Common methods for obtaining workpiece dimensional accuracy in cnc machining

Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the part after machining are consistent with the ideal geometric parameters specified in the drawing. The higher the degree of consistency, the higher the machining accuracy.

During machining, due to the influence of various factors, it is actually impossible to machine every geometric parameter of the part to be completely consistent with the ideal geometric parameters, and there will always be some deviations. This deviation is the machining error.

1. Methods for obtaining part dimensional accuracy

(1) Trial cutting method

That is, first cut out a small part of the machining surface, measure the size obtained by the trial cutting, adjust the position of the tool cutting edge relative to the workpiece according to the machining requirements, then try cutting, and then measure again. After two or three trial cuttings and measurements, when the machined size meets the requirements, cut the entire surface to be machined.

The trial cutting method is repeated through "trial cutting-measurement-adjustment-retrial cutting" until the required dimensional accuracy is achieved. For example, trial boring machining of box hole system.

The precision achieved by the trial cutting method may be very high, and it does not require complex equipment. However, this method is time-consuming (requires multiple adjustments, trial cutting, measurement, and calculation), inefficient, and dependent on the worker's technical level and the accuracy of the measuring instrument. The quality is unstable, so it is only used for single-piece small batch production.

As a type of trial cutting method, matching is a method of processing another workpiece that matches the processed part based on the processed part, or combining two (or more) workpieces together for processing. The requirements for the final processed size in matching are based on the matching requirements with the processed part.

(2) Adjustment method

Pre-adjust the accurate relative positions 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 for trial cutting 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 tool setting device on the machine tool or the pre-adjusted 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.

Feeding the tool according to the dial and then cutting on the machine tool is also a kind of adjustment method. This method requires first determining the scale on the dial according to the trial cutting method. In mass production, fixed-distance blocks, samples, templates and other tool setting devices are often used for adjustment.

The adjustment method has better processing accuracy stability than the trial cutting method, has higher productivity, and does not require high requirements for machine tool operators, but 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 workpiece to be processed is called the fixed-size method. It uses standard-sized tools for processing, and the size of the processed surface is determined by the tool size. That is, using tools with certain dimensional accuracy (such as reamers, reamer drills, drill bits, etc.) to ensure the accuracy of the workpiece to be processed (such as holes). If you don't want to work at the bottom level, want to get out of the current situation, and want to learn UG programming, you can find a teacher from Dianguan Education to learn CNC numerical control technology.

The sizing method is easy to operate, has high productivity, and has relatively stable processing accuracy. It is almost independent of the worker's technical level and has high productivity. It is widely used in various types of production. For example, drilling and reaming.

(4) Active measurement method

During the processing, the processing dimensions are measured while processing, and after comparing the measured results with the dimensions required by the design, the machine tool is either allowed to continue working or stopped. This is the active measurement method.

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

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

(5) Automatic control method

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

A series of tasks such as dimension measurement, tool compensation adjustment, cutting processing, and machine tool parking are completed automatically to automatically achieve the required dimensional accuracy. For example, when processing on a CNC machine tool, the parts are controlled by various instructions of the program to control the processing sequence and processing accuracy.

There are two specific methods of automatic control:

① Automatic measurement, that is, the machine tool has a device that automatically measures the size of the workpiece. When the workpiece reaches the required size, the measuring device issues a command to automatically retract the tool and stop working.

② Digital control, that is, the machine tool has a servo motor, a ball screw nut pair, and a complete set of digital control devices that control the precise movement of the tool holder or the worktable. The acquisition of the size (the movement of the tool holder or the worktable) is automatically controlled by a pre-programmed program through a computer digital control device.

The initial automatic control method was completed using active measurement and mechanical or hydraulic control systems. At present, program-controlled machine tools that are pre-programmed according to processing requirements and work by issuing instructions from the control system or digitally controlled machine tools that work by issuing digital information instructions from the control system are widely used, as well as adaptive control machine tools that can adapt to changes in processing conditions during processing, automatically adjust processing parameters, and optimize the processing process according to specified conditions for automatic control processing.

The automatic control method has stable processing quality, high productivity, good processing flexibility, and can adapt to multi-variety production. It is the current 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 movement to form the shape of the processed surface. Ordinary turning, milling, planing and grinding all belong to the tool tip trajectory method. The shape accuracy obtained by this method mainly depends on the accuracy of the forming movement.

(2) Forming method

The shape of the processed surface is obtained by using the geometric shape of the forming tool to replace certain 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) Generating method

The shape of the machined surface is obtained by using the envelope surface formed by the developing motion of the tool and the workpiece, such as gear hobbing, gear shaping, gear grinding, and spline knurling. The shape accuracy obtained by this method mainly depends on the shape accuracy of the blade and the accuracy of the developing motion.

3. Methods for obtaining position accuracy

In mechanical processing, the position accuracy of the machined surface relative to other surfaces mainly depends on the clamping of the workpiece.

(1) Direct alignment clamping

This method is a clamping method that uses a dial indicator, a marking disk or visual inspection to directly align the position of the workpiece on the machine tool.

(2) Marking alignment clamping

This method is to first mark the center line, symmetry line and processing lines of each surface to be processed on the blank according to the part drawing, then install the workpiece on the machine tool, and align the clamping position of the workpiece on the machine tool according to the marked lines.

This clamping method has low productivity and precision, and requires high technical skills of workers. It is generally used to process complex and bulky parts in single-piece small batch production, or in situations where the tolerance of the blank size is large and it is impossible to clamp it directly with a fixture.

(3) Clamping with a fixture

The fixture is specially designed according to the requirements of the processing process. The positioning elements on the fixture can quickly enable the workpiece to occupy the correct position relative to the machine tool and the tool, and the clamping and positioning accuracy of the workpiece can be guaranteed without alignment. Clamping with a fixture has high productivity and high positioning accuracy, but it requires the design and manufacture of special fixtures. It is widely used in batch and mass production.