Thread Standards Guide
A complete reference for thread standards: ISO Metric coarse/fine, UNC/UNF, BSP/BSPT, NPT, Acme/Trapezoidal, aerospace MJ/UNJ, tap drill size charts, tapping speeds, bolt torque and property classes, and a full thread type comparison guide.
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🔩Threads — Thread Standards & Reference
Threads — 30+ global standards. Angle, pitch, tolerance — the three critical parameters.
ISO Metric Coarse — Coarse (Most Common)
ISO 724DIN 13Most common worldwide
StandardISO 724 / DIN 13 / JIS B0205
DesignationM10 (M + nominal ⌀)
Angle60° Included Angle
M3Pitch=0.5 | ⌀₂=2.459 | ⌀₂min=2.350
M6Pitch=1.0 | ⌀₂=4.917 | Tap Drill=5.0mm
M10Pitch=1.5 | ⌀₂=8.376 | Tap Drill=8.5mm
M20Pitch=2.5 | ⌀₂=17.294 | Tap Drill=17.5mm
Tap Drill = D - P (approximation!). Exact: D - 1.0825×P
Tolerance: 6H (Nut) / 6g (Bolt) = Standard Fit
✓ The global standard. Calculate: D-P = Tap Drill
✓ 4H/4g = Close Fit | 7H/7e = Free Fit
▶ YouTubeTolerance: 6H (Nut) / 6g (Bolt) = Standard Fit
✓ The global standard. Calculate: D-P = Tap Drill
✓ 4H/4g = Close Fit | 7H/7e = Free Fit
ISO Metric Fine — Fine
ISO 724VibrationPrecision
M6×0.75Fine. vs Coarse 1.0. ↑ tapping strength
M10×1.0Fine 1.0 vs Coarse 1.5
M10×1.25Medium fine
M12×1.5Fine — automotive, hydraulics
M14×1.5Fine — hydraulics
M20×2.0Fine vs Coarse 2.5
UseInstruments, hydraulic bolts, adjustment mechanisms, automotive
Fine = ↓ Pitch → ↑ TPI → ↑ tapping strength + ↓ self-loosening
✓ ↑ vibration resistance + ↑ precise adjustment
✗ ↓ dust and contamination resistance
▶ YouTube✓ ↑ vibration resistance + ↑ precise adjustment
✗ ↓ dust and contamination resistance
Tolerance Classes — ISO Tolerance Grades
6H/6g4H/4g7H/7g
6H (Nut)Standard internal. Free running
6g (Bolt)Standard external. Free running
4H / 4hClose Fit. ↓ Clearance. ↑ Strength
7H / 7eFree Fit. ↑ Clearance. ↓ ↑ Assembly
5H / 4hTight Fit. Gauge / Precision Applications
Plated6g → before plating → 6H fit OK (↓ undersize)
Rule: Bolt Tolerance Letter = lowercase (g,e,d)
Nut = uppercase (H,G,E)
✓ 6H/6g = 99% of applications
✓ ↓ number = ↓ tolerance = ↑ precision
▶ YouTubeNut = uppercase (H,G,E)
✓ 6H/6g = 99% of applications
✓ ↓ number = ↓ tolerance = ↑ precision
| M | Pitch Coarse | Pitch Fine | Tap Drill Coarse | Tap Drill Fine 1 |
|---|---|---|---|---|
| M3 | 0.5 | 0.35 | 2.5mm | 2.65mm |
| M4 | 0.7 | 0.5 | 3.3mm | 3.5mm |
| M5 | 0.8 | 0.5 | 4.2mm | 4.5mm |
| M6 | 1.0 | 0.75 | 5.0mm | 5.25mm |
| M8 | 1.25 | 1.0 | 6.8mm | 7.0mm |
| M10 | 1.5 | 1.0 | 8.5mm | 9.0mm |
| M12 | 1.75 | 1.25 | 10.2mm | 10.75mm |
| M16 | 2.0 | 1.5 | 14.0mm | 14.5mm |
| M20 | 2.5 | 1.5 | 17.5mm | 18.5mm |
| M24 | 3.0 | 2.0 | 21.0mm | 22.0mm |
UNC — Unified National Coarse
ASME B1.1SAEUSA/Canada
StandardASME B1.1 / SAE J475
Designation1/4-20 UNC (⌀ inches - TPI - UNC)
Angle60° (like ISO!)
