1.1 Coordinates systems
CNC machining uses nine standard universal axes. The primary linear movements of X, Y, and Z. To identify arc or circular movements such as a programmable turntable, lathe spindle, or an articulating, wrist action milling head (rotary motion) three primary rotary axes (A, B, and C) are used. The auxiliary linear axes (U, V, and W) are the three secondary straight-line motions.
Figure 1.1 - Primary linear axes of X, Y, and Z.
A flat plane is defined as a combination of any two primary axis lines. There are three planes: X-Y, X-Z, and Y-Z (Fig. 1.2). View of a part placed on a vertical milling machine, the table represents the X-Y plane, whereas a lathe object is viewed in the X-Z plane.
When the machine control is capable of cutting curves in more than one of these three discrete planes, the programmer must add a code word to define in which plane the motion is to occur.
Figure 1.2 – Primary planes X-Y, X-Z, and Y-Z.
Figure 1.3 – The primary axes as they apply to three machines
Z-axis parallel to the main spindle; X-axis usually parallel to the floor; Y-axis perpendicular to X and Z. To identify any axes apply the righthand rule.
Some CNC machines feature programmable axes that rotate or articulate. Each is identified by the central primary linear axis around which it pivots. A axis rotates around a line parallel to X, B axes around Y, C around Z.
Articulating mill heads machines are equipped with a spindle head that can rotate in one or two planes during a cut (A or A 1 B). Bridgeport type mill has similar head rotations (Fig. 1.4).
Figure 1.4 – A/B axis tilting mill head
Rotary axis cutting and indexing in CNC machine also can be done by moving the part instead of the cutter head. It is accomplished on a programmable A axis accessory (Fig. 1.5), or in the middle of the table axis.
Figure 1.5 – Programmable rotary axes can be add-on accessories or built-in parts of the machine.
1.2 The Secondary Linear Axes u, V, and w
To add auxiliary tool slides or boring quills and other machining functions CNC machines have secondary, straight-line axes.
Secondary Linear Rule
By determining the primary linear parallels (X, Y, or Z) the secondary linear axes can be identified. If the secondary axis is parallel to X, it is the U axis; Y - V axis; Z - W axis.
For instance, a vertical milling machine (Fig. 1.6) has its programmable quill designated as the Z-axis. Thus, the knee becomes the “W” axis because also moves vertically.
Figure 1.6 – W axis is the knee in the programmable milling machine because it moves parallel to the Z axis
1.3 Coordinate Systems and Points
Absolute coordinates are values based on the origin of the axis set, so called the program reference zero (PRZ). It refers to master origin for program and part geometry and chosen by the programmer. Cartesian coordinates are rectangular coordinate set that refers points to reference axes. Moving the cutter to a defined position corresponding to the part geometry, then setting axis registers to show its position is called coordinating (CNC axes). An expected or preset value or mode is called the default in CNC condition. Entity is identified as a straight line or curved arc with a start and end point coordinate. Meaning of the full floating reference lies on the ability to place the PRZ anywhere within the work envelope.
Homing of the CNC machine is driving it to a fixed position on the machine or also called machine home. In addition, it refers to the never-changing utility position used for safe parking, accuracy refreshment, and setup purposes.
Coordinates which had the previous entry corresponds to the incremental (relative) value. Every entry, which is based on the last, is called jump-to-jump values. Previously set a command, mode, or coordinate is called the null entry. Employing radial and angular displacement from the origin we can define the polar coordinates. One of four possible 900 segments lying on a flat plane, created by the intersection of the two axes that define the plane is called quadrant.
Assessing questions:
1. What is the main purpose of using CNC machine?
2. Explain the architecture of NC machine tools?
3. What kind of coordinate system uses CNC machining?
4. How can be identified any axes using the right-hand rule?
5. Identify the secondary linear axes?
6. What does coordinating (CNC axes) mean?