CNC Prototypes Manufacturing 1 : Programming and Optimization

The Importance of Prototypes

The three-dimensional design software used in modern industrial design can intuitively express the three-dimensional model of the product on the screen, perform simulation assembly and motion simulation, and even static and dynamic analysis.

But the screen is flat after all, and the size is limited, it can not be compared with the real thing, various design errors may still occur. On the contrary, the physical model can truly show the structure, form, color, texture and other elements of the product design from all directions.

The relationship between product assembly and movement can be fully tested. Consistent materials can directly express the strength and rigidity of the product. The defects in product design will be fully exposed in the product model. The most important thing is that the customer’s feelings are more real, so the prototypes are still It is a condition of many companies’ decision-making and ordering.

cnc machine
CNC Machining

CNC Prototyping

CNC Prototyping is a product of modern industry that integrates advanced technologies such as computer-aided design (CAD) and computer-aided manufacturing (CAM). The CAD three-dimensional modeling is excavated from a whole piece of material on a CNC machine tool, so it is called CNC Prototyping.

CNC Prototyping not only has high precision, but also has good material consistency and strong sense of reality. The surface quality can reach a high level. After polishing, sandblasting, painting, silk screen, glazing, electroplating and other subsequent processing, the production effect can be completely the same as that of the mold. The products produced are comparable or even higher.

With the large-scale application of CNC machine tools, the cost of CNC machining is rapidly reduced. People find that CNC machine tools are used to process prototypes not only with high dimensional accuracy, fast speed, but also competitive in cost, making it possible to produce prototypes in large quantities.

On the other hand, with the development of numerical control technology, many small structures that could not be machined can be completed by numerical control processing. At the same time, with the improvement of living standards and the diversification of products, the speed of product development is increasing. It is under this situation that CNC Prototyping manufacturing industry stands out, becomes a relatively independent industry and flourishes.

The Advantages of CNC Prototyping

  • The processing speed is fast, the cost is low, and high processing accuracy can be achieved. The surface quality and some small parts can be well compensated in the post-processing stage of processing and production;
  • Prototypes produces a wide range of materials, which can meet the needs of different customers, ensure the consistency of Prototypes materials and batch parts materials, and can complete the manufacture of transparent parts that are difficult to achieve with rapid prototypes;
  • The processing equipment is simple, and it can be produced by using a general three-dimensional CNC milling machine or even an engraving machine, which is economical and quick to promote.
CNC Machining
CNC Machining

1. CNC Prototyping programming and optimization

CNC programming software uses Mastercam, which is a widely used CAD/CAM software developed by CNC Soft-ware in the United States and runs on Windows or Windows NT. The three-dimensional modeling of the software is a bit poor, but the programming function is superior.

Its processing options give users greater flexibility, such as multi-surface radial cutting and projecting tool paths onto an unlimited number of surfaces. The software also includes a C-axis programming function, which can smoothly combine milling and turning. Other functions, such as diameter and end face cutting, automatic C-axis horizontal drilling, automatic cutting and tool plane setting, etc., contribute to efficient parts production.

The post-processing program supports milling, turning, wire cutting, laser processing, and multi-axis processing. In addition, Mastercam provides a variety of graphic file interfaces, such as SAT, IGES, VDA, DXF, etc.

Mastercam is a professional automatic programming software. The NC program files generated by the post-processing are very accurate and reliable. Judging from the dozens of CNC machine tools of the Wanguo brand in our processing base, no machine tool has had any problems due to the post-processing of Mastercam. Moreover, the post-processing file of Mastercam is a text file, which can be easily modified as needed to suit the special requirements of different processing tasks.

1.1 Data model conversion

The production of Prototypes has been paperless. Customers transfer the 3D data model to the prototypes manufacturing company, and the prototypes manufacturing company directly uses the automatic programming software to carry out CNC programming based on the 3D model.

Generally, the software designed is mainly UG, PRO/E, SOLID WORK, CATIA and other software, and the automatic programming of CNC machining is mainly based on MASTERCAM, CIMATRO, POWERMILL and other software.

There are two ways to read files, one is direct conversion. Three-dimensional software has a file conversion function, which can directly read the modeling of other three-dimensional software, but in fact, due to the sequence of program development, only old version files of other three-dimensional software can be read, and the mainstream version of the current application is often not recognized. For example, MASTERCAM9.0 can recognize PRO/E2001, but it cannot recognize PRO/E Wildfire 3.0, so standard files must be used.

