A set screw is a headless, fully threaded fastener that locks one part to another by pressing directly against a surface, usually to secure a pulley, gear, collar, or knob onto a shaft. You tighten it into a threaded hole until its tip digs into or presses against the mating part, and the friction at that contact point prevents the two pieces from moving relative to each other. The process is straightforward once you understand how to choose the right point style, prepare the shaft, and tighten correctly.
How a Set Screw Works
Unlike a standard bolt that clamps two parts together by pulling them with a nut, a set screw works by pushing. It threads into a tapped hole in one component (like a collar or hub) and its exposed tip presses against the surface of a second component (like a shaft). That pressure creates friction between the screw’s tip and the shaft, which is the only thing resisting movement. This means the holding power of a set screw depends entirely on how much force the tip applies and how well it grips the mating surface.
Because the grip relies on a single small contact point, set screws are best suited for light to moderate loads. They’re common in door hardware, machinery couplings, motor pulleys, and anywhere a component needs to be positioned and locked on a shaft without drilling all the way through it.
Choosing the Right Point Style
Set screws come with different tip shapes, and each one is designed for a specific situation. Picking the wrong point style is one of the most common reasons a set screw fails to hold.
- Cup point: The most common type. The tip is a small hollow cup with a sharp edge that bites into the shaft surface. Best for permanent or semi-permanent installations where you don’t plan to remove and reinstall frequently. The sharp rim digs a small ring into the shaft, which adds mechanical grip beyond just friction.
- Cone point: Tapers to a sharp point and is designed to seat into a matching dimple or hole drilled in the shaft. This style resists vibration better than any other, even in poorly tapped holes, and can contact surfaces at an angle. Ideal when vibration loosening is a concern.
- Flat point: Has a smooth, flat tip. Use this when you need to tighten against a ground flat on the shaft without marring the surface, or when the set screw backs a soft metal or plastic plug.
- Dog point: Features a cylindrical extension that fits into a slot or groove machined into the shaft. This provides positive mechanical engagement rather than relying on friction alone, making it a good choice for alignment and for loads that exceed what friction can handle.
- Knurled cup: Similar to a cup point, but the inside of the cup has a textured, knurled pattern. It grips well but can be readjusted without damaging the shaft surface as badly as a standard cup point. Good for applications where you’ll need to reposition the part.
Preparing the Shaft
Tightening a set screw against a round, polished shaft is the weakest possible setup. The small contact area between a curved tip and a curved surface limits how much friction you can generate. If the part you’re securing will experience any real torque or axial load, you should mill or file a small flat spot on the shaft where the set screw will contact it.
A flat gives the screw tip a larger, more stable surface to press against. For cup point screws, the sharp rim can bite into the flat and create a slight indent that resists rotation. For cone point screws, drilling a shallow dimple into the flat gives the tip a pocket to seat into, dramatically increasing holding strength. Dog point screws need a groove or slot machined into the shaft to function at all. Even a quick pass with a file to create a small flat makes a noticeable difference in how securely the part holds.
Tools You’ll Need
Most set screws use a hex socket drive, which is a hexagonal recess in the top of the screw. You’ll need an Allen wrench (hex key) that matches the socket size exactly. Using a slightly undersized key will round out the socket, making future adjustments difficult or impossible. L-shaped Allen wrenches work fine for most jobs, but a T-handle version gives you more control and torque for larger sizes.
Some set screws use a Torx drive instead, which has a six-pointed star pattern. Torx drives transfer torque more efficiently and are less likely to cam out (slip) under heavy tightening, but they require Torx-specific drivers that aren’t as commonly found in basic tool kits. If you’re buying set screws for a new project, hex socket is the safer default unless you already own Torx tools. Slotted set screws, driven with a flat-head screwdriver, also exist but are less common and harder to torque precisely.
Step-by-Step Installation
Start by sliding the component (collar, pulley, gear, knob) onto the shaft and positioning it exactly where you want it. Make sure the threaded hole in the component lines up with the flat or dimple on the shaft if you’ve prepared one.
