8 Important Tips about CNC Machining Process Analysis

Programmers should have machine lathe manual, programming manual, Table of cutting amount, standard cutters, fixture manual and other information when conducting process analysis, select the appropriate machine lathe according to the material, contour shape, machining accuracy of the workpiece being machined, formulate machining plan, determine the machining order of the parts, cutters used in each process, fixtures and cutting dosage, etc. In addition, programmers should continuously summarize and accumulate practical experience in process analysis to write high-quality CNC machining programs.

1, Reasonable selection of machine lathe

    • When machining parts on a CNC lathe, there are generally two scenarios. The first case: there is a part drawing and blank, we just need to choose the CNC lathe suitable for machining the part. The second case: there is already a CNC lathe, we need to select the parts suitable for machining on that lathe. In either case, the main factors to consider are the material and class of the blank material, the complexity of the part profile shape, size, machining accuracy, number of parts, heat treatment requirements, etc. In summary, there are three points: (i)To ensure that the technical requirements of the processed parts, the processing of qualified products. (ii) Contribute to increased productivity. (iii) Minimizing production costs (processing costs).

2、Technological analysis of CNC machining parts

    • CNC machining process analysis is very extensive, here only from the possibility and convenience of CNC machining analysis.
    • (1)Dimensional data on part drawings should be given in accordance with the principle of convenient programming
      • i).Dimensioning methods on part drawings should be adapted to the characteristics of CNC machining, in the CNC machining parts drawing, should be the same reference cited size or directly give the coordinate size.  This method of labelling method both enable programming and dimensional coordination, and provides great convenience in maintaining consistency between the design reference, process reference, inspection reference and programming origin settings. As part designers generally take into account the use of characteristics such as assembly in the dimensioning, they have to adopt a locally dispersed labelling method, which will cause a lot of inconvenience to the process arrangement and CNC machining. Because of the high precision of CNC machining and repeatability of positioning, the characteristics of use will not be damaged by the large accumulation of errors, so you can change the local dispersion marking method to the same reference citation size or directly give the coordinate size marking method
      • ii).The conditions for the geometrical elements that make up the contours of the part should be adequate
      • Calculate the base point or node coordinates when programming manually. In automatic programming, all geometric elements that make up the part’s profile need to be defined. Therefore, when analyzing part drawings, need to analyze the adequacy of the conditions for the geometric elements. Such as the arc and the straight line, the arc and the arctangent on the drawing, but according to the dimensions given on the drawing, when calculating the tangent conditions, becomes intersecting or separated from the state. Inadequate conditions that make up the geometric elements of the part make it impossible to get started when programming. When this situation is encountered, it should be resolved in consultation with the part designer.
    • (2)The structural manufacturability of each processing parts should meet the characteristics of CNC machining
      • 1)The internal cavity and shape of the parts are preferable of uniform geometric type and size. This can reduce the cutter specifications and the number of cutter changes, making programming easier and improving production efficiency.
      • 2)The size of the internal groove fillet determines the size of the cutter diameter, so the internal groove fillet radius should not be too small. The quality of the part’s manufacturability is related to the height of the processed contour and the size of the transition arc radius.
      • 3)When milling the bottom of a part, the radius(r) of the bottom of the groove should not be too large.
      • 4)A unified reference positioning should be used. In CNC machining, if there is no unified reference positioning, the repositioning of the workpiece will lead to the inconsistency of the contour position and size of the two surfaces after processing. Therefore, to avoid the above-mentioned problems and ensure the accuracy of the relative position after two clamping processes, a unified reference positioning should be adopted.
      • It is better to have a suitable hole on the part as a positioning reference hole. If not, a process hole should be set as the positioning reference hole (such as adding a process lug on the blank or setting a process hole on the margin to be milled in the subsequent process). If the process hole cannot be made, at least the finished surface should be used as a unified reference to reduce the error caused by the two clampings.
      • In addition, it also needs to analyze whether the required machining accuracy and dimensional tolerances of the parts can be guaranteed, whether there are extraneous dimensions that cause conflicts, or closed dimensions that affect the process arrangement.