1/4-2020 TPI. Tap Drill #7 (5.105mm)
3/8-1616 TPI. Tap Drill 5/16 (7.938mm)
1/2-1313 TPI. Tap Drill 27/64 (10.716mm)
1"-88 TPI. Tap Drill 7/8 (22.225mm)
UNC = most common in the US and Canada. Coarse = fast + contamination-resistant
Tolerance: 2A (External) / 2B (Internal) = Standard
✓ ↑ strength ↑ vibration resistance
✗ Not compatible with ISO Metric!
▶ YouTubeTolerance: 2A (External) / 2B (Internal) = Standard
✓ ↑ strength ↑ vibration resistance
✗ Not compatible with ISO Metric!
UNF — Unified National Fine
ASME B1.1Automotive/Defense
Designation1/4-28 UNF
1/4-2828 TPI. Tap Drill #3 (5.487mm)
3/8-2424 TPI. Tap Drill Q (10.516mm)
1/2-2020 TPI. Tap Drill 29/64 (11.509mm)
UseAutomotive bolts, hydraulics, weapons, aerospace
vs UNC↑ TPI → ↑ strength + ↓ self-loosening
+OK+ ↑ Shear Strength per Length
+OK+ ↓ Risk Self-Loosening
+NO+ ↓ Contamination Tolerance
▶ YouTube+OK+ ↓ Risk Self-Loosening
+NO+ ↓ Contamination Tolerance
UNEF + UN Special
ASME B1.1Extra Fine
UNEFExtra Fine. High TPI. Precision instruments
1/4-32 UNEF32 TPI — very fine!
UN Special⌀ × TPI × UN — any combination
UNSSpecial — non-standard
UseMedical devices, optics, watches
+OK+ ↓ Pitch → ↑ Load per Turn
+NO+ High cost. Not readily available
▶ YouTube+NO+ High cost. Not readily available
| Size | TPI UNC | TPI UNF | Tap Drill UNC | Tap Drill UNF |
|---|---|---|---|---|
| #4 (0.112") | 40 | 48 | #43 | #42 |
| #6 (0.138") | 32 | 40 | #36 | #33 |
| #8 (0.164") | 32 | 36 | #29 | #29 |
| #10 (0.190") | 24 | 32 | #25 | #21 |
| 1/4" | 20 | 28 | #7 | #3 |
| 5/16" | 18 | 24 | F | I |
| 3/8" | 16 | 24 | 5/16" | Q |
| 1/2" | 13 | 20 | 27/64" | 29/64" |
| 5/8" | 11 | 18 | 17/32" | 37/64" |
| 3/4" | 10 | 16 | 21/32" | 11/16" |
BSPP — British Standard Pipe Parallel
ISO 228G ThreadHydraulics
StandardISO 228-1 / BS 2779
DesignationG1/2 (G + pipe size in inches)
Angle55° Whitworth — ≠ 60° ISO!
G1/4TPI=19 | OD=13.157mm | Tap=11.8mm
G1/2TPI=14 | OD=20.955mm | Tap=19.0mm
G3/4TPI=14 | OD=26.441mm | Tap=24.5mm
G1"TPI=11 | OD=33.249mm | Tap=30.5mm
SealingInternal O-Ring / bonded seal — not from the thread itself!
BSPP = Parallel = not self-sealing! O-Ring mandatory!
55° ≠ 60° ISO = doesn't mate with ISO Metric!
✓ Common in European hydraulics/pneumatics
✗ Not for direct NPT connections — ≠ compatible
▶ YouTube55° ≠ 60° ISO = doesn't mate with ISO Metric!
✓ Common in European hydraulics/pneumatics
✗ Not for direct NPT connections — ≠ compatible
BSPT — British Standard Pipe Taper
BS 21Self-SealingR Thread
StandardBS 21 / ISO 7-1
DesignationR1/2 (external) / Rc1/2 (internal)
Taper1:16 (1.7899°) = taper seal
R1/4TPI=19 | OD Start=13.157mm
R1/2TPI=14 | OD Start=20.955mm
R1"TPI=11 | OD Start=33.249mm
SealingTaper = self-sealing. PTFE tape adds security
R = Male External Taper. Rc = Female Internal Taper
Self-Sealing: threading in → compression → mechanical seal!
✓ Good sealing without an O-Ring
✗ ↓ disassembly cycles vs. BSPP
▶ YouTubeSelf-Sealing: threading in → compression → mechanical seal!
