First, the proposed tool compensation:
When using an end mill on a CNC machine to machine a workpiece, it becomes evident that the motion of the tool center does not align with the actual contour of the workpiece. This is because the workpiece's shape is formed by the envelope of the end mill's movement. The center of the end mill is referred to as the tool point (or tool position vector in 4th and 5th axis CNC machines), and its path represents the tool's movement. In CNC machining, programs can be written based on either the workpiece contour or the tool point’s movement path, depending on the situation.
CNC Machine Tool End Milling
Full-featured CNC machines come equipped with tool compensation functions, which allow programming based on the workpiece contour. Once the tool compensation is activated, the system automatically adjusts the tool position to match the intended movement path. This makes programming simpler, as the program is based on the workpiece dimensions. Even if the tool wears or is replaced, the program remains unchanged, making the process efficient and user-friendly.
Economical CNC machines, while more affordable and widely used in production, often lack built-in tool compensation. Therefore, the machining program must be based on the tool point's movement path, which requires calculating the tool path from the workpiece dimensions and the tool diameter. This increases the complexity of calculations and necessitates reprogramming when tools are worn or replaced.
Second, Tool Compensation in Full-Function CNC Systems:
1. Tool Compensation on CNC Lathes
CNC lathes typically include two types of tool compensation: tool position compensation and tool nose radius compensation. These are specified using the T function in the program. The first two digits represent the tool number, and the last two digits indicate the compensation number, such as T0202. If the compensation number is 00, the compensation is disabled.
(1) Tool Position Compensation: This accounts for changes in the tool's position due to wear or reinstallation. Once enabled, the program doesn’t need to be modified. Instead, the tool's measured position is input into memory, and during execution, the system replaces the original position with the actual one.
For example, without compensation, a tool might move from point 0 to point 1, as in N60G00C45X93T0200. With X=3 and Z=4 stored in memory, the tool would move to point 2 instead, resulting in N60G00X45Z93T0202.
(2) Tool Nose Radius Compensation: When programming a lathe, the tool tip is treated as a theoretical point (P point). However, in reality, the cutting edge is often rounded, which can cause errors. By using radius compensation, these issues are resolved. The parameters defining the tool's geometry, such as orientation (0–9), are stored and used for accurate compensation.
(3) Tool Compensation Parameters: Each compensation number corresponds to values for tool position (X, Z) and radius (R, T). These values are entered into memory before machining and displayed on the CRT. During operation, the system automatically corrects the tool position and applies radius compensation based on these values.
2. Tool Compensation on Machining Centers and CNC Milling Machines
Machining centers and CNC milling machines support various types of tool compensation, including radius compensation, angle compensation, and length offset.
(1) Tool Radius Compensation (G41, G42, G40): The tool radius is stored in memory (HXX), and the system automatically compensates for it. G41 is for left-side compensation, and G42 is for right-side compensation. G40 cancels the compensation. It is important to use G00 or G01 when enabling or disabling compensation; G02/G03 should not be used. Negative values can swap the effect of G41 and G42.
There are two modes: B function and C function. While B function only compensates within a single block, C function handles transitions between blocks smoothly, making it more suitable for complex contours. Most modern systems use the C function.
(2) Angle Compensation (G39): This is used to prevent overcutting at sharp corners where two planes meet. It is non-modal and must be used immediately after G41 or G42. It is only effective within the same block.
(3) Tool Length Offset (G43, G44, G49): This allows compensation for tool length changes without modifying the program. G43 adds the offset to the coordinate, while G44 subtracts it. G49 or H00 cancels the offset. For example, N80G43Z56H05 with H05 set to 16 means the final position is 72mm.
The compensation value can be pre-entered via MDI or DPL, or directly set using G10P05R16.0.
Third, Calculating the Tool Path on Economical CNC Machines:
On economical CNC machines without built-in compensation, the tool path must be manually calculated and programmed accordingly. Local adjustments may also be necessary to ensure accuracy and proper machining results.
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Tag: CNC Machine Tool, End Milling, Tool Compensation
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