Unified Miniature Thread (UNM) ISO 1501 Dimensions Chart
The Unified Miniature Screw Thread (UNM) series—standardized under ASME Geometric & Mechanical Standards (ASME B1.10M) and ISO International Metrology Standard (ISO 1501)—establishes 60° micro-fastener specifications from 0.30 UNM to 1.40 UNM. Basic thread depth is h = 0.52 × P, featuring shallower, highly rounded root truncations designed specifically for micro-machining, horology, and surgical implants.
Unified Miniature Thread (UNM) ISO 1501 & ASME B1.10M Dimensions Chart
Compute 60° miniature watch and medical screw thread pitch diameters, tap drill sizes, 3-wire microscope measurements, and safe micro-tapping torque limits across 0.30 UNM to 1.40 UNM.
What Are Unified Miniature (UNM) Screw Threads and ISO 1501 Standards?
Miniature threaded fasteners used in luxury mechanical timepieces, biomedical implants, optical assemblies, and aerospace instrumentation require specialized geometric standards. Below a major diameter of 0.060" (approximately 1.5 mm), standard Unified coarse (UNC) and fine (UNF) thread geometries become impractical because standard 60° root depths (0.6134 × P) severely weaken small-diameter screw blanks and lead to frequent tap breakage during tapping operations.
To solve this micro-manufacturing challenge, the American Society of Mechanical Engineers and the International Organization for Standardization unified miniature screw standards into ASME B1.10M and ISO 1501. The UNM series encompasses 14 standard nominal sizes ranging from 0.30 UNM (Ø0.300 mm / 0.0118") up to 1.40 UNM (Ø1.400 mm / 0.0551"). Unlike standard metric or UN threads, the UNM thread form uses a modified basic depth of h = 0.52 × P with a generous root radius of 0.120 × P to prevent stress concentrations in micro-fasteners.
| Designation | Pitch P (mm) | External Major Ø (mm) | External Pitch Ø d_2 (mm) | Internal Minor Ø D_1 (mm) | Tap Drill Ø (75% Eng.) | Best Wire Ø (W = 0.577P) |
|---|---|---|---|---|---|---|
| 0.30 UNM | 0.080 mm | 0.280 – 0.300 | 0.235 – 0.248 | 0.207 – 0.224 | Ø 0.23 mm (#98 drill) | 0.046 mm |
| 0.35 UNM | 0.090 mm | 0.328 – 0.350 | 0.278 – 0.292 | 0.246 – 0.265 | Ø 0.27 mm (#94 drill) | 0.052 mm |
| 0.40 UNM | 0.100 mm | 0.376 – 0.400 | 0.320 – 0.335 | 0.284 – 0.305 | Ø 0.31 mm (#91 drill) | 0.058 mm |
| 0.50 UNM | 0.125 mm | 0.470 – 0.500 | 0.400 – 0.418 | 0.355 – 0.381 | Ø 0.39 mm (#83 drill) | 0.072 mm |
| 0.60 UNM | 0.150 mm | 0.566 – 0.600 | 0.482 – 0.502 | 0.426 – 0.457 | Ø 0.47 mm (#77 drill) | 0.087 mm |
| 0.70 UNM | 0.175 mm | 0.660 – 0.700 | 0.563 – 0.585 | 0.498 – 0.533 | Ø 0.55 mm (#74 drill) | 0.101 mm |
| 0.80 UNM | 0.200 mm | 0.756 – 0.800 | 0.645 – 0.670 | 0.569 – 0.609 | Ø 0.63 mm (#71 drill) | 0.115 mm |
| 0.90 UNM | 0.225 mm | 0.852 – 0.900 | 0.727 – 0.754 | 0.640 – 0.685 | Ø 0.71 mm (#69 drill) | 0.130 mm |
| 1.00 UNM | 0.250 mm | 0.948 – 1.000 | 0.810 – 0.838 | 0.711 – 0.761 | Ø 0.78 mm (#67 drill) | 0.144 mm |
| 1.20 UNM | 0.250 mm | 1.148 – 1.200 | 1.010 – 1.038 | 0.911 – 0.961 | Ø 0.98 mm (#60 drill) | 0.144 mm |
| 1.40 UNM | 0.300 mm | 1.340 – 1.400 | 1.173 – 1.205 | 1.054 – 1.114 | Ø 1.14 mm (#56 drill) | 0.173 mm |
How Are Tap Drill Sizes Selected for Miniature Micro-Tapping?