✓ Good sealing without an O-Ring
✗ ↓ disassembly cycles vs. BSPP
NPT/NPTF — National Pipe Taper (USA)
ASME B1.20.160°Dryseal
StandardASME B1.20.1 (NPT) / B1.20.3 (NPTF)
Designation1/2-14 NPT (size-TPI-NPT)
Angle60° (vs BSPT=55°)
Taper1:16 = same as BSPT! But ≠ compatible
NPTFDryseal = sealing without PTFE tape
1/4-18 NPT18 TPI | Tap Drill=11/32"
1/2-14 NPT14 TPI | Tap Drill=23/32"
NPT vs BSPT: both 1:16 taper but 60° vs 55° = not compatible!
NPTF (Dryseal) = ↑↑ sealing without PTFE
✓ USA standard. Common in American industry
✗ Not compatible with BSPT! Different angle = leakage
▶ YouTubeNPTF (Dryseal) = ↑↑ sealing without PTFE
✓ USA standard. Common in American industry
✗ Not compatible with BSPT! Different angle = leakage
Rc vs G — Which to Choose?
GR/Rc
G = BSPPParallel. O-Ring seal. ↑ reusable
R/Rc = BSPTTaper. Self-seal. PTFE tape
Practical ruleG: for frequently disconnected fittings
Practical ruleRc: for permanent connections with ↓ disassembly
+OK+ G = ↑ reuse + reliable O-Ring seal
+OK+ Rc = self-seal + ↓ extra components
▶ YouTube+OK+ Rc = self-seal + ↓ extra components
| Thread | TPI | OD mm | Tap Drill mm | Depth of Engagement mm |
|---|---|---|---|---|
| G1/8 | 28 | 9.728 | 8.8 | 4.0 |
| G1/4 | 19 | 13.157 | 11.8 | 6.0 |
| G3/8 | 19 | 16.662 | 15.25 | 6.4 |
| G1/2 | 14 | 20.955 | 19.0 | 8.2 |
| G3/4 | 14 | 26.441 | 24.5 | 9.5 |
| G1" | 11 | 33.249 | 30.5 | 10.4 |
| G1¼" | 11 | 41.910 | 38.9 | 12.7 |
| G1½" | 11 | 47.803 | 44.7 | 12.7 |
| G2" | 11 | 59.614 | 56.7 | 15.9 |
| R1/4 (BSPT) | 19 | 13.157 | 11.5 | — |
| R1/2 (BSPT) | 14 | 20.955 | 18.6 | — |
| R1" (BSPT) | 11 | 33.249 | 30.1 | — |
| 1/4-18 NPT | 18 | 13.716 | 11.1 | — |
| 1/2-14 NPT | 14 | 21.336 | 18.3 | — |
| 3/4-14 NPT | 14 | 26.670 | 23.8 | — |
| 1"-11.5 NPT | 11.5 | 33.401 | 29.4 | — |
Acme Thread — Lead Screw (USA)
ASME B1.529°Lead Screw
StandardASME B1.5 / ASME B1.8 (Stub Acme)
Angle29° Included
Designation1"-5 Acme (⌀-TPI-Acme)
UseLead screws, vises, presses, jacks
G AcmeGeneral Purpose — 2G Fit standard
C AcmeCentralizing — ↑ precision
Stub Acme↓ tooth height. ↓ volume. Stub ACME
Acme → power transmission. Not fastening!
29° = a compromise between V (60°) and square (0°)
✓ ↑ wear resistance ↑ easier assembly vs square
✗ ↓ efficiency vs. ball screw (40% vs 90%)
▶ YouTube29° = a compromise between V (60°) and square (0°)
✓ ↑ wear resistance ↑ easier assembly vs square
✗ ↓ efficiency vs. ball screw (40% vs 90%)
Trapezoidal Tr — European Metric
ISO 2901DIN 103CNC
StandardISO 2901 / DIN 103
Angle30° Included (vs Acme=29°)
DesignationTr10×2 (Tr + ⌀ × Pitch)
Tr10×2Lead=2mm. ⌀₂=8.5mm
Tr16×4Lead=4mm. ⌀₂=13.5mm
Multi-LeadTr16×4(P2): Lead=4, Pitch=2, 2 Starts
UseCNC lathes, presses, positioning mechanisms
30° (Tr) vs 29° (Acme) = NOT INTERCHANGEABLE!
