Cnc machining nm09/2

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4.2 Program zero points

A program zero is simply the point which a programmer uses as a datum or reference point from which movement instructions originate. These points can be anywhere on the workpiece or in fact off the workpiece. The figures below show some of the options which can be used.

Figure 4.5	Figure 4.6
Figure 4.7	Figure 4.8

Once a program zero reference has been established and Cartesian coordinates applied the tool path instruction will adopt sense of direction based on positive and negative X, Y and Z coordinates. Generally the program zero selected for a milling job is the bottom left hand corner of the work so allowing tool movement across the work to be in the (+) X, (+) Y coordinates quadrant.

Figure 4.9

For programming on a CNC lathe the program zero can be set either at the face of the chuck jaws, or the end of the workpiece. Once again to allow programming in the (+) X +Z coordinates the preferred program zero point is the face of the chuck jaws.

Figure 4.10 Figure 4.11

Note

In programming tool paths for both lathes and machining centres the programmer calculates the coordinates on the assumption that a point on the tool will move from one coordinate to another. This is done even through on a machining centre X and Y movement is created by table movement.

4.3 Absolute and incremental co-ordinate programming

4.3.1 Incremental (G91)

The word ‘incremental’ may be defined as a dimension or a movement with respect to the preceding point in a prescribed sequence of points. Each positioning move is described quantitatively in distance and in direction from a previous point rather than from a fixed zero reference point. In incremental mode all moves are with respect to the last position reached.

Figure 4.12 Tool path

4.3.2 Absolute (G90)

The data in the absolute system describes the next location always in terms of its relationship to the fixed zero point (0,0). The zero point when used as a program datum is known as the program origin. The G90 code sets the control up in ABSOLUTE mode. All moves are performed with respect to the axes zero.

Figure 4.13 Tool path

4.4 Machine home and work zero

4.4.1 Principal programming points

1. Machine reference or home position is the datum from which all machine motions are taken. Most machines require movement to this position during the power-up procedure so that this datum is established. This is known variously as homing, machine referencing, zero return etc.

2. Program origin is a datum position on the workpiece or the fixture holding it. Basing program co-ordinates from a Datum on the workpiece is far more convenient than trying to base part co-ordinates on the machine home position, so a variety of techniques have been adopted by control manufacturers to allow setting program origins at any position within the machine movements.

Three systems are in general use, some controls are capable of all three, some only two of them. They are:

By manually zeroing each axis via a button on the operating panel while the machine is at the appropriate position.
By achieving the same result as above but via a program code.
By offsetting (or shifting) the machine datum to the required workpiece position via program codes, or keyboard entry on the machine control unit (MCU).

Once the program origin has been established by one of these methods, all machine motions are referenced to the program origin and will remain so until cancelled.

4.5 CNC calculations — Basic trigonometry

The assumption is made that students have the required maths competence for this unit.

The following section is designed to re-aquaint all CNC Machining students with the geometry and trigonometry which are relevant to this field of technology.

4.5.1 Basic geometry

Example Exercise

Answers:

A = 130°

B = 120°

C = 50°

4.5.2 Circles, radii and tangents

A line drawn from the centre of an arc or circle to the circumference will strike any tangent line at 90°

Similar triangles

Any triangle drawn within a circle using the diameter as the hypotenuse with the points of intersection at the circumstances will be a right angled triangle.

4.5.3 Laws of triangles

An Isosceles triangle is one which has two lengths and two adjacent angles equal.

Isosceles triangles are most common in any calculation which involves circles, radii and chords.

Any line drawn from the vertex of an isosceles to strike the base at 90° will:

Bisect the base
Bisect the vertex angle and,
Form two right angled triangles

An equilateral triangle is one which has all sides of equal length and all angles equal to 60°.

A line which bisect the angles of an equilateral triangle pass through the centre of the triangle.

The centre of an equilateral triangle is also the centre for any circle to be drawn inside or outside of the triangle.

Congruent triangles are those which are the same in all respects.

Note: where two radii blend it is often a requirement that you must first solve for triangles not directly associated with the problem in order to get relevant information.