Taking Tool Offset on Lathe Machine and Proving Selected Program

Taking Tool Offset on Lathe Machine

Tool offset setting is one of the very important things in CNC operation. The machine must be informed how much is the offset between the machines zero point and the component zero point.  The machine zero point (origin of the machine coordinate system)  is  pre-defined  on  a  machine.  It is situated at one extreme position of the tool. The component zero point is defined by the user. It is the origin of the coordinate system with respect to which all distances are measured and used in the program. Obviously, the machine must know where this point is located. This is done by measuring the distance between the two origins and specifying it as tool offset distance. Since the coordinates of the chosen tool tip are displayed on the machine's screen (Visual display unit), one can easily read the distance and type it in the tool offset table. Different machines may have different methods for specifying tool offsets. The tool offsets must be correctly specified for all the tools which are to be used during machining. Any error will result in a defective component, even though the part program may be perfectly correct. Most machines also provide the facility of saving the offset details, so that in case of power failure, the operator may not have to repeat the whole process of tool offsetting all over again. Instead, he may simply load the relevant offset file into the memory of the machine.

On a lathe, the component zero point is usually taken at the centre of the right face of the work piece, as shown in figure "Tool Offset on Lathe Machine". It is virtually impossible to place the tool tip at this point accurately for measuring the offset distance, because it is not a sharp point. So, we normally use an indirect method for locating this point. First, the tool is made to just touch the face of the job by jogging in small steps from right to left as shown in figure "Tool Offset on Lathe Machine" (a). With spindle rotating, the tool leaves a mark on the face when it touches it. The final jog steps should be very small to ensure accuracy. The tool is, thus, accurately placed at Z=0 and we fill the tool offset table accordingly.

Now, the tool is retracted and made to touch the cylindrical surface of the job by jogging it radially towards the job as shown in figure "Tool Offset on Lathe Machine" (b). In this position, the tool is at X=d, where d is the diameter of the job, which can be measured by a micrometer. Thus, X=0 is indirectly, but accurately, located. The offset table is now edited for the X value. This completes the offset setting. If radius compensation is desired to the used, we will also have to fill the value of the nose radius and the direction of tool approach (the relative position of the tool and the job, which is identified by a digit given in respective machine manuals) in the offset table. In one offset table, tool offsets for all the tools can be stored. Obviously, for work pieces of different sizes, all tool offsets may be different, and new offset tables will have to be created. All offset tables, which correspond to different jobs, can be saved on disk for future use. For a particular job, the corresponding offset table needs to be loaded from the disk before machining starts.

Proving Selected Program

After making a program, it must be proved for its correctness. If it is used directly and if there is any error in program it may cause serious damage to tool, job and machine. There are many ways of checking the program, few of them are explained below.

Dry Run. Though the simulation can verify the tool paths, it has certain limitations depending upon its capability. For example, it may not be able to check for interference between the billet and the body of the tool. It is because the actual dimensions of the tool may be different from those being used by the software for simulation purpose. It is expected that more advanced software’s will be developed to simulate the machining operations more accurately. In view of the limitations discussed above, it is advisable to first dry run a machine before actually producing the part. In the dry run, everything necessary for machining is done, but before giving the execute command, the job is taken out, and thus, the machining is performed in the air. The operator must carefully verify the tool paths and check for any undesirable interference. He can use feed override switch to regulate the speed of the tool movement. In case of any doubt, very small feed can be used to observe the tool movement closely. Coordinates of the tool tip are also displayed, so complete verification is possible. If everything is found in order, the billet may now be inserted and machined.

Single Block. If automatic mode is used operator will not have any control over the machine. So before the job is produced in automatic mode, first few pieces must be produced in single block mode. In this mode, on pressing the switch “cycle start” only one block will be executed and machine will stop. If operator satisfies with that block execution, he can again press the cycle start switch. In this way all blocks are to be checked before automatic mode is selected.

MDI Automatic Mode. Another way of checking for correctness of a program or a block of a program is manual data input mode method. In this mode one block is entered. When cycle start switch is pressed machine will execute that block and stops. After that another block is entered and executed.

Automatic Mode Block Search. Few CNC systems come with a unique advantage of warning the operator about an error in a program well before that particular block comes to execution. It also indicates the block number. However, it will warn about programming errors only. If depth of cut or length of cut is given more it will not warn. This sort of errors can only be found out in other methods.

This way a part program may be verified. However, the cutting parameters such as spindle speed, depth of cut and feed along With the type of the cutting tool (its shape, material etc.) have to be selected by the programmer. So, the programmer must have a good knowledge of workshop practices, without which a CNC machine cannot perform the way it is supposed to.