Part Rotating and Indexing
The second way a machining center or CNC mill might achieve rotary axis cutting and indexing is by moving the part rather than the cutter head. This can be accomplished on a programmable A axis accessory (more common), as shown on the right of Fig. 4.7, or a built-in axis in the middle of the table. There are also several other versions of programmable part index and rotate accessories.
Figure 4.7 – Programmable rotary axes can be add-on accessories or built-in parts of the machine
Part Indexing. A very efficient way to index parts is a foursided, tombstone table. Here parts are loaded on four sides of a sturdy vertical, work-holding tower. Each side can be indexed and locked toward the spindle.
The Secondary Linear Axes U, V, and W
CNC machines occasionally receive secondary, straight-line axes to add auxiliary tool slides or boring quills and other machining functions to their capability.
Secondary Linear Rule
To identify the secondary linear axes, determine the primary linear parallels (X, Y, or Z). If the secondary axis is parallel to
X, it is the u axis; y, it is the V axis; z, it is the w axis
For example, the small vertical milling machine shown in Fig. 4.8 has its programmable quill designated as the Z axis, therefore the knee, which also moves vertically, becomes the “W” axis.
Figure 4.8 – The programmable milling machine knee would be called a w axis since it moves parallel to the z axis
Coordinate Systems and Points
Absolute coordinates. Values based on the origin of the axis set—the PRZ.
Cartesian coordinates. Rectangular coordinate set that refers points to reference axes.
Coordinating (CNC axes). Physically moving the cutter to a known position relative to the part geometry, then setting axis registers to represent that position.
Default (CNC condition). An expected or preset value or mode.
Entity. A straight line or curved arc with a start and end point coordinate.
Full floating reference. The ability to place the PRZ anywhere within the work envelope (and sometimes beyond in special situations).
Homing (the CNC machine). Driving the machine to a fixed position on the machine; machine home.
Incremental value (relative value). Coordinates based on the previous entry. Jump-to-jump values. Sometimes called relative coordinates because each entry is based on the last.
Machine home (M/H). The never-changing utility position used for safe parking, accuracy refreshment, and setup purposes.
Null entry. A command, mode, or coordinate that has been set previously and can be optionally omitted from a program statement.
Polar coordinates. Point identification using radial and angular displacement from the origin.
Program reference zero (PRZ). Master origin for program and part geometry. Chosen by the programmer, PRZ is coordinated by the machinist during setup.
Quadrant. One of four possible 900 segments lying on a flat plane, created by the intersection of the two axes that define the plane.
Absolute and Incremental Value Coordinates
There are two different ways to use both Cartesian and polar coordinates.
Absolute Value Coordinates. An absolute coordinate is one where each entry represents the point’s distance from the master origin: X 5 0, Y 5 0, and Z 5 0.
Incremental Value Coordinates. These refer to a coordinate set where each entry represents the identified point’s distance from the previous point. These points can be envisioned as jumps from the present location to the next.
References: Gen [1-3].
Assessing questions:
What is the World Axis Standards?
How can be defined Three Primary Planes?
What the difference between the Primary Rotary Axes and The Mill Rotary Axes?
Features of the Secondary Linear Axes?
Coordinate Systems and Points?
Lecture 5. CNC systems. Five axis drive components. Industrial machining and turning systems. Vertical machining centers. Tech-specs for CNC turning centers
Turning and related operations
Turning is a machining process in which a single-point tool removes material from the surface of a rotating workpiece, as illustrated in Figure 5.1. Turning is traditionally carried out on a machine tool called a lathe, which provides power to turn the part at a given rotational speed and to feed the tool at a specified rate and depth of cut.