3、Selection of processing method and finial determining about processing plan

    • (1)Selection of processing methods
      • The selection principle of the processing method is to ensure the processing precision and surface roughness of the processed surface. Since there are generally many processing methods to obtain the same level of accuracy and surface roughness, in the actual selection, it is necessary to consider the shape, size and heat treatment requirements of the parts. For example, boring, reaming, grinding and other processing methods can be used for holes with IT7 accuracy, but the holes on the box are generally boring or reaming, and grinding is not suitable. Generally, reaming holes are selected for small-sized box holes, and boring holes should be selected when the hole diameter is large. In addition, the requirements of productivity and economy, as well as the actual conditions of the factory’s production equipment, etc. should also be considered. The economical processing accuracy and surface roughness of common processing methods can be found in the relevant process manual.
    • (2)Principles for determining the processing plan
      • The machining of relatively precise surfaces on parts is often achieved gradually through roughing, semi-finishing and finishing. It is not enough to select the corresponding final processing method for these surfaces according to quality requirements, and the processing scheme from blank to final forming should also be correctly determined
      • When determining the processing plan, first of all, should be according to the requirements of the accuracy and surface roughness of the main surface, the processing methods required to meet these requirements should be initially determined. For example, for a hole with a small IT7 grade accuracy, when the final machining method uses fine reaming, it is usually processed by drilling, reaming and rough reaming before finishing reaming.

4、Division of process and process steps

    • (1) The divesion of Process
      • When processing parts on CNC machine lathes, the process can be concentrated, and most or all of the processes can be completed in one clamping. First of all, should be according to the part drawing, consider whether the processed parts can complete the processing of the whole part on a CNC machine lathe. If this is not possible, it should be decided which part is machined on the CNC lathe and which part is machined on the other machine, it’s the division of process steps. Generally, the process steps of machining can be divided as below: (2) Division of process steps, The division of working steps is mainly considered from two aspects of processing accuracy and efficiency. It is often necessary to use different cutters and cutting amounts to process different surfaces in one process. In order to facilitate the analysis and description of more complex processes, they are further subdivided into process steps. The following uses the machining centre as an example to illustrate the principle of division of working steps:
      • 1)Rough machining, semi-finishing and finishing are performed on the same surface in sequence, or all the machined surfaces are processed separately after roughing and then finishing
      • 2)For parts that have both milled faces and boring holes, then can be milled first and then bored. Dividing the working steps according to this method can improve the accuracy of the hole. Because the cutting force is large during milling, the workpiece is prone to deform. Milling the surface first and then boring the hole to allow it to recover for a period of time, reducing the impact of the deformation on the accuracy of the hole.
      • 3)Divide the working steps according to the cutter. For some machine lathes, the rotation time of the worktable is shorter than the cutter change time. The steps can be divided according to the cutter to reduce the number of cutter changes and improve the processing efficiency.
      • In short, the division of working procedures and working steps should be comprehensively considered according to the structural characteristics and technical requirements of specific parts.

5、Parts installation and fixture selection

    • (1)Basic principles of positioning installation
      • 1)Strive to unify the benchmarks of design, process and programming calculation.
      • 2)Minimize the number of clampings as much as possible. Position and clamp one time as much as possible, then process all the surfaces to be processed.
      • 3)Avoid the use of manual adjustment of the machining program to give full play to the efficiency of CNC machine lathe.
    • (2)Basic principles for selecting fixtures
      • The characteristics of CNC machining put forward two basic requirements for the fixture: one is to ensure that the coordinate direction of the fixture is relatively fixed with the coordinate direction of the machine lateh; the second is to coordinate the dimensional relationship between the parts and the machine lathe coordinate system. In addition, the following four points must be considered:
        • 1)When the mass production of machining parts are not large, combined fixtures, adjustable fixtures and other general fixtures should be used as much as possible to shorten the production preparation time and save production costs.
        • 2)The special fixture is only considered in batch production, and to ensure the structure is simple.
        • 3)The loading and unloading of parts should be fast, convenient and reliable to shorten the standstill time of the machine lathe.
        • 4)The parts on the fixture should not hinder the machining of the surfaces of the parts by the machine lathe, that is, the fixture must open its positioning, and the clamping mechanism components must not affect the cutting in the process (such as collision, etc.).