There are many standard formats of data files, such as STEP format and IGES format, which can easily convert the data format, but there are many data losses and errors. Most of the information can be read, but some lines and surfaces will happen after being read in. Deviations are manifested as surface fractures and surface undulations. This is also related to the working habits of the designer. In fact, many 3D software files have many options when converting, but generally they are executed according to the original default options.

The current commonly used IGES surface model format file conversion error is relatively large, the maximum error of the same curve is 0.02mm, the error of the surface boundary is larger, and the two adjacent surface boundaries of the surface model cannot be well overlapped. And because of the different formats, the curved surface becomes more and more fragmented as it turns, which increases the difficulty of subsequent processing.

1.1.1 Curve generation and processing

Curve boundaries
Curve boundaries

The generation of curves has very important meaning in automatic programming: the efficiency of two-dimensional processing with curves as processing objects far exceeds that of three-dimensional processing. If we can transfer three-dimensional processing to two-dimensional processing, the processing efficiency will be greatly improved;

Curve boundaries are used in 3D machining. Although the blank is square or bar during model machining, it is not necessary to remove all materials by cutting. If we can limit the tool path to a suitable range during 3D machining , You can get twice the result with half the effort.

Although there are curve retention options when importing 3D standard files, they are just some control lines on the surface. Because different 3D software has different formats for surface and entity descriptions, the surface is broken a lot after the standard format file is transferred, and the original whole piece is Large curved surfaces are often broken into many small curved surfaces.

If you select the curve option when transferring to a standard format file, there will be many curves. Sometimes it is better to regenerate from the vast curve group, and the lines in the original wireframe model may not be in these curve groups. Generation and utilization of boundary curves

The boundary curves of the surface can be generated all at once, or the boundaries can be generated one by one, but all generations are prone to errors, and some are not necessary. When the hole boundary is generated, you can choose the top surface of the hole as the reference surface, or the cylindrical surface as the reference surface, but the generated two semicircles, as shown in the following figure:

It must be noted that the common boundary curves generated on different surfaces are not the same curve. Their spatial positions are different, but they overlap within the error range, which is difficult to detect with the naked eye. We will find the problem if we enlarge it. . The picture shows two boundary lines generated by the cylindrical surface and the top surface respectively. After zooming in, you will find two lines.

Curve space misalignment
Curve space misalignment

Although the error is in the control, it interferes greatly with the programming and processing results, and it will be interrupted when the curves are generated in series. Many technicians are often at a loss because they do not find the problem. Therefore, try to generate the required curves on the same side of the surface. The points can be enlarged and connected by curve welding or trimming. Skills of curve series processing
Several cases of breakpoints
Several cases of breakpoints

Due to the relationship between file conversion errors and surface curve operations, the actual boundary lines generated when there are many surfaces often have breakpoints and repetitions. These points are invisible to the naked eye directly on the screen, but they seriously affect the subsequent programming and processing. 

The simplest method is to increase the allowable error value of the series connection. As long as the distance of the breakpoint is less than the allowable value, the system will ignore the breakpoint, but this will affect the accuracy and continuity of the system’s processing. The following describes the techniques of serial processing.

  1. First use the option of deleting duplicate pixels to delete duplicate curves.
  2. Use the analysis command to select the seemingly continuous curve. The system will use different colors and arrows to indicate the problematic points. The places that fail to pass are the problems. You can enlarge and connect, but you should avoid direct connections.
Bad connection
Bad connection

Poor connection will cause difficulty in offset operation in the future. In CNC machining, the feed mechanism will decelerate or even temporarily stop for commutation, which will seriously affect the processing efficiency. It is generally believed that the broken points can be connected by curve welding, and the starting point and the falling point should be selected farther to make the connection smooth.

Curved welding
Curved welding

 In fact, because the actual straight line distance is very small, only a few micrometers, it is of little significance to follow the curve, which suddenly increases the burden of CNC system programming and machine tool processing. Therefore, the breakpoints can be manually connected and trimmed to become a straight line connection, but no matter what kind of connection All methods should pay attention to the natural transition and reduce the burden of the CNC machining system.

