Machining accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of the part after processing conform to the ideal geometric parameters specified in the drawing. The higher the degree of this conformity, the higher the processing accuracy.
Today we will discuss from the following 3 aspects:
- How to obtain the dimensional accuracy of parts
- Method of obtaining shape accuracy
- Method of obtaining position accuracy
How to obtain the dimensional accuracy of parts
(1) Trial cutting method
That is, first try to cut out a small part of the machined surface, measure the size of the test cut, adjust the position of the cutting edge of the tool relative to the workpiece according to the processing requirements, and then try to cut, and then measure, so after two or three trial cuts and measurements, when it is processed After the size meets the requirements, cut the entire surface to be machined.
The trial cut method is through “trial cut-measurement-adjustment-trial cut again”, and repeats until the required dimensional accuracy is reached. For example, the trial boring of the box hole system.
The accuracy achieved by the trial cutting method may be very high, it does not require complicated devices, but this method is time-consuming (requiring multiple adjustments, trial cuts, measurements, and calculations), low efficiency, and depends on the skill level of the workers and the accuracy of the measuring instruments , The quality is unstable, so it is only used for single-piece small batch production.
As a type of trial-cutting method-matching, it is based on the processed part, processing another matched workpiece, or combining two (or more) workpieces together for processing. The requirements for the final processed dimensions in the matching work are based on the matching requirements with the processed parts.
(2) Adjustment method
Use sample or standard parts to adjust the accurate relative positions of machine tools, fixtures, cutting tools and workpieces to ensure the dimensional accuracy of the workpieces. Because the size is adjusted in advance, there is no need to try cutting 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 machine fixture, the position of the tool is determined by the tool block. The essence of the adjustment method is to use the fixed distance device or the tool setting device on the machine tool or the pre-set 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.
The machine tool feeds and then cuts according to the dial, which is also a kind of adjustment method. This method requires first to determine the scale on the dial according to the trial cut method. In mass production, tool setting devices such as fixed-range stoppers, prototypes, and prototypes are often used for adjustment.
The adjustment method has better processing accuracy and stability than the trial cutting method, and has higher productivity. It does not require high machine tool operators, but it has high requirements for machine tool adjusters. It is often used in batch production and mass production.
(3) Fixed size method
The method of using the corresponding size of the tool to ensure the size of the processed part of the workpiece is called the sizing method. It uses standard-sized tools for machining, and the size of the machined surface is determined by the tool size. That is, a tool with a certain dimensional accuracy (such as a reamer, a reaming drill, a drill bit, etc.) is used to ensure the accuracy of the workpiece to be processed (such as a hole).
The sizing method is easy to operate, has high productivity, and has relatively stable processing accuracy. It has almost nothing to do with the technical level of workers. It has high productivity and is widely used in various types of production. For example, drilling, reaming, etc.
(4) Active measurement method
During the machining process, the machining dimensions are measured while machining, and the measured results are compared with the dimensions required by the design, or the machine tool can continue to work or stop the machine tool. This is the active measurement method.
At present, the value in active measurement can be displayed digitally. The active measurement method adds the measurement device to the process system (that is, the unity of machine tools, cutting tools, fixtures and workpieces), which becomes its fifth factor.
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 combines the measuring, feeding device and control system into an automatic processing system, and the processing process is automatically completed by the system.
A series of tasks such as size measurement, tool compensation adjustment, cutting processing, and machine stoppage are automatically completed to automatically achieve the required dimensional accuracy. For example, when processing on a CNC machine tool, the parts are processed through various instructions of the program to control the processing sequence and processing accuracy.
There are two specific methods of automatic control
It means that the machine tool has a device for automatically measuring the size of the workpiece. When the workpiece reaches the required size, the measuring device will issue an instruction to automatically retract the machine tool and stop working.
It means that there are servo motors, rolling screw nut pairs and a complete set of digital control devices in the machine tool to control the precise movement of the tool post or worktable. The size acquisition (the movement of the tool post or the movement of the worktable) is determined by a pre-programmed program. Automatic control by computer digital control device.
The initial automatic control method was completed by using active measurement and mechanical or hydraulic control systems. At present, pre-arranged programs according to processing requirements have been widely used. The program controlled by the control system to perform work, or the digital control machine tool issued by the control system to perform digital information instructions, and can adapt to changes in processing conditions during the processing process, automatically Adjust the processing amount, and realize the optimization of the processing process according to the specified conditions. The adaptive control machine tool performs automatic control processing.
The automatic control method has stable quality, high productivity, good processing flexibility, and adaptability to multi-variety production, which is the current development direction of mechanical manufacturing and the basis of computer-aided manufacturing (CAM).