need to remove and reposition the screw without damaging the part underneath.
- Cone point: A pointed tip that wedges into softer materials like brass or aluminum. Provides a very secure hold but leaves a permanent indentation.
- Dog point (extended): Features a protruding cylindrical tip that fits into a pre-drilled hole or slot in the mating part. This physically locks the two pieces together rather than relying on friction alone, making it the most secure option for alignment-critical applications.
- Knurled cup point: Serrations on the cup edge grip the surface more aggressively than a standard cup. Helpful in high-vibration environments where extra holding power is needed.
How Set Screws Differ From Regular Screws
Regular screws have a visible head, either slotted or cross-shaped, designed to be driven with a standard screwdriver. They pass through one part and thread into another, clamping the pieces together. Set screws work differently. They sit entirely below the surface and apply force in one direction, pressing a tip against a mating part rather than pulling two pieces together.
Because they lack a protruding head, most set screws use an internal hex socket driven by an Allen key. This recessed design provides a non-slip, deformation-resistant connection that lets you apply firm torque even in tight spaces. Some set screws use a Bristol spline drive, which features small internal teeth for precise torque control in delicate assemblies. External hex heads do exist on certain set screws, giving them a bolt-like appearance, but the headless internal-drive version is far more common.
Materials and Where They Matter
Most set screws are alloy steel, which provides high strength and resistance to deformation under clamping force. For environments involving moisture, chemicals, or salt exposure, stainless steel is the better choice. Standard 18-8 stainless steel handles most corrosive conditions, while 316 stainless steel offers stronger protection against chemicals and saltwater.
Plastic set screws fill a niche role. They’re lightweight, chemically resistant, and nonconductive, making them useful in electrical assemblies or applications where metal contact could cause damage. One trade-off: plastic can expand slightly when exposed to moisture, which may affect fit over time.
Keeping Set Screws From Loosening
Vibration is the main enemy of any set screw. Over time, repeated vibration can gradually back a set screw out of its hole, especially in rotating equipment. Several strategies help prevent this.
Milling a small flat spot on the shaft gives the screw tip a defined seat, preventing it from sliding around a curved surface. This alone significantly improves holding power. Thread-locking adhesive applied to the threads before installation adds chemical resistance to loosening, though it makes future removal harder and can reduce the clamping force you achieve at a given torque. For applications that need periodic disassembly, a second set screw (called a jam screw) threaded in behind the first creates a mechanical lock between the two, similar to how jam nuts work on bolts.
Dog-point set screws paired with a matching hole in the shaft offer the most vibration-resistant connection, since the interlocking geometry physically prevents movement rather than relying on friction.
How to Remove a Stuck or Stripped Set Screw
Stripped set screws are a common frustration, especially with smaller sizes where the hex socket is easy to round out. If the damage is mild, place a rubber band or a piece of steel wool over the stripped socket and press your Allen key firmly into it. The added friction often provides enough grip to turn the screw. This low-tech fix works surprisingly well for light damage.
For more serious stripping, a screw extractor kit is the most reliable tool. One end of the extractor drills into the damaged socket, and then the bit is flipped so its reverse threading backs the screw out as you turn it. Extractor sets designed for micro screws can handle fasteners as small as 2 to 3 millimeters in diameter.
When a set screw is completely destroyed or seized from corrosion, drilling it out may be the only option. Use a drill bit slightly smaller than the screw shaft and work carefully through the center. Applying penetrating oil beforehand helps loosen corrosion, and gentle heat from a soldering iron can expand the surrounding metal enough to free a stubborn screw. A left-handed drill bit sometimes catches the screw threads and backs it out during drilling, saving you an extra step. If the socket is stripped but there’s still material on top, cutting a new slot with a rotary tool and using a flathead screwdriver is another effective approach.