CNC Technology – Page 37 – CNC Machining & Turning, Milling, Metal parts

CNC machining experience sharing (Part 1)

A number of years engaged in CNC processing experience, today to share with you.

The speed of white steel knife should not be too fast.
Copper roughing less white steel knife, more use flying knife or alloy knife.
When the workpiece is too high, it should be sliced with different length of knife.
After roughening with a big knife, use a small knife to remove the remaining material to ensure that the allowance is consistent.
The plane should be processed with flat bottom cutter and less ball cutter to reduce the processing time.
When the copper worker cleans the angle, first check the size of R on the angle, and then determine how large the ball knife is used.
The four corners of the calibration plane should be smooth.
If the slope is an integer, use the slope cutter to process, such as pipe position.
Before each process, think about the remaining allowance after the previous process, so as to avoid empty tool or excessive machining
Try to take simple cutting path, such as shape, grooving, single side, less walking around the same height.
When taking wcut, if you can walk finish, don’t go rough.
When the shape of the light knife is smooth, first rough and then fine. When the workpiece is too high, polish the edge first and then the bottom.
Set tolerance reasonably to balance machining accuracy and computer calculation time. The tolerance is set to 1 / 5 of the allowance for roughening and 0.01 for smooth knife.
Do more procedures to reduce the time of empty knife. Think more and reduce the chance of making mistakes. Make more auxiliary lines and auxiliary surfaces to improve the processing condition.
Establish a sense of responsibility and check each parameter carefully to avoid rework.
For CNC processing, diligent in learning, good at thinking, continuous progress. When milling non flat surface, use ball cutter more, use less end knife, do not be afraid to connect knife; small knife angle cleaning, big knife finishing; don’t be afraid of patching, proper patching can improve the processing speed and beautify the processing effect.
High hardness of blank material:

Better up milling, lower hardness of blank material: better forward milling, better machine tool precision, good rigidity, finish machining: more suitable for forward milling, vice versa, more suitable for finish machining of inner corner of parts by up milling. It is strongly recommended to use forward milling. Rough machining: reverse milling is better, finish machining is better: down milling is better, tool material toughness is good, hardness is low: it is more suitable for rough machining (large cutting amount processing), tool material toughness is poor, hardness is high: it is more suitable for finishing (small cutting amount processing).

Copper processing method

Before writing the cutter path, after the three-dimensional drawing is finished, the graphic center should be moved to the coordinate origin, and the highest point should be moved to Z = 0 before machining. The copper male spark position can process negative reservation. Before processing, it is also necessary to check whether the clamping direction of the workpiece is the same as the graphic direction in the computer, whether the arrangement in the mold is correct, whether the fixture hinders the processing, and whether the direction of the front and rear dies match. Also check whether the tools you use are complete, and check the benchmark in the calibration table. Matters needing attention in copper processing: for the determination of spark position, the reserved amount of young male (i.e. fine male) is 0.05 ~ 0.15 and coarse male is 0.2 ~ 0.5. The specific spark position can be determined by the mold maker. Copper public has no dead corner that cannot be processed. Whether it is necessary to dismantle one more loose one. Generally, the cutter path of copper processing is as follows: big knife (flat knife) roughening – small knife (flat knife) angle clearing – ා 0; smooth knife uses spherical knife to smooth curved surface. In general, teachers use a flat knife instead of a ball knife. After a big knife, they use a small knife to make a rough cut. Then they use a large ball knife to smooth the surface, and then use a small ball knife to smooth the surface. It is not easy to use a small knife to process the dead angle that the big knife can’t pass for some small angles. It is advisable to limit the range of the knife’s travel so as to avoid too many empty knives. Copper, especially Yougong, requires high precision. The tolerance is generally 0.005 ~ 0.02 and the step distance is 0.05 ~ 0.3. When the copper is exposed to be thick, the tool passing position of the ball cutter position should be reserved, that is, the copper male shape should be opened to a depth of a knife radius. In general, the copper worker must have three datum planes when calibrating three surfaces (upper, lower, left and right). Copper material is easy to process. The feed speed and rotation speed can be faster. When roughing, the machining allowance is 0.2 ~ 0.5. Depending on the size of the workpiece, the machining allowance is large, so the cutting can be fast and the efficiency can be improved. Note: the brackets are the parameters of high-speed steel knife for steel roughening, and the above cutting speed refers to that the required profile f = 300 ~ 500, and the steel smooth cutter f = 50 ~ 200.

