CNC Machining to Obtain the Workpiece Size Accuracy of the Common Method

Workpiece Size Accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of the part after cnc machining are in line with the ideal geometric parameters specified in the drawings. The higher the degree of this conformity, the higher the machining accuracy.

During 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 machining error.

We will discuss it from the following three aspects:

  1. Method for obtaining dimensional accuracy of parts
  2. Method of obtaining shape accuracy
  3. Method for obtaining position accuracy

1. Method for Obtaining Dimensional Accuracy of Parts

1)Trial Cutting

That is, first try to cut a small part of the machined surface, measure the size obtained by 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 measure again, so after two or three trial cuts and measurements, when the machine is processed After the required size is reached, the entire surface to be machined is cut.

The trial cutting method is repeated until the required workpiece size accuracy is achieved through “trial cutting-measurement-adjustment-retry-cutting”. For example, the trial boring processing of the box hole system.

 CNC machining parts manufactured by YIJIN Hardware

Custom CNC machining parts manufactured by YIJIN Hardware

The precision achieved by the trial cutting method may be very high, 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 workpiece, processing another workpiece that matches it, or combining two (or more) workpieces for cnc machining. The requirements for the final workpiece size accuracy in the matching process are based on the matching requirements with the processed parts.

2)Adjustment Method

Adjust the accurate relative position of the machine tool, fixture, cutter, 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, during processing, there is no need to try cutting, 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 the fixed-range device or the tool setting device on the machine tool or the pre-adjusted tool holder to make the tool reach a certain positional accuracy relative to the machine tool or fixture, and then process a batch of workpieces.

Feeding the tool on the machine tool according to the dial and then cutting is also a kind of adjustment method. This method needs to first determine the scale on the dial according to the trial cut method. In mass production, the tool setting devices such as fixed-range stops, samples, and templates are often used to adjust.

The adjustment method has better machining 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 adjustment workers. It is often used in batch production and mass production.

 CNC machining parts manufactured by YIJIN Hardware

Custom CNC machining parts manufactured by YIJIN Hardware

3) Fixed Size Method

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

The sizing method is easy to operate, has high productivity, and has 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

In the cnc machining, measuring the processing size while processing, comparing the measured results with the size required by the design, or making the machine tool continue to work, or stop the machine tool, is the active measurement method.

Currently, values in active measurement can be displayed digitally. The active measurement method adds the measurement device to the processing system (ie, the unity of the machine tool, the tool, the fixture, and the workpiece), which becomes its fifth factor.

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

 CNC machining parts manufactured by YIJIN Hardware

YIJIN Hardware parts machining

5) Automatic Control Method

This method is composed of a measuring device, feeding device, and control system. It is an automatic processing system composed of measuring, feeding devices, and control systems, 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 is automatically completed to automatically achieve the required dimensional accuracy. For example, when machining on a CNC machine tool, the parts control the machining sequence and machining accuracy through various instructions of the program.

There are two specific methods of automatic control:

①Automatic measurement means that there is a device for automatically measuring the size of the workpiece on the machine tool. When the workpiece reaches the required size, the measuring device sends an instruction to make the machine tool automatically retract and stop working.

②Digital control means that the machine tool has a servo motor, a rolling screw nut pair, and a complete set of digital control devices that control the precise movement of the tool rest, or worktable. Automatic control by computer numerical control device.

The initial automatic control method is completed using active measurement and mechanical or hydraulic control systems. At present, pre-arranged programs according to processing requirements have been widely used, program-controlled machine tools that are issued by the control system to work, or digitally-controlled machine tools that are issued by the control system to work with digital information commands, and can adapt to changes in processing conditions during processing. Adjust the processing amount and realize the automatic control processing of the adaptive control machine tool that optimizes the processing process according to the specified conditions.

The automatic control method processing has stable quality, high productivity, good processing flexibility, and can adapt to multi-variety production. It is the current development direction of machinery manufacturing and the basis of computer-aided manufacturing (CAM).

2. Method of Obtaining Shape Accuracy

1) Trajectory Method

This processing method uses the trajectory of the tooltip movement to form the shape of the machined surface. Ordinary turning, milling, planing, and grinding belong to the tool nose trajectory method. The shape accuracy obtained with this method mainly depends on the accuracy of the forming movement.

2) Forming Method

The shape of the machined surface is obtained by using the geometry of the forming tool to replace some of the 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 cutting edge.

3) Expansion Method

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

3. Method for Obtaining Position Accuracy

In machining, the accuracy of the position of the machined surface to other surfaces is mainly determined by the clamping of the workpiece.

1) Directly Look for the Formal Clip

This method is a clamping method in which the position of the workpiece is directly located on the machine tool with a dial indicator, ascribing disc, or a visual inspection.

2) Mark the Line to Find the Correct Clamp

This method is to first draw the centerline, the symmetry line and the processing line of each surface to be machined 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 drawn line.

This clamping method has low productivity, low precision, and a high technical level of workers. It is generally used for processing complex and heavy parts in single-piece small batch production, or when the dimensional tolerance of the blank is large and cannot be directly clamped with a fixture.

3) Clamp with A Jig

The fixture is specially designed according to the requirements of the process to be processed. The positioning elements on the fixture can quickly occupy the correct position of the workpiece relative to the machine tool and the tool, and the clamping and positioning accuracy of the workpiece can be ensured without alignment. The positioning accuracy is high, but it needs to design and manufacture special fixtures, which are widely used in batch and mass production.


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