Milling Machine Table T-Slot Dimensions Chart (DIN 650, ISO 299 & ASME B5.1M)
Machine tool table T-slots are standardized by their nominal throat width (a) under DIN 650 / ISO 299 (metric) and ASME B5.1M (inch). To prevent casting break-out when tightening hold-down studs, a DIN 508 T-nut always uses a thread diameter one standard size smaller than the slot throat width (for example, an M12 stud in a 14 mm T-slot, or a 1/2"-13 stud in a 5/8" / 16 mm Bridgeport/Haas T-slot).
Milling Machine Table T-Slot & DIN 508 T-Nut Dimensions Calculator
Lookup exact DIN 650 / ISO 299 and ASME B5.1M T-slot throat width (a), undercut width (b), undercut height (c), and clamping stud size.
How Are DIN 650 / ISO 299 Machine Table T-Slots Dimensioned and Toleranced?
Every CNC vertical machining center (VMC), horizontal boring mill, knee mill, rotary indexing table, and cast-iron subplate relies on inverted T-slots to anchor precision vises, tombstones, and toe clamps. Under ISO International Metrology Standard (ISO 299) and German standard DIN 650, a machine T-slot is defined by four primary geometric dimensions: the upper vertical Throat Width (a), the lower horizontal Undercut Head Width (b), the lower Undercut Head Height (c), and the Total Depth (h) from the table top surface to the bottom of the slot.
On precision CNC machine tables, the center T-slot (the Master Reference Slot) has its upper throat width a precision ground to an ISO H8 tolerance (for example, 14.000 to 14.027 mm on a 14 mm slot, or 18.000 to 18.027 mm on an 18 mm slot) so fixture alignment keys and sine plates align parallel to the X-axis travel within 0.005 mm without dial-indicating! The remaining parallel flanking T-slots are milled to an ISO H12 tolerance solely for clamping clearance.
| Nominal Throat Width a (H8 / H12) | Clamping Stud Thread (DIN 508) | Undercut Width b (Min – Max) | Undercut Height c (Min – Max) | Total Slot Depth h (Min – Max) | DIN 508 T-Nut Base Width × Height |
|---|---|---|---|---|---|
| 10 mm T-Slot | M8 × 1.25 | 16.0–18.0 mm (0.630"–0.709") | 7.0–8.0 mm (0.276"–0.315") | 17.0–21.0 mm (0.669"–0.827") | 15.0 mm × 6.0 mm (M8 T-Nut) |
| 12 mm T-Slot | M10 × 1.50 | 19.0–21.0 mm (0.748"–0.827") | 8.0–9.0 mm (0.315"–0.354") | 20.0–24.0 mm (0.787"–0.945") | 18.0 mm × 7.0 mm (M10 T-Nut) |
| 14 mm T-Slot | M12 × 1.75 | 23.0–25.0 mm (0.906"–0.984") | 9.0–11.0 mm (0.354"–0.433") | 23.0–28.0 mm (0.906"–1.102") | 22.0 mm × 8.0 mm (M12 T-Nut) |
| 16 mm T-Slot (Haas VF) | M14 / 1/2"-13 | 26.0–28.0 mm (1.024"–1.102") | 11.0–13.0 mm (0.433"–0.512") | 27.0–32.0 mm (1.063"–1.260") | 25.0 mm × 10.0 mm (M14 / 1/2" T-Nut) |
| 18 mm T-Slot | M16 × 2.00 | 30.0–32.0 mm (1.181"–1.260") | 12.0–14.0 mm (0.472"–0.551") | 30.0–36.0 mm (1.181"–1.417") | 28.0 mm × 11.0 mm (M16 T-Nut) |
| 22 mm T-Slot | M20 × 2.50 | 37.0–40.0 mm (1.457"–1.575") | 16.0–18.0 mm (0.630"–0.709") | 38.0–45.0 mm (1.496"–1.772") | 35.0 mm × 14.0 mm (M20 T-Nut) |
| 28 mm T-Slot | M24 × 3.00 | 46.0–50.0 mm (1.811"–1.969") | 20.0–22.0 mm (0.787"–0.866") | 48.0–56.0 mm (1.890"–2.205") | 44.0 mm × 18.0 mm (M24 T-Nut) |
| 36 mm T-Slot | M30 × 3.50 | 56.0–60.0 mm (2.205"–2.362") | 25.0–28.0 mm (0.984"–1.102") | 61.0–70.0 mm (2.402"–2.756") | 54.0 mm × 22.0 mm (M30 T-Nut) |
When machining custom subplates or tombstones with DIN 650 T-slots on a vertical machining center, never plunge or slot the undercut throat at full radial engagement in a single pass without evacuating chips from the central guide throat. Program a roughing end mill to open the vertical throat width (a) to full depth (h) plus 0.5 mm clearance first, then engage the Woodruff-style T-slot cutter in two alternating climb-milling side passes with high-pressure flood coolant directed into the slot entry to prevent chip recutting and carbide tooth chipping.
