Mounting tools in the tool changer

6.3 Offsets

There are a few variations to the way tool length offsets can be used, such as:

1. 
   Each tool has an offset equal to the distance the Z-axis must move from home position to bring that tool to the work surface (which is usually taken as the Z program origin).
   
2. 
   A reference tool (usually the longest tool) is used to set the Z program origin by entering the tool length offset value in the workpiece zero offset register instead of the tool length offset register. This results in the Z-axis moving towards the work by the offset amount each time the workpiece zero offset is commanded. Any additional movement towards the work to make up for shorter tools is accomplished by initiating the tool length offset for that tool, the offset amount being equal to remaining motion required.
   
3. 
   By using the program commands available on most controls to reset a machine position to represent a workpiece position. The codes used between controls may be different and the terminology used to describe the technique may be different but the results are similar.
   
4. 
   Using the gauge line on the machine spindle in conjunction with a known length setting bar.
   

6.4 Tool change position

Tool changing should take place at a position that removes the risk of any tool colliding with any part of the machine or fixturing during the tool change sequence.

On some machines, the position may vary, on others the position may be fixed. Your teacher can supply this information for the machine at your college.

6.5 Workpiece datum

As with tool offsets, there are a few variations to the method of locating the three dimensional workpiece zero (program origin) within the total motions of the machine, the differences mainly depend on the control executive program and are basically:

1. 
   By entering the XY machine positions where the program origin is desired into an offset register known as a zero offset or a Workpiece Coordinate System register, or similar.
   
2. 
   By using the program commands to reset a machine position to represent a workpiece position usually used with a G50 or G92 code, depending on the control.
   
3. 
   By utilising the tool offset register on those controls that will accept X and Y offsets as well as Z offsets.
   

Again, your teacher can assist you in this regard.

6.6 Workpiece and machine checks before and after machining

Before and after machining, a range of checks should be carried out on both the material and the machine to ensure that all is in readiness for the machining operation to be performed safely and in the most efficient manner with regards to the quality of the part being produced and care of the machine tool itself.

These checks will include the following points:

6.6.1 Workpiece checks before machining

1. 
   Correct material specification
   
2. 
   Correct size allowing for initial roughing cuts
   
3. 
   Pre-machining of datums carried out if required
   
4. 
   Castings and forgings checked for clean and removal of excess flash
   
5. 
   Cored holes in castings and forgings clear and in correct position
   

6.6.2 Workpiece checks after machining

1. 
   Sizes correct to gauge or manual measurements taken
   
2. 
   Surface finish specification
   
3. 
   Burrs and sharp edges removed where necessary.
   

6.6.3 Machine checks before machining

1. 
   Machine lubrication system levels correct
   
2. 
   Tooling correctly loaded in tool changer with position numbers matching program
   
3. 
   Machine referenced (machine home)
   
4. 
   Workpiece datum set
   
5. 
   Tool offsets set (length and radius)
   
6. 
   Safety guards in position
   
7. 
   Coolant levels checked.
   

6.6.4 Machine checks after machining

1. 
   Machine returns to safe position for loading next component
   
2. 
   Program returns to start (production sequence)
   

6.7 Dry running program

Dry running a CNC program is a procedure that allows the program to be run with or without tool movements in order to check that the program will actually run on the machine.

The initial dry runs are usually done without tool or axis movement so as to verify that the program can be understood by the machine.

After the initial dry run a trial work piece may be machined using machinable wax, foam or timbers specifically developed for machine trialling.

During a dry run certain alarms may come up because the machine cannot carry out the commands given to it by the program. These alarms must be cleared and the program edited to remove the cause of the alarm.

After the program is proved to run correctly on the machine, a dry run may be made with the Z axis set a safe distance above the Z zero so that the tools will not come in contact with the workpiece and all tools and axis movements used. Your teacher will assist you to carry out the dry run.

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