§1The workholding interface
The T-slot is the universal anchoring interface of machine tools — inverted-T channels machined into the table, into which a matching bolt head or nut is captured and can slide anywhere along the slot.
Mills, drills, shapers, planers and grinders all carry them, and their genius is that they are a continuous fixing: a T-bolt or T-nut can be positioned at any point along the slot’s length, so any workpiece or fixture footprint can be clamped without a grid of tapped holes to line up with. The slot’s cross-section does the mechanical work. The narrow throat at the table surface passes the bolt shank but not its head; the wider cavity beneath captures the head or nut; and the two lips of table material between them carry the entire clamping load in bending — which is why the lips, not the bolt, turn out to be the part that sets the limit (§4). Everything clamped to a machine table — vices, angle plates, chucks on rotary tables, and the strap-clamp setups of §3 — ultimately hangs from these two lips of cast iron.
Contents§2The T-slot kit
Three anchor styles and a small supporting cast make up every T-slot setup, and the choice among them is mostly about convenience and thread protection.
The T-bolt is the original: a bolt forged or machined with a T-shaped head that drops into the slot (through an open end, or a slot’s entry recess) and slides to position, its shank rising through the throat to take a nut on top. The T-nut with a stud inverts the arrangement — a rectangular nut captured in the cavity, a stud screwed down into it, and the clamping nut on top of the stud — and is what clamping kits supply, because studs of many lengths pair with one set of nuts, and a damaged thread costs a stud, not a special bolt. Around them: step blocks and packing to support the outer end of a strap at any work height, strap clamps of various reaches, flange nuts and washers to spread load on the strap, and extension and coupling nuts for tall setups. One detail of the T-nut repays attention: its thread is deliberately staked (deformed) at the bottom so a stud cannot be screwed right through. If it could, the stud would bottom on the slot floor and further tightening would jack the nut upward against the lips — prying the slot open from inside rather than clamping the work. The staked thread is a small piece of designed-in protection for the table, and it is why a stud that will not pass through a T-nut is working correctly, not defective.
| Part | What it does | The rule it obeys |
|---|---|---|
| T-bolt | drops and slides in the slot, nut on top | matched to the slot exactly, never improvised (§6) |
| T-nut + stud | captured nut, interchangeable studs | staked bottom thread — the stud must not pass through |
| Step blocks & packing | support the strap’s outer end at any height | level with or a shade above the work (§3) |
| Strap clamps | the lever that delivers the force | bolt as close to the work as the setup allows (§3) |
| Flange nuts & washers | spread the load into the strap | a washer under every nut that bears on a strap (§5) |
| Extension & coupling nuts | reach for tall setups | full thread engagement throughout (§5) |
| Sold as a boxed clamping kit in one slot size, these parts are a system, not a drawer of spares: every rule in the right-hand column is one of this page’s equations wearing overalls, and the kit performs only when all of them are respected together. | ||
§3The strap is a lever
A strap clamp is a simple lever: bolt in the middle, work under one end, packing under the other — and where the bolt sits along it decides how much of the bolt’s force actually reaches the work.
Treat the strap as a beam pivoting on the packing (the beams page’s statics, one line of it): taking moments about the packing, the share of the bolt force delivered to the work is (L − a)/L. Put the bolt in the middle (a/L = 0.5) and the work receives 50.0% — the other half is spent pressing the packing block into the table. Put the bolt two-thirds of the way to the packing (a/L = 0.667) and the work gets only 33.3%. Slide it to one-third (a/L = 0.333) and the work gets 66.7% — moving the bolt from two-thirds to one-third doubles the clamping force from the identical bolt at the identical torque. At a/L = 0.25 the work takes 75.0%. The rule this arithmetic produces is the oldest one in the toolroom: the bolt goes as close to the work as the setup allows, and the packing merely reacts the remainder. It also explains the companion rule that the packing should stand level with or a shade higher than the work: a strap tilted slightly down toward the work bears on the work first and keeps its line of action honest, where packing set low tips the strap the other way and delivers the force to the packing instead.