6、Selection of cutting tools and determination of cutting amount

    • (1)Cutting tool’s selection
      • The choice of the cutter is one of the important contents in the CNC machining process. It not only affects the machining efficiency of the machine lathe, but also directly affects the machining quality. When programming, the choice of cutter usually needs to consider factors such as the machine lathe’s processing capability, process content, and workpiece material.
      • Compared with traditional machining methods, CNC machining has higher requirements for cutting tools. Not only requires high precision, good rigidity and high durability, but also requires stable dimensions and easy installation and adjustment. This requires the use of new high-quality materials to manufacture CNC machining lathe, and optimize tool parameters.
      • When selecting a cutter, the size of the cutter should be adapted to the surface size and shape of the workpiece to be processed. In production, end mills are often used to process the contours of planar parts. When milling a flat surface, you should choose a hard alloy blade milling cutter; when machining a boss or a groove, choose a high-speed steel end mill; when machining a rough surface or rough machining hole, you can choose a hard alloy corn milling cutter. When selecting an end mill, the relevant parameters of the tool are recommended to be selected based on empirical data. Surface machining often uses ball-end milling cutters. However, when machining flat parts of the curved surface, the cutter is cut with the tip of the ball head, and the cutting conditions are poor, so a ring cutter should be used. In single-piece or small batch production, in order to replace multi-coordinate linkage machine lathes, drum cutters or cone cutters are often used to process some variable bevel parts on the aircraft plus serrated disc milling cutters, which is suitable for five-coordinate linkage CNC machine lathes. When machining some spherical surfaces, the efficiency is nearly ten times higher than that with ball-end milling cutters, and good machining accuracy can be obtained.
      • In the machining center, various cutters are installed in the cutter magazine, and the cutter selection and cutter change work can be carried out at any time according to the procedures. Therefore, there must be a set of connecting rods for connecting ordinary cutters, so that the standard cutters for drilling, boring, expanding, reaming, milling and other processes can be quickly and accurately installed on the machine cutters spindle or cutter magazine. As a programmer, you should understand the structural dimensions and adjustment methods of the arbour used on the machine lathe, and the adjustment range to determine the radial and axial dimensions of the cutters during programming. At present, the machining center in China adopts the TSG tool system. Its shank has two types: straight shank (three specifications) and tapered shank (four specifications), which includes 16 different kinds of knives.
    • (2)Determination of cutting amount
      • Cutting usage includes spindle speed (cutting speed), back-feeding amount, and feed amount. For different processing methods, different cutting amounts need to be selected and should be included in the program list.
      • The principle of reasonable choice of cutting amount is that when roughing, productivity is generally the main factor, but economic and processing costs should also be considered; when semi-precision finishing and precision finishing, the cutting efficiency should be taken into account while ensuring the quality of processing. Economical and processing costs. The specific value should be based on the machine lathe manual, cutting dosage manual, and combined with experience.