Multi-Start: ↑ Lead = ↑ speed. Lead=Pitch×Starts
✓ ISO/DIN. Common in Europe and machinery
▶ YouTubeMulti-Start: ↑ Lead = ↑ speed. Lead=Pitch×Starts
✓ ISO/DIN. Common in Europe and machinery
Ball Screw
Efficiency 90%±0.003mmCNC
PrincipleSteel balls in a track → ↓↓ friction
Efficiency90-95% (vs Acme=30-50%)
Lead AccuracyC5: ±0.023mm/300mm | C3: ±0.012mm
BacklashPreloaded = 0 backlash!
UseCNC machining centers, robotics
ManufacturersTHK, NSK, SKF, Hiwin, KURODA
+OK+ ↑↑ efficiency + ↓ backlash + ↓ wear
+OK+ Preload = ±0 backlash (mandatory for CNC!)
+NO+ Expensive. Needs lubrication. Not self-locking
▶ YouTube+OK+ Preload = ±0 backlash (mandatory for CNC!)
+NO+ Expensive. Needs lubrication. Not self-locking
Square Thread
Max EfficiencyLegacy
Angle0° — completely square!
Efficiency↑↑↑ — minimal radial friction
DisadvantageHard to manufacture + not self-locking → ↓ usage
UseAntique manual presses, special heavy equipment
TodayReplaced by Acme / ball screw
+OK+ Maximum theoretical efficiency
+NO+ Very difficult to manufacture. Not standardized
▶ YouTube+NO+ Very difficult to manufacture. Not standardized
| Thread | Pitch mm | Lead mm | ø₂ mm | ø Minor mm | Ball Screw equiv. |
|---|---|---|---|---|---|
| Tr8×1.5 | 1.5 | 1.5 | 7.25 | 6.5 | — |
| Tr10×2 | 2.0 | 2.0 | 9.0 | 8.0 | — |
| Tr12×3 | 3.0 | 3.0 | 10.5 | 9.0 | — |
| Tr16×4 | 4.0 | 4.0 | 14.0 | 12.0 | — |
| Tr20×4 | 4.0 | 4.0 | 18.0 | 16.0 | — |
| Tr24×5 | 5.0 | 5.0 | 21.5 | 19.0 | — |
| Tr32×6 | 6.0 | 6.0 | 29.0 | 26.0 | — |
| Tr40×7 | 7.0 | 7.0 | 36.5 | 33.0 | — |
| 1/2"-10 Acme | 2.54 | 2.54 | 11.8 | 10.4 | — |
| 3/4"-6 Acme | 4.23 | 4.23 | 17.8 | 15.5 | — |
| 1"-5 Acme | 5.08 | 5.08 | 23.8 | 21.0 | — |
| BS 1250×10 BallScrew | 10 | 10 | — | — | ø25mm |
| BS 1600×10 BallScrew | 10 | 10 | — | — | ø16mm |
Buttress Thread — Unidirectional Load
7°/45°Unidirectional Load
StandardASME B1.9
Angle7° Load Flank + 45° Trail Flank
UsePressure screws, large diameters, presses, gun breeches, oil pipes
Advantage↑↑ load resistance in one direction
Saw-Tooth≠ Buttress. Load=0° ↑ efficiency
+OK+ Maximum unidirectional resistance
+OK+ ↑↑ strength vs. V-Thread
+NO+ Not for bidirectional loads
▶ YouTube+OK+ ↑↑ strength vs. V-Thread
+NO+ Not for bidirectional loads
NPSC/NPSM — Pipe Thread Straight
ASMECoupling
NPSMStraight Mechanical — O-Ring seal
NPSCStraight Coupling — compatible with NPT external
NPSHHose Coupling Straight
NPSLLoose Fit Straight
+OK+ Straight = ↑ reusability vs. taper
+OK+ NPSC is compatible with external NPT
▶ YouTube+OK+ NPSC is compatible with external NPT
Spark Plug Thread
14mm18mmSAE
M14×1.25Most common — most automotive engines
M18×1.5Large engines
M10×1.0Motorcycle engines, small
M12×1.25Certain European/Japanese engines
StandardSAE J548 / ISO 28239
+OK+ A thread unique to high heat and pressure
+OK+ Gasket seat / conical seat
▶ YouTube+OK+ Gasket seat / conical seat
Watchmaker Threads
0.25-2mm60°
SwissM0.3-M1.6 ISO Metric
BABritish Association. 0BA=6mm. ↓ BA=↓ ⌀
0BA6.0mm / 1.0mm pitch
8BA1.3mm / 0.22mm pitch
UseLuxury watches, cameras, instruments
+OK+ Maximum precision. ↑ TPI
+NO+ Special manufacturing. An extremely fine tap
▶ YouTube+NO+ Special manufacturing. An extremely fine tap
Worm Gear Thread
AGMAGear
StandardAGMA 6022 / ISO 3408
Angle20° Pressure Angle (most)
Lead Angle5-30° — determines self-locking
Self-LockLead Angle < Friction Angle → self-lock
UseDrive wheels, opening/closing mechanisms, gearboxes
+OK+ High reduction ratio (10:1 to 100:1)
+OK+ Self-locking when desired
▶ YouTube+OK+ Self-locking when desired
ACME-Stub + Left-Hand
Left HandStub
Left-HandL.H. marking. Threads left. Lathe leadscrew, gas cylinders
Stub AcmeDepth=0.3P vs. full Acme=0.5P. ↓ height ↑ strength
O2 CylindersRH. H2/acetylene cylinders=LH — ↑ safety!
