Machine Screws, Cap Screws and Set Screws — Part 4

Set Screws — Where the practitioner's Journey Really Begins

After the pulley incident, the practitioner became obsessed with set screws. He studied every standard, memorized the point styles, and learned to calculate holding power before specifying a fastener. Here is everything he learned — and everything you need to know.

The Six Point Styles

Every set screw terminates in one of six point configurations. The point style determines how the screw engages the shaft and directly affects holding power, removability, and shaft damage.

FLAT POINT        CUP POINT         CONE POINT
  ┌──┐              ┌──┐              ┌──┐
  │  │              │  │              │  │
  │  │              │╲╱│              │ /│
  └──┘              └──┘              │/ │
 ════                ⌣                └──┘
                                       ▽

DOG POINT         HALF DOG POINT    OVAL POINT
  ┌──┐              ┌──┐              ┌──┐
  │  │              │  │              │  │
  │  │              │  │              │  │
  ├──┤              ├──┤              └──┘
  │  │              │  │                ⌢
  └──┘              └──┘

• Cup Point — The default. A hardened, knurled ring bites into the shaft for maximum holding power. Best for permanent or semi-permanent installations. Damages the shaft.
• Flat Point — A smooth, flat end. Least shaft damage. Best where frequent adjustment is needed. Lowest holding power.
• Cone Point — A pointed tip that seats into a pre-drilled dimple or center-drilled hole in the shaft. Highest holding power when properly aligned. Essentially permanent.
• Dog Point — A cylindrical extension that fits into a hole, slot, or groove machined into the shaft. Provides positive location — the screw physically locks into a specific position.
• Half Dog Point — A shorter version of the dog point. Used where the full dog length is impractical.
• Oval Point — A rounded, convex end. Provides a pivot or bearing contact. Commonly used where the screw must press against a hardened surface or provide adjustable pressure.

Slotted Headless Set Screws (ANSI/ASME B18.6.2-1998)

These are the simplest set screws — a threaded cylinder with a screwdriver slot at one end and a point at the other. Available in sizes #0 through 3/4 inch.

Dimensional Notes:

• Crown Radius (I): Equals the basic screw diameter to three decimal places
• Oval Point Radius (R): Size 0 = .045; Size 1 = .055; Size 2 = .064; Size 3 = .074; Size 4 = .084; Size 5 = .094; Size 6 = .104; Size 8 = .123; Size 10 = .142; Size 12 = .162; 1/4 = .188; 5/16 = .234; 3/8 = .281; 7/16 = .328; 1/2 = .375; 9/16 = .422; 5/8 = .469; 3/4 = .562
• Cone Point Angle (Y): 90° ± 2° for longer screws; 118° ± 2° for shorter screws (threshold varies by size)
• Point Angle (X): 45° (+5°, −0°) for longer screws; 30° minimum for shorter screws

All dimensions in inches. Threads are Unified Standard Class 2A; UNC and UNF Series.

Hexagon and Spline Socket Set Screws (ANSI/ASME B18.3-1998)

These are the modern standard for set screw applications. The internal hex or spline drive allows installation below the surface of the workpiece with no protruding head. Thread class: Unified Standard, Class 3A, UNC and UNF series.

Length Increments:

• Sizes #0 through #3: 0.06" increment
• Lengths 1/8 through 1": 1/8" increment
• Lengths 1 through 2": 1/4" increment
• Lengths 2 through 6": 1/2" increment
• Lengths 6" and longer: 1" increment

Length Tolerances:

• Up to 5/8" long: ± 0.01"
• Over 5/8 to 2": ± 0.02"
• Over 2 to 6": ± 0.03"
• Over 6": ± 0.06"

All dimensions in inches. Cone point angle: 90° ± 2° for nominal lengths equal to or longer than the value shown in the last column; 118° ± 2° for shorter lengths.

Optional Cup Points — Seven Manufacturer Variations (ANSI/ASME B18.3-1998)

Not all cup points are created equal. Seven standardized variations (Types A through G) are available from various manufacturers. The standard cup point (Type A, column C) is the default. Types B through G offer different diameters and counterbored options for specialized applications.

All dimensions in inches. Type C (C2) diameter may be counterbored.

Square Head Set Screws (ANSI/ASME B18.6.2-1998)

The square head set screw is the traditional "visible" set screw — the protruding head provides wrench purchase and positive identification. Unless otherwise specified, square head set screws are supplied with cup points.

Designation example: 1/4-20 × 3/4 Square Head Set Screw, Flat Point, Steel, Cadmium Plated

Thread: Unified Standard Class 2A; UNC, UNF, and 8 UN Series. Full form threads extend over the entire length not affected by the point.

Head Dimensions

Point Dimensions (Square Head Set Screws)

Cone Point Angle Rules: For the following nominal lengths (or longer) shown by screw size, the cone point angle is 90° ± 2° — for example: size #10 at 1/4" length; 1/4 at 5/16"; 5/16 at 3/8"; 3/8 at 7/16"; and so on up through 1-1/2 at 1-3/4". For shorter screws, the cone point angle is 118° ± 2°.

Hexagon and Spline Socket Dimensions — The Master Reference

Hexagon Socket Width Across Flats (ANSI/ASME B18.3-1998)

This table tells you the exact socket opening size for every nominal hex socket designation. Where sockets are chamfered, the depth of chamfer must not exceed 10% of the nominal socket size for sizes up to and including 1/16", and 7.5% for larger sizes.

