Looking for a reliable way to fasten components without protruding heads? The 4-40 set screw is a compact yet essential hardware piece, favored by DIY enthusiasts, hobbyists, and professionals alike. This guide will help you navigate the options, ensuring you find the ideal 4-40 set screw for mechanical, electronic, or custom projects—unlocking precision and durability in every assembly.
Comparison Table: Types and Variations of 4-40 Set Screws
| Type | Material | Drive Style | Point Style | Common Lengths | Typical Applications | Corrosion Resistance |
|---|---|---|---|---|---|---|
| Hex Socket Headless | Stainless Steel | Hex (Allen) | Cup | 1/8″, 3/16″, 1/4″, 5/16″ | Machinery, automotive, electronics | Excellent |
| Hex Socket Headless | Alloy Steel | Hex (Allen) | Cup | 1/8″, 3/16″, 1/4″, 5/16″ | Power transmission, robotics | Moderate |
| Hex Socket Headless | Titanium | Hex (Allen) | Cup | 3/16″ | Aerospace, marine, specialized | Superior |
| Slotted Headless | Stainless Steel | Slotted | Flat | 1/8″, 3/16″ | Electronics, light-duty | Excellent |
| Hex Socket Headless | Steel (Plain) | Hex (Allen) | Cup | 1/4″ | General fastening, fixtures | Low |
Understanding 4-40 Set Screws: Types, Compatibility & Safety
What is a 4-40 Set Screw?
A 4-40 set screw is a small, fully-threaded fastener, typically headless, featuring a #4 diameter with 40 threads per inch (TPI). Unlike standard screws, it sits below or flush with the part surface. These set screws are designed to secure one component inside or against another, providing a compact solution for holding power in tight or recessed spaces.
Common Types of 4-40 Set Screws
Set screws come in various configurations tailored to different applications. Let’s break down the most significant differences:
By Drive Style
- Hex (Allen) Socket: The most prevalent for 4-40 set screws, requiring a hex key (Allen wrench). Provides a firm grip and resists stripping.
- Slotted: Uses a flathead screwdriver. Classic choice but easier to strip, best for low-torque situations.
- Phillips: Cross-shaped, less common in this size but occasionally used for electronics.
By Point Style
- Cup Point: The most widely used. Features a slightly recessed tip that digs into the mating surface for solid holding power. Ideal for shaft collars, gears, or pulleys.
- Flat Point: The end is flat, providing minimal surface damage to the mating part. Suitable when you need to avoid leaving marks.
- Cone Point: Tapered tip penetrates the material for maximum holding. Works well for permanent or semi-permanent applications.
- Dog Point: Flat tip with an unthreaded section, used as a locating pin in assemblies needing precise alignment.
- Knurled Cup Point: Offers more bite and resistance in high-vibration environments.
By Material
- Stainless Steel: Best for corrosion resistance. Essential in humid, marine, or outdoor applications.
- Alloy Steel: Higher strength, suited for areas needing maximum load capacity and mechanical stability.
- Titanium: Offers the highest resistance to corrosion and the best strength-to-weight ratio, often used in demanding industries like aerospace.
- Plain Steel: Standard strength, cost-effective choice for indoor or non-corrosive settings.
Compatibility: Will a 4-40 Set Screw Fit Your Project?
Proper selection and compatibility are the keys to a successful fastener installation. Here’s what you should consider:
Diameter: The ‘4’ in 4-40 refers to the screw’s diameter (approx. 0.112 inches), while ‘40’ designates 40 threads per inch. Match this precisely with your tapped hole or fittings to ensure proper threading.
Thread Fit: Always use a compatible 4-40 threaded hole, nut, or insert. Too tight a fit risks stripping; too loose a fit undermines holding power.
Material Match: Use stainless steel for outdoor jobs or contact with moisture. For maximum mechanical strength—such as with heavy gears—consider alloy steel. For extremely lightweight or anti-magnetic needs, titanium is best.
Component Material: Softer mating materials (like aluminum) pair best with flat or cup points. For harder surfaces, cone or dog points may be more secure.
Length: Choose a set screw just long enough to fully engage the threads without protruding excessively. Common lengths—including 1/8″, 3/16″, 1/4″, and 5/16″—suit most applications.
Drive Tool: Confirm you have the appropriate driver (Allen key, slotted, etc.) for installation and future servicing accessibility.
Safety Tips for DIY and Household Repair
- Tool Safety: Always use the correct size driver to prevent cam-out and stripping, which can make removal difficult and damage your fastener or component.
- Protective Gear: For power tools or working with metal filings, wear safety goggles and gloves.
- Prevent Over-Tightening: Set screws are typically designed for moderate torque. Over-tightening can strip the threads or distort the components.
- Thread Locker Use: In vibrating environments, apply a medium-strength thread locker to prevent loosening over time.
