What Is CNC Machining?

CNC machining refers to machining in which the control system issues instructions to make the tool perform various movements that meet the requirements, and expresses the technical requirements and machining technology requirements such as the shape and size of the workpiece in the form of numbers and letters. It generally refers to the process of machining parts on CNC machine tools.

CNC machine tool is a machine tool controlled by a computer. The computer used to control the machine tool, whether it is a special computer or a general-purpose computer, is collectively called a CNC system. The motion and auxiliary actions of CNC machine tools are controlled by instructions issued by the CNC system.

The instructions of the CNC system are compiled by the programmer based on the material of the workpiece, machining requirements, characteristics of the machine tool, and the instruction format (CNC language or symbols) specified by the system. The CNC system sends run or stop information to servo devices and other functional components according to program instructions to control various movements of the machine tool. When the part machining program ends, the machine tool will automatically stop. Any CNC machine and metal etching tool cannot work if there are no program instructions input into its CNC system.

The controlled actions of the machine tool generally include the starting and stopping of the machine tool; the starting and stopping of the spindle, the change of rotation direction and speed; the direction, speed, and mode of the feed motion; the selection, length and radius compensation of the tool; the replacement and cooling of the tool Opening and closing of liquid, etc.

2. Characteristics of CNC machining

From the beginning, CNC machine tools selected aircraft parts with complex profiles as machining objects to solve the key problems that are difficult to solve with ordinary machining methods. The biggest feature of CNC machining is the use of perforated tape (or magnetic tape) to control the machine tool for automatic machining.

1. Process concentration

CNC machine tools generally have tool holders and tool magazines that can automatically change tools. The tool changing process is automatically controlled by the program, so the process is relatively centralized. Centralization of processes brings huge economic benefits:

(1) Reduce the area occupied by machine tools and save factory buildings.

(2) Reduce or eliminate intermediate links (such as intermediate inspection of semi-finished products, temporary storage and transportation, etc.), saving both time and manpower.

2.Automation

When CNC machine tools are processed, there is no need to manually control the tool, and the degree of automation is high. The benefits are obvious.

(1) Reduced requirements for operating workers: A senior worker on an ordinary machine tool cannot be trained in a short time, but a CNC worker who does not need programming can be trained in a very short time (for example, a CNC lathe worker can take a week, and can also write simple machining procedures). Moreover, the parts processed by CNC workers on CNC machine tools are more precise and take less time than the parts processed by ordinary workers on traditional machine tools.

(2) Reduces the labor intensity of workers: cnc machining china workers are excluded from the machining process most of the time, which is very labor-saving.

(3) Stable product quality: The machining automation of CNC machine tools eliminates human errors such as fatigue, carelessness, and estimation of workers on ordinary machine tools, and improves product consistency.

(4) High machining efficiency: The automatic tool change of CNC machine tools makes the machining process compact and improves labor productivity.

3. High flexibility

Although traditional general-purpose machine tools have good flexibility, they are low in efficiency; while traditional special-purpose machine tools, although very efficient, have poor adaptability to parts, high rigidity and poor flexibility, making it difficult to adapt to the fierce competition in the market economy. Products are frequently modified. As long as the program is changed, new parts can be processed on CNC machine tools, and they can be operated automatically, with good flexibility and high efficiency. Therefore, CNC machine tools can well adapt to market competition.

4. Strong ability

The machine tool can accurately process various contours, and some contours cannot be processed by ordinary machine tools. CNC machine tools are particularly suitable for the following occasions:

    (1) Parts that are not allowed to be scrapped.
    (2) New product development.
    (3) machining of urgently needed parts.

