Zero Offset and Tool Offset on CNC Machine

On each CNC machine, zero points and reference points are defined. The part program for any component is developed relative to these points.

Zero Offset.  The machine zero point is at the origin of the coordinate measuring system of the machine. The machine zero point is fixed and cannot be shifted. The machine zero point is also called 'Home position'.

Tool Offset. The machine must be informed the amount the machine zero and component zero point. The component of offset between zero points is the starting point of the machining operation on the work piece at X, Y and Z axis.

Tool Nose/Cutter Nose Radius Compensation

An interesting feature of a modern CNC machine is automatic tool nose compensation. While writing a program, the programmer assumes that the tool has a sharp  and  pointed  cutting edge,  and  he  programs the  movement  of  this  edge. However, in practice, the nose has a finite radius because of which the actual surface obtained after machining will be somewhat different from the desired profile. CNC machines have the provision to automatically correct the programmed tool paths in order to obtain the desired profile as shown in "Tool Nose Radius Compensation" figure below. In fact, exactly the same surface profile can be obtained by tools having different nose radii. The operator has to only specify the nose radius. Corrective calculations are done by the machine.

Without compensation, the imaginary tool tip follows the programmed path resulting in undercutting (the shaded area), i.e., an over-sized job. For compensation, the centre of the tool nose (C) is made to follow the programmed path with tool nose radius compensation (right) command active (G42 on a FANUC machine).

In milling operation, the part program is developed for the cutter path with reference to the centre of the tool rather than the point on the perriphery where the actual cutting takes place. At the time of writing a part program, a cutter of suitable diameter is selected and program is developed for centre line of the cutter. But when actual machining is done, if a cutter of smaller diameter is used, it will result in a larger work piece and if a cutter with larger diameter is used, it will result in a smaller work piece.

The difference in the programmed diameter of the cutter and the diameter of the actual cutter is accounted for by cutter radius compensation. The difference in the diameter of the cutter is entered into the control system. The control system will then generate a new cutter path. The new path will be separated from the programmed cutter path by difference in the radius of programmed cutter and the actual cutter. It is necessary to indicate whether the compensation to be made is to the right or to the left of the tool when machining. The following three G-codes used for cutter radius compensation are:

(a) G-41 :- Compensation applied to shift the programmed cutter path to the left

(b) G-42 :- Compensation applied to shift the programmed cutter path to the right

(c) G-40 :- Cancel cutter radius compensation

ISO designation of Tool Holder, Boring Bars

Tool Inserts. Carbides and other harder tool materials are very costly. Moreover, they cannot be machined. So, only tool tips are made for such materials using powder metallurgy technique. In this method, the tool material is taken in a powder form. It is mixed with a suitable binder (in powder form) and compressed in the shape of an insert which may have a hole in the centre for clamping to the shank. Sometimes, it is even brazed to the shank. To ensure proper binding and strength in the compacted powdery mixture, it is kept at an elevated temperature for a long time. This process is called sintering.

Inserts are available in various shapes such as triangle, square, rectangle, pentagon, hexagon, octagon, diamond shaped and circle.  They cannot be re-sharpened, but they have a number of cutting edges. For example, a rectangular insert has 4 edges on each face. i.e., total 8 edges. A round has a large number of cutting edges, but its application is limited to such cases where radius does not affect the machining (such as straight turning). Triangular is the weakest and circular is the strongest, but of all types, triangular is the most versatile. They can be very conveniently used for both turning and facing even a complex job. In diamond-type, normally only acute angle edges are used. Hence, they have only four cutting edges. They can be used for a variety of operations like a triangular insert.

Inserts are produced in various sizes and thicknesses. Smallest possible size is chosen to produce the desired depth of cut. Thickness of an insert affects its strength. Hence, for a large depth of cut and feed, a thicker insert is chosen.

The choice of an insert depends on a particular operation. Choosing an insert and its holder requires familiarity with the machining operations and the machine being used.  The insert manufacturers and ANSI have devised a system for identifying tool holders compatible with the inserts. The tool holder specification describes method of holding the insert, compatible insert shape and size, left hand or right hand tool, geometrical features etc. A typical insert and tool holder combination is shown in "Insert and Tool Holder Combination" figure below.

Parameter programming

Parameter programming is one type of programming executed in CNC system. As in CNC Lathe, various machining cycles are already stored in the form of software by the manufacturers.  It depends as per the system adopted by the company.  The Lathe cycles include parameters – these parameters (Sinumaric System) denoted by R .R parameters of the software is in the form of machine language already stored in the computer. Once the parameter values are put, the computer automatically recognizes and store as per program. R parameters are described and written in single block. There are 01 to 199 parameters available; it may differ as per manufacturers. These parameters are declared in two ways i.e. direct declaration and indirect as per system. Parameters are freely assigned by the control for the purpose of arithmetical calculation in a part program. Trigonometric operations are also possible with R parameters. Parameter programming plays a vital role in CNC machines which are as follows:

(a) It helps an operator to program in short time.

(b) It is very easy to understand.

(c) It saves valuable time for programmer.

(d) It used for complicated job of special contour with greater accuracy.

(e) It used for mass production.

Sub Routine Programming

In CNC lathe, subroutine programming is done where a simple operation is repeated. While programming, it is denoted by letter L (for example L0366). The first two digits 03 denotes the number of repetitions (01- 99) times as per the capacity of computer software, the next two digits 66 denote the program number. Suppose the program No. 10 is repeated for three times it is written as L0310. The advantage of using this subroutine is that it eliminates the need for rewriting a repetitive operation, thereby saving the memory space of the computer and time of the programmer.