The contour of a part - also known as a profile - is normally programmed for milling applications by establishing the depth in the Z axis first, then moving the cutting tool individually along the X axis, Y axis, or both axes simultaneously. For turning applications, either the X axis or the Z axis, or both axes can be used to face, turn or bore a contour. For both types of machining, each contour element contour requires one block of nutting motion. These motions between contour change points can be programmed in inches or millimeters and they can use an absolute value position or an incremental distance. In either case, keep in mind that this type of programming uses the center line of the spindle as the X and Y or X and Z tool movements. Although the center line programming is a very convenient method for program development, it is also a method unacceptable for machining. During contact with the material, the edge of the cutting tool must touch the programmed part contour, not its center line.
The tool path for all contouring operations is always equivalent to the cutting tool motion. Whether used on a CNC machining center or on a CNC lathe, the cutting tool edge must always be tangent to the contour, which means the tool motion has to create a path where the center point of the cutter is always at the same distance from the con-tour of the part. This is called the equidistant tool path.
The illustration in Figure below shows two types of a tool path. One is nut compensated, the Whet is compensated. Both am applied to a particular contour, with the cutter diameter shown as well, including its positions.
MANUAL CALCULATIONS
Some realities should become apparent from the Figure above. The most noticeable observation is that the machined contour must always take place with the tool path compensated by its radius, which means its center point must be located in positions shown in the lower example. This machining requirement is not matched by the reality of the engineering drawing. In a drawing, all dimensions refer to the part contour, not the contour of the tool center. In fact, the drawing is dimensioned to the tool positions illustrated in the upper example of the illustration. The question is -how do the tool center positions get from a drawing to the part contour?
The answer is - they have to be calculated. Actually, they do not have to be, if the CNC system is equipped with an advanced built in feature called the cutter radius compensation or cutter radius offset. On the CNC turning systems, this feature is called the tool nose radius compensation or the tool nose radius offset. This advanced and common control feature enables the programmer to apply the offset command, program the part contour as per drawing dimensions and let the control do all the necessary calculations and adjustments automatically.
At this point, the current chapter could continue and strictly concentrate on the automatic method of programming, using this exceptional feature. After all, all modem CNC machines do have a cutter radius offset built-in, Once several basic rules are followed, the feature is easy to use.
In order to automate something, we have to first under-stand how it works. If something is automated already, the knowledge of how it works makes the job so much easier, particularly when encountering a difficulty that has to be resolved very quickly. To really understand cutter radius offset - many programmers and machine operators do not -it is important to understand the principles built in the system, principles that are very much based on basic mathematical calculations, including the often unpopular trigonometry calculations. A very simple drawing is shown in Figure below for that purpose.
The program zero will he selected at the lower left corner of the part. Since the cutting will be external, in a climb milling mode, the tool will start along the Y direction first. At the moment, the start and end tool position is not important. only calculations of the individual contour points at intersections and tangency points.
Note that there arc five points on the drawing, one at each contour change. These points are either intersections or points of tangency. As each point has two coordinates, total of ten values will be required.
The drawing always offers some points that need no calculations. It is a good idea to get well organized and mark the points from the drawing first. Then, make a chart in the order of tool path. Study next Figure carefully - it shows all five points and all the values that need no calculation, perhaps some addition or subtraction only.
Out of the ten values required, nine of them are given. The missing Y value for P3 is not expected on the drawing. Regardless of whether the cutter radius offset is used or not, some calculations will always be necessary and this is one of them. After all, manual programming is done by hand. Figure below shows the trigonometry method used.
All five points can be summed up in a small table:
Once all the coordinates are completed, there is enough data to start the tool path, but only if the cutter radius offset feature is used. However, that is not the intention at the moment. to illustrate, a whole new set of points has to be found - coordinates for the center of the cutter.
Offset Commands
In order to program one or the other mode of cutting (cut-ting direction), there are two preparatory commands avail-able to select the cutter radius offset direction:
G4 I or G42 mode is canceled by the 040 command:
In terms of the milling method, G41 command is applied to the climb milling mode, G42 command is applied to the conventional milling mode. This is true only if the spindle rotates with M03 function active (spindle CW) and the cutter is right hand. If the cutter is left hand, the spindle must rotate with MQ4 function active (spindle CCW) and all rules applying to cutter radius offset are the exact opposite of those discussed here. There is no cutter radius offset ap-plied when 040 command is in effect.
figure below shows the 041 command as a climb milling mode and the G42 command as a conventional milling mode. Climb milling mode is the most common in CNC milling, particularly in contour milling.
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