What is a fourth axis?

A fourth axis rotates the workpiece about one axis. Rotating about X, it is called the A axis in the usual axis nomenclature; about Y, the B axis. On a vertical machining centre it is usually a CNC rotary table or indexer mounted on the machine table, often with a tailstock or support at the far end. If the axis only positions and the machine then mills with three axes, that is indexing or 3+1. If it turns during the cut, that is simultaneous 4-axis work, for cams, helical grooves or engraving.

What does a fourth axis reach?

On a horizontal machining centre a rotary table about the vertical axis, the B axis, is common. One rotary axis reaches the sides around that axis. Faces angled in two directions and features in the end faces, such as axial holes, cannot be reached with the tool axis normal to them; a free end face can still be milled with the side of the cutter. How many setups remain also depends on the workholding, the part length, the support at the far end, the tool length and the tolerances between features. The relative position of the sides depends on the indexing accuracy of the axis and how accurately its centre line has been located.

How does a fourth axis work in practice?

A part with features on four sides around one axis can take four setups on a 3-axis machine, and fewer with a fourth axis. That saves setup time and can hold the sides more accurately in position relative to each other. If the rotary table is not permanently fitted, mounting it, aligning it and locating its centre line add setup time of their own. With a tombstone or multiple fixtures on the axis, several parts can be clamped per cycle. Whether it is cheaper depends on quantity, hourly rate and whether the axis is already on the machine.

What is a fourth axis often confused with?

A fourth axis is not 3+2: with one rotary axis you bring the sides around that axis square to the tool, but not faces angled in two directions. Nor is every dividing head a CNC axis: a manual dividing head indexes in fixed steps and needs a hand on the machine at every position.

How do you see a fourth axis in Blake?

In your machine park you record a 3-axis machine with a rotary table as a four-axis machine, with its hourly rate and the envelope left once the table is fitted. The first check per machine shows whether a part fits and which limit decides. You enter per route how many setups remain with the fourth axis. More on the factory.