N1 G21 T0100 N2 G96 $300 M03 N3 GOO X138.0 73.0 T0101 M08 N4 G01 X154.0 2-5.0 FO.2 N5 Z-10.0 N6 GO2 X164.0 Z-15.0 R5.0 N7 G01 X184.0 NS GO3 x194.0 Z-20.0 R5.0 N9 G01 Z-34.0 N10 X188.0 Z-40.0 N11 GOO 110.0 N12 X250.0 Z50.0 T0100 N13 M30
The next entity is a diameter, and since the geometry offset was set on a diameter, every contour diameter will be as per drawing. Note that the six o’clock position of the insert is the same as that of geometry setup:
The previous chapter addressed the major issues of lathe offsets sufficiently. In this chapter, the previous subject of lathe offsets will continue, with the special focus at the tool nose radius offset, and some issues that both the CNC Programmer and the CNC operator have to deal with, at least once in a while.
For reference, the geometry offset for both external and internal tool is set to the imaginary point (virtual point) – not to the nose radius center (TNR):
The cutting insert used has the common nose radius of 0.8 mm (0.0313 inch):
As per drawing
The inside arc will also be wrong, for the same reason as the one described for the front chamfer, resulting in an insufficient cut and much larger radius:
In order to understand how tool nose radius offset works, a simple contour will be evaluated with and with out the offset programmed.
Tool nose radius offset is nothing more for a lathe than a cutter radius offset for a mill. It’s just a different name for the same control feature. Some CNC lathe manufacturers prefer to separate these terms, to suggest that there are some differences between radius offset in milling and turning. This distinction is more superficial than practical, because the internal functions are the same for both machine types. That is not to say that differences do not exist – they do.
In order to understand the difference between the two types of cutter radius offsets, you have to understand the differences in tooling. For both types of machining applications, cutter radius offset is used mostly for semi finish and finish contouring. For both types of machining, it is important that the physical edge of the cutting tool follows the contour machined. Any contouring tool for milling can be represented by a circle that has the size of the tool diameter.
The previous chapter introduced four major subjects:
• Wear offset
Imaginary tool point
• Tool tip orientation number
TOOL PATH EVALUATION For the evaluation, an external contour will be used:
D = 55°
Keep in mind one very important fact – it is the imaginary tool point that was the point of reference during geometry offset setup. During tool path, it will be this point that will be located at the endpoint of each contour entity. The next seven illustrations will show the effect in detail, individually for each entity.
Just like diameters, when machining walls, faces and shoulders that are perpendicular to the lathe center line, there will be no dimensional error, as the Z-axis geometry offset had been set from a face. Note that the nine o’clock position of the insert is the same as that of geometry setup:
For the front chamfer, not enough material has been re- moved, resulting in insufficient cutting:
When it comes to actual setup, lathe tools are generally set as a distance between the cutting edge and part zero. This is a major difference from a milling cutter, where the setup is always measured to the cutter center, For milling applications, work offsets G54-G59 are used for this purpose. On CNC lathes, the equivalent measurement is called Geometry offset. Work offsets can be used on CNC lathes as well, but that is not a common practice. Think of the geometry offset as being the lathe version of the work offset.
The contour has all geometrical features of a typical contour machined on CNC lathes: i Horizontal lines i Vertical line
Angular lines CW and CCW arcs
Standard D-style 55° tool will be used, and only fin ishing toolpath.
As per drawing
CNC Control Setup for Milling and Turning
TOOL NOSE RADIUS OFFSET
The outside arc radius will also be larger than the radius specified in the drawing, resulting in an insufficient material removal:
Program Listing – With Offset
In the corrected program, only a small but significant change has been made, causing the part to turn on size: N1 G21 T0100 N2 G96 8300 M03 N3 G42 GOO X138.0 23.0 T0101 MO8 N4 G01 X154.0 2-5.0 F0.2 N5 2-10.0 N6 GO2 X164.0 Z-15.0 R5.0 N7 G01 X184.0 N8 GO3 x194.0 Z-20.0 R5.0 N9 G01 Z-34.0 N10 X188.0 Z-40.0 N11 GOO 110.0 N12 G40 X250.0 250.0 T0100 N13 M30
Another diameter of the contour will be machined correctly and to drawing dimension:
Two preparatory commands G42 and G40 have been added to the program, in blocks N3 and N12 respectively.
Compare the ‘before’ and ‘after’insert positions:
As per drawing Although insufficient cutting results in a part that does not match drawing dimensions, there is no danger to the cutting tool itself:
From the detailed evaluation it is clear that without tool nose radius offset the machined part will not be correct and will result in scrap. The only exception will be if the contour does not include angular or round entities at all, which is not likely for virtually all lathe jobs.
Yet, including the G41 or G42 command is a very simple addition to the program.
For the tool nose radius offset to work, G41 or G42 has to be in the program and the R and T set in the control:
I z RT