3. How to divide machining procedures

The division of CNC machining processes can generally be carried out according to the following methods:

    1. The tool centralized sorting method is to divide the process according to the tools used, and use the same tool to process all the parts that can be completed on the part. Use the second knife and the third knife to finish other parts they can finish. This can reduce the number of tool changes, compress idle time, and reduce unnecessary positioning errors.
    2. Using the machining part sorting method, for parts with a lot of machining content, the machining part can be divided into several parts according to their structural characteristics, such as inner shape, outer shape, curved surface or plane, etc. Generally, planes and positioning surfaces are processed first, and then holes are processed; simple geometric shapes are processed first, and then complex geometric shapes are processed; parts with lower precision are processed first, and then parts with higher precision requirements are processed.
    3. Use the roughing and finishing sequential method for parts that are prone to machining deformation. Due to the possible deformation after roughing, deformation needs to be corrected. Therefore, generally speaking, the processes must be separated for roughing and finishing.

To sum up, when dividing the process, we must consider the structure and craftsmanship of the parts, the function of the machine tool, the amount of CNC machining content of the parts, the number of installations and the production organization status of the unit. It is also recommended to adopt the principle of process concentration or decentralization, which should be determined according to the actual situation, but it must be reasonable.

4. CNC machining process design principles

1. The principle of maximum concentration of processes and one-time positioning

Generally, when machining parts on CNC machine tools, especially on machining centers, the processes can be concentrated to the maximum extent, that is, the parts should be processed in one clamping to complete most or all of the processes that the CNC machine tool can process. CNC machining tends to concentrate processes, which can reduce the number of machine tools and the number of workpiece clampings, reduce unnecessary positioning errors, and achieve high productivity. For the machining of hole systems with high coaxiality requirements, all machining of the coaxial hole system should be completed through sequential and continuous tool changes after one installation, and then holes at other coordinate positions should be processed to eliminate the influence of repeated positioning errors. , improve the coaxiality of the hole system.

2. The principle of coarse first and fine later

When performing CNC machining, when dividing the process according to the machining accuracy, stiffness, deformation and other factors of the part, the process should be divided according to the principle of separation of roughing and finishing, that is, roughing is completed first and then semi-finishing and finishing are performed. For a certain machined surface, it should be completed in the order of roughing – semi-finishing – finishing. During rough machining, the performance of the machine tool and the cutting performance of the tool should be fully utilized, and a larger cutting depth should be used as much as possible while ensuring the machining quality, tool durability, and the rigidity of the machine tool-fixture-tool-workpiece process system. , fewer cutting times to obtain a machining condition where the allowances of each part before finishing are as uniform as possible, that is, most of the machining allowance can be quickly removed during roughing, reducing the number of tool passes as much as possible, and shortening the roughing time.

During finishing, the accuracy and surface quality of the parts are mainly ensured, so usually the final contour of the part should be continuously finished with the last cut. In order to ensure the machining quality, under normal circumstances, it is appropriate to leave a finishing allowance of 0.2~0.6mm. It is best to leave a period of time between roughing and finishing so that the deformation of the parts after roughing can be fully restored, and then finish machining to improve the machining accuracy of the parts.

3. The principle of near first and then far, face first and then hole.

According to the distance between the machining part and the tool setting point, in general, the parts close to the tool setting point are processed first, and the parts far away from the tool setting point are processed later, in order to shorten the tool movement distance and reduce the idle travel time. For turning, approaching first and then far is also helpful to maintain the rigidity of the blank or semi-finished product and improve its cutting conditions. For the machining of parts with both milled planes and boring holes, the order of milling the planes first and then boring can be carried out. Because the cutting force is large when milling the plane, the parts are prone to deformation. Milling the surface first and then boring it will allow it to recover for a period of time, and then boring it after it recovers its deformation will help ensure the machining accuracy of the hole. Secondly, if boring is done first After the hole is milled flat, burrs and flash will occur at the hole, which will affect the assembly of the hole.

4. Principle of internal first, then external, internal and external crossover

For parts that have both inner surfaces (inner molds, inner cavities) and outer surfaces that need to be processed, when arranging the machining sequence, the inner surface should usually be processed first, and then the outer surface. The inner and outer surfaces should be roughed first, and then processed. Internal and external surface finishing. Usually in one clamping, you must not process a certain part of the surface (outer surface or inner surface) of the part before machining other surfaces (inner surface or outer surface) of the part.