Problems of front die opening:

First of all, rotate the Copper drawing 180o in the front view or side view to turn it into the front mold drawing, and of course, add the pillow position and PL surface. If the original body needs to be reserved, do not use the mirror body method to change the Copper drawing into the front model drawing, which may be wrong (when the X direction and Y direction of the Copper drawing are asymmetric). There are two difficulties in front mold processing: the material is relatively hard; the front mold can not be easily welded and wrong. The principle of using a knife for roughening the front die is similar to that of a copper tool. However, a large knife should be used as far as possible for the front die, rather than a too small knife. It is easy to snap the knife. Generally, the handle () is used for roughening, and the round nose knife is also used for the smooth knife. Because this kind of knife is large and powerful, there is usually a problem when processing the front die with parting surface The parting surface should be accurate because of the impact of the machine, and the cavity should be left 0.2 ~ 0.5 machining allowance (reserved for spark). In this way, the surface of the die cavity can be corrected by 0.2 ~ 0.5 towards the positive direction, and the machining allowance can be set to 0 when the face is writing the tool path. When the front die is roughened or polished, it is usually necessary to limit the cutting range. Remember that the range you set is the range of the tool center, not the range of the tool boundary, not the range of the tool processing, but a larger tool radius. Front die opening is commonly used

Fault curve and characteristics of NC machining

The fault curve can conceptually show the relationship between failure rate and time in NC machining. Because the shape of the curve is bathtub shaped, it is often called “bathtub curve”. It divides the service life of CNC machine tools into three stages: early failure period, accidental failure period and loss failure period.

At the beginning, the failure rate is very high, but with the increase of operation time, the failure rate decreases quickly and enters the stage of constant failure rate. There are many reasons for the high early failure rate, such as design defects, manufacturing and installation errors, hardware parts processing and operation errors and so on. After running in, find out the cause of the fault and eliminate it, the failure rate will gradually tend to be stable. This stage is called the commissioning period or running in period.

The early failure period not only occurs in the initial stage of the new machine tool, but also occurs when the parts processed by the CNC machine tool are repaired or replaced and put into use again.

The accidental failure period is also called effective failure period, in which the failure rate is the lowest and the failure rate is constant. In general, this stage should not be the main fault. It is the best working period of hardware parts processing. However, due to improper use, misoperation or other unexpected reasons, some faults can also be caused. In addition, if the safety factor of some components is small and the upper limit of load is encountered in use, it may be damaged and fail due to overload. Therefore, we should pay special attention to rational use, strengthen maintenance, avoid operational errors, and extend the effective life of the machine tool as far as possible.

After entering the wear failure period, the failure rate rises again due to the normal wear, chemical corrosion, physical and electrical performance changes, and material fatigue and other aging processes. For CNC machine tools, if we can use all kinds of diagnostic techniques, master the wear law of parts, and take corresponding maintenance or replacement measures before the parts enter the failure and loss period, we can control the occurrence of loss faults, thus extending the actual life of CNC machine tools

Key points of improving CNC machining quality

Reasonable machining tool with CNC

The smooth knife for processing steel and copper should be used strictly, and the allowance of the smooth knife should be reasonable, so that the smoothness of the workpiece and the service life of the tool will be better.

