Offsets

For a generic cylindrical cutting tool movements are relative to the centre of the tool tip, this is the coordinate 0,0,0

When a CNC machine tool is first started all the drive axis are ‘homed’ – driven to their limits to determine the actual position of the centre and end of the spindle in the machine envelope. This defines the the machine coordinate system.

It is important you know where the MCS is defined on your machine tool, getting it wrong can be expensive and very embarrassing – see here.  Commonly the MCS is the centre of the bottom face of the spindle.

We need to know where the stock material is in this envelope and it would be impractical to define material removal movements within the stock material relative to this position, the zero reference position is therefore moved, this is the Workpiece Coordinate System or WCS and is defined in the Workpiece Offsets. You will also see this referred to as the Fixture Offset.

It is generally easy to grasp the X and Y offsets, but what about the Z position – the position of the tool tip……

Tools offsets

When we execute the tool movements in the program, but what happens when we change the tool? are all the tools the same length? As tools generally vary in length we need to make an adjustment to the reference point based on the difference in length, this is the TLO, the Tool Length Offsets. Tool lengths can be measured on or off the machine.

Workpiece offsets

A position is determined on the job from which the tool movements in the program are defined. This position may be on the workpiece or on a fixture, jig or vice. Consider the repeatable of this position should it need to be redefined, the top face of the workpiece may be an obvious option but what about when it is machined and therefore no longer available for referencing? The horizontal surface of the fixture may be more appropriate.

Multiple WCS can be stored in the machine, these are stored in the work offset page and are given G codes commonly from G54 to G59. If multiple programs are applied to a job then the job may be in different positions in the work envelope, each position is defined, stored on the machine and called in the program.

Offset methods and management

So the WCS and the TLO are combined to adjust the current reported position of the tool tip. There are many different systems and schools of thought when it comes to defining and managing offsets, the most important factors are consistency of your method and repeatability.

Some questions to be asked first:

• Are the tools (in their holders) being used on multiple machines?
• Are different operators setting the same machines?
• Do you have a single fixture setup or is it likely to change with each job?

Also see discussion of positive or negative offsets and ‘gage line’. These discussion favour logical dimensions – easy to visualise and check the tool position – versus safety issues – if a positive or negative sign are mixed up how likely is there to be a crash? Excellent discussion here