# 3 Compensation Methods Commonly Used In CNC Machining

Although compensation is applied to different machine types for different reasons, all forms of compensation allow CNC users to consider unpredictable conditions related to the tool.

Before discussing how compensation is applied to CNC usage, let us first look at compensation from a general perspective.

Tool compensation determines the machining accuracy and quality. There are tool length compensation, tool radius compensation and fixture offset compensation in the CNC system. These three types of compensation can basically solve the path problem caused by the shape of the tool in the machining.

## Tool Length Compensation:

### 1. The concept of tool length

Tool length is a very important concept. When we program a part, we must first specify the programming center of the part, and then establish the workpiece programming coordinate system, and this coordinate system is just a workpiece coordinate system, and the zero point is generally on the workpiece. The length compensation is only related to the Z coordinate. It is not like the programmed zero point in the X and Y planes, because the tool is positioned by the spindle taper hole without changing, and the zero point of the Z coordinate is different. The length of each knife is different. For example, we need to drill a hole with a depth of 50mm, and then tap a hole with a depth of 45mm. We use a drill with a length of 250mm and a tap with a length of 350mm. First use a drill to drill a hole with a depth of 50mm. At this time, the machine tool has set the workpiece zero point. When tapping is replaced, if both tools start processing from the set zero point, the tap is longer than the drill and the tap is too long, which will damage the tool. And artifacts. If the tool compensation is set at this time, the length of the tap and the drill will be compensated. At this time, after the machine zero point is set, even if the length of the tap and the drill are different, due to the existence of compensation, the zero point Z coordinate has automatically moved to Z+ when the tap is called. The length of the tap is compensated to ensure the correct processing zero point.

### 2. Tool length compensation work

The use of tool length compensation is achieved by executing commands containing G43 (G44) and H, and at the same time we give a Z coordinate value, so that the tool moves to a distance Z from the workpiece surface after compensation. Another command G49 cancels the G43 (G44) command. In fact, we don’t need to use this command, because each tool has its own length compensation. When changing the tool, the G43 (G44) H command is used to give its own tool length compensation. The length compensation of the previous tool is automatically cancelled.

### 3. Two ways of tool length compensation

#### 1) Use the actual length of the tool as the tool length compensation (this method is recommended).

To use the tool length as compensation is to use the tool setter to measure the length of the tool, and then input this value into the tool length compensation register as tool length compensation.

The reasons for using tool length as tool length compensation are as follows:

First of all, using the tool length as tool length compensation can avoid constantly modifying the tool length offset in different workpiece processing. Such a tool is used on different workpieces without modifying the tool length offset. In this case, you can file each tool according to a certain tool numbering rule, and write the relevant parameters of each tool with a small sign, including the length and radius of the tool. In fact, many large machinery Processing enterprises adopt this method for tool management of CNC machining equipment. For those companies that have specialized tool management departments, there is no need to tell the tool parameters face-to-face with the operator. At the same time, even if the tool is removed due to the capacity of the tool magazine and the next time it is reinstalled, it is only necessary to The tool length value on the label is used as tool length compensation without measuring.

Secondly, the use of tool length as tool length compensation allows the machine tool to perform machining operations while measuring the length of other tools on the tool setter, instead of occupying machine tool running time due to tool setting on the machine tool, which can give full play to processing The efficiency of the center. In this way, when the spindle moves to the programmed Z coordinate point, it is the Z coordinate value of the spindle coordinate plus (or subtracting) the tool length compensation.

#### 2) Use the distance (positive or negative) between the tool tip in the Z direction and the programmed zero point as the compensation value.

This method is suitable for use when there is only one person operating the machine tool and there is not enough time to measure the length of the tool with the tool setter. In this way, it is necessary to re-set the tool length compensation when machining another workpiece with one tool. When using this method for tool length compensation, the compensation value is the tool nose movement distance when the spindle moves from the machine tool Z coordinate zero point to the workpiece programming zero point, so this compensation value is always negative and large.

### 1. The concept of tool radius compensation

Just as the tool length compensation is used in programming, the length of the tool is basically not considered. Because of the tool radius compensation, we can not consider too much tool diameter when programming. Tool length compensation is applicable to all tools, while tool radius compensation is generally only used for milling cutters. When the milling cutter is processing the outer or inner contour of the workpiece, the tool radius compensation is used, and when the end face of the workpiece is processed with the end milling cutter, only the tool length compensation is required. Because tool radius compensation is a relatively difficult instruction to understand and use, many people do not want to use it in programming. But once we understand and master it, using it will bring great convenience to our programming and processing.

When the programmer prepares a program for processing the shape of a workpiece with a milling cutter, first of all, it is necessary to calculate the coordinate value carefully according to the shape of the workpiece and the radius of the tool to clarify the path of the tool center. The radius of the tool used at this time is only the radius value of the milling cutter. After working hard to program the milling cutter, it is found that this milling cutter is not suitable for changing to other diameters. The programmer has to recalculate the tool center without hesitation. The coordinate value of the route taken. This is not too big a problem for a simple workpiece, and it is simply too difficult to recalculate for a mold with a complicated shape.

The contour machining of a workpiece is divided into rough machining and finishing, so that the rough machining is completed after the rough machining program is compiled. Because after rough machining, the workpiece dimensions have changed, and then the coordinate value of the tool center for finishing must be calculated, and the workload is even greater. At this time, if the tool radius compensation is used, these troubles are all solved. We can ignore the tool radius and program according to the workpiece size, and then put the tool radius as radius compensation in the radius compensation register. Whether the milling cutter is temporarily replaced or rough and finished, we only need to change the tool radius compensation value to control the size of the workpiece shape, and basically not modify the program.

### 2. Use of tool radius compensation

The use of tool radius compensation is executed by commands G41 and G42. Compensation has two directions, that is, the compensation is carried out along the left and right sides perpendicular to the cutting feed direction of the tool, which conforms to the left-hand rule. G41 is left compensation and conforms to the left-hand rule. G42 is right compensation and conforms to the right-hand rule. G41 is used 2. When G42 performs radius compensation, special attention should be paid to the tool movement direction and coordinates for effective compensation. The starting position of the tool radius compensation is very important. It is easy to make mistakes if the path processed by the improper tool is used.

## Fixture Offset Compensation

Just as the tool length compensation and radius compensation allow the programmer to not consider the length and size of the tool, the fixture offset allows the programmer to use the fixture offset regardless of the position of the workpiece fixture.

When a machining center is processing small workpieces, the tooling can clamp several workpieces at a time. The programmer does not need to consider the coordinate zero point of each workpiece during programming, but only needs to program according to the respective programmed zero point, and then use the fixture Offset to move the programmed zero point of the machine tool on each workpiece. The fixture offset is executed using fixture offset commands G54～G59. Another method is to use G92 command to set the coordinate system. After one workpiece is processed, use G92 to reset the new workpiece coordinate system when processing the next workpiece.

The above are 3 kinds of compensation commonly used in CNC machining, which bring great convenience to our programming and machining, and can greatly improve work efficiency.