Before CNC machining, use the calibration table to check whether the tool swing is within the allowable tolerance range. Before machining, the tool head and lock nozzle should be blown clean with air gun or cleaned with cloth before installing the tool. Too dirty will have a certain impact on the accuracy and quality of the workpiece.
When clamping, pay attention to see whether the name and model of CNC machining workpiece and program list are the same, whether the material size is matched, whether the clamping height is high enough, and use caliper to check.
The CNC machining program sheet should be consistent with the reference angle direction marked on the mold, and then check whether it is aligned with the upper 3D drawing, especially for the workpiece with water transportation, it is necessary to see whether the 3D drawing is consistent with the water transportation of the workpiece. If there is any ambiguity, it is necessary to timely feed back to the programmer or find the fitter to check the 2D drawing to see whether the 2D and 3D datum angles are consistent.
The program list of CNC machining documents should be normalized, including die number, name, program name, processing content, tool size, feed amount, especially the safe length of tool clamping, the reserved allowance of each program, and the smooth knife. The place where the R-plane should be connected with the plane should be indicated on the program sheet. The operator should increase 0.02-0.05mm during machining, and the Gong should be cut several times When you stop to see if it is connected smoothly, touch it with your hand to see if it rises, and if it doesn’t, lower the Gong again.
Before processing, it is necessary to understand the contents of the CNC processing program sheet. There must be 2D or 3D drawings on the program list, and the hexagonal data of “x length, y width, Z height” should be marked,

If there is a plane, it should be marked with “Z” value. The operator can easily check whether the data is correct after processing. If there is tolerance, the tolerance data should be indicated.

The processing speed of machine tool should be strictly controlled by the operator. The rotation speed of F-speed and s-spindle should be adjusted reasonably. When F-speed is fast, it should be accelerated relative to s-spindle, and the feed speed should be adjusted in different areas. After processing, check the quality of the machine before it can be off the machine, to achieve a perfect processing.

Speed control of CNC machining

CNC machining is developing towards high speed and high efficiency. This includes the speed control of CNC machining. If we want to make full use of the effective working stroke, we must make the moving parts accelerate to the high speed stroke in a very short time, and stop in the high speed stroke instantaneously. This is why CNC machining speed control.

According to the open control idea, we put forward a method of automatic acceleration and deceleration control for the motion of CNC machine tool according to any curve. This method pushes the automatic acceleration and deceleration control from the traditional fixed mode to the new flexible mode, and explores a new way to effectively improve the dynamic performance of CNC machine tools.

Flexible acceleration and deceleration control

In CNC machining, the specific automatic speed control function is usually realized by the system program directly. In this way, it is necessary to change the acceleration and deceleration characteristics of the system or to control the addition and subtraction to modify the NC program, so ordinary users can not make the CNC machine tool have the best acceleration and deceleration performance according to their own will. Therefore, the flexible acceleration and deceleration control method is based on the principle of database. The acceleration and deceleration control is divided into acceleration and deceleration description and implementation, and the acceleration and deceleration description is separated from the system program. In the software of numerical control system, a general control channel is designed which has nothing to do with the contents of acceleration and deceleration database, which can complete acceleration and deceleration calculation and trajectory control independently.

Flexible automatic acceleration control

The acceleration curve, analytic curve and non analytical curve are set and stored in the acceleration and deceleration curve library as templates in the form of data table.

Flexible automatic deceleration control

As for acceleration control, it is stored in the acceleration and deceleration curve Library in the form of data table. Reasonable automatic acceleration and deceleration control is an important link to ensure the dynamic performance of CNC machine tools. Due to the lack of flexibility, the traditional automatic acceleration and deceleration control based on fixed curve is not easy to ensure that the acceleration and deceleration process is compatible with the performance of the machine tool, so it is difficult to achieve the best dynamic characteristics of the machine tool

Attention details of CNC external grinder

12 details to be paid attention to in maintenance of CNC external grinder

before grinding, please check the balance of the positive wheel of the CNC external grinder.
the grinding wheel must be selected carefully according to the material and hardness of the work.
the main shaft end and the grinding wheel flange