First, the proposed tool compensation:
When using an end mill on a CNC machine to cut a workpiece, it becomes evident that the motion of the tool center does not align with the workpiece contour. This is because the workpiece's shape is created 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 movement path represents the actual tool movement. In CNC machining, programming can be done either based on the workpiece contour dimensions or according to the tool point’s movement path. The choice depends on the specific situation and requirements.
CNC Machine Tool End Milling
In full-featured CNC machines, the CNC system typically includes a tool compensation function, allowing programs to be written based on the workpiece's dimensions. Once the tool compensation is activated, the system automatically adjusts the tool position to match the intended movement path. This makes the process more user-friendly, as the program remains unchanged even if the tool wears or is replaced.
However, economical CNC machines, which are more cost-effective and widely used in production environments, often lack this advanced tool compensation feature. Without it, the machining program must be written based on the tool point's movement path. This requires calculating the tool path dimensions from the workpiece contour and the tool diameter, leading to more complex calculations. Additionally, any tool wear or replacement necessitates recalculating the tool path and reprogramming the entire sequence, which can be time-consuming and inefficient.
Second, Tool Compensation in Full-Functional CNC Systems:
1. Tool Compensation on CNC Lathes
CNC lathes usually have two types of tool compensation: tool position compensation and tool nose radius compensation. These are specified using the T function in the machining program. The first two digits of T***X indicate the tool number, while the last two represent the compensation number, such as T0202. If the compensation number is 00, the tool compensation is disabled.
(1) Tool Position Compensation: This compensates for changes in the tool position due to wear or reinstallation. Once enabled, the program doesn't need to be rewritten. Instead, the tool's measured position is entered into a designated memory. When the tool compensation command is executed, the actual tool position replaces the original one.
For example, without compensation, a tool might move from point 0 to point 1, as in N60G00C45X93T0200. But with X=3 and Z=4 stored in the compensation memory, the tool would move to point 2 instead, resulting in N60G00X45Z93T0202.
(2) Tool Nose Radius Compensation: During programming, the turning tool tip is considered a single point (P point). However, in reality, the cutting edge is often rounded to improve tool life and reduce surface roughness. This can lead to shape errors, which can be corrected using tool nose radius compensation. Parameters like R and T are used to define the tool's geometry, and these values are stored in memory for automatic correction during machining.
(3) Tool Compensation Parameters: Each compensation number corresponds to both position (X, Z) and radius (R, T) values. These parameters are input into memory before machining and displayed on the CRT. During operation, the CNC system automatically adjusts the tool's position and applies the necessary radius compensation based on the stored values.
2. Tool Compensation on Machining Centers and CNC Milling Machines
Machining centers and CNC milling machines also support various forms of tool compensation, including radius compensation, angle compensation, and length compensation.
(1) Tool Radius Compensation (G41, G42, G40): The tool's radius is stored in HXX memory, where XX is the address. G41 is used for left-side compensation, and G42 for right-side compensation. G40 cancels the radius compensation. It's important to note that when setting or canceling compensation, only G00 or G01 commands should be used, not G02 or G03. Also, negative values can reverse the effect of G41 and G42.
There are two modes of radius compensation: B function and C function. While B function only considers the current block, C function allows for smooth transitions between blocks, making it more suitable for complex contours. Most modern CNC systems use the C function for better accuracy and ease of programming.
(2) Angle Compensation (G39): This is used when two planes intersect at an angle, potentially causing overcutting or undercutting. G39 helps prevent such errors but is non-modal and only valid within the block it is used in. It must follow G41 or G42 commands.
(3) Tool Length Offset (G43, G44, G49): This compensates for variations in tool length without changing the program. G43 adds the offset value to the endpoint coordinate, while G44 subtracts it. G49 or H00 cancels the offset. For example, N80G43Z56H05 with H05 storing 16mm means the final Z-coordinate is 72mm.
Compensation values can be pre-set via MDI or DPL, or directly programmed using G10P05R16.0.
Third, Tool Path Calculation on Economical CNC Machines:
On economical CNC machines, if no tool compensation is available, the tool path must be calculated manually based on the tool point's movement. This can be done through direct programming or partial compensation. Although less efficient than full-featured systems, this method is still viable for simpler operations.
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Tag: CNC Machine Tool, End Milling, Tool Compensation
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