Seven common tool setting methods in CNC machining

Tool setting is a major operation and an important skill in numerical control machining. Under certain conditions, the accuracy of tool setting can determine the machining accuracy of parts. Meanwhile, the efficiency of tool setting also directly affects the efficiency of CNC machining. It is not enough to merely know the tool setting methods. One also needs to understand the various tool setting Settings of the numerical control system and the calling methods of these methods in the processing program. At the same time, one should be aware of the advantages and disadvantages of each tool setting method, as well as their application conditions, etc.

I. Principle of Tool Setting

The purpose of tool setting is to establish the workpiece coordinate system. Intuitively speaking, tool setting is to determine the position of the workpiece in the machine tool's worktable. In fact, it is to find the coordinates of the tool setting point in the machine tool coordinate system. For CNC lathes, the first step before processing is to select the tool setting point. The tool setting point refers to the starting point of the tool's movement relative to the workpiece when processing it with a CNC machine tool. The tool setting point can be set either on the workpiece (such as the design reference or positioning reference on the workpiece), or on the fixture or machine tool. If it is set at a certain point on the fixture or machine tool, that point must maintain a certain dimensional relationship with the positioning reference of the workpiece.

When setting the tool, the tool pointing point should coincide with the tool setting point. The so-called tool pointing point refers to the positioning reference point of the tool. For turning tools, the tool pointing point is the tool tip. The purpose of tool setting is to determine the absolute coordinate values of the tool setting point (or the origin of the workpiece) in the machine coordinate system and measure the tool position deviation value. The accuracy of tool alignment directly affects the machining precision. When actually processing workpieces, using one cutting tool generally cannot meet the processing requirements of the workpiece. Usually, multiple cutting tools are used for processing. When using multiple turning tools for processing, if the tool change position remains unchanged, the geometric position of the tool tip point will vary after the tool change. This requires that different tools can ensure the normal operation of the program when starting the processing from different initial positions. To solve this problem, the numerical control system of the machine tool is equipped with the function of tool geometric position compensation. By using the tool geometric position compensation function, as long as the position deviation of each tool relative to a pre-selected reference tool is measured in advance and input into the specified group number in the tool parameter correction column of the numerical control system, the T command can be used in the processing program. It can automatically compensate for the tool position deviation in the tool path. The measurement of tool position deviation also needs to be achieved through tool setting operations.

Ii. Method of Setting the knife

In numerical control machining, the basic methods of tool setting include trial cutting, tool setting with a tool setting instrument, and automatic tool setting, etc. This article takes a CNC milling machine as an example to introduce several commonly used tool setting methods.

1. Try cutting the correct knife technique

This method is simple and convenient, but it will leave cutting marks on the surface of the workpiece and has relatively low tool setting accuracy. As shown in Figure 1, taking the tool setting point (which coincides with the origin of the workpiece coordinate system) at the center of the workpiece surface as an example, a bilateral tool setting method is adopted.

(1) Tool alignment in the x and y directions.

① Place the workpiece on the worktable through the fixture. When clamping, leave space for tool setting on all four sides of the workpiece.

② Start the spindle to rotate at medium speed, quickly move the worktable and the spindle, and let the tool move rapidly to a position close to the left side of the workpiece at a certain safe distance. Then reduce the speed and move it close to the left side of the workpiece.

When approaching the workpiece, switch to a fine-tuning operation (generally 0.01mm) to get close. Let the tool slowly approach the left side of the workpiece until it just touches the left surface of the workpiece (observe, listen to the cutting sound, look at the cut mark, and look at the chip. If any one of these situations occurs, it indicates that the tool has touched the workpiece), and then retracted by 0.01mm. Record the coordinate values displayed in the machine tool coordinate system at this time, such as -240.500.

④ Withdraw the tool along the Z-positive direction until it is above the workpiece surface. Approach the right side of the workpiece in the same way and record the coordinate values displayed in the machine coordinate system at this time, such as -340.500.

Based on this, the coordinate values of the origin of the workpiece coordinate system in the machine tool coordinate system can be obtained as

{-240.500+ (-340.500)}/2=-290.500.

Similarly, the coordinate values of the origin of the workpiece coordinate system in the machine tool coordinate system can be measured.

(2) Z-direction tool alignment.

① Quickly move the cutting tool above the workpiece.

② Start the spindle to rotate at medium speed, quickly move the worktable and the spindle, and let the tool move rapidly to a position close to the upper surface of the workpiece at a certain safe distance. Then reduce the speed and move to bring the end face of the tool close to the upper surface of the workpiece.

③ When approaching the workpiece, switch to fine-tuning operation (generally 0.01mm) to approach, allowing the end face of the tool to gradually approach the surface of the workpiece (note that when cutting the tool, especially the end mill, it is best to cut at the edge of the workpiece, and the area of the end face of the tool in contact with the surface of the workpiece should be less than a semi-circle. Try not to cut the center hole of the end mill on the surface of the workpiece), so that the end face of the tool exactly touches the upper surface of the workpiece Raise the axis again and record the z value in the machine tool coordinate system at this time, which is -140.400. Then, the coordinate value of the origin W of the workpiece coordinate system in the machine tool coordinate system is -140.400.

(3) Input the measured x, y, and z values into the storage address G5* of the machine tool workpiece coordinate system (generally, G54 to G59 codes are used to store the tool setting parameters).

(4) Enter the Panel Input mode (MDI), type "G5*", press the start button (in automatic mode), and run G5* to make it effective.

(5) Check whether the tool setting is correct.