5. Principle of minimum number of tool calls

In CNC machining, in order to reduce the number of tool changes and compress the idle time, the processes and steps should be divided according to the tools used. Parts can be processed using a tool-focused process. In order to reduce the tool changing time, the same tool process should be concentrated as much as possible, and the same cutting part on the surface of the part should be processed with the same tool as much as possible to avoid multiple calls and installations of the same tool. That is, in one clamping, use the same tool as much as possible to process all parts of the finished piece that need to be processed with this tool, and then use a second tool to process other parts.

6. Principle of minimum number of attachment calls

That is, on the premise of ensuring the machining quality, after calling the accessory once, the machining and cutting will be carried out to the maximum extent to avoid multiple calls and installations of the same accessory.

7. The principle of shortest path

On the premise of ensuring the machining quality, making the machining program have the shortest tool path can not only save machining time, but also reduce some unnecessary tool wear and other consumption. The selection of the tool path mainly depends on the determination of the tool path for rough machining and idle stroke, because the tool path of the finishing cutting process is basically carried out in sequence along the contour of the part. Under normal circumstances, if the tool starting point and tool change point can be reasonably selected, and the idle stroke connection between each path can be reasonably arranged, the length of the idle stroke can be effectively shortened.

8. Principle of minimum program segments

In the preparation of machining programs, it is always hoped that the parts can be processed with the minimum number of program segments, so as to make the program concise, reduce the chance of errors, improve the efficiency of programming work, and reduce the time and computer input of program segments. The number of memory capacity occupied.

9. Principles of connection between CNC machining procedures and ordinary procedures

CNC machining processes are generally interspersed with other ordinary processes. If the connection is not good, conflicts may easily arise. The best way is that each process needs to establish status requirements for each other. Each process must be considered both before and after, and comprehensively considered, such as: Do not leave a machining allowance, how much should be left; the accuracy requirements of the datum plane and the hole, the heat treatment status of the blank, etc., the purpose is to meet the machining requirements, and the quality objectives and technical requirements are clear; there is a basis for handover and acceptance of each process.

10. Principles of special handling of special circumstances

The above principles are not static. For some special situations, the process design needs to adopt flexible and variable solutions according to the actual situation. These rely on programmers’ continuous accumulation and learning of actual machining experience.

5. CNC machining route

1. CNC lathe machining route:

The CNC lathe turning end face machining route is as shown in the figure below: A-B-Op-D, where A is the tool change point, B is the entry point, C–0p is the tool cutting trajectory, 0p is the cut-out point, and D is the tool withdrawal point.

CNC lathe turning end face machining route CNC lathe turning end face machining route

The machining route of CNC lathe turning outer circle is shown in the figure below A-B-C-D-E-F, where A is the tool change point, B is the entry point, C–D–E is the tool cutting trajectory, E is the cut-out point, and F is the tool withdrawal point. Quality new space

CNC lathe turning outer circle machining route CNC lathe turning outer circle machining route

2. CNC milling machine machining route:

When milling the outer contour of a flat part with the side edge of an end mill, it should be cut in or out along the tangential extension of the outer contour curve to avoid cutting marks and ensure a smooth transition of the part’s curved surface.CNC machining china routes for external contour milling machining routes for external contour milling

When milling the closed inner contour surface, the tool must also advance and retract along the tangential direction of the contour line, as shown in the figure below, A-B-C is the tangential cutting path of the tool into the contour, C-D-C is the path of the closed inner contour of the workpiece cut by the tool, and C-E-A is The tool cuts the contour path tangentially.CNC machining routes for internal contour milling machining routes for internal contour milling

3. Hole machining positioning route:

Pay attention to the consistency of the positioning direction of each hole, that is, use a one-way approach positioning method. This positioning method avoids positioning errors caused by backlash in the transmission system and improves the position accuracy of the holes. As shown below

Hole machining positioning route Hole machining positioning route

6. How to determine the cutting amount

When programming CNC, the programmer must determine the cutting amount for each process and write it into the program in the form of instructions. Cutting usage includes spindle speed, back cutting amount and feed speed, etc. For different machining methods, different cutting amounts need to be selected. The selection principle of cutting amount is: ensure the machining accuracy and surface roughness of the parts, give full play to the cutting performance of the tool, ensure reasonable tool durability, and give full play to the performance of the machine tool to maximize productivity and reduce costs.