LH StandardISO: M10×1.5-LH. USA: 1/2-13 UNC-LH
+OK+ LH = ↓ risk of self-loosening
+OK+ Cylinder safety: O2=RH, Fuel=LH!
▶ YouTube+OK+ Cylinder safety: O2=RH, Fuel=LH!
| Thread | Std | TPI/Pitch | OD mm | Angle | Use |
|---|---|---|---|---|---|
| Buttress 1"-5 | ASME B1.9 | 5 TPI | 25.4 | 7°/45° | Presses, jacks |
| 0BA | BA | 25.4 TPI | 6.0mm | 47.5° | Precision watches |
| 2BA | BA | 31.9 TPI | 4.7mm | 47.5° | Cameras, instruments |
| 4BA | BA | 38.5 TPI | 3.6mm | 47.5° | Luxury watches |
| 6BA | BA | 47.9 TPI | 2.8mm | 47.5° | Luxury watches |
| 8BA | BA | 59.1 TPI | 2.2mm | 47.5° | Microscopes |
| M14×1.25 Spark | SAE J548 | 1.25 | 14.0 | 60° | Most automotive engines |
| M18×1.5 Spark | SAE J548 | 1.5 | 18.0 | 60° | Large engines |
| M10×1.0 Spark | SAE J548 | 1.0 | 10.0 | 60° | Motorcycles |
| LH M10×1.5-LH | ISO 724 | 1.5 | 10.0 | 60° | Gas, O2 cylinders |
| Stub Acme 1"-4 | ASME B1.8 | 4 TPI | 25.4 | 29° | ↓tooth height |
UNJ — Unified Inch (Aerospace)
MIL-S-8879Root Radius↑ Fatigue
StandardASME B1.15 / MIL-S-8879
DifferenceRoot Radius = 0.15875×P (vs. V-Sharp in UN!)
Advantage↑40% fatigue life! ↑ fatigue strength
Designation1/4-28 UNJF
MaterialsTi, Inconel, high strength steel
UseF-35, F-22, C-130 — all aerospace/landing gear structure
Root Radius = stress concentration ↓ → fatigue ↑↑!
UNJ ≠ UN: cannot use a UN thread gauge
✓ ↑40% fatigue = mandatory for aerospace/defense
✗ Expensive. Unique gauge. Non-interchangeable
▶ YouTubeUNJ ≠ UN: cannot use a UN thread gauge
✓ ↑40% fatigue = mandatory for aerospace/defense
✗ Expensive. Unique gauge. Non-interchangeable
MJ — Metric Aerospace Thread
ISO 5855Root RadiusAirbus/Boeing
StandardISO 5855 / NAS 619 / ASME B1.21M
DifferenceRoot Radius ≥0.125×P (vs. ISO sharp root)
DesignationMJ10×1.5 (MJ + ⌀×Pitch)
Advantage↑ fatigue life vs. ISO regular
UseAirbus A320/A350/A380, Boeing 737/777/787
StandardsEN 3821-3827 / NAS 618
MJ = ISO Metric + Root Radius = aerospace grade
✓ ↑ fatigue. Airbus/Boeing standard
✗ MJ Nut/Bolt ≠ ISO regular — different gauge!
▶ YouTube✓ ↑ fatigue. Airbus/Boeing standard
✗ MJ Nut/Bolt ≠ ISO regular — different gauge!
