Machine Screws
What Are Machine Screws?
Machine screws are straight, uniform-diameter screws made to mate with a nut or a tapped hole in metal, plastic, or wood with inserts. They are tightened with a driver or socket and are stocked in many head styles, drive types, materials, and finishes to suit production assembly, maintenance, and OEM work.
Common Types of Machine Screws
- Standard machine screws: General-purpose fasteners in pan, flat (countersunk), button, oval, truss, and round heads with slotted, Phillips, combo, Torx/6-lobe, or hex-socket drives.
- SEMS (captive-washer) screws: Pre-assembled with one or more washers under the head for faster, consistent assembly and improved bearing area.
- Tamper-resistant machine screws: Security drives (pin-in Torx, spanner, tri-wing, one-way) to deter unauthorized removal.
- Thread-cutting / thread-forming machine screws: Cut or form mating threads in thin sheet metal or plastics where tapping is impractical.
- Specialty styles: Sidewalk/hurricane-panel bolts, captive panel screws, and floorboard screws for specific applications.
Materials & Finishes
- 18-8 / 304 stainless: dependable corrosion resistance for most indoor/outdoor uses.
- 316 stainless: upgraded corrosion resistance for marine, chloride, and chemical environments.
- Carbon steel, zinc plated: economical and clean appearance for dry interiors.
- Black oxide steel: matte black appearance; light corrosion resistance—use indoors or with protective oil.
- Brass / silicon bronze: non-magnetic, conductive, and corrosion-resistant options for electrical and architectural work.
Sizes, Threads & How to Specify
Order machine screws by diameter × thread × length. Examples: #6-32 × 1″, 1/4″-20 × 2″, M4-0.7 × 20 mm. Inch series use UNC (coarse) or UNF (fine) threads; metric uses ISO pitches (M2–M12 common). Length is measured from under the head to the tip—except countersunk flat heads, which are measured overall.
Where Machine Screws Are Used
- Metal fabrication, enclosures, and sheet-metal assemblies
- Electronics, appliances, HVAC, and automotive service
- Fixtures, furniture, and architectural hardware
- Plastic parts with threaded inserts; wood parts with inserts or T-nuts
Installation Basics
- Confirm the thread size and pitch match the nut or tapped hole.
- For tapped holes, use the correct tap-drill size and full thread depth; for clearance holes, drill per the clearance chart.
- Use washers to protect surfaces or increase bearing area; consider lock washers, prevailing-torque nuts, or threadlocker where vibration is present.
- In plastics or thin sheet, use thread-forming/cutting screws or install threaded inserts for stronger joints.
- Apply appropriate torque—avoid over-tightening, which can strip threads or deform thin materials.
Related Hardware
- Finished machine screw nuts, keps (external-tooth) nuts, flange nuts, nylon-insert lock nuts
- Flat, fender, lock, and finishing washers
- Threaded inserts for plastic/wood, T-nuts, binding posts (sex bolts)
Machine Screws FAQs
See the accordion below for answers about sizing, head styles, materials, torque, and when to use SEMS, security, or thread-forming screws.
Disclaimer: This page provides general guidance. Always follow the product datasheet and applicable standards.
A uniform-diameter screw that mates with a nut or tapped hole. It’s tightened with a driver or socket and used in metal, plastic (with inserts), and prepared wood.
The terms overlap, but machine screws are typically smaller diameters and driven with a screwdriver/bit; bolts are often larger and installed with a wrench. Either may be used with a nut or tapped hole when the thread matches.
For inch sizes, #6-32; means a #6 diameter with 32 threads per inch. For metric, M4-0.7 means 4 mm diameter with 0.7 mm thread pitch.
Measure from under the head to the tip. The exception is flat (countersunk) heads—measure overall.
UNC (coarse) is most common and more forgiving in assembly. UNF (fine) offers finer adjustment and is used when the mating parts are tapped fine.
Only if the mating part isn’t tapped. If you have a tapped hole or a threaded insert, you don’t need a nut.
When you want speed and consistency. SEMS screws arrive with captive washers (flat, lock, or combo) already installed.
A machine-screw nut with a captive external-tooth lock washer. It speeds assembly and resists loosening.
Thread-cutting screws remove material to create threads in thin metal or hard plastics; thread-forming screws displace material (no chips) and are common in softer plastics. Use the style specified for your base material.
Yes, use thread-forming screws designed for plastic or install threaded inserts and use standard machine screws. Avoid over-torque to prevent cracking or creep.
Yes, with a threaded insert or T-nut for the wood side. For direct wood fastening, choose wood or sheet-metal screws instead.
Pan/button give a low, wide bearing surface; flat heads sit flush in a countersink; truss has extra-wide bearing for thin materials. Choose based on clearance and appearance.
Torx/6-lobe and hex-socket provide better torque transfer and reduced cam-out compared with Phillips. Phillips/combo are common where universal tools are needed.
18-8/304 is typically low-magnetic; some magnetism can appear from cold-working. 316 is also generally low-magnetic.
Zinc-plated is economical for dry interiors. 18-8/304 suits most outdoor/indoor use. Choose 316 for marine or aggressive environments.
Torque depends on size, material/grade, lubrication, and joint design. Use the product’s torque table or your engineering spec; in plastics and thin sheet, lower torque is usually required.
Most follow ASME/ANSI or ISO dimensional standards. Always match the thread series and pitch to the mating parts.
Common causes are mismatched thread sizes, shallow or oversize tapped holes, or over-torque. In stainless-to-stainless assemblies, use proper lubrication to reduce galling.
Often yes if the head and threads are undamaged and the joint is non-critical. Replace screws that are worn, bent, or have damaged drives.