Wednesday, 2 October 2013

Machine and Tool Offsets..

As we know it is difficult to place a vise in the exact same position on the machine each time, the distance from Home to the WCS is usually not known until the vise is set and aligned with the machine. 

Machine set up is best done after the program is completely written, because it is expensive to keep a CNC machine idle waiting for the CNC programming to be done. To complicate matters further, different tools extend out from the machine spindle different lengths, also a value difficult to determine in advance. For example, a Ø20 Ramping Cutter extends further from the spindle face than a stub length a drill or any other milling cutter. 

If the tool wears or breaks and must be replaced, it is almost impossible to set it the exact length out of the tool holder each time. Therefore, there must be some way to relate the Machine C oordinate system to the part WCS and take into account varying tool lengths. This is done using Machine Tool and Fixture Offsets

There are many offsets available on CNC machines. Understanding how they work and to correctly use them together is essential for successful CNC machining.

  • Fixture Offset XY:

Fixture offsets provide a way for the CNC control to know the distance from the machine home position and the part WCS. In conjunction with Tool Offsets, Fixture Offsets allow programs to be written in relation to the WCS instead of the Machine Coordinates. They make setups easier because the exact location of the part in the machine envelop does not need to be known before the CNC program is written.

As long as the part is positioned where the tool can reach all machining operations it can be located anywhere in the machine envelope. Once the Fixture Offset values are found, entered into the control, and activated by the CNC program, the CNC control works behind the scene to translate program coordinates to WCS coordinates.

Notice in Figure below how Fixture Offsets (+X, -Y) are used to shift the centerline of the machine spindle directly over the WCS.

Fixture Offset

  • Tool Length Offset (TLO):

The tools loaded in the ATC (automatic tool changer) unit are of varying length. The length of each tool from the spindle varies according to requirement. Also due to tool wear the length of tool varies hence it becomes nearly impossible to set the tool in same position for each operation. Hence in CNC programming we use Tool Length Offset. It can be found out using following methods.

  1. Generally the the TLO can be found out by directly touching the tool tip on the workpiece surface as shown in 1st method in figure below. Here the tool is made to travel from machine home Z-position to the part Z-zero position. In CNC machine controller settings the TLO settings are provided. This value of tool travel length is registered for each tool in that controller register. But during this method as we have to touch the workpiece with tool tip material removal takes place there. Hence this method is not preferable
  2. In second method a sample block of some exact standard dimensions is made called as 1-2-3 block whose height is maintained precisely to a predefined value. Here the tool tip is made to touch the block surface and then TLO is calculated..
  3. In 3rd method we simply use a Touch Probe which is connected to the machine controller. Here as soon as the tool tip touches the probe it send tool offset distance to the machine controller and the controller automatically registers the corresponding tool offset values.
Ways To Set Tool Offset

So in this module we have seen how to consider fixture offset and tool offsets so as to get precise machining. In next module we will discuss about the types of tools used with CNC machines. 

Images Courtesy : CNC Handbook HSMWorks