Tool Nose Radius Compensation (TNRC) in CNC

 

Tool Nose Radius Compensation (TNRC) in CNC

In CNC turning, the cutting tool typically has a rounded tip, known as the tool nose radius (TNR). While programming, we often consider the tool tip as a sharp point for simplicity. However, this simplification leads to inaccuracies when machining contours, tapers, and radii. Tool Nose Radius Compensation (TNRC) is a function in the CNC control that accounts for the actual radius of the tool nose, ensuring the programmed path results in the desired workpiece geometry.  

Here's a detailed breakdown:

What it is:

• TNRC is a software-based compensation method.
• It instructs the CNC machine to automatically adjust the programmed tool path.  
• The adjustment ensures that the cutting edge of the tool, rather than the theoretical sharp point, follows the intended part profile.
• To use TNRC, the programmer needs to:
  1. Include specific G-codes (G41 for compensation to the left, G42 for compensation to the right, and G40 to cancel compensation) in the CNC program.  
  2. Enter the tool nose radius value in the machine's tool offset table.
  3. Specify the tool orientation (the direction in which the tool nose radius is offset relative to the programmed path) in the tool offset table.  

How it works:

When TNRC is activated (using G41 or G42), the CNC control performs real-time calculations based on the programmed path, the entered tool nose radius, and the tool orientation. It then commands the machine axes to move in a compensated path, ensuring the desired contour is achieved by the tool's cutting edge. The machine display will show the compensated tool position, which will differ from the programmed coordinates.

Why it is Necessary:

Without TNRC, significant errors can occur in the machined part, especially when dealing with non-straight cuts:

• Inaccurate Contours: On tapers and curves, the tool nose radius will cause the tool to either leave extra material (undercutting) or cut into the material (overcutting) if not compensated. This leads to deviations from the intended shape.
• Incorrect Chamfers: When machining chamfers, the angle and length will be incorrect without TNRC. A programmed chamfer might end up being smaller or larger than intended.
• Radii Errors: Machining radii without compensation will result in a different radius than programmed. Inspection methods that rely on diameter measurements might still show seemingly correct values, masking the actual radius error.
• Mismatched Features: When a part has a combination of straight lines, tapers, and radii, the transitions between these features will not be smooth or accurate without TNRC. This can lead to visible steps or mismatches in the profile.
• Scrapped Parts: The cumulative effect of these inaccuracies can lead to parts that do not meet specifications and have to be rejected, resulting in material waste and increased production costs.  

Benefits of using TNRC:

• Improved Accuracy: TNRC ensures the finished part matches the programmed geometry, especially for complex contours.
• Enhanced Surface Finish: By accurately following the programmed path, TNRC contributes to a smoother and more consistent surface finish.
• Consistent Part Dimensions: It guarantees that critical dimensions, including tapers, chamfers, and radii, are within the required tolerances.  
• Reduced Errors and Rework: By compensating for the tool nose radius, the likelihood of machining errors and the need for rework are significantly reduced.
• Simplified Programming: Programmers can focus on the desired part geometry without having to manually calculate and incorporate offsets for the tool nose radius at every point.
• Increased Tool Life: By ensuring the correct engagement of the cutting edge, TNRC can help prevent excessive tool wear.

In essence, Tool Nose Radius Compensation is a crucial function in modern CNC turning that enables the production of accurate and high-quality parts with complex geometries. It eliminates the errors associated with treating a rounded tool tip as a sharp point, leading to better dimensional control and reduced manufacturing costs.