On a lathe the cutting tool stays still and the workpiece turns. The spindle rotates the part; the carriage carries the tool along Z and across X. That reversal — compared with a mill, where the tool spins and an arm swings the changer clear of the work zone — is why a turning centre has to keep every tool it needs inside the working area at once.
Machine builders have spent decades trying to fit more tools in there and change between them faster. A manual lathe makes do with a quick-change tool post holding three or four. Every modern CNC turning centre uses a turret instead.
A turret is a disc on a base mounted to the X-axis carriage. Its stations hold cutting tools, tool holders and driven blocks, and a tool change is simply the disc indexing round — each tool stays where it was set. Nothing is lifted, swapped or carried, which is why this arrangement won: fast changes, high tool capacity, and repeatable concentricity every time the disc turns.
Early on, every machine builder designed its own disc geometry and its own way of clamping the tool. What survived standardisation are three interfaces, plus combined VDI/BOT discs:
Which one your machine has decides every holder you will ever buy for it, so it is worth being certain before ordering.
In a VDI turret the tool always mounts through a block. The disc has cylindrical bores, each with an internal toothed rack driven by a clamping screw brought out to the turret surface. Every VDI block therefore has a cylindrical shank with matching teeth. Size is the shank diameter in millimetres — VDI16, VDI20, VDI30, VDI40, VDI50, VDI60.
This is the fastest interface to change: slacken one screw and the whole block comes out, tool and all. Because the forms are standardised, choosing a block takes only a handful of parameters.
The standard forms:
VDI holders have a hand. Right and left versions exist and they are not interchangeable — a detail that costs a week when it is discovered after delivery rather than before ordering.
VDI turrets can be specified with driven tooling. The trade-offs against BOT are clamping force, which is lower because the load goes through one screw and a toothed shank, and price.
In a BOT turret, OD tools bolt straight onto the disc with no holder at all. Slots are milled into the turret face and the tool is clamped by a strap across it. ID tooling — boring bars, drills — mounts through blocks on the disc face, usually on four bolts.
The case for BOT is simple: fewer parts, nothing between the tool and the turret, and no holder to buy for external work. That makes it rigid and cheap.
The two costs are real, though. A BOT turret generally cannot be equipped with driven tooling. And the bolt pattern is often specific to the machine builder — SKT and others each have their own grid, written as, for example, BOT 66 x 90 mm. There is no catalogue of universal BOT blocks the way there is for VDI, so blocks are made to the pattern.
BMT took the rigidity of BOT and the standardisation of VDI:
Tools always mount through holders. The stations sit on the turret face and carry four threaded holes for the mounting screws, four locating slots, and a central bore. That bore does double duty: additional location, and — its main job — accepting the drive shank of a live block.
The bore diameter in millimetres is the size designation: BMT40, BMT45, BMT55, BMT60, BMT65. Unlike VDI, BMT blocks have no handedness — the same block works either way round.
One thing the size alone does not tell you. BMT60 exists in machine-specific forms that are not interchangeable with each other — the Okuma LB3000EX pattern, the LB4000EX pattern and the DMG pattern are all BMT60 and all different. Ordering by the number alone is how a shop ends up with a block that will not bolt on.
The drawback of BMT is cost, both for the turret and for the holders.
Look at the disc face rather than the paperwork. Round bores with a screw at the surface mean VDI. Milled slots with a clamping strap for OD tools mean BOT. A flat face with a four-bolt pattern around a central bore means BMT.
Then get the exact designation from the machine manual — VDI30 and VDI40 look alike in a photograph, and BMT60 comes in the machine-specific forms above. If you are not certain, send us the machine model and a photo of the turret face.
We supply static tool holders and driven tool holders for BMT and VDI turrets from stock in Canada, and make blocks to pattern for bolt-on turrets that no catalogue covers. Prices and stock are on every product page and you can order online without a quote.
For the wider picture — chucks, live centres, inserts and the rest — see our guide to tools for CNC lathes.