§4The lip is the limit
Work the numbers and the bolt is not the weak link in a T-slot setup — the cast-iron lips of the slot are, and they run out of strength at about the bolt’s ordinary preload.
Take an M12 class 8.8 stud in a representative 14 mm slot. The bolt side, from the metric fasteners page: As = 84.3 mm², yield 640 × 84.3 = 53.9 kN, and the standard 75% preload target is 40.4 kN. The slot side: with a head-cavity width of 28 mm, each lip is a shelf (28 − 14)/2 = 7.0 mm wide; take the lip 9 mm thick and the T-nut 25 mm long. Each lip carries half the preload, 20.2 kN, acting at about the middle of its bearing land — a 3.5 mm moment arm — so the bending moment at the lip root is 20 200 × 3.5 ≈ 70 700 N·mm, on a section modulus (the beams page’s Z = bt²/6) of 25 × 9²/6 = 338 mm³. The bending stress is 70 700 ÷ 338 ≈ 210 MPa — and grey cast iron’s tensile strength is only some 200–250 MPa. The lip is at its limit at the bolt’s normal preload. That is the punchline of the whole page: the system is deliberately matched, with nothing in reserve on the table side, so over-torquing a T-slot stud does not break the stud — it breaks the table, cracking out a lip and ruining a slot that cannot be economically repaired. It is why T-slot work is tightened firmly but never with a pipe on the spanner, why the load should be spread along the slot with the longest T-nut available, and why a nut must never be drawn down over an unsupported strap end.
§5Setting up well
Good T-slot practice is five habits, each of which exists because of the mechanics in §3 and §4.
Bolt close to the work — the lever fractions of §3; every millimetre toward the work is free clamping force. Packing level or a shade high, and solid: a step block or machined packing, never a springy stack, since the packing carries up to half the bolt force and any give in it is lost preload. Full thread engagement: the nut fully on the stud, the stud fully into the T-nut down to the stake — a half-engaged thread strips at a fraction of its rating, exactly as the machine screws page’s nut-height rule warns. Spread the slot load: long T-nuts, and for heavy work more clamps at moderate force rather than one at heroic force, because §4’s lip stress scales directly with the load each nut carries. And washers under every nut that bears on a strap, so the tightening torque is not spent galling the strap’s slot. None of these is folklore; each is one of this page’s equations applied — which is the pattern with workholding generally: the setups that hold are the ones whose statics were respected.
Contents§6Sizes and fit
T-slots are named by their throat width, and every part of the kit — bolt, nut, stud — is matched to that one number.
A “14 mm slot” has a 14 mm throat, and takes the T-nut and T-bolt made for a 14 mm slot with the thread size standardised to suit (an M12 in the representative slot of §4). The fits are deliberately loose — the anchor must slide freely along a slot that accumulates chips and coolant — so a T-nut a size small will enter, cock over, and bear on one lip only: half the bearing, double the lip stress, and §4’s margin gone. The matching rule is therefore absolute: the kit matches the slot, exactly, and nothing is improvised. Two further cautions close the page. Do not run an ordinary hex-head bolt inverted in a slot as a makeshift T-bolt: its head is the wrong shape, bears on its corners, and concentrates the lip load §4 just computed onto two points. And keep the slots clean and lightly oiled — a chip packed under a T-nut tips it exactly as the undersized nut does, and the slot lips collect the damage. The T-slot is the one part of the machine that every job loads and no job can replace; the whole discipline of this page amounts to treating it that way.
Contents§7Quick reference
The working core of the page on one card rack.
Interface
throat · cavity · two lips
lips carry everything, in bending
Lever
R = F(L − a)/L
bolt at ⅓ vs ⅔ → 2× the force
Limit
M12 preload 40.4 kN
lip ≈ 210 MPa ≈ cast iron limit
Rules
bolt near work · packing high
long T-nuts spread the load
Protection
staked T-nut thread
over-torque breaks the table