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ASME B18.11-1961 (2017) — Miniature Screws (Slotted Head Series)
🔬 ASME B18.11-1961 (2017) (originally ASA B18.11-1961) establishes the head types, dimensions, and length series of slotted head miniature screws, whose threads conform to the American Standard Unified Miniature Screw Threads (ASA B1.10-1958, the UNM series), covering nominal diameters from 0.0118 inch (0.3 mm) to 0.0551 inch (1.4 mm). The Standard grew out of the needs of the watch industry and the subminiaturization of control mechanisms, instruments, and electronic devices; it was reaffirmed in 1983, 2000, and 2017 and remains the authoritative reference for specifying and accepting miniature screws in precision assemblies.
💡 Although miniature screws are tiny, the Standard’s provisions on head eccentricity, slot depth, bearing-surface squareness, body diameter, and thread runout are remarkably detailed — these micron-level requirements decide the reliability and adjustability of precision assemblies.
1. Scope and General Framework 📌
Four slotted head types are covered: fillister head, pan head, flat head, and binding head. The flat head has a conical bearing surface of approximately 100° included angle; the binding head is shorter but wider than the pan head and is intended for applications that would otherwise require washers. The Standard is based on research of the major user industries and notes that the inclusion of four head types and the given thread sizes does not imply that all are stock production items — preferred diameter-pitch combinations are shown in bold in the data tables, and users should consult manufacturers’ catalogs for current stock.
⚠️ The bearing surface, slot geometry, and head geometry of miniature screws directly govern insertion and locking behavior. The Standard limits the bearing surface to within 2° of perpendicularity to the body axis and the maximum permissible slot concavity to 3 percent of the mean head diameter — deviations cause driver slip and loss of torque control.
2. Technical Requirements and Dimensions 📏
Head heights in the dimensional tables represent the metal measurement after slotting. Slot depth is measured from the bearing surface to the intersection of the slot bottom with the head diameter for fillister, pan, and binding heads, and from the flat top surface to the intersection with the bearing surface for heads with conical bearing surfaces (flat head). The Standard also sets limits on head eccentricity, slot eccentricity, underhead fillet radius, body diameter, and length tolerance.
| Size Designation (UNM) | Basic Major Dia (in) | Basic Major Dia (mm) | Typical Uses |
|---|---|---|---|
| 30 UNM | 0.0118 | 0.30 | Precision instruments, micro relays |
| 60 UNM | 0.0236 | 0.60 | Optical devices, control mechanisms |
| 100 UNM | 0.0394 | 1.00 | Electronic instruments, watch movements |
| 140 UNM | 0.0551 | 1.40 | Instrument housings, contact assemblies |
Heads shall not be eccentric with the bodies by more than 2 percent of the maximum head diameter or 0.001 inch, whichever is greater; slots shall not be eccentric by more than 5 percent of the nominal body diameter. The underhead fillet radius shall not exceed one-half the thread pitch for perpendicular bearing-surface heads, or twice the pitch for conical bearing-surface heads. The unthreaded body diameter shall be no greater than the maximum major diameter and no less than the minimum pitch diameter of the thread.
| Head Type | Features | Bearing Surface | Table |
|---|---|---|---|
| Fillister head | Flat top (oval crown optional), cylindrical sides | Perpendicular to axis | 1 |
| Pan head | Flat top, cylindrical sides; lower height, slightly larger diameter | Perpendicular to axis | 2 |
| Flat head | Flat top, conical bearing surface (~100°) | Conical | 3 |
| Binding head | Shorter and wider than pan head; washer-less applications | Perpendicular to axis | 4 |
Length is measured from the bearing surface (perpendicular bearing-surface heads) or from the top of the head (conical bearing-surface heads) to the extreme end; preferred lengths and tolerances are given in Table 5. Screws with a length no greater than four times the nominal body diameter must be threaded to within two threads of the bearing surface; longer screws must have complete threads for a minimum of four diameters. Screws are regularly supplied with flat ends chamfered at approximately 45° to the minor diameter of the thread as a minimum depth.
