Fixture Plates & Tooling Plates
The fixture plate — the tooling plate, the sub-plate — is the foundation that the other workholding of this wiki’s workholding group stands on. It is a thick, precision plate that bolts to the machine table or to a tombstone, drilled and tapped over its surface with a grid of accurately positioned holes, and it provides the mounting surface on which vices, clamps and fixtures are bolted. The plate turns the bare table of the machining centre — a plain surface with a few T-slots — into a regular grid on which any workholder can be set at a known, repeatable position; and because the plate is indicated square once and left in place, everything mounted on it inherits the plate’s truth. Where the tombstone entry described the tower that carries workholding round the machine, this entry describes the plate that carries it flat.
What a fixture plate is
A fixture plate is a plate made to be machined on, and its construction is all about accuracy and wear. It is a thick slab of steel or cast iron, ground flat on its top surface, with two sets of holes: tapped holes, spread in a regular grid across the face, into which the studs and bolts of clamps and fixtures screw; and a smaller number of location holes, precision reamed holes that take dowels or bushings, so that a vice or a fixture can be placed on the plate not merely where the tapped holes allow, but at an exact grid position, repeatable every time it is mounted. The plate is drilled and tapped on the grid, and its top face is hardened or protected, because it takes the wear that would otherwise mark the machine table. Bolted to the table by its own fixing holes, or to a pallet as a sub-plate, the fixture plate becomes the working surface of the machine — the surface that the shop bolts its workholding to and does not wish to drill into the machine itself.
The plate as the reference
The plate’s greatest value is that it can be made the reference for the whole setup, and the setting is done once. When a fixture plate is first fitted, it is indicated true: swept with a dial indicator so that its top face is square to the spindle and its grid is square to the machine’s axes, then bolted down and left. From then on, workholding mounted on the plate is square because the plate is square: a vice bolted to the plate at a known grid position, with its jaws set to the grid, needs no more than a light check before it cuts true. The grid also gives the setup its coordinates: because the holes sit at known positions, a fixture or a vice mounted on them puts the part at a known place in the machine’s coordinate system, which is how the shop relates the fixture to the part’s datums. The plate turns every new setup into a matter of bolting workholding to a reference that has already been made true, instead of indicating each vice and each clamp afresh.
Setting up on the plate
A job on a fixture plate is set up in a deliberate order, and the order follows the principles of this group. The plate is cleaned first — its tapped holes freed of chips, its face wiped, for a chip under a vice or a fixture throws the whole setup out — and the workholding is bolted on: a vice at one end of the grid for the batch’s first operation, a modular fixture or a set of clamps at another for the parts that need them. Each piece of workholding is located where the grid and the locating pins put it, and the parts are machined, the clamps holding them in the positions the plate established. Because the plate carries several vices or fixtures at once, the shop machines a whole row of parts in one run, moving from one end of the plate to the other; and because the plate and its grid are recorded — its hole pattern, its position on the table, the workholding that lives on it — a familiar job is set up again by bolting the same vices to the same holes, no indicating from scratch.
Types of plates
The plates come in the forms that suit the machines they serve, and each has its place. The table plate is the fixture plate bolted directly to the table of a vertical machining centre, covering the table with its grid and carrying the workholding of the everyday job. The sub-plate is the plate that mounts on a pallet — the tray that the machine or a pallet system exchanges — so that a whole setup, workholding and all, is prepared off the machine, loaded on its pallet, and swapped in and out as a unit; this is the plate that makes off-machine setup and quick changeover work. And the tooling plate in its fixture-building sense is the plate (often of aluminium) that a dedicated or modular fixture is built on, drilled and machined to the fixture’s own layout. Between the table plate and the sub-plate, the plates of the shop carry its workholding and protect its machine tables, and the tombstone that stands the work up is their vertical cousin on the horizontal machine.
The plate in the shop
In the shop the fixture plate is the quiet constant beneath the changing setups, and it pays for itself in changeover time and in protection. It spares the machine table the drilling, the tapping and the wear of a thousand setups, taking those marks on a plate that can be resurfaced when it is worn. It makes the setup repeatable: the same plate, the same grid, the same recorded hole positions, so that a job run again is set up again with the confidence that the workholding sits where it sat before. And it makes the setup movable: a sub-plate lifted with its vices and its fixtures intact and carried to another machine or stored until the job returns, holding its accuracy through the move. The fixture plate is not a workholder itself but the surface all workholders share — the precision grid that carries the vices, clamps and fixtures of the shop, square to the machine and true to the part’s datums, job after job.