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Tool_Length_Offset : Methods for the Tool Length Offset
AA_Siemens_808d - Offset - Tool_Techniques -
ref: P 79 Contemporary CNC.doc Rules of TLO Correction If the tool travels negative into the work to take off stock, then a more negative tool length offset will take off more stock. If the tool travels positive into the work to take off stock, then a more positive tool length offset will take off more stock. TLO is changed in order to achieve a tolerance and normally the change is set in the wear register of the D-code. TLO Wear The D-code structure supports geometry and wear. Geometry is what you measure. Wear is a correction to account for tool wear. Wear is a catchall for any reason the tool does not achieve the nominal dimension. Stay in G17 For a mill, rotate the G17 system so that the G17 Z-negative is always the infeed direction.
SET UP METHODS

Variable length imagined tool

TLO: Negative G54:0 Summary: basically , move Z to zero (fully up), insert tool, measure distance from tip to work, enter this as a negative value in offset.
Method: When the Z-machine=0 position is "up" as shown in the figure right, we imagine there is a tool in the spindle whose tip touches the desired Z-work=0 surface. This imagined tool is our reference tool. Since its length is dependent on the work plane we choose to make Z-work=0, this method of machine setup is called the variable length imagined tool method. TLO Measurement: The actual tool in the spindle is shorter than this tool by the distance labeled TLO in the figure. This distance is set as a negative number in the L1 Geometry register of the tool's D1. The operator finds this distance by positioning the tip of the actual tool to the Z0 work surface. The distance is the number he sees in the Z-register of the machine position display. Disadvantage: Have re-settup each time a new hight work piece is used.

Fixed length imagined tool

TLO: Negative G54: POSITIVE Work height Summary: Same as above except measure all the way to the machine bed. Now the TLO are good for the machine. Once setup you are good for life ( ex tool wear). Only measement you need is the work piece height (input G54 Z-value as the workpiece height).
Imaginary tool is measured from Z top all the way to the machine bed. However, in this case, ZofG54 is not zero. It is some positive value measured from the table top to the desired Z-work=0 surface. Therefore G54 Z-value just need to be the thickness of the workpiece. TLO Measurement: Same as above but to machine bed.

Zero length reference

TLO: Positive G54: NEGATIVE Zero-point to top work Summary: Imagined tool is of zero lenght. Tool is measured (+) from end to this imaginary zero-point. G54 is from zero-point to top work surface.
Advantage: The advantage of this method is that it supports preset tooling (tools pre-measured). Clearly the tool length offsets are properties of the tool and not the machine or job. To get the Z-axis "up" to its machine zero position one would program G0 SUPA Z0 D0 provided "up" is the machine zero position.

Zero length reference in Base Zero G500

TLO: Positive G54: POSITIVE base zero to top work Summary: Similar to above but use G500 to add the offset from the bed to the zero-point. Now all tools are referenced to the zero-point and the zero-point is referenced to the machine bed. Hence all is needed is a +G54 offset from base zero to top work.
Notes: SUPA G0 Z0 D0 returns the spindle "up" to the machine zero position/point. If you were to program G53 G0 Z0 D0 your would be in Crash City because G53 does not cancel the Base zero offset.