Machine screws are straight shank fasteners with a blunt end and machine threads that run the entire length of the shank. They are designed to go through a hole or machine screw nut that is pre-tapped to form a mating thread for the screw. Machine screws are often described as a small diameter bolt because of their blunt end, and some styles are called stove bolts. Machine screw heads come in binding undercut, button, fillister, flat, indented hex, indented hex washer, oval, pan, round, and truss. You can read more on each style below. Machine Screw drive styles come in hex slotted/unslotted, one way tamper proof, phillips, phillips pin head, phillips/slotted combo, six-love, slotted, spanner tamper proof, and triangular slotted. You can also read more on each of these drive styles below.
SEMS machine screws are pre-assembled fasteners consisting of the screw and washer, making them both cost-effective and efficient in most applications.
AFT Fasteners is a machine screw supplier, offering almost 5,000 different types and sizes, including metric and inch. Search our selection or contact us today for a custom quote – our experienced sales team is available to help.
Machine Screw Head Styles
A binding undercut machine screw has a rounded top surface with slightly tapered sides. The bearing surface is flat and the slotted type has an annular undercut adjacent to the shank. The binding head style is the preferred design for making a firm electrical connection.
Button head machine screws have a domed top that protrudes above the fastening surface.
Fillister (or Cheese)
Fillister machine screws have a rounded top surface, cylindrical sides, and a flat bearing surface. The greater side height is what distinguishes a fillister head from a pan head. A fillister machine screw is the preferred style for use in counterbored holes. This head style is commonly known as a cheese head style.
Flat head machine screws have a countersunk head with a flat top surface and a cone-shaped bearing surface with either an 82 degree or 100 degree head angle. Flat undercut is similar to an 82 degree flat head except that the head is undercut 70% of its normal head height. Flat head machine screws are used in applications where protrusion of the screw above the mating surface is not wanted.
Indented hex machine screws have an indented top surface, six flat sides, and a flat bearing surface. They are preferred in high-volume assembly applications where pneumatic equipment is used to drive the screw. The indented hex style can transmit higher tightening torque levels than other head styles. The heads may be slotted for driving with a screw driver or unslotted for driving with a socket or wrench.
Indented Hex Washer
Indented hex washer machine screws have an indented top surface, six flat sides, and a flat washer which protrudes beyond the sides and provides a flat bearing surface. The washer and hex head are formed together as one piece. Indented hex washer machine screws offer greater protection to the mating surface than a standard indented hex head. The heads may be slotted for driving with a screw driver or unslotted for driving with a socket or wrench.
Oval head machine screws have a countersunk head with a rounded top surface and a cone-shaped 82 degree bearing surface. The oval style is often preferred over a flat head in conical applications, or when a more decorative look is required. Oval undercut machine screws are similar to the standard oval head style except that the head is undercut 70% of its normal side height. The oval undercut style is standard in shorter lengths because it allows greater length of threads.
Slotted pan head machine screws have a flat or gently rounded top surface, cylindrical sides, and a flat bearing surface. Phillips and Torx pan head machine screws have a rounded top, cylindrical sides, and a flat bearing surface. The pan head style is considered for general purpose uses and can be substituted in most applications for round, truss, or binding heads.
Round head machine screws have a semi-elliptical top surface and a flat bearing surface. They are often preferred over the pan head because of its smooth surface and appearance.
Truss head machine screws have a low, rounded top surface with a flat bearing surface greater in area than a round-head screw of the same nominal size. They are not as strong as pan or round heads but often preferred in applications where minimal clearance exists above the head. The truss head provides a trim and finished appearance.
Machine Screw Drive Styles
Hex Slotted/ Hex Unslotted
A hex head machine screw can be tightened or loosened with a hex wrench. The slotted drive is added to make it easier to remove.
One Way Tamper Proof
The one way drive is found on security screws used in permanent applications. It can be installed with a standard slotted screwdriver but requires a special one-way removal tool.
The phillips drive is the most recommended drive type as it provides good control in driving. Be sure to always use a driver bit in good condition.
Phillips Pin Head
The phillips pin head drive features a traditional phillips drive with a tamper-resistant pin added for security. It must be installed and removed with a special removal tool.
The combo drive style accepts both phillips and standard blade screwdrivers. It’s often used in applications where the machine screw is expected to be driven and backed-out several times.
Six-Lobe (Torx Tamper Proof)
This star shaped, 6 lobe drive provides a high level of security without sacrificing accessibility. It must be removed with a special removal tool.
The slotted drive requires less downward pressure to drive slotted parts, and accepts standard blade screwdrivers. Be sure to use a proper fitting blade to minimize slippage.
Spanner Tamper Proof
This unique design is found on security screws and must be installed and removed with a special spanner tool.
This unique design is found on security screws and must be installed and removed with a special security tool.