Locking Features
Prevailing TorqueSelf-Lock
Nylon InsertNylock — ISO 7042. Reuse ↓ strength after 5 cycles
All-Metal NutStover/oval distorted — ↑ high temp
Safety WireMIL-DTL-83140. → no torque check required
Nordlock2 toothed surfaces. ↑↑ vibration. AMS 5613
Loctite 243M3-M20. Medium. ↓ 250C/5min
Loctite 263Strong. Non-reversible without heat
+OK+ Safety wire = the gold standard → aerospace
+OK+ Nordlock = severe vibration
+NO+ Nylock = not for T° > 120C!
▶ YouTube+OK+ Nordlock = severe vibration
+NO+ Nylock = not for T° > 120C!
Thread Inserts
HelicoilKeensertAl Repair
Helicoil (STI)Stainless spring insert. M3-M30. NASM 21209
KeensertSolid key-locking insert. M6+
E-Z LokThreaded insert → special for plastic/Al
RecoilAustralia/New Zealand. ≈ Helicoil
Tap DrillHelicoil: larger than standard! (per table)
UseThread repair, Al → ↑ strength, plastic-to-metal joints
Helicoil in Al: ↑ pull-out vs. base! Higher strength than original
✓ Fast repair. Strength equal to/↑ original
✗ Different tap drill — per NASM table!
▶ YouTube✓ Fast repair. Strength equal to/↑ original
✗ Different tap drill — per NASM table!
| Thread | Std | TPI/Pitch | Root Radius | vs Standard | Use |
|---|---|---|---|---|---|
| 1/4-28 UNJF | MIL-S-8879 | 28 TPI | 0.0160" | UNF Sharp | F-35, F-22 |
| 3/8-24 UNJF | MIL-S-8879 | 24 TPI | 0.0213" | UNF Sharp | Aircraft structure |
| 1/2-20 UNJF | MIL-S-8879 | 20 TPI | 0.0257" | UNF Sharp | Landing gear |
| 3/4-16 UNJF | MIL-S-8879 | 16 TPI | 0.0320" | UNF Sharp | Aerospace forgings |
| MJ6×1.0 | ISO 5855 | 1.0mm | 0.125P | ISO Sharp | Airbus A320 |
| MJ8×1.25 | ISO 5855 | 1.25mm | 0.156P | ISO Sharp | Boeing 737 |
| MJ10×1.5 | ISO 5855 | 1.5mm | 0.188P | ISO Sharp | A350/787 |
| MJ12×1.75 | ISO 5855 | 1.75mm | 0.219P | ISO Sharp | Aircraft engine |
| M10×1.5 Helicoil | NASM 21209 | 1.5mm | STI Tap: 10.5mm | — | Al repair |
| M12×1.75 Keensert | NASM 33537 | 1.75mm | Solid Insert | — | M6+ rigid |
Tapping — Hand/Machine
Taper/Plug/BottomHSS/Carbide
Taper Tap6-7 chamfer threads. Gentle entry. For through holes
Plug Tap3-5 chamfer threads. Most common. Through and blind holes
Bottom Tap1-1.5 chamfer threads. For deep blind holes
Spiral PointGun Tap — chip forward. Fast. Through holes
Spiral FluteChip back. For blind holes. ↓ chip jam
Form TapNo cutting! Cold forming → ↑ strength. Al/Cu/SS
Form Tap = cold form → ↑↑ thread strength vs. cut tap
Tap Drill = D - P (approximation). More precise: 75% thread engagement
✓ Form Tap = ↑↑ strength in Al/Cu
✗ Form Tap not for cast iron / hard steel
▶ YouTubeTap Drill = D - P (approximation). More precise: 75% thread engagement
✓ Form Tap = ↑↑ strength in Al/Cu
✗ Form Tap not for cast iron / hard steel
Thread Milling
CNCInternal+External↑ Accuracy
PrincipleEnd mill → helical path → thread profile
AdvantageOne tool → every ⌀! No need for a tap per size
Accuracy±0.01mm — ↑ than tapping
UseM3-M200+. Deep holes. Blind. Hard materials
Ti/InconelThread milling ↓ tool engagement → ↓ force
+OK+ One tool for every ⌀. ↑ accuracy. Al/Ti/Inconel
+OK+ No chip jam. Left+right hand. Any size
+NO+ CNC only. ↑ cycle time vs. tapping
▶ YouTube+OK+ No chip jam. Left+right hand. Any size
+NO+ CNC only. ↑ cycle time vs. tapping
Drill + Tap Sizes — Quick Guide
75% ThreadISO
M3×0.5Drill 2.5mm | 75% engage