- Flush Installation: When possible, ensure the set screw sits slightly below or flush with the surface to avoid catching on moving parts or presenting a snag hazard.
- Periodic Checks: Inspect set screws periodically, especially in assemblies subject to vibration or movement.
Practical Tips & Best Practices for Choosing and Using a 4-40 Set Screw
- Measure Twice: Confirm thread size, pitch, and hole depth before choosing your screw length and type.
- Match the Application: For rotary applications (like securing pulleys or collars), cup or knurled cup points grip best; for delicate or decorative housings, flat points reduce marking.
- Corrosion Check: Use stainless steel or titanium set screws for outdoor, damp, or marine environments.
- Avoid Mixing Materials: To prevent galvanic corrosion, use compatible materials between the set screw and mating part (e.g., stainless with stainless, steel with steel).
- Use Proper Insertion Technique: Start threading by hand to avoid cross-threading, then finish with the correct tool. Never force a set screw—if resistance is high, back out and realign.
- Apply Lubricant: For high-torque installations or when working with stainless steel, use a dab of thread lubricant to reduce galling and simplify removal.
- Inventory Smartly: For frequent use, buy set screws in bulk for cost efficiency. Pay attention to required purchase multiples (some suppliers sell in batches of 12, 20, or 25).
- Test Fit: In precision settings, fit the screw and test component security before final assembly—especially with moving parts.
- Documentation: Label spare parts and keep documentation of screw size, type, and placement for easier repairs in the future.
- Replacement: If a set screw shows rounding or wear in the drive socket after removal, replace it to avoid problems during future maintenance.
Specification Table: Technical Features of Common 4-40 Set Screws
| Feature | Example 1 | Example 2 | Example 3 |
|---|---|---|---|
| Thread Size | #4-40 | #4-40 | #4-40 |
| Overall Length | 1/8 inch | 1/4 inch | 3/16 inch |
| Point Style | Cup | Cup | Cup |
| Drive Type | Hex Socket (Allen) | Hex Socket (Allen) | Hex Socket (Allen) |
| Head Type | Headless | Headless | Headless |
| Material | Stainless Steel (18-8) | Alloy Steel | Titanium (Grade 5) |
| Corrosion Resistance | Excellent | Moderate | Superior |
| Cost per Unit | ~$0.36 (bulk) | ~$0.19 (bulk) | ~$12.15 (premium/specialty) |
| Package Quantity | 20 | 25 | 1 |
| Intended Use | Electronics, general machinery | Fixtures, general fabrication | Aerospace, marine |
Related Video
Conclusion
Choosing the right 4-40 set screw means understanding your application’s size, strength, and durability requirements. Key decisions include selecting the appropriate material, length, point style, and drive type to ensure a flush, secure, and long-lasting result. Always factor in compatibility, environmental factors like corrosion, and the need for easy installation or removal.
With careful selection and proper installation, 4-40 set screws are a reliable solution for everything from precision electronics to robust mechanical assemblies. Following best practices ensures both safety and long-term success in your projects.
FAQ
-
What does the “4-40” designation mean in a set screw?
The “4” stands for the screw’s diameter (approximately 0.112 inches), and “40” means it has 40 threads per inch. This is a standard imperial thread size. -
Do 4-40 set screws always have a hex (Allen) drive?
No. While hex (Allen) is the most common drive type, 4-40 set screws can also come in slotted or Phillips drives, depending on application and manufacturer. -
How do I know what length set screw to use?
Select a length that fully engages the tapped hole’s threads without protruding excessively. Common lengths are 1/8″, 3/16″, 1/4″, and 5/16″. -
Is stainless steel always the best material choice?
Stainless steel offers great corrosion resistance and is ideal for most environments. However, alloy steel is stronger for high-stress uses, and titanium provides specialized benefits such as extreme corrosion resistance and light weight. -
Can I use a 4-40 set screw to fasten anything other than metal?
Yes; they can secure plastic or composite parts, but you should match the point style to avoid damage. Flat points are best for softer or delicate materials. -
What point style is best for holding power?
Cup and cone points offer the best grip in rotary or high-vibration applications. Cup points are versatile, while cone points are for permanent or semi-permanent installations. -
How do I prevent a 4-40 set screw from loosening over time?
Apply a medium-strength thread locker (such as Loctite) during installation, especially in applications subject to vibration or repeated movement. -
What tools do I need to install or remove a 4-40 set screw?
Use the matching driver for your set screw’s drive type—most often a 0.05” Allen key for hex socket screws. Always use the correct tool size to avoid stripping. -
Can I reuse a 4-40 set screw?
If the threads and drive socket are in good condition, yes. However, replace any set screws that show rounding, distortion, or damage. -
Are 4-40 set screws available in bulk quantities?
Yes, many suppliers offer discounts for bulk purchases in quantities such as 12, 20, 25, or more per package, making them more cost-effective for frequent users.