Manual tool change : TLO procédure
This procedure, although strongly discouraged, is nevertheless the oldest and the most effective for implementing machining requiring the use of several tools. Contrary to assertions made on some forums, the so-called "TLO" method does not trigger any soft limit alerts and does not modify the machining reference system (WCS). This bad judgment that is made to this tool change procedure comes from the very broad interpretation of the term TLO and the ignorance of its implementation.
As a reminder TLO = Tool Length Offset. In no case is it the difference in length between two tools, but the length of the tool that is used for a particular operation. In CNC, TLO is not a generic term that can be used to define any offset. TLO can only be associated with the length of a cutting tool, and nothing else.
The Basics: Driven Point
From the beginning of CNCs, the programming of interpolations was done on the profile to be obtained, disregarding the tooling to be implemented to achieve this result. The point controlled by the program is therefore located in the axis of the spindle for the X and Y interpolations, and at the level of the spindle taper gauge plane for the Z axis (tool position reference).
The operator, when choosing his tool, will bring in a correction device according to the radius of the tool (functions G41 and G42) and its length (G43 and derivatives). The controlled point will then be corrected according to the characteristics of the tool implemented.
Grbl allows tool length offset (TLO) with function G43.1.
TLO: How does it work?
TLO, (Tool Lenght Offset) is the centerpiece of the tool change mechanism. The N.I.S.T lists 3 ways to refer to it: G43 Hxxx → Function reserved for CNCs equipped with an ATC and a magazine with a database listing all the characteristics of the tools. G43.1 Zxxx → positive dynamic tool length compensation G43.2 Zxxx → negative dynamic tool length offset.
Grbl, running on Arduino, not having sufficient memory capacity to manage a tool magazine with database, opted for the G43.1 correction.
G43.1 is a modal function that remains active until revoked.
Revocation: either by calling function G49, or by resetting the software.
Mode of action
During machining bCNC sequentially transmits the lines of code to grbl which analyzes them, before transmitting the X, Y, or Z commands to the axis control module. In Grbl, the procedure responsible for converting Zxxx commands into Z axis control pulses is associated with a register containing an offset value (TLO). The association is of the form Z = Z + TLO (always positive correction).
At grbl startup TLO is initialized to 0.
The TLO correction is only active on Z movements in work coordinates. A G53 Z0 order (Machine Coordinates) with a TLO of 40.00 will not trigger a soft limits alert on Z!
Tool length correction is handled entirely by gbrl and does not affect the WCS working repository.
The role of bCNC is limited to measuring the length of the tool that has just been mounted and transmitting it to grbl using the G43.1 Zxxx function. On tool change, the current TLO is canceled (call G49) before being loaded with the new value.
Implementation on the machine
TLO represents the length of the tool, not a difference in length between two tools as is the case with the typical WCS procedure. TLO is always positive! ! !
Tool length It is measured between the gauge plane of the mounting cone on the spindle and the tip of the tool.
Measurement reference
Without length correction (TLO = 0) the point controlled by the Gcode is located in the axis of the spindle, at the level of the gauge plane. For the Z axis, it is the gauge plane that serves as a reference for measuring the length of the tool, the calibration for the tool change and of course for the definition of the Program Origin (WCS Z0).
On the machine, the gauge plane is materialized by the spindle nose.
Calibration procédure
The procedure described is permanent as long as the bed of the machine is not modified or the measurement sensor is not replaced.
Note: see thread #1775 #1827
