Tooling and hardware
T-slot dimensions and T-nut sizes, inch and metric
A 14 mm T-slot (ISO 299) is 23 to 25 mm wide in the head space and 9 to 11 mm deep under the lip, 23 to 28 mm deep overall, and takes an M12 T-nut. A 5/8 in table slot takes a 1/2-13 or 3/8-16 T-nut.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Source: ISO 299 (machine tool tables, T-slots and corresponding bolts) and DIN 650 (T-slots), as adopted in IS 2013 and JIS B 0952; DIN 508 (T-slot nuts) as published by Halder; T-slot cutter sizes from ICS Cutting Tools catalog data; inch T-nut sizes from Te-Co catalog data. ASME B5.1M (T-slots, their bolts, nuts and tongues) inch slot dimensions are not listed yet.
| Slot width A, mm (in) | Head space width B, min to max, mm (in) | Head space depth C, min to max, mm (in) | Total depth H, min to max, mm (in) | T-nut thread, DIN 508 |
|---|---|---|---|---|
| 6 (0.2362) | 11 to 12.5 (0.4331 to 0.4921) | 5 to 6 (0.1969 to 0.2362) | 11 to 13 (0.4331 to 0.5118) | M5 |
| 8 (0.3150) | 14.5 to 16 (0.5709 to 0.6299) | 7 to 8 (0.2756 to 0.3150) | 15 to 18 (0.5906 to 0.7087) | M6 |
| 10 (0.3937) | 16 to 18 (0.6299 to 0.7087) | 7 to 8 (0.2756 to 0.3150) | 17 to 21 (0.6693 to 0.8268) | M6 or M8 |
| 12 (0.4724) | 19 to 21 (0.7480 to 0.8268) | 8 to 9 (0.3150 to 0.3543) | 20 to 25 (0.7874 to 0.9843) | M10 |
| 14 (0.5512) | 23 to 25 (0.9055 to 0.9843) | 9 to 11 (0.3543 to 0.4331) | 23 to 28 (0.9055 to 1.1024) | M12 |
| 18 (0.7087) | 30 to 32 (1.1811 to 1.2598) | 12 to 14 (0.4724 to 0.5512) | 30 to 36 (1.1811 to 1.4173) | M16 |
| 22 (0.8661) | 37 to 40 (1.4567 to 1.5748) | 16 to 18 (0.6299 to 0.7087) | 38 to 45 (1.4961 to 1.7717) | M20 |
| 28 (1.1024) | 46 to 50 (1.8110 to 1.9685) | 20 to 22 (0.7874 to 0.8661) | 48 to 56 (1.8898 to 2.2047) | M24 |
| 36 (1.4173) | 56 to 60 (2.2047 to 2.3622) | 25 to 28 (0.9843 to 1.1024) | 61 to 71 (2.4016 to 2.7953) | M30 |
A is the throat (slot opening) width, B the head space width under the lip, C the head space depth, H the depth from the table top to the slot floor. Inch values are mm / 25.4, to 4 places. Slot width tolerance: H8 for locating (reference) slots, H12 for clamping slots.
| Table slot width (in) | Table slot width (mm) | Stud threads T-nuts are made in |
|---|---|---|
| 5/16 (0.3125) | 7.94 | 1/4-20 |
| 3/8 (0.3750) | 9.53 | 1/4-20, 5/16-18 |
| 7/16 (0.4375) | 11.11 | 5/16-18, 3/8-16 |
| 1/2 (0.5000) | 12.70 | 5/16-18, 3/8-16, 7/16-14 |
| 9/16 (0.5625) | 14.29 | 3/8-16, 1/2-13 |
| 5/8 (0.6250) | 15.88 | 3/8-16, 1/2-13 |
| 11/16 (0.6875) | 17.46 | 1/2-13, 5/8-11 |
| 3/4 (0.7500) | 19.05 | 1/2-13, 5/8-11 |
| 13/16 (0.8125) | 20.64 | 5/8-11, 3/4-10 |
| 7/8 (0.8750) | 22.23 | 3/4-10 |
| 1 (1.0000) | 25.40 | 3/4-10, 7/8-9 |
| 1 1/16 (1.0625) | 26.99 | 3/4-10, 7/8-9, 1-8 |
| 1 1/8 (1.1250) | 28.58 | 7/8-9, 1-8 |
| 1 1/4 (1.2500) | 31.75 | 1-8 |
| 1 5/16 (1.3125) | 33.34 | 7/8-9, 1-8 |
| 1 3/8 (1.3750) | 34.93 | 1 1/4-7 |
Slot width is the throat (slot opening). The fraction shows its decimal to 4 places; mm = in x 25.4, to 2 places. One slot width takes nuts in more than one stud thread.
