Machine Screws, Cap Screws and Set Screws: Head Type #6

Head Type #6: Slotted Binding Head

The Wiring Specialist

The slotted binding head has a unique profile — a low oval crown with an undercut recess beneath the head rim. That undercut is the key feature: it creates a channel that captures and secures wire leads, cable lugs, or bus bars under the screw head.

Use it when:

• You are making electrical connections where wire must be trapped under the screw head
• Terminal blocks, junction boxes, or switchgear assemblies require binding-post functionality
• The oval head provides a large bearing surface for clamping flat connectors

ANSI Dimensional Data — Slotted Binding Head (Inch Series)

Per ANSI B18.6.3-1972 (R1991). All dimensions in inches.

Note: Unless otherwise specified, slotted binding head machine screws are not furnished with the undercut. You must explicitly call for the undercut on drawings and purchase orders.

Head Type #7: Slotted Round Head — The Legacy Standard (Not Recommended)

Still on Your Drawings? Here's Why to Change

The slotted round head features a semi-elliptical top surface and a flat bearing surface. It was the original general-purpose above-surface machine screw head — and it has been formally superseded.

ANSI B18.6.3-1972 states explicitly: "Not recommended, use Pan Head machine screws."

The round head appears in the Appendix of the standard, not the main body. It is maintained solely for legacy support of existing designs.

Why it was replaced:

• The pan head offers superior slot driving characteristics — less tendency for the driver to slip
• Cross-recessed pan heads and round heads overlap in dimensions, making substitution straightforward
• The pan head's flat top provides better reference surface for automated assembly

Dimensional Data — Slotted Round Head (Inch Series, Appendix)

Per ANSI B18.6.3-1972 (R1991) Appendix. All dimensions in inches.

Head Type #8: Machine Screw Cross Recesses — The Four Types You Must Know

Beyond "Phillips": A Precision Drive System

When the practitioner's production line was stripping Phillips heads, part of the problem was that he didn't understand there are four distinct cross recess types specified by ANSI for machine screws. Each has different geometry, torque characteristics, and cam-out resistance.

The four cross recess types per ANSI B18.6.3-1972:

Dimensions for recess diameter (M), width (N), and depth (T) together with recess penetration gaging depths are given in the full ANSI standard. The critical takeaway for design engineers:

Type III (square recess) offers the highest torque transfer and zero cam-out. If your assembly can use Robertson drivers, this is always the superior choice for production efficiency.

Cross Recesses Can Replace Slots on ANY Head Type

All slotted head types covered in this guide — flat countersunk, oval countersunk, pan, fillister, truss, binding, and round — are available with cross recesses in lieu of slots. The dimensional standards ensure interchangeability of the head envelope regardless of drive type.

Head Type #9: Square Recessed Flat Countersunk Head (Metric)

The Flush-Mount Champion for High-Torque Assembly

The square-recessed (Type III / Robertson) flat countersunk head combines the flush-mounting capability of a countersunk profile with the superior torque transfer of a square recess. This is the optimal choice for automated assembly lines that demand consistent torque with zero cam-out.

ANSI Metric Dimensional Data — Flat Countersunk Head (Style A and Style B)

Per ANSI/ASME B18.6.7M-1985. All dimensions in millimeters.

Style A is standard for all screws except heat-treated high-strength variants. Style B adds a mandatory underhead shoulder and is required for all recessed head heat-treated screws of property class 9.8 or higher.

Style B Shoulder Dimensions (for High-Strength Screws)

Head Type #10: Oval Countersunk Head (Metric)

The Decorative Flush-Mount with a Raised Crown

The metric oval countersunk head mirrors its inch-series counterpart: a conical bearing surface combined with a rounded raised crown. It provides the centering advantage of a countersink with a finished decorative appearance.

ANSI Metric Dimensional Data — Oval Countersunk Head

Per ANSI/ASME B18.6.7M-1985. All dimensions in millimeters.

Head Type #11: Square Recessed Pan Head (Metric)

Maximum Drive Efficiency in the Most Popular Profile

The square-recessed pan head combines the universal utility of the pan head profile with the torque-transfer superiority of the Type III (Robertson) square recess. This is the metric equivalent of the most efficient general-purpose machine screw head available.

The dimensional envelope matches the standard metric pan head (see Table in Pan Head section above), with the square recess replacing the slot or cross recess.

Head Type #12: Header Points

What Your Machine Screws Look Like Before Threading

Header points define the screw blank geometry before threads are rolled or cut. Understanding header points matters because they affect insertion into pilot holes, thread starting reliability, and overall fastener quality.

Machine screws normally have plain sheared ends. However, when specified, header points can be furnished with controlled geometry.

Key rules for header points:

• Edges of the point may be rounded
• End of point need not be flat nor perpendicular to the shank axis
• Header points apply only to nominal lengths at or shorter than the specified maximum (longer screws may require machining)

Header Point Dimensions — Metric Machine Screws

Per ANSI/ASME B18.6.7M-1985. All dimensions in millimeters.