⚠️ Miniature screw designation has a fixed format: the thread size designation per ASA B1.10, followed by the nominal length in units of 1/1000 inch (omitting the decimal point) and the head type — for example, 60 UNM × 040 FIL HD, 100 UNM × 080 PAN HD, 120 UNM × 120 FLAT HD, and 140 UNM × 250 BIND HD. Orders and drawings must use this complete format to avoid ambiguity.
3. Material, Finish, and Process Requirements 🔧
- 🧪 Material and coating: screws are generally supplied in ferrous and nonferrous materials, coatings, and heat treatments specified by the user; coatings, when required, are limited to electroplating or chemical oxidation to avoid plugging fine threads.
- 🔍 Thread accuracy: threads must conform to ASA B1.10-1958 Unified Miniature Screw Threads in form and tolerance; ordinary UNC/UNF threads cannot be substituted, because at these tiny sizes profile deviations are greatly amplified in strength and fit.
- 📐 Machined finish: roughness of machined head surfaces must not exceed 63 micro-inches arithmetical average (per ASA B46.1, Surface Texture), determined by visual comparison with roughness comparison specimens.
- 🛠️ Assembly torque: because shank diameters are so small, use calibrated micro torque tools and verify effective thread engagement length against the runout requirements to prevent stripping and shank breakage.
4. Engineering Practice Essentials 🛠️
Miniature screws are widely used in aerospace instruments, navigation equipment, relays, potentiometers, optical mounts, and watch movements. Key practices: always specify the full UNM size designation and head type; prefer the bold (recommended) diameter-pitch combinations for availability; for flush installations, match the flat head’s 100° countersink angle and depth; under vibration, prefer binding heads or add thread-locking compound; and for coating-sensitive fits, verify the effect of plating thickness on thread engagement.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between UNM miniature threads and ordinary inch threads?
A: UNM (Unified Miniature) threads are defined by ASA B1.10 and are far smaller than the conventional UNC/UNF series; sizes are designated by number (30–140 UNM), where the number divided by 2540 gives the nominal major diameter in inches. Miniature screws must be threaded to UNM, never mixed with ordinary series.
A: UNM (Unified Miniature) threads are defined by ASA B1.10 and are far smaller than the conventional UNC/UNF series; sizes are designated by number (30–140 UNM), where the number divided by 2540 gives the nominal major diameter in inches. Miniature screws must be threaded to UNM, never mixed with ordinary series.
❓ Q: Why does the Standard limit head and slot eccentricity?
A: With body diameters of only 0.3–1.4 mm, head eccentricity causes wobble on insertion, driver slip, and poor head appearance; slot eccentricity disturbs symmetric torque transfer. The 2%/5% relative limits balance manufacturability with assembly performance.
A: With body diameters of only 0.3–1.4 mm, head eccentricity causes wobble on insertion, driver slip, and poor head appearance; slot eccentricity disturbs symmetric torque transfer. The 2%/5% relative limits balance manufacturability with assembly performance.
🔍 Q: Why does the flat head have a conical bearing surface?
A: The conical bearing surface (about 100°) mates with a countersunk hole so the head sits flush or slightly below the surface, avoiding protrusion while providing self-centering; length and slot-depth measurements for such heads are therefore taken from the top of the head.
A: The conical bearing surface (about 100°) mates with a countersunk hole so the head sits flush or slightly below the surface, avoiding protrusion while providing self-centering; length and slot-depth measurements for such heads are therefore taken from the top of the head.
❓ Q: Does the Standard guarantee that all listed sizes are in stock?
A: No. The Standard states that listing does not imply stock production status; bold (recommended) combinations are more readily available, and special sizes require confirmation of availability and minimum order quantities with the manufacturer.
A: No. The Standard states that listing does not imply stock production status; bold (recommended) combinations are more readily available, and special sizes require confirmation of availability and minimum order quantities with the manufacturer.