Which T-nut fits your slot
Matched slot width
in
difference = w - A = 0.627 - 0.6250 = 0.0020 in
5/8 in slot = 0.6250 in (15.88 mm). Difference from the reading: 0.0020 in (0.05 mm). Nearest metric slot: 14 mm (0.5512 in), 0.0758 in off.
T-nut stud threads for this slot
T-nuts are sold by slot width; pick the stud thread. Inch slot head space and depth are not listed yet; check the nut height against your slot.
Measure your slot before ordering nuts; table makers do not all follow the standard sizes.
| Bolt size (in) | Cutter diameter (in) | Face width (in) | Neck diameter (in) | Shank (in) |
|---|---|---|---|---|
| 1/4 | 9/16 (0.5625) | 15/64 (0.2344) | 17/64 (0.2656) | 1/2 (0.5000) |
| 5/16 | 21/32 (0.6563) | 17/64 (0.2656) | 21/64 (0.3281) | 1/2 (0.5000) |
| 3/8 | 25/32 (0.7813) | 21/64 (0.3281) | 13/32 (0.4063) | 3/4 (0.7500) |
| 1/2 | 31/32 (0.9688) | 25/64 (0.3906) | 17/32 (0.5313) | 3/4 (0.7500) |
| 5/8 | 1 1/4 (1.2500) | 31/64 (0.4844) | 21/32 (0.6563) | 1 (1.0000) |
| 3/4 | 1 15/32 (1.4688) | 5/8 (0.6250) | 25/32 (0.7813) | 1 (1.0000) |
| 1 | 1 27/32 (1.8438) | 53/64 (0.8281) | 1 1/32 (1.0313) | 1 1/4 (1.2500) |
| 1 1/4 | 2 7/32 (2.2188) | 1 3/32 (1.0938) | 1 9/32 (1.2813) | 1 1/4 (1.2500) |
Fractions with their decimals to 4 places. The cutter diameter and face width set the head space width and depth the cutter leaves; the throat is cut first with an end mill.
How the lookup works
w_in = w_mm / 25.4; difference = w - A
| Symbol | Meaning | Unit |
|---|---|---|
| w | Measured slot opening (throat) width | in or mm |
| w_mm, w_in | The same reading in mm and in inches | mm, in |
| A | Standard slot width from the tables above | in or mm |
| difference | Reading minus the slot size; the match is the slot with the smallest difference, within 0.010 in (0.25 mm); within that of an inch and a metric slot, both are shown | in or mm |
The lookup checks your reading against every inch slot and every metric slot in the tables and reports the closer one. When the reading is within 0.010 in (0.25 mm) of both an inch slot and a metric slot, it shows both and asks for the head space (undercut) width, which tells them apart. A reading more than 0.010 in (0.25 mm) from every slot is not a standard size. An inch nut is never matched to a metric slot, or the other way.
Worked example: 0.627 in reading
Throat width measured at 0.627 in.
- Nearest inch slot: 5/8 in = 0.6250 in. Difference: 0.627 - 0.6250 = 0.0020 in.
- Nearest metric slot: 14 mm = 14 / 25.4 = 0.5512 in, 0.0758 in off.
- 0.0020 in is under 0.010 in, and closer than the metric slot.
Result: 5/8 in slot; T-nuts in 3/8-16 or 1/2-13.
Worked example: 14.05 mm reading
Throat width measured at 14.05 mm.
- In inches: 14.05 / 25.4 = 0.5531 in.