Head Type #13: Hex and Hex Flange Head — Wrench-Driven Power

When a Screwdriver Is Not Enough

The hex head and hex flange head machine screws represent a fundamental shift in drive philosophy: from driver-in-recess to wrench-on-flats. This delivers dramatically higher torque capacity and is the standard choice for structural, high-vibration, and automated assembly applications.

Hex Head

The hex head has a flat or indented top surface, six flat sides, and a flat bearing surface. Heads may be indented, trimmed, or fully upset at the manufacturer's option.

Critical design note on slotted hex heads: ANSI warns that slotted hex heads require a secondary operation that often produces burrs at the slot extremity, interfering with socket wrench engagement. Since the wrenching capability of the hex far exceeds that of the slot, slotted hex heads are not recommended for new designs.

Hex Flange Head

The hex flange head adds an integral washer-like flange that projects beyond the hex flats. This flange provides a larger bearing surface and eliminates the need for a separate washer in most applications.

Flange characteristics:

• Top surface may be straight or slightly rounded (convex) upward
• Contour at the flange periphery is optional provided minimum flange thickness is maintained
• The flange distributes clamping force, reducing bearing stress on soft materials

ANSI Metric Hex Head Machine Screw Dimensions

Per ANSI/ASME B18.6.7M-1985. All dimensions in millimeters.

Note on M10: The M10 size with 15 mm width across flats is not ISO Standard. Unless M10 screws with 15 mm width are specifically ordered, M10 with 16 mm width across flats shall be furnished.

ANSI Metric Hex Flange Head Machine Screw Dimensions

Per ANSI/ASME B18.6.7M-1985. All dimensions in millimeters.

ANSI Inch-Series Hex Head Machine Screw Dimensions

Per ANSI B18.6.3-1972 (R1991). All dimensions in inches.

Plain and Slotted Hexagon Head (Regular and Large Head)

Hex Washer Head Machine Screw Dimensions (Inch)

Per ANSI B18.6.3-1972 (R1991). All dimensions in inches.

Design Warning: Slotted hex washer heads require a secondary slotting operation that often produces burrs interfering with socket wrench engagement. Since the hex wrenching capability far exceeds the slot, slotted hex washer heads are not recommended for new designs.

Head Type #14: British Standard Countersunk Head (Metric, BS 4183)

The International Standard for Flush Mounting

British Standard BS 4183:1967 defines metric machine screws with a 90° countersunk head — a 2° wider angle tolerance than ANSI (90° +2°). This standard covers sizes from M1 through M20.

Key Dimensional Relationships (BS 4183)

The British Standard uses elegant proportional relationships:

• Head Diameter (max, theoretical sharp): $D = 2d$ (where $d$ = nominal screw diameter)
• Head Diameter (min): $D_{min} = 1.75d$
• Head Height (max): $k = 0.5d$
• Head Height (min): $k_{min} = 0.45d$
• Slot Width (max): $n = 0.3d$
• Slot Width (min): $n_{min} = 0.2d$

BS 4183 Slotted Countersunk Head Dimensions

All dimensions in millimeters.

Flushness Requirement

BS 4183 explicitly states that countersunk head screws should fit into the countersunk hole with as great a degree of flushness as possible. The maximum head size is controlled by a theoretical sharp corner diameter and the minimum head angle of 90°. The minimum head size is controlled by the minimum head diameter, a maximum head angle of 92°, and a flushness tolerance. The edge of the head may be flat or rounded, but not sharp-edged.

Head Type #15: British Standard Raised Countersunk Head (Metric, BS 4183)

The raised countersunk head adds a domed crown above the countersink, identical in function to the ANSI oval countersunk. Dimensional proportions follow the same base as the countersunk head with the addition of the raised crown height (f) and crown radius (R).

Head Type #16: Slotted Pan Head (Metric, BS 4183)

BS 4183 Pan Head Dimensions

All dimensions in millimeters.

Head Type #17: Slotted Cheese Head (Metric, BS 4183)

The Cylindrical Workhorse of British Engineering

The cheese head is a distinctly British head type — a flat-topped cylinder with straight sides and a flat bearing surface. It provides good slot depth relative to head height, with a clean, industrial appearance.

The cheese head is covered in BS 4183:1967 for sizes M1 through M10 in metric series, and in BS 450:1958 for BSW/BSF thread forms.

Cheese Head vs. Fillister Head

The cheese head and fillister head serve similar niches — both are cylindrical above-surface heads with good slot depth. The primary differences:

Machine Screw Nuts: The Other Half of the Joint

No machine screw guide is complete without covering the mating nut. Machine screw nuts are available in both square and hexagon configurations.

ANSI Square and Hexagon Machine Screw Nuts

Per ANSI B18.6.3-1972 (R1991). All dimensions in inches.

Key specifications:

• Hexagon nuts have tops flat and chamfered; diameter of top circle should equal max width across flats within –15% tolerance
• Bottoms are flat but may be chamfered if specified
• Square nuts have tops and bottoms flat without chamfer
• Thread class: UNC or UNF, Class 2B (hex); UNC Class 2B (square)