Five methods for measuring the dimensional accuracy of machined parts

There are mainly the following methods to obtain the dimensional accuracy of workpiece in machining:

(1) Trial cut method

That is, first try to cut a small part of the machined surface, measure the size obtained by the test cutting, properly adjust the position of the cutting edge of the tool relative to the workpiece according to the processing requirements, try to cut again, and then measure. After two or three times of test cutting and measurement, when the machined size meets the requirements, cut the entire surface to be machined.

The trial cutting method is repeated until the required dimensional accuracy is reached through “trial cutting – measurement – adjustment – trial cutting”. For example, the trial boring of the box hole system.

The precision achieved by the trial cutting method may be very high, and it does not require complex devices. However, this method is time-consuming (it requires multiple adjustments, trial cutting, measurement and calculation), inefficient, depends on the technical level of workers and the accuracy of measuring instruments, and the quality is unstable, so it is only used for single piece small batch production. WeChat of metal processing, good content, worthy of attention!

As a type of trial cutting method – matching, it is a method of processing another matching workpiece or combining two (or more) workpieces for processing based on the processed workpiece. The final size to be machined during matching is subject to the matching requirements with the machined parts.

(2) Adjustment method

Accurately adjust the relative positions of the machine tool, fixture, cutter and workpiece with samples or standard parts in advance to ensure the dimensional accuracy of the workpiece. Because the size is adjusted in place in advance, it is unnecessary to cut again during processing. The size is automatically obtained and remains unchanged during the processing of a batch of parts. This is the adjustment method. For example, when using a milling fixture, the position of the tool is determined by the tool setting block. The essence of the adjustment method is to use the fixed distance device or tool setting device on the machine tool or the preset tool holder to make the tool reach a certain position accuracy relative to the machine tool or fixture, and then process a batch of workpieces.

It is also a kind of adjustment method to feed and cut according to the dial on the machine tool. This method needs to determine the scale on the dial according to the trial cutting method. In mass production, tool setting devices such as fixed distance stop, sample piece and sample plate are often used for adjustment.

The adjustment method is more stable than the trial cutting method in processing accuracy, has higher productivity, and has lower requirements for machine tool operators, but has higher requirements for machine tool adjusters, which is commonly used for mass production and mass production.

(3) Sizing method

The method of using the corresponding size of the cutter to ensure the size of the part to be processed is called sizing method. It is processed with standard size tools, and the size of the machined surface is determined by the tool size. In other words, tools with certain dimensional accuracy (such as reamers, reamers, drills, etc.) are used to ensure the accuracy of the parts (such as holes) to be processed.

The sizing method is easy to operate, with high productivity, stable processing accuracy, almost independent of the technical level of workers, and high productivity. It is widely used in various types of production. Such as drilling, reaming, etc.

(4) Active measurement

In the process of processing, measure the processing size while processing, and compare the measured results with the size required by the design, or make the machine tool continue to work, or make the machine tool stop working, this is the active measurement method.

At present, the values in the active measurement can be displayed digitally. The active measurement method adds the measuring device to the process system (i.e. the unity of machine tool, tool, fixture and workpiece), and becomes the fifth factor.

The active measurement method has stable quality and high productivity, which is the development direction.

(5) Automatic control method

This method is composed of measuring device, feeding device and control system. It is an automatic processing system composed of measuring device, feeding device and control system, and the processing process is automatically completed by the system.

A series of work such as dimension measurement, tool compensation adjustment, cutting and machine tool stop are automatically completed, and the required dimension accuracy is automatically achieved. For example, when machining on a CNC machine tool, parts control the machining sequence and accuracy through various instructions of the program.

There are two specific methods of automatic control:

① Automatic measurement means that the machine tool is equipped with a device to automatically measure the size of the workpiece. When the workpiece reaches the required size, the measuring device will send a command to make the machine tool automatically retract and stop working.

② Digital control means that there are servo motors, rolling lead screw nut pairs and a complete set of digital control devices in the machine tool to control the precise movement of the tool rest or workbench. The acquisition of dimensions (the movement of the tool rest or the movement of the workbench) is automatically controlled by the computer digital control device through the pre prepared program.

The initial automatic control method is completed by using active measurement and mechanical or hydraulic control systems. At present, it has widely adopted the program pre arranged according to the processing requirements, the program control machine tool that the control system sends instructions to work, or the digital control machine tool that the control system sends digital information instructions to work, and the adaptive control machine tool that can adapt to the changes of processing conditions in the processing process, automatically adjust the processing amount, and optimize the processing process according to the specified conditions to automatically control the processing.

Automatic control processing has stable quality, high productivity, good processing flexibility, and can adapt to multi variety production. it is the development direction of machine manufacturing and the basis of computer aided manufacturing (cam).