M4×0.7Drill 3.3mm
M5×0.8Drill 4.2mm
M6×1.0Drill 5.0mm
M8×1.25Drill 6.8mm
M10×1.5Drill 8.5mm
M12×1.75Drill 10.2mm
DepthMin: 1.5×D. Recommended: 2×D
75% thread engagement = optimal! 100% ↑ >15% strength, ↑↑ tap break risk
✓ 75% = ↓ tap breakage + adequate strength
▶ YouTube✓ 75% = ↓ tap breakage + adequate strength
Lubrication + Speeds
SFMCutting Oil
Al / BrassVc=30-60 m/min. Kerosene / soluble
C-SteelVc=8-15 m/min. Sulphurized oil / Tap Magic
SS 304/316Vc=3-8 m/min. Tap Magic SS / Molykote
TiVc=3-5 m/min. TiCN tap. Flood coolant
Blind Hole↓25% speed + spiral flute tap
Form Tap↓30% vs. cut tap
+OK+ Tap Magic SS = best for SS
+NO+ SS dry = tap break! Lubrication mandatory
▶ YouTube+NO+ SS dry = tap break! Lubrication mandatory
| Material | Speed Vc m/min | Lubricant | Tap Type | Note |
|---|---|---|---|---|
| Al 6061 | 30-60 | Kerosene/Soluble | HSS/Carbide | High speed OK |
| Al Cast | 20-40 | Kerosene | HSS | Form tap excellent |
| Brass/Bronze | 20-50 | Dry/Oil | HSS | Form tap OK |
| C-Steel <0.3%C | 8-15 | Sulphurized Oil | HSS | Tap Magic |
| C-Steel >0.3%C | 5-10 | Cutting Oil | HSS/Co | ↓Speed |
| SS 304/316 | 3-8 | Tap Magic SS | Co/Carbide | ↓Speed mandatory! |
| SS 17-4PH | 2-5 | Tap Magic SS | Carbide | Very hard |
| Titanium Ti64 | 3-5 | Flood Coolant | TiCN Carbide | Spiral flute |
| Inconel 718 | 1-3 | Flood Coolant | Carbide | Very hard |
| PEEK/PA | 20-40 | Dry/Air Blast | HSS | Sharp edge |
| Cast Iron | 8-15 | Dry/Air Blast | HSS | Brittle — ↓Feed |
| Hardened >HRC45 | — | — | Thread Mill | No tapping! |
Torque — Tightening Torque
K FactorClamp ForceTension
FormulaT = K × F × D
TTorque (N·m)
KNut Factor: 0.11-0.22 (depends on lubricant)
FClamp Force (N) — what you actually want!
DNominal Diameter (m)
K DryK=0.20-0.22
K OiledK=0.13-0.15
K MoS2K=0.10-0.11 (↓↓ friction)
T = K×F×D: lower K = ↓torque → ↑clamp force!
↑K dry → most torque goes to friction, not clamp!
✓ MoS2 = ↓↓ torque, ↑ clamp. ↑ overtorque risk
✗ Wrong K = ±30% error in clamp force
▶ YouTube↑K dry → most torque goes to friction, not clamp!
✓ MoS2 = ↓↓ torque, ↑ clamp. ↑ overtorque risk
✗ Wrong K = ±30% error in clamp force
Property Classes — Metric Bolt Grades
ISO 898-18.8/10.9/12.9Grade
4.6RM=400 MPa | Rp0=240. General cheap bolts
4.8RM=420 MPa | Rp0=336. ↑ proof load
5.8RM=520 MPa | Rp0=420
8.8RM=800 MPa | Rp0=640. ↑↑ standard grade. Black
10.9RM=1040 MPa | Rp0=940. ↑↑↑ alloy steel. HRC≥32
12.9RM=1220 MPa | Rp0=1080. HRC≥39. ↑↑↑↑
8.8: 8 = RM/100 | 8 = Yield/RM×10. So: RM=800, Rp0=640
✓ 8.8 = the gold standard for most
✗ 12.9 HRC>39 → hydrogen embrittlement risk! Cadmium plate only
▶ YouTube✓ 8.8 = the gold standard for most
✗ 12.9 HRC>39 → hydrogen embrittlement risk! Cadmium plate only
SAE Bolt Grades — USA
SAE J429Grade 5/8
Grade 2RM=380-450 MPa. Low carbon. Non-critical
Grade 5RM=830 MPa. ≈ 8.8 metric. 3 radial lines
Grade 8RM=1040 MPa. ≈ 10.9. 6 radial lines
Grade B7ASTM A193. Alloy B7. Stud bolts. Flanges
Grade L9Alloy. RM=1170. Aircraft
Metric MarkM8.8 = 3 RL, M10.9 = 5 RL, M12.9 = 6 RL
SAE grade lines: Grade 2=0 | Grade 5=3 | Grade 8=6
✓ B7 stud = ASME flange standard
▶ YouTube✓ B7 stud = ASME flange standard
VDI 2230 — Bolt Joint Analysis
VDI 2230Fatigue
PurposeComprehensive calculation: clamp force → fatigue life
EmbeddingSettling: ↓5-15% clamp force!