Selecting tap drill diameters for miniature UNM threads requires extreme dimensional discipline. Because miniature taps have core web diameters smaller than a human hair, tapping at standard 75% thread engagement often generates excessive cutting torque that snaps the tap flush with the workpiece. Precision Swiss machining standards published in Machinery's Handbook (Industrial Press) recommend targeting 65% to 70% thread engagement for stainless steels and titanium, and 70% to 75% for brass and aluminum alloys.
For a standard 1.00 UNM thread with a 0.250 mm pitch, the nominal tap drill is Ø0.78 mm (using a standard #67 wire gage drill), which provides 73.9% thread engagement. For a sub-millimeter 0.50 UNM thread with a 0.125 mm pitch, the recommended hole is Ø0.39 mm (using a #83 micro-drill), leaving a minor bore tolerance window of 0.355 mm to 0.381 mm. Tapping speeds must be held strictly between 15 and 30 SFM using specialized high-lubricity synthetic micro-tapping oils—cross-reference with our Master Tap Drill Chart and 4-Way Drill Bit Size Converter.
| Thread Size | Minor Hole Ø | Spindle RPM @ 50 SFM | Max Allowable TIR Runout | Max Tapping Torque (mN·m) | Typical Application |
|---|---|---|---|---|---|
| 0.30 – 0.40 UNM | 0.23 – 0.31 mm | 20,000 – 30,000 RPM | ≤ 0.002 mm (0.00008") | 0.08 – 0.18 mN·m | Watch balance wheels & jewels |
| 0.50 – 0.60 UNM | 0.39 – 0.47 mm | 15,000 – 20,000 RPM | ≤ 0.003 mm (0.00012") | 0.35 – 0.65 mN·m | Medical hearing aids & optics |
| 0.70 – 0.90 UNM | 0.55 – 0.71 mm | 10,000 – 14,000 RPM | ≤ 0.004 mm (0.00016") | 1.10 – 2.20 mN·m | Aerospace gyroscopes & sensors |
| 1.00 – 1.40 UNM | 0.78 – 1.14 mm | 6,000 – 9,000 RPM | ≤ 0.005 mm (0.00020") | 3.20 – 7.80 mN·m | Precision instrument mechanisms |
Why Are Three-Wire Checks Conducted Under Toolmaker Microscopes?
Measuring external pitch diameter d_2 on miniature screw threads cannot be performed with standard handheld micrometer anvils because the physical pitch is as small as 0.080 mm (317.5 TPI on 0.30 UNM). Under NIST (National Institute of Standards and Technology) metrology guidelines, calibration labs utilize calibrated tungsten carbide micro-wires measured under optical toolmaker tool-inspection microscopes or laser non-contact micrometers. The best wire diameter satisfies W = (P / 2) × sec(30°) = 0.57735 × P, placing wire contact precisely along the theoretical pitch cylinder.
For 1.00 UNM (P = 0.250 mm), the ideal wire size is Ø0.144 mm (0.00567"), and the pitch diameter must fall within 0.810 mm to 0.838 mm. Maximum allowable tapping torque on a 1.00 UNM tap is a delicate 3.20 mN·m (0.45 oz-in)—exceeding 4.5 mN·m causes instant torsional shear failure. Compare with our Three-Wire Thread Measurement Calculator and Thread Engagement Calculator.