And continuing for larger sizes:

All dimensions in inches.

Hexagon and Spline Keys and Bits — Applicability Cross-Reference

This table answers the question: "Which Allen key fits which screw?" It cross-references every nominal key/bit size against cap screws (1960 series), flat countersunk head cap screws, button head cap screws, shoulder screws, and set screws.

Selected Hexagon Key Sizes:

Holding Power of Set Screws — The Calculation That Could Have Saved the practitioner's Pulley

This is the section the practitioner wishes he had read before that Tuesday morning. The holding power of a set screw depends on its diameter, the shaft diameter, and the rotational speed of the shaft. Get this wrong, and components slip — or worse, become projectiles.

Safe Holding Force in the supplied reference

Experiments have established the following approximate safe holding forces:

Critical note: These values vary with the physical properties of both the set screw and the shaft, surface finish, material hardness, and point style. Use these as starting points, not absolutes.

The Horsepower Formula

The power that can be safely transmitted by a set screw:

$$P = \frac{D \times N \times d^{2.3}}{50}$$

Where:

• P = Power transmitted (horsepower)
• D = Shaft diameter (inches)
• N = Shaft speed (RPM)
• d = Set screw diameter (inches)

The Torque Formula

$$T = 1250 \times D \times d^{2.3}$$

Where:

• T = Torque transmitted (inch-pounds)
• D = Shaft diameter (inches)
• d = Set screw diameter (inches)

Worked Example 1: the practitioner's Pulley (Reconstructed)

Problem: How many 1/2-inch diameter set screws are required to transmit 3 horsepower at a shaft speed of 1,000 RPM if the shaft diameter is 1 inch?

Solution using the power formula:

$$P = \frac{1 \times 1000 \times (0.5)^{2.3}}{50}$$

$$P = \frac{1000 \times 0.2031}{50} = 4.1 \text{ hp}$$

A single 1/2-inch set screw can safely transmit 4.1 hp. the practitioner's 3 hp requirement is well within the capacity of one properly installed 1/2-inch set screw. His problem was using a 1/4-inch screw — let's check that:

$$P = \frac{1 \times 1000 \times (0.25)^{2.3}}{50} = \frac{1000 \times 0.0408}{50} = 0.8 \text{ hp}$$

A 1/4-inch set screw transmits only 0.8 hp on a 1-inch shaft at 1,000 RPM. the practitioner was asking it to carry 3 hp — nearly four times its capacity. The failure was inevitable.

Worked Example 2: Downsizing the Screw

Problem: In the previous example, how many 3/8-inch diameter set screws would be required?

$$P = \frac{1 \times 1000 \times (0.375)^{2.3}}{50}$$

$$P = \frac{1000 \times 0.1072}{50} = 2.1 \text{ hp}$$

A single 3/8-inch set screw transmits 2.1 hp. Since we need 3 hp:

$$\text{Number of screws} = \lceil 3 \div 2.1 \rceil = 2$$

Two 3/8-inch set screws are required. Space them 90° or 120° apart around the hub for balanced clamping.

Quick-Reference: Horsepower Capacity Per Set Screw

On a 1-inch shaft at 1,000 RPM:

Scale linearly for different shaft diameters and speeds: $$P_{\text{actual}} = P_{\text{table}} \times \frac{D_{\text{actual}}}{1} \times \frac{N_{\text{actual}}}{1000}$$

British Standard Specifications

BS 4168 Hexagon Socket Head Cap Screws — Metric Series

The British Standard provides a comprehensive metric series of socket head cap screws in stainless steel and alloy steel. Alloy steel screws have Class 12.9 mechanical properties per BS 6104:Part 1.

Designation Example: Hexagon socket head cap screw BS 4168 M5 × 20-12.9

This means: thread size M5, nominal length 20 mm, property class 12.9.

Finish: Alloy steel → black oxide (thermal or chemical). Stainless steel → plain.

All dimensions in millimeters.

BS 4168 Hexagon Socket Set Screws — Metric Series

Parts 2 through 5 of BS 4168:1994 cover flat-point, cone-point, dog-point, and cup-point set screws respectively. Available in steel (class 45H per BS 6104:Part 3) or stainless steel (per BS 6105).

Designation Example: Hexagon socket set screw flat point BS 4168 M6 × 12-45H

Point Angle Rules:

• Flat point: 120° angle mandatory for short-length screws (length ≈ diameter)
• Cup point: 45° angle applies only to the portion below the root diameter
• Cone point: 120° for certain short lengths; 90° for all other lengths

Popular nominal lengths: 2, 2.5, 3, 4, 6, 8, 10, 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, 60 mm

All dimensions in millimeters.

BS 4168 Countersunk and Button Head Screws — Metric Series

Countersunk Head Screws

Button Head Screws

All dimensions in millimeters.

Bright Square Head Set Screws — BSW and BSF (With Flat Chamfered Ends)

The British Whitworth (BSW) and British Standard Fine (BSF) series provides three standards of square head set screw, each with progressively larger head dimensions.

All dimensions in inches. Depth of Head for No. 1, No. 2, and No. 3 Standards is the same as Width Across Flats for No. 1 Standard.

The Set Screw Selection Decision Matrix

the practitioner built this matrix after his incident and taped it inside his toolbox lid. It saved him — and his colleagues — from dozens of potential mistakes.