Automatic axis tracking (C-axis tracking)

Machining operations, such as cutting various materials, necessitate guidance of the tool used which is generally associated with the C-axis in such a manner that the tool is always aligned tangentially with respect to the path followed.

It must be noted that the tangent is not unique at each point of the path (break points). Consequently, a solution necessitates strategies for handling block transitions which do not feature constant tangents.

One typical application is the technology sector for glass cutting. This involves machining level contours with the aid of cutting tools in the form of carbide metal cutting wheels using CNC machines. The level workpiece is slit slightly at the machining point in accordance with the programmed contour (closed contour, e.g. ellipse). The required contour can then be broken out from the glass workpiece.

Automatic axis tracking (C-axis tracking) 1:
Fig. 11.14: Tracking the rotational C-axis tangentially with respect to the x-y contour

The C-axis can also be guided tangentially with respect to the path by explicit programming. The NC-commands described below do, however, simplify programming substantially.

#CAXTRACK ON [ [ [ANGLIMIT<expr>] [OFFSET<expr>] [OPTALIGN<expr>] ] ] (modal)
#CAXTRACK OFF [ [ANGPOS<expr>] ] (modal)

ANGLIMIT<expr> Limiting angle in degrees (°).
This parameter is only considered for non-tangential continuous contour sections. Tangential continuous contour sections for example are created by the contouring function G61.
If the angle between the tangents to the contour exceeds the limit angle on block transition, the path movement is halted and the dressing movement is performed by an inserted motion block at rapid traverse rate. The inserted movement forms one unit in this case with the following, second block. This means that PLC synchronization events in conjunction with the movement (M-functions etc.) are possible only before or after this motion unit.
If the transition angle to second block is smaller than the limiting angle, the dressing movement will start immediately with transition to second block. Because of limited axis acceleration in general the path transition velocity to second block is reduced by dynamic monitoring. If this behavior is not acceptable, the axis dynamic monitoring of the trailing axis can be excluded by NC command and path motions are not affected. (e.g. G116 C1, see chap.).

OFFSET<expr> Angle offset in degrees (°).
This serves to specify an angle offset to orient the tool opposite to the tangent against the contour.

OPTALIGN<expr> At selection the alignment is automatically optimized, if the alignment distance is greater than the defined angle value.

This parameter is only considered for active automatic aligning according to P-CHAN-00101 and rotatory linear axes with limited moving area (no modulo axes). It is only effective during the automatic alignment process on the first contour element. After selection of the tangential tracking function with automatic aligning the position of the tracking axis lays without any offset in the area of -180..+180 degree. The parameter allows, that the position of the tracking axis before selection of the tangential tracking function is considered during the automatic aligning process. The function is only useful, if the tracking axis approximately has the correct position to the first contour element already before selection of the automatic aligning, but for example has a pre orientation of +-360 degree. If the internal calculated alignment angle value exceeds the programmed angle value, alternative solutions for the alignment angle are taken into consideration. Then the smallest alignment distance of the solutions determs the alignment angle. (*)

Automatic axis tracking (C-axis tracking) 2:

(*) For modulo axes the automatic alignment process always is executed on shortest way.

ANGPOS<expr> Position at deselection in degrees (°).

During the deselection the tracking axis can be positioned additional. The positioning for rotational axes is executed on shortest way.

The tracking axis is defined in the channel parameters [1]-34. Automatic tracking of the axis is performed with the correct sign, relative to the last position in accordance with the resulting contour transition angle.

Dependend on the parametrization [1]-37 the aligning of the tracking axis to the required orientation (generally parallel to the contour) is done as follows:

If the tracking axis has already the correct orientation at selecting, for active parameter [1]–40 the path motion is continued with the first relevant motion block without interruption.

Tracking commences with the transmission of the first to the second relevant motion block after activation with #CAXTRACK ON. Automatic axis tracking operates in the main plane of circular interpolation (1st + 2nd main axis). This must be defined prior to activation (G17 / 18 / 19, #PUT AX / #CALL AX / #SET AX).

Automatic axis tracking (C-axis tracking) 3:

In synchronous operation a tracking axis my not be operated neither as master nor as slave axis!

Programming example

N10 G00 G90 X0 Y0 Z0 C0 
N20 X5 Y5 C45 (Straight line 45° to the X-axis, C-axis (parallel to the contour 
N20 #CAXTRACK ON [ANGLIMIT 3, OFFSET 0] (Activation of axis tracking, (Limit angular 3°,)
(Angular offset 0°) 
N30 X10 Y10 (Primary movement block, no tracking 
N40 X20 (Angular to the previous block: -45° > (Limit angular -> Block is inserted: (End position of C = 0 
N50 M99 X30 (If M-function synchronization before (Movement -> First synch., then movem. C (on 0, then X on 30. 
(If Sync. after block-> Movem. C on 0 (then X on 30, then Sync. 
N60 X40 G60 (Transitional angular 0°, Exact stop 
N70 X30 (Transitional angular 180°, no tracking 
N80 Y10 X29.9 (Transitional angular -89.427°, new Pos. (C = -89.427° (clockwise) 
N90 #CAXTRACK OFF (Deactivation of axis tracking 
M30