Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the part after machining conform to the ideal geometric parameters specified in the drawing. The higher the degree of conformity, the higher the machining accuracy.
In processing, due to the influence of various factors, it is actually impossible to process every geometric parameter of the part to be completely consistent with the ideal geometric parameter, and there will always be some deviations. This deviation is the processing error.
Discuss from the following three aspects:
1. Methods to obtain the dimensional accuracy of parts
2. Methods for obtaining shape accuracy
3. How to get position accuracy
1. Methods to obtain the dimensional accuracy of parts
(1) Trial cutting method
That is, first try to cut out a small part of the processed surface, measure the size obtained from the test cut, adjust the position of the cutting edge of the tool relative to the workpiece according to the processing requirements, then try to cut, and then measure, so after two or three times of test cutting and measurement, when the processed After the size meets the requirements, the entire surface to be processed is then cut.
The trial cutting method is repeated through "trial cutting-measurement-adjustment-re-trial cutting" until the required dimensional accuracy is achieved. For example, the trial boring process of the box hole system.
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The precision achieved by the trial cutting method may be very high, and it does not require complicated devices, but this method is time-consuming (requires multiple adjustments, trial cutting, measurement, calculation), low efficiency, and depends on the technical level of workers and the accuracy of 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 piece, processing another matching workpiece, or combining two (or more than two) workpieces for processing. The requirements of the final processed size in the matching are based on the matching requirements with the processed parts.
(2) Adjustment method
Pre-adjust the exact relative position of the machine tool, fixture, tool and workpiece with samples or standard parts to ensure the dimensional accuracy of the workpiece. Because the size is adjusted in place in advance, there is no need to try cutting during processing, the size is automatically obtained, and it 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 setting block. The essence of the adjustment method is to use the positioning device or the tool setting device or the pre-adjusted tool holder on the machine tool 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 the tool according to the dial on the machine tool and then cut it. This method needs to first determine the scale on the dial according to the trial cutting method. In mass production, fixed stoppers, samples, templates and other tool setting devices are often used to adjust.
Compared with the trial cutting method, the adjustment method has better machining accuracy stability and higher productivity. It does not have high requirements for machine tool operators, but has high requirements for machine tool adjustment workers. 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 workpiece to be processed is called the sizing method. It is processed using standard size tools, and the size of the processing surface is determined by the size of the tool. That is, tools with certain dimensional accuracy (such as reamers, reamers, drills, etc.) are used to ensure the accuracy of the workpiece to be processed (such as holes).
The sizing method is easy to operate, has high productivity, and relatively stable machining accuracy. It has almost nothing to do with the technical level of workers, and has high productivity. It is widely used in various types of production. Such as drilling, reaming, etc.
(4) Active measurement method
During the processing, measure the processing size while processing, and compare the measured result with the size required by the design, or make the machine tool continue to work, or stop the machine tool, this is the active measurement method.
Currently, values in active measurements can be displayed numerically. The active measurement method adds the measuring device to the process system (that is, the unity composed of machine tools, tools, fixtures and workpieces), becoming its 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, feeding device and control system, and the processing process is automatically completed by the system.
A series of tasks such as dimensional measurement, tool compensation adjustment, cutting processing and machine tool parking are automatically completed to automatically achieve the required dimensional accuracy. For example, when processing on a CNC machine tool, the parts control the processing sequence and processing accuracy through various instructions of the program.
There are two specific methods of automatic control:
① Automatic measurement means that there is a device on the machine tool to automatically measure the size of the workpiece. When the workpiece reaches the required size, the measuring device will issue an instruction to automatically retract the tool and stop working.
②Digital control means that the machine tool has a servo motor that controls the precise movement of the tool holder or worktable, a rolling screw nut pair, and a complete set of digital control devices. The size (movement of the tool holder or worktable) is obtained by a pre-programmed program Automatically controlled by computer numerical control device.
The initial automatic control method was accomplished by using active measurement and control systems such as mechanical or hydraulic pressure. At present, programs pre-arranged according to processing requirements have been widely used, program-controlled machine tools issued by the control system to work, or digitally controlled machine tools issued by the control system to issue digital information instructions to work, and can adapt to changes in processing conditions during the processing process. Adjust the amount of processing, according to the specified conditions to achieve the optimization of the processing process, the adaptive control machine tool performs automatic control processing.
The automatic control method has stable quality, high productivity, good processing flexibility, and can adapt to multi-variety production. It is the development direction of mechanical manufacturing and the basis of computer-aided manufacturing (CAM).
2. Methods for obtaining shape accuracy
(1) Trajectory method
This processing method uses the trajectory of the tool tip to form the shape of the machined surface. Ordinary turning, milling, planing and grinding all belong to the tool nose trajectory method. The shape accuracy obtained by this method mainly depends on the accuracy of the forming movement.
(2) Forming method
The machined surface shape is obtained by using the geometric shape of the forming tool instead of some forming movements of the machine tool. Such as forming turning, milling, grinding, etc. The shape accuracy obtained by the forming method mainly depends on the shape of the blade.
(3) Development method
The shape of the machined surface is obtained by using the envelope surface formed by the generating motion of the tool and the workpiece, such as gear hobbing, gear shaping, grinding, knurling, etc., all belong to the generating method. The shape accuracy obtained by this method mainly depends on the shape accuracy and generative motion accuracy of the blade.
3. How to get position accuracy
In mechanical processing, the position accuracy of the machined surface to other surfaces mainly depends on the clamping of the workpiece.
(1) Direct alignment and clamping
This method is a clamping method to directly find the position of the workpiece on the machine tool by using a dial indicator, a marking plate or visual inspection.
(2) Scribing and aligning the clamping
This method is to first draw the center line, symmetry line and the processing line of each surface to be processed on the blank according to the part drawing, then load the workpiece on the machine tool, and then find the clamping position of the workpiece on the machine tool according to the drawn line.
This clamping method has low productivity, low precision, and requires high technical level of workers. It is generally used to process complex and heavy parts in single-piece small batch production, or where the blank size tolerance is large and cannot be clamped directly with fixtures.
(3) Clamping with a fixture
The fixture is specially designed according to the requirements of the processed process. The positioning elements on the fixture can make the workpiece quickly occupy the correct position relative to the machine tool and the tool. It can ensure the clamping and positioning accuracy of the workpiece without alignment, and the productivity of clamping with the fixture is high. The positioning accuracy is high, but special fixtures need to be designed and manufactured, which are widely used in batch and mass production.
