MachPro Lathe Tool Nose Radius Compensation
Tool Nose Radius Compensation
Overview
Tool nose radius compensation (TNRC) corrects toolpath for the rounded tool tip.
Every turning tool has a finite tip radius.
That radius shifts the real cutting contact point away from the programmed tip point.
Key terms
Tool nose radius: the arc radius at the tool tip, given in inches or millimeters.
Programmed tip point: the point your program uses as the tool reference.
Cutting contact point: the point where the tool edge touches the workpiece.
Why TNRC matters
A CNC control moves the tool to the programmed tip point.
The workpiece is cut by the contact point, not the programmed point.
The offset between the programmed and contact points depends on the nose radius and tool orientation.
Without correction, corners and tapers can cut undersize or oversize.
How TNRC works
TNRC shifts the toolpath so the contact point follows the intended geometry.
The control applies the shift using the stored nose radius value.
The control also uses the tool orientation to choose the correct offset direction.
This method keeps the programmed geometry consistent across different insert radii.
Typical workflow
Most CAM systems account for the nose radius during toolpath generation.
You usually only verify that the tool radius data matches the installed insert.
You only use TNRC directly when you write G-code by hand.
On many controls, you enable it with commands such as G41 or G42.
You then set the radius value in the tool offset table.
Illustration explanation
The illustration exaggerates the nose radius to make the offset easy to see.
The red diamond marks the programmed tip position.
The purple area marks the actual cutting edge.
These locations differ, so the control must compensate to cut the correct profile.
Example
Suppose you wish to cut this profile (with a tool tip of type 3):
If the cutter had a perfect point, then the commanded path would be precisely along the desired geometry path.
For a real tool, however, the nose of the cutting tool is some distance away from the point on the desired geometry. If we commanded positions along the path of the desired geometry, we would not achieve the desired geometry. The actual geometry achieved is illustrated by the blue path. Compare that with the commanded positions, illustrated with red diamonds.
A real tool will not produce the desired geometry without some compensation. A compensated path (illustrated in red) would put the rounded edge of the tool nose at the correct position to produce the desired geometry (illustrated in black).
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