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Some common methods for obtaining dimensional accuracy of workpieces in machining!

Mar 20, 2020

In processing, due to the influence of various factors, it is actually impossible to process every geometric parameter of the part completely to the ideal geometric parameter, and some deviations will always occur. This deviation is the processing error.

Explore from the following three aspects:

1Method for obtaining dimensional accuracy of parts

2Method for obtaining shape accuracy

3Method for obtaining position accuracy

1Method for obtaining dimensional accuracy of parts

(1)Trial cut method

That is, first try to cut out a small part of the processed surface, measure the size of the trial cut, adjust the position of the cutting edge of the tool relative to the workpiece according to the processing requirements, and then try cutting, and then measure, so after two or three trial cuts and measurements, when being processed After reaching the required size, the entire surface to be machined is cut.

machining tools

The trial cutting method is carried out through "trial cutting-measurement-adjustment-re-trial cutting" repeatedly until the required dimensional accuracy is achieved. For example, the trial boring of the box hole system.

The precision achieved by the trial cutting method may be very high. It does not require complicated equipment, but this method is time-consuming (requires multiple adjustments, trial cutting, measurement, calculation), low efficiency, and depends on the technical level of the worker 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—combination, it is based on the processed part, processing another matching workpiece, or combining two (or more) workpieces for processing. The requirements of the final processed size in the formulation are based on the cooperation requirements with the processed parts.

(2)Adjustment method

Adjust the exact relative positions of the machine tool, fixture, tool and workpiece with sample or standard parts in advance to ensure the dimensional accuracy of the workpiece. Because the size is adjusted in place 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 setting block. The essence of the adjustment method is to use a fixed-range device or a tool setting device on the machine tool or a 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 and cutting according to the dial on the machine tool is also an adjustment method. This method requires the trial cut method to determine the scale on the scale. In large-scale production, adjustments to the knife device such as fixed-range stoppers, prototypes, and templates are often used.

The adjustment method has better stability of processing accuracy than the trial cutting method, has higher productivity, and 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)Sizing method

The method of using the corresponding size of the tool to ensure the size of the part being processed is called the fixed size method. It is processed using standard size tools, and the size of the working surface is determined by the size of the tool. That is, a tool with a certain dimensional accuracy (such as a reamer, a reamer, a drill, etc.) is used to ensure the accuracy of the processed part (such as a hole) of the workpiece.

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

(4)Active measurement method

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

At present, the values 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, tools, fixtures, and workpieces) as its fifth factor.

Active measurement is of stable quality and high productivity, which is the development direction.

(5) Automatic control method

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

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

There are two specific methods of automatic control:

Automatic measurement 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 sends a command to automatically retract the tool and stop the work.

Digital control means that the machine tool has a servo motor that controls the precise movement of the tool post or table, a rolling screw nut pair and a complete set of digital control devices. The size (movement of the tool post or table) is obtained by a pre-programmed program. Automatically controlled by computer digital control device.

The initial automatic control method was completed using active measurement and control systems such as mechanical or hydraulic. At present, programs pre-programmed 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 send digital information instructions to work, and can adapt to changes in processing conditions during the process, automatically Adjust the amount of processing and adapt the control machine to optimize the processing process according to the specified conditions for 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 current development direction of mechanical manufacturing and the basis of computer-aided manufacturing (CAM).

2Method for obtaining shape accuracy

1)Trajectory method

This machining method uses the trajectory of the tip movement to form the shape of the surface to be machined. Common turning, milling, planing, and grinding 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 geometry of the forming tool is used instead of some forming movements of the machine tool to obtain the shape of the machining surface. Such as forming turning, milling, grinding and so on. The shape accuracy obtained by the forming method mainly depends on the shape of the blade.

3)Construction method

The shape of the machining surface is obtained by using the enveloping surface formed by the tool and the workpiece in the forming motion, such as hobbing, inserting, grinding, knurling, etc., all of which are forming methods. The shape accuracy obtained by this method mainly depends on the shape accuracy of the blade edge and the accuracy of the spreading motion.

3Method for obtaining position accuracy

In machining, the accuracy of the position of the machined surface on other surfaces depends mainly on the clamping of the workpiece.

1) Directly locate the clamp

This method uses a dial indicator, a scribing dial, or visual inspection to directly clamp the workpiece on the machine.

2)Crossing to find the clamp

This method is to first draw the center line, symmetry line and processing lines of each surface to be processed on the blank according to the part drawing, then load the workpiece on the machine tool, and correct the clamping position of the workpiece on the machine tool according to the drawn line.

This clamping method has low productivity, low accuracy, and high technical requirements for workers. It is generally used in the processing of complex and cumbersome parts in single-piece small batch production, or in situations where the tolerance of the blank is too large to directly clamp with clamps.

3)Clamp with clamp

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. The positioning accuracy of the workpiece can be ensured without alignment. The positioning accuracy is high, but special fixtures need to be designed and manufactured, which are widely used in batch and mass production.

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