2. Tool alignment methods using feeler gauges, standard mandrels and gauge blocks

This method is similar to the trial cutting tool setting method, except that the spindle does not rotate during tool setting. A feeler gauge (or standard mandrel, gauge) is added between the tool and the workpiece, with the feeler gauge not being able to move freely as the standard. Note that when calculating the coordinates, the thickness of the feeler gauge should be subtracted. Since the spindle does not need to rotate for cutting, this method will not leave marks on the workpiece surface, but the tool setting accuracy is not high enough either.

3. Use tools such as edge finders, eccentric rods and shaft setters for tool setting

The operation steps are similar to those of the trial cutting and tool alignment method, except that the tool is replaced with an edge finder or an eccentric rod. This is the most commonly used method. High efficiency and capable of ensuring tool setting accuracy. When using an edge finder, one must be careful to ensure that the steel ball part makes slight contact with the workpiece. At the same time, the workpiece being processed must be a good conductor, and the positioning reference surface must have a good surface roughness. The Z-axis setter is generally used for transferring (indirect) tool alignment.

4. Transfer (indirect) the knife technique

When processing a workpiece, more than one tool is often required. The length of the second tool is different from that of the first tool, and it needs to be reset to zero. However, sometimes the zero point is removed during processing and cannot be directly retrieved, or it is not allowed to damage the already processed surface. There are also some tools or situations where direct tool setting is not feasible. In such cases, an indirect zeroing method can be adopted.

(1) The first knife.

When using the first knife, still use the trial cutting method, feeler gauge method, etc. first. Record the machine coordinate z1 of the workpiece origin at this time. After the first tool is processed, stop rotating the spindle.

② Place the tool setter on the flat surface of the machine tool's worktable (such as the large surface of a vise).

③ In handwheel mode, use the hand crank to move the worktable to a suitable position, then move the spindle downward. Press the bottom of the tool against the top of the tool setter. The pointer on the dial should rotate within one full circle. Record the reading of the axis setter at this time and reset the relative coordinate axes to zero.

④ Raise the main shaft accurately and remove the first knife.

(2) The second strike.

① Install the second knife.

② In handwheel mode, move the spindle downward, press the bottom end of the knife against the top of the knife tool, and the pointer on the dial will rotate, pointing to the same reading A position as the first knife.

③ Record the value z0 corresponding to the relative coordinates of the axis at this time (with a positive or negative sign).

④ Raise the spindle and remove the tool setter.

⑤ Add z0 (with positive and negative signs) to the z1 coordinate data in the G5* of the original first knife to obtain a new coordinate.

This new coordinate is the actual coordinate of the machine tool corresponding to the workpiece origin of the second tool to be found. Input it into the G5* working coordinate of the second tool. In this way, the zero point of the second tool is set. The alignment method of the remaining knives is the same as that of the second knife.

Note: If several tools use the same G5*, then in steps 5) and 6), instead of storing z0 in the length parameter of the second tool, when using the second tool for processing, simply call the tool length correction G43H02.

5. Top-notch swordsmanship

(1) Tool alignment in the x and y directions.

① Load the workpiece onto the machine tool's worktable through the fixture and replace it with the center.

② Quickly move the worktable and the spindle to bring the center point closer to the workpiece. Find the center point of the line drawn on the workpiece and reduce the speed to bring the center point closer to it.

③ Switch to fine-tuning operation, allowing the center point to gradually approach the center point of the workpiece's drawing line until the tip of the center point aligns with the center point of the workpiece's drawing line. Record the x and y coordinate values in the machine coordinate system at this time.

(2) Remove the center, install the milling cutter, and obtain the Z-axis coordinate values by other tool setting methods such as the trial cutting method or the feeler gauge method.

6. Dial indicator (or micrometer) tool setting method (generally used for setting circular workpieces)

(1) Tool alignment in the x and y directions.

Install the mounting rod of the dial indicator on the tool holder or attach the magnetic seat of the dial indicator to the spindle sleeve. Move the worktable to approximately align the centerline of the spindle (i.e., the center of the tool) with the center of the workpiece. Adjust the length and Angle of the telescopic rod on the magnetic seat so that the contact of the dial indicator touches the circumferential surface of the workpiece (the pointer rotates approximately 0.1mm). Slowly rotate the spindle by hand Rotate the contact of the dial indicator along the circumferential surface of the workpiece, observe the movement of the dial indicator pointer, slowly move the shaft of the worktable and the shaft, and repeat this process several times. When the main shaft is rotated, the pointer of the dial indicator is basically at the same position (when the dial indicator rotates one full circle, the pointer's jump is within the allowable tool setting error, such as 0.02mm). At this point, it can be considered that the center of the main shaft is the axis and the origin of the axis.

(2) Remove the dial indicator and install the milling cutter. Use other tool setting methods such as trial cutting or feeler gauge method to obtain the Z-axis coordinate values.

7. Special tool setting method

The traditional tool setting methods have disadvantages such as poor safety (for example, using a feeler gauge for tool setting, where the hard tool tip is prone to damage), high machine time occupation (for instance, trial cutting requires repeated measurement several times), and large random errors brought by human factors. They can no longer adapt to the rhythm of CNC machining and are even less conducive to the full play of the functions of CNC machine tools. Using a dedicated tool setter for tool setting has advantages such as high tool setting accuracy, high efficiency, and good safety. It simplifies the cumbersome tool setting work that relies on experience, ensuring the full play of the high efficiency and high precision characteristics of CNC machine tools. It has become an indispensable special tool for solving tool setting on CNC machining machines