3.Even if the original physical concept is a flat boundary curve, the system considers it as a spatial curve due to the conversion error, so the generated boundary curve may not be on the same plane, and some curves seem to intersect, and the system always prompts If they do not intersect, it is recommended that if conditions permit, project them on a standard plane before processing. In terms of programming efficiency and processing efficiency, two-dimensional processing is undoubtedly much more efficient than three-dimensional processing, so it is better to project the boundary curve onto a standard plane. Establishment of processing boundary
Establishment of processing boundary
Establishment of processing boundary

After the surface boundary is generated, some can be used directly. Sometimes, for convex surface processing, the size of the tool must be considered. That is, the actual surface boundary must be offset by a tool diameter to get the processing boundary, as shown in the figure, otherwise the surface The peripheral edge may not be processed. Independent addition of curves

Sometimes the curve can also be added artificially as needed to limit the processing range to an appropriate part. As shown in the figure, the part of the front end of the model with a larger area can not be processed by external processing, so add a rectangular box and make a cut. Sometimes in order to ensure the natural transition of the processing edges of different curved surfaces, a larger boundary is added. Uniform coordination standards

For surfaces or lines with matching requirements, because they are distributed on different parts, different appearance errors will occur. The error of ±0.02mm may exceed the allowable processing error. Therefore, the same appearance must be selected, which can be divided in the assembly model. Before dismantling, project or copy to the mounting surface of the counterpart, as shown in the following figure: The flange and the groove are processed with the same boundary curve.

Consistency of processing boundaries
Consistency of processing boundaries

1.1.2 Supplements for machining curved surfaces Datum plane added

After file conversion, all curved surfaces and curves have errors. The general CAD conversion standard of ±0.02mm is acceptable for curved surfaces, but it is obviously not enough for plane datums. Therefore, if the selected surface is to be used as the reference, it is best to regenerate a surface at the given coordinate position. Add blank side

The three-dimensional processing of the processing software is for surface processing, the blank space of the modeling file is not processed, and the blanks in the model processing are generally square materials. When the surface is processed, the programming software will treat the blank space as a blank movement without resistance, which will cause the knife to hit, so all the blank spaces should be filled.

Some structures such as holes and grooves cannot be processed without a bottom surface in curved surface processing, and become convex. Contrary to our original intention, even if there is a bottom surface, it is unreasonable to use curved surface processing with a ball cutter, and it will affect the continuous surface processing. Therefore, some holes and grooves that affect three-dimensional processing must be sealed, and the holes can be processed after the overall surface processing is completed.

Hole closure
Hole closure

The prerequisite for the hole to be closed is that the hole must be inside the curved surface. This curved surface is not a visual single curved surface, but a single curved surface defined by the system. Like the hole at the bottom of the picture, because there are tiny gaps at the edge formed by adjacent curved surfaces, the hole cannot By surface closure, it is necessary to generate a curved surface instead of the generated boundary.

There is a principle here, the hole should be closed before the hole boundary is generated, otherwise, the boundary will be gone and subsequent processing cannot be carried out. Surface extension

When processing the blank, unless the material outside the boundary of the surface is cut off before the surface processing, the tool may have problems when processing the edge of the surface, so the surface must be extended. Generally, the surface cannot be extended after trimming, and it can be copied by the boundary curve. Extend the surface physically, as shown in the figure below, to ensure that the surface can be processed completely.

Curved extension
Curved extension

1.1.3 Prototypes spin-off

Many plastic parts are hollow, and ordinary three-axis machining tools cannot be inserted. Multi-axis machining is not economical or not necessarily possible. Therefore, complex prototype models need to be processed separately by the parting surface, and then spliced, and some need to be divided. Several parts are processed. Generally, the mold opening processing conditions have been considered in the design of general parts, so there are few hand model parts that must be disassembled.

Another situation of the split of the prototype model is that the protruding part of the model is relatively large, because the prototype model processing is a single piece of production, so the blank is usually a square material or a bar material, and the cutting allowance is large. In order to save materials and reduce processing time, if the accuracy permits, the necessary long extension parts are cut off and processed independently, and then spliced.

In order to save materials, some researchers use software for automatic splitting, which can be automatically split according to the thickness of the material. However, for the counterpart model company, splicing will inevitably reduce the accuracy and strength. In order to ensure the dimensional accuracy and strength of the model, it should be done as much as possible. Avoid disassembly, even for larger hollow parts.

When splitting, you must consider the reference of the combination, and avoid the plane contact that is not positioned. When the hollow model can only be divided, do not divide it according to a datum plane. The installation datum should be added, as shown in the figure below.

Prototypes spin-off
Prototypes spin-off
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