1. Determine the spindle speed

The spindle speed should be selected based on the allowed cutting speed and workpiece (or tool) diameter. The calculation formula is: n=1000 v/7 1D where: v? Cutting speed, unit is m/m, determined by the durability of the tool; n – spindle speed, unit is r/min, D is the diameter of the workpiece Or tool diameter, unit is mm. To calculate the spindle speed n, we must finally select the speed that the machine tool has or is close to.

2. Determine the feed speed

Feed rate is an important parameter in the cutting amount of CNC machine tools. It is mainly selected based on the machining accuracy and surface roughness requirements of the part as well as the material properties of the tool and workpiece. The maximum feed rate is limited by the stiffness of the machine tool and the performance of the feed system. The principle of determining the feed speed: When the quality requirements of the workpiece can be guaranteed, in order to improve production efficiency, a higher feed speed can be selected. Generally selected within the range of 100-200mm/min; when cutting, machining deep holes or machining with high-speed steel tools, it is advisable to choose a lower feed speed, generally selected within the range of 20-50mm/min; when machining accuracy, surface When the roughness requirements are high, the feed speed should be selected smaller, usually within the range of 20–50mm/min; when the tool is idle, especially when “returning to zero” at a long distance, the machine tool CNC system settings can be set the maximum feed speed.

3. Determine the amount of knife on the back

The amount of back cutting is determined based on the stiffness of the machine tool, workpiece and tool. If the stiffness allows, the amount of back cutting should be equal to the machining allowance of the workpiece as much as possible. This can reduce the number of tool passes and improve production efficiency. In order to ensure the quality of the machined surface, a small amount of finishing allowance can be left, generally 0.2 to 0.5mm. In short, the specific value of the cutting amount should be determined by analogy based on the performance of the machine tool, relevant manuals and practical experience.

At the same time, the spindle speed, cutting depth and feed speed can adapt to each other to form the optimal cutting amount.

The cutting amount is not only an important parameter that must be determined before machine tool adjustment, but whether its value is reasonable or not has a very important impact on machining quality, machining efficiency, production cost, etc. The so-called “reasonable” cutting amount refers to the cutting amount that makes full use of the cutting performance of the tool and the dynamic performance (power, torque) of the machine tool to obtain high productivity and low machining cost while ensuring quality.

7. Precautions for CNC machining

    1. At present, my country’s economical CNC lathes generally use ordinary three-phase asynchronous motors to achieve stepless speed change through frequency converters. If there is no mechanical deceleration, the spindle output torque is often insufficient at low speeds. If the cutting load is too large, it is easy to get bored. , but some machine tools have gear gears that solve this problem very well;
    2. Try to enable the tool to complete the machining of one part or one working shift. Pay special attention to the finishing of large parts to avoid changing tools midway to ensure that the tool can be processed in one go;
    3. When turning threads with a CNC lathe, use a higher speed as much as possible to achieve high-quality and efficient production;
    4. Use G96 as much as possible;
    5. The basic concept of high-speed machining is to make the feed exceed the heat conduction speed, thereby discharging the cutting heat with the iron chips to isolate the cutting heat from the workpiece to ensure that the workpiece does not heat up or heats up less. Therefore, high-speed machining requires a very high cutting speed. Match the speed with high feed and select a smaller amount of back cutting;
    6. Pay attention to the compensation of tool tip R.

CNC machining is among our specialties here at Be-Cu.com, and we have been offering it since we started. Through the years, we’ve witnessed how technological advancements—from sophisticated software to faster multi-axis machines—have impacted CNC rapid prototyping.

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