ScatterTorque wrench ±30% clamp!
FatigueAlternating bolt load → S-N curve
UseAutomotive, aerospace, pressure equipment
+OK+ VDI 2230 = the most comprehensive bolted-joint analysis
+OK+ Embedding loss is mandatory in the calculation!
▶ YouTube+OK+ Embedding loss is mandatory in the calculation!
| Class/Grade | RM MPa | Rp0.2 MPa | M6 Torque Nm | M10 Torque Nm | M12 Torque Nm |
|---|---|---|---|---|---|
| 4.6 | 400 | 240 | 3.2 | 12.5 | 21.5 |
| 5.8 | 520 | 420 | 5.5 | 17.0 | 29.5 |
| 8.8 | 800 | 640 | 9.9 | 48 | 84 |
| 10.9 | 1040 | 940 | 14 | 68 | 118 |
| 12.9 | 1200 | 1080 | 16 | 80 | 140 |
| SAE Gr.5 | 830 | — | 8.1 | 46 | 80 |
| SAE Gr.8 | 1040 | — | 13 | 67 | 116 |
| A2-70 SS | 700 | 450 | 7.0 | 40 | 68 |
| A4-80 SS | 800 | 600 | 8.5 | 48 | 84 |
| Ti Grade 5 | 1000 | — | 5.0 | 28 | 48 |
* K=0.18 (light oil) | 75% proof load | 6H/6g fit
For SS bolts: -30% torque. For MoS2: an additional -15% torque.
For SS bolts: -30% torque. For MoS2: an additional -15% torque.
📊 Thread Type Comparison
| Type | Standard | Angle | Main Use | Compatibility | Note |
|---|---|---|---|---|---|
| ISO Metric | ISO 724 | 60° | Global standard. M3-M300+ | ISO only | M+⌀+Pitch |
| UNC/UNF | ASME B1.1 | 60° | USA/Canada. Fasteners | UN only | Not compatible with ISO! |
| BSPP (G) | ISO 228 | 55° | European hydraulics | BSP only | O-Ring seal |
| BSPT (R) | BS 21 | 55° | Fixed piping | BSP only | Self-seal |
| NPT | ASME B1.20.1 | 60° | USA pipe | NPT only | PTFE tape |
| Acme | ASME B1.5 | 29° | Lead screw | Acme only | Power trans. |
| Tr | ISO 2901 | 30° | CNC/lathe | Tr only | Power trans. |
| Ball Screw | ISO 3408 | — | CNC high precision | Ball nut | 90% eff. |
| UNJ | MIL-S-8879 | 60°+R | Aerospace high fatigue | UNJ gauge | ↑40% fatigue |
| MJ | ISO 5855 | 60°+R | Airbus/Boeing | MJ gauge | Aerospace |
| Buttress | ASME B1.9 | 7/45° | Unidirectional load | Buttress | Press/jack |
📌 Selection Guide
General connection? ISO Metric 6H/6g
USA-made? UNC/UNF
European hydraulics? G (BSPP) + O-Ring
Piping/steam? Rc (BSPT) / NPT
Machine drive? Tr / Acme / ball screw
Aerospace/defense? MJ / UNJ (root radius!)
Vibration resistance? Fine pitch + Nordlock/safety wire
USA-made? UNC/UNF
European hydraulics? G (BSPP) + O-Ring
Piping/steam? Rc (BSPT) / NPT
Machine drive? Tr / Acme / ball screw
Aerospace/defense? MJ / UNJ (root radius!)
Vibration resistance? Fine pitch + Nordlock/safety wire