A programmed tool motion creates a pattern, contour or a pocket that can be rotated about a defined point by specified angle. With this control feature, there are many opportunities to make the programming process much more flexible and equally efficient. This very powerful programming feature, usually a special control option, is called the Coordinate System Rotation, or just Coordinate Rotation.
One of the most important applications of coordinate ro-tation is a program that is defined in an orthographic orien-tation but machined at an angle (as required by the drawing specifications). Orthographic mode defines only horizontal and vertical orientation, which means that the tool motion takes place parallel to the machine axes. To program ortho-graphic mode is much simpler than calculating tool posi-tions for many contour change points in an angular orienta-tion. Compare the two rectangles shown in Figure
The above figure (a) shows an orthogonal orientation of a rectangle, the figure below (b) shows the same rectangle, rotated by 10° in the counterclockwise direction. Manually, it is much easier to program the tool path for figure (a) and let the control system change it to a tool path represented in figure (b). The coordinate rotation feature is a special op-tion and must be the part of the control system.
Mathematically, the coordinate rotation is a feature that requires only three items to define a rotated part - the center of rotation, the angle of rotation, and the tool path to rotate.
ROTATION COMMANDS
The coordinate rotation uses two preparatory commanas to turn this feature ON or OFF. The two G commands con-trolling the coordinate rotation are:
G68 Coordinate system rotation ON
G69 Coordinate system rotation OFF
The G68 command will activate the coordinate system rotation, based on the center of rotation (also known as the pivot point) and the degrees of notation:
G68 X.. Y.. R..
where:
X = Absolute X coordinate of the center of rotation
Y = Absolute Y coordinate of the center of rotation
R = The angle of rotation
♦Canter of Rotation
The XY coordinates are normally the center of rotation (pivot point). This is a special point about which the rota-tion takes place - a point that can be defined by two different axes, depending on the selected working plane. X and Y is the absolute rotation center selection for the G 17 active plane. G18 will use XZ as the rotation point coordinates and G 19 will use YZ as the rotation point coordinates. The plane selection command G17, G18 or G19 must be entered into the program anytime before the rotation command G68 is issued.
If the X and Y coordinate locations are not specified with the G68 command as the center of rotation (in the GI7 plane), the current tool position will be used as the default center of rotation. This method is neither a practical nor recommended approach in any circumstances.
♦Radius of Rotation
The G68 angle leplescillativii is specified by the alum it of R. The units are degrees, measured from the defined cen-ter The number of decimal places of the R amount will be-come the amount of the angle. Positive R defines a CCW rotation, negative R defines a CW rotation - Figure below
For a basic programming example, we use a simple part shape that is easy to visualize, such as a rectangular shape with a fillet corner radius
Part oriented as per engineering drawing specification The actual tool path, including the approach towards the part and the departure from the part, is not normally included in the engineering drawing. Be careful here - if the approach and/or departure motions are included in the rotation, the program zero may also be rotated. In the Figure, the orientation of the part is 15° counterclockwise based on the lower left corner.
For a moment, ignore the rotation angle and program the part as if it were oriented in an orthogonal position, that is perpendicular to the axes, as shown earlier in Figure
For actual cutting, decide whether the approach tool motions will be included in the rotation or not. This is a very important decision. In Figure 42-5 are the two possibilities and the effect of coordinate rotation on program zero. In both cases, the approach tool path starts and ends at the same location of X-1.0 and Y-1.0 (clearance location).
♦ Coordinate Rotation Cancel
Command G69 cancels the coordinate rotation function and returns the control system to its normal orthogonal condition.
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