- Nearest metric slot: 14 mm. Difference: 14.05 - 14.00 = 0.05 mm (0.0020 in).
- Nearest inch slot: 9/16 in (0.5625 in), 0.0094 in off.
- 0.05 mm is under 0.25 mm, and closer than the inch slot. The 9/16 in slot is 0.24 mm off, also under 0.25 mm, so the lookup lists it second; a head space 23 to 25 mm wide confirms the 14 mm slot.
Result: 14 mm slot, the closer match; T-nut M12 (DIN 508; M10 nuts are also made for 14 mm slots); head space 23 to 25 mm wide, 9 to 11 mm deep.
Worked example: 14.15 mm reading, two candidates
Throat width measured at 14.15 mm.
- In inches: 14.15 / 25.4 = 0.5571 in.
- 14 mm slot: 14.15 - 14.00 = 0.15 mm, under 0.25 mm.
- 9/16 in slot (14.29 mm): 14.15 - 14.29 = -0.14 mm (-0.0054 in), also under 0.25 mm.
- Both slots are within 0.25 mm of the throat reading, so measure the head space width: 23 to 25 mm on a 14 mm slot.
- A 22.15 mm reading is the same case: 0.15 mm over 22 mm and 0.08 mm (0.0030 in) under 7/8 in (22.23 mm); a 22 mm slot is 37 to 40 mm wide in the head space.
Result: 9/16 in or 14 mm (the closer slot is listed first); order nuts (3/8-16 or 1/2-13 for 9/16 in, M12 for 14 mm) only after the head space width confirms the slot.
How to measure your slot
- Blow out chips and wipe the slot.
- Measure the throat width with inside caliper jaws or gauge pins at three places along the slot; use the smallest reading.
- Measure from the table top to the slot floor with a depth gauge; that is the total depth H.
- Measure the lip thickness, table top to the underside of the lip, with a hook rule or a small square; H minus the lip thickness is the head space depth.
- Enter the throat width in the lookup; check that the T-nut height fits under the lip with clearance above the floor.
Spot checks against the sources
- 14 mm slot: B 23 to 25 mm, C 9 to 11 mm, H 23 to 28 mm (IS 2013, ISO 299). A DIN 851 cutter for a 14 mm slot is 25 mm in diameter and 11 mm wide (cutter maker data), matching B max and C max. T-nut M12: Halder marks it as the DIN 508 size, and Te-Co lists M12 nuts for a 14 mm slot.
- 5/8 in slot: Te-Co lists T-nuts in 1/2-13 and 3/8-16. A 1/2 in bolt T-slot cutter is 31/32 in in diameter with a 25/64 in face (ICS catalog data).
Shop notes
- T-nuts are sold by table slot width; the stud thread is a separate choice. A 9/16 in slot takes 3/8-16 or 1/2-13 nuts.
- Metric T-nut tongues are made under the slot width: 13.6 mm for a 14 mm slot, 11.6 mm for 12 mm.
- Locating slots are held to H8, clamping slots to H12; only a locating slot is good for keying a fixture with tenons.
- The tap drill for the stud thread is on the tap drill chart.
FAQ
What size T-nut fits a 14 mm T-slot?
An M12 T-nut per DIN 508; M10 nuts are also made for 14 mm slots.
What are the dimensions of a 14 mm T-slot?
Head space 23 to 25 mm wide and 9 to 11 mm deep, 23 to 28 mm deep overall (ISO 299).
What T-nut fits a 5/8 in table slot?
A 5/8 in slot T-nut, in 1/2-13 or 3/8-16.
What T-slot cutter for a 1/2 in bolt?
31/32 in diameter, 25/64 in wide, 17/32 in neck, on a 3/4 in shank.
What is the difference between locating and clamping T-slots?
Tolerance on the slot width: H8 for a locating slot, H12 for a clamping slot.
Is a 14 mm slot the same as a 9/16 in slot?
No. 14 mm is 0.5512 in, 0.0113 in under 9/16 in (0.5625 in); measure the slot and order the nut for that system. A reading such as 14.15 mm is within 0.25 mm of both, so measure the head space width too: 23 to 25 mm on a 14 mm slot.
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