Pallet Changer
Pallet changer is the mechanism that swaps the whole workholding table of a machining centre for another, and it answers a simple, costly problem: a machine cannot cut while it is being set up. Every job on a machining centre has two phases — the cutting, when the spindle is making parts, and the setting, when the operator loads work into the fixture, clamps it, proves it and starts the next cycle. On a machine without a pallet changer, those phases happen on the same table: the spindle stops while the operator unloads the finished parts and sets the new ones, and the machine — the most expensive thing in the shop — sits idle while its operator works. The pallet changer separates the two. The work is set up on a pallet, a removable table that stands at a loading station beside the machine, while the machine cuts the pallet already on its table; when the cycle ends, the changer swaps the finished pallet for the loaded one in a minute, and the spindle is cutting again while the operator sets up the next pallet outside the machine. This entry sets out what a pallet changer is and why it is one of the great productivity devices of the machine shop.
Pallets and why they pay
The pallet changer is built on the pallet, and the pallet is the key to the whole idea. A pallet is a rigid, accurate plate that carries the workholding — the vice, the fixture, the tombstone — and that locates on the machine’s table with precision. Each pallet is a complete, self-contained setup: the fixture is bolted to it, the parts are clamped in the fixture, and the pallet’s position on the machine is fixed by locating pins and a coupling so that it seats exactly the same way every time. Because the pallet is set up off the machine, at a loading station where the operator has room and time, the setting work that used to stop the spindle happens beside it. And because pallets are interchangeable, one machine can own several — the operator sets up pallet two while the machine cuts pallet one, and the machine’s only idle moment is the minute it takes to swap them. The pallet is what makes the separation physical: the setup leaves the machine, and the machine is free to do what it is for.
How the change happens
The mechanism that moves the pallets is the changer itself, and it comes in the forms that suit the machine. The simplest is the two-pallet changer: two pallets side by side, one on the machine’s table and one at the loading station, exchanged by a shuttle or a rotary mechanism that slides the finished pallet out and the loaded pallet in. The exchange is precise and quick: the machine finishes its cycle, the pallet is unclamped from the table, the changer draws it out to the loading position, slides the waiting pallet onto the table, and clamps it home on its coupling — the whole swap taking a minute or two, far less than any manual change of work. Beyond the two-pallet changer lies the pallet pool: a rack or a carousel holding many pallets, from which the machine draws any one by program, each pallet a different job or a different part, so the machine moves from job to job — changing its whole setup in minutes — without an operator between. On the largest systems the pallet changer becomes part of a flexible manufacturing cell, pallets travelling on rails between machines, each carrying its own work to whichever spindle is free.
The tombstone and the loaded pallet
The pallet changer is at its most powerful with the workholding that packs a pallet full, and that is the tombstone this wiki describes. A tombstone is the four-sided carrier that stands on a pallet, its faces each carrying fixtures and parts, so one pallet presents many parts to the spindle; the machine indexes the pallet’s rotary position or the tombstone’s faces, cutting part after part before the pallet need be changed. Set a tombstone pallet up at the loading station while the machine finishes the previous one, and the machine runs through a whole tombstone of parts — minutes or hours of cutting — before it needs the next exchange. The pallet changer and the tombstone together are the classic pairing of the production machine shop: the pallet carries a full load of work, the tombstone multiplies the parts on it, and the changer keeps the spindle cutting from one full pallet to the next with only a minute’s pause between. It is the machine-tool version of the assembly line, applied to the single machining centre.
The changer and automation
The pallet changer is also the bridge from the manned machine to the automatic one, and it is what makes a machining centre able to run without an operator for long stretches. A machine with a pallet pool can run lights-out: it cuts pallet one to completion, changes to pallet two that was set up earlier, cuts it, changes to pallet three — each pallet a fresh setup of parts, loaded while the machine was busy — and runs on through the pool until the work is done or the tools are worn. In the manned shop the changer gives the operator the same gift in miniature: the time to set up the next job carefully, outside the machine, while the machine earns its keep cutting the last one. And alongside the automatic tool changer — which frees the machine from waiting on tools — the pallet changer frees it from waiting on work: the tool changer keeps the spindle cutting through every operation, and the pallet changer keeps it cutting through every job. Between them they turn the machining centre into the machine that this wiki’s job cycle describes, cutting nearly without pause.
The pallet changer’s place
The pallet changer completes the machining centre’s escape from idle time, and it belongs with the other devices that keep the spindle busy. The machining centre is at its best when it is cutting, and every minute it stands still — waiting for a tool, waiting for a part, waiting for an operator — is a minute of its cost with no parts to show. The tool changer ended the wait for tools; the pallet changer ends the wait for work, holding the loaded setups ready at the machine’s side and swapping them in the minute the spindle goes quiet. On the modern machine the pallet changer is standard equipment wherever parts are made in runs — the pallet carrying its tombstone of parts, the changer keeping the spindle fed — and it is the quiet device that lets a machine cut part after part, pallet after pallet, with the operator’s hands busy setting up the next load of work while the machine, unchanging, does the cutting it was bought to do.