Insert the set screw into the threaded hole by hand, turning it clockwise until it’s finger-tight. Then use your hex key to continue tightening. You’ll feel the resistance increase as the tip contacts the shaft surface. From that point, tighten firmly, but don’t overtorque. Overtightening can strip the threads in the host component (especially if it’s aluminum or another soft metal), deform the screw tip, or crack the part. For most applications, snug plus a quarter turn past contact is a reasonable starting point.
After tightening, try to move the secured component by hand. Twist it, push it, pull it along the shaft. If it shifts, you may need a longer set screw, a different point style, a flat on the shaft, or a second set screw offset 90 or 120 degrees from the first. Using two set screws is common practice on shafts that carry meaningful loads.
Preventing Loosening From Vibration
Vibration is the number one enemy of set screws. Because they rely on friction, any repeated shaking or cycling can gradually back them out. There are a few ways to address this.
Applying a threadlocking compound before installation is the most reliable method. For set screws you’ll need to remove later for maintenance, use a low-strength (purple) or medium-strength (blue) threadlocker. Both can be broken free with standard hand tools when you need to make adjustments. Low-strength formulas work well for small threads up to about M12 (roughly 1/2 inch), while medium-strength is better for threads up to M18. High-strength (red) threadlockers require heat to remove and are overkill for most set screw applications.
To apply threadlocker, put a small drop on the threads of the set screw before inserting it. The liquid will cure between the threads and the tapped hole, creating a bond that resists vibration loosening while still allowing removal. Cone point set screws seated into a drilled dimple also resist vibration well on their own, since the mechanical interlock supplements friction.
Choosing the Right Material
Set screws are typically available in alloy steel, carbon steel, and stainless steel. Alloy and carbon steel screws are harder and stronger, which means their tips bite into shaft surfaces more effectively and hold higher loads. The tradeoff is that they corrode if exposed to moisture. Stainless steel resists corrosion well and looks better, but it’s softer and weaker. A stainless set screw pressing against a stainless shaft can also gall, meaning the two surfaces weld together at a microscopic level during tightening, seizing the screw in place.
For indoor machinery, alloy steel is the standard choice. For outdoor or wet environments, stainless steel with a thread lubricant to prevent galling is the better option. If you’re working with a soft shaft material like aluminum or brass, nearly any set screw material will bite in easily, but you’ll want to be careful not to overtighten and damage the shaft.
How to Remove a Stuck or Stripped Set Screw
If the hex socket has rounded out and your Allen key just spins, you have a few options. Start by spraying penetrating oil into and around the screw and letting it soak for 10 to 15 minutes. If the screw still won’t budge, apply localized heat with a soldering iron to expand the metal around the screw, which can break the grip of corrosion or old threadlocker.
For more severely damaged screws, a screw extractor kit is the most reliable solution. You drill a small hole into the center of the stuck screw using a left-handed drill bit (which sometimes backs the screw out on its own during drilling). Then you insert the extractor bit, which has aggressive reverse threads. As you turn it counterclockwise, it bites into the drilled hole and unscrews the set screw. These kits are inexpensive and available at any hardware store.
If the set screw is small and you can’t drill into it, try tapping a slightly larger Torx bit into the rounded hex socket with a hammer. The star points can sometimes catch enough material to get the screw turning. As a last resort, you can drill the screw out entirely and retap the hole to the next size up.
Getting the Size Right
Set screws are measured by thread diameter, thread pitch, and length. Metric sizes list the pitch as the distance between threads in millimeters (for example, M6 x 1.0 means a 6mm diameter screw with 1mm between threads). Imperial sizes list pitch as threads per inch, using either UNC (coarse) or UNF (fine) standards.
The screw diameter needs to match the tapped hole in your component. Length matters because the screw needs to be long enough to extend through the wall of the component and press firmly against the shaft, but not so long that it bottoms out in the threads before making contact. If you’re replacing a set screw, bring the old one to the hardware store or measure both the diameter and pitch carefully. A thread pitch gauge (sometimes called a thread detective) lets you quickly match both metric and imperial thread patterns and takes the guesswork out of the process.