7、Determination of cutter setting point and cutter change point

    • When programming, the position of “cutter setting point” and “cutter change point” should be correctly selected. “cutter setting point” is the starting point of cutter movement relative to the workpiece when machining parts on a CNC machine lathe. Since the program block is executed from this point, the cutter setting point is also called “program starting point” or “starting point”.
    • The selection principle of cutter setting point is: 1. It is easy to use digital processing and simplify programming; 2. It is easy to find on the machine lathe, and it is easy to check during processing; 3. The processing error caused is small.
    • The cutter setting point can be selected on the workpiece or outside the workpiece (such as on a fixture or machine lathe) but must have a certain dimensional relationship with the positioning reference of the part.
    • In order to improve the machining accuracy, the cutter setting point should be selected as much as possible on the design basis or process basis of the part. For example, if the workpiece is positioned with a hole, the center of the hole can be used as the cutter setting point. The position of the cutter is aligned with this hole, so that the “cutter position point” and the “cutter setting point” coincide. The factory’s common method of aligning is to install the dial indicator on the machine lathe spindle, and then turn the machine lathe spindle to make the “cutter point” consistent with the cutter setting point. The better the consistency, the higher the accuracy of cutter set. The so-called “knifepoint” refers to the tip of the turning cutter and boring cutter; the tip of the drill; the center of the bottom of the end mill and end mill, and the center of the ball mill.
    • After the part is installed, the workpiece coordinate system and the machine lathe coordinate system have a certain dimensional relationship. After the workpiece coordinate system is set, the coordinate value of the first block from the cutter setting point; the coordinate value of the cutter setting point in the machine cutter coordinate system is (X0, Y0). When programming by absolute value, regardless of whether the cutter setting point and the workpiece origin coincide, both are X2 and Y2; when programming by incremental value, the cutter setting point
    • When it coincides with the origin of the workpiece, the coordinate values of the first block are X2 and Y2, and when they do not coincide, it is (X1 + X2), Y1 + Y2).
    • The cutter setting point is both the starting point and the endpoint of the program. Therefore, the repeat accuracy of the cutter setting point should be considered in batch production. This accuracy can be checked by the coordinate values (X0, Y0) of the cutter setting point from the machine origin.
    • The so-called “machine origin” refers to a fixed limit point on the machine. For example, for a lathe, it refers to the intersection of the turning center of the lathe spindle and the end surface of the chuck.
    • When the cutter needs to be changed during processing, the cutter change point should be specified. The so-called “cutter change point” is the position when the cutter holder is indexed and changed. This point can be a fixed point (such as a machining center machine where the position of the cutter change manipulator is fixed) or an arbitrary point (such as a lathe). The cutter change point should be set outside the workpiece or fixture, subject to the workpiece and other parts when the cutter holder is indexed. The set value can be determined by actual measurement method or calculation.

8、Determination of processing route

    • In CNC machining, the trajectory of the cutter position relative to the workpiece is called the machining route. When programming, the principles for determining the processing route mainly include the following:
      • 1)The processing route should ensure the precision and surface roughness of the processed parts, and the efficiency is high.
      • 2)Make a numerical calculation simple to reduce programming workload.
      • 3)The processing route should be the shortest, so that both the program segment and the empty cutter time can be reduced. Degree, etc., determine whether it is a single pass, or multiple passes to complete the processing, and whether to use down milling or up milling in milling.
    • For CNC machine lathes with point control, only high positioning accuracy is required, and the positioning process is as fast as possible, and the movement path of the cutter relative to the workpiece is irrelevant, so this type of machine lathe should arrange the cutter path according to the shortest free path. In addition, the axial movement dimension of the cutter must be determined. Its size is mainly determined by the hole depth of the machined part, but some auxiliary dimensions should also be considered, such as the introduction distance and the overshoot of the cutter.
    • When threading a CNC machine lathe, the z-direction feed along the pitch direction should maintain a strict speed ratio relationship with the rotation of the machine lathe spindle, so cutting during acceleration or deceleration of the feed mechanism should be avoided. For this purpose, the introduction of distance δ1 exceeds the distance δ2. The value of and is related to the dynamic characteristics of the machine lathe drag system, and to the thread pitch and thread accuracy. Generally, it is 2-5mm, and take a large value for large pitch and high-precision threads; Generally, it takes about 1/4. If there is no retreat at the end of the thread, the shape of the end is related to the CNC system, and the end is generally 45o.
    • When milling flat parts, the side edges of the end mill are generally used for cutting. In order to reduce the cutter connection marks and ensure the surface quality of the parts, the cutting-in and cutting-out procedures for the cutters need to be carefully designed. When milling the outer surface contour, the cut-in and cut-out points of the milling cutter should be tangentially cut in and out of the part surface along the extension line of the part contour curve, and should not be cut directly into the part in the normal direction to avoid scratches on the processed surface Ensure smooth outline of parts.
    • When milling the inner contour surface, the cut-in and cut-out cannot be extended. At this time, the milling cutter can cut in and cut out along the normal direction of the part contour, and select the cut-in and cut-out points at the intersection of two geometric elements of the part contour In the state where the workpiece, cutter, fixture, and machine lathe system are balanced and elastically deformed, when the feed is stopped, the cutting force is reduced, which will change the balance state of the system, and the cutter will leave scratches on the surface of the part where the feed is stopped. In contour machining, feed stop should be avoided.
    • When the surface is curved, the commonly used ball head knife is processed by the “row cutting method”. The so-called line cutting method means that the tangent point trajectory of the cutter and the part contour is line by line, and the distance between lines is determined according to the requirements of the machining accuracy of the part.

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