Why DIN 508 T-Nuts Step Down One Thread Size Below the Throat Width (a)
Beginners frequently try to thread an M16 stud into a 16 mm T-slot or an M14 stud into a 14 mm T-slot—only to discover that no commercial T-nut exists with those proportions. Under DIN 508 and Machinery's Handbook (Industrial Press), the unthreaded neck of a T-nut must slide smoothly inside the throat width a with at least 1.5 to 2.0 mm of wall thickness around the tapped female threads so the neck cannot expand and jam inside the throat under tensile load! Therefore, a 10 mm T-slot takes an M8 stud; a 12 mm T-slot takes an M10 stud; a 14 mm T-slot takes an M12 stud; a 16 mm (5/8") T-slot takes an M14 or 1/2"-13 stud; and an 18 mm T-slot takes an M16 stud.
| Inch Throat Width (a) | Standard T-Slot Bolt / Stud | Head Space Width (b) | Head Space Depth (c) | Total Slot Depth (h) | Common Milling Machine Fitment |
|---|---|---|---|---|---|
| 3/8" (0.375" / 9.53 mm) | 3/8"-16 UNC | 21/32" (16.67 mm) | 7/32" (5.56 mm) | 11/16" (17.46 mm) | Benchtop mills, Sherline / Taig / Mini-VMCs |
| 1/2" (0.500" / 12.70 mm) | 1/2"-13 UNC | 7/8" (22.23 mm) | 9/32" (7.14 mm) | 29/32" (23.02 mm) | Tormach PCNC 1100/770, compact toolroom mills |
| 9/16" (0.5625" / 14.29 mm) | 1/2"-13 UNC | 1.000" (25.40 mm) | 11/32" (8.73 mm) | 1-1/16" (26.99 mm) | Older Cincinnati and Kearney & Trecker knee mills |
| 5/8" (0.625" / 15.88 mm) | 1/2"-13 or 5/8"-11 | 1-1/8" (28.58 mm) | 13/32" (10.32 mm) | 1-7/32" (30.96 mm) | Bridgeport Series I & II, Haas VF-1 to VF-5 (16 mm / 5/8") |
| 3/4" (0.750" / 19.05 mm) | 3/4"-10 UNC | 1-5/16" (33.34 mm) | 17/32" (13.49 mm) | 1-15/32" (37.31 mm) | Large bed mills, Haas VF-6 to VF-11, horizontal boring mills |
| 1" (1.000" / 25.40 mm) | 1"-8 UNC | 1-3/4" (44.45 mm) | 11/16" (17.46 mm) | 1-15/16" (49.21 mm) | Heavy gantry mills and planer-mill tables |
Three Golden Rules to Prevent Cracking Cast-Iron Machine Table T-Slot Lips
Because CNC milling machine tables are cast from grey flake-graphite cast iron (typically GG-25 / Class 40, which has high vibration damping but low tensile fracture toughness), improper T-nut usage can snap the upper cast-iron lip right off the table. Follow the three rules established by ASME Geometric & Mechanical Standards (ASME B5.1M) and NIST (National Institute of Standards and Technology) machine shop practice: (1) Never let a clamping stud bottom out against the floor of the T-slot—a stud threaded all the way through the bottom of the T-nut acts as a jackscrew that pries the table lip upward; (2) use full-length flanged DIN 508 T-nuts rather than standard hex bolts slid into the slot; and (3) never clamp directly over the open gap at the end of a T-slot. Check your clamping stud preload on our Bolt Torque & Preload Calculator, size vise locating keys on our ANSI B17.1 & DIN 6885 Keyway Calculator, verify fixture dowel holes on our Dowel Pin Reamer Tolerance Chart, and check shaft tolerances on our ISO 286 Hole & Shaft Tolerance Calculator.