Turret Milling Head

Write down what a cutter must be able to do: reach anywhere in X, Y and Z, and arrive at any angle. That is six freedoms, and a fixed head offers three.

Glue a 300 mm plywood upright to a plywood base. This is the column, and it must be square to the base — check it.

Screw a plywood disc to the top of the column with a single machine screw so it rotates. That is the turret.

Cut a 200 mm plywood arm and fit it so it slides in and out across the turret. That is the overarm, or ram.

Pivot a small block on the end of the ram with a screw so it tilts. Mark a dowel stub in it as the spindle.

Draw a degree scale around the head pivot with the protractor, zero at vertical.

Lay a 250 mm bar on the base. Reach the spindle to each end using only ram and turret — the workpiece never moves.

Lock the ram and turret and try again. Now the part must be re-clamped to reach the far end. Count the re-clampings.

Swivel the turret 20° and tilt the head 30°. Record both settings. Two rotations compose into one arbitrary approach direction.

Return both to zero and check the spindle against the square. Any error is what a real machine calls tramming, and it must be re-checked after every head move.

On the benchtop mill, sweep the dial indicator in a circle across the table and record the readings at four points. Equal readings mean the spindle is square to the table.

Set the milling vise on the table and indicate its fixed jaw parallel to the table travel. A swivelling head is worthless above a crooked vise.

List what the ram-and-turret buys (reach, angles, small footprint) against what it costs (rigidity, and re-tramming after every move). Heavy cuts still go on fixed-bed machines.

US 2,275,291, “Machine Tool Operating at Universal Angles in Overall Locations”, Rudolph F. Bannow, Bridgeport, Connecticut. Filed 4 April 1939, granted 3 March 1942. The claims cover a two-section articulated support — an overarm that both slides longitudinally and rotates, carried on a swivelling turret head, with worm-and-wheel angular adjustment — so a cutter can work on “extreme end portions of a long piece of work” without repositioning it.

The dates, in order. Bannow (1897-1962) conceived the improvement in 1936. He and Magnus Wahlstrom, both Swedish immigrants, had founded Bridgeport Machines in 1929 and were already selling a high-speed milling attachment. The complete knee-and-column vertical mill was first built in 1938; the patent was applied for in 1939 and issued in 1942. Sources that give a single date for “the Bridgeport” are picking one of four.

Why it took over the world's small shops. One machine of modest size and cost could drill, bore, face, slot and cut at compound angles — work that previously needed several dedicated machines or a horizontal mill and a lot of setup skill. For a jobbing shop that never makes the same thing twice, flexibility beats rigidity, and this machine is the flexible one.

What it cost. Everything that swivels can be out of square. A Bridgeport-type head must be trammed, and re-trammed after any head movement — steps 10 and 11 are that discipline. Production shops cutting one part in quantity still choose fixed-bed machines, which cannot be angled and do not need to be checked.

The direct line to CNC. Fit the knee mill's three lead screws with servo motors and you have the first generation of small CNC machining centres — which is exactly what happened in the 1970s, often to second-hand Bridgeports converted in place. The machine in this blueprint is the immediate ancestor of the desktop CNC mill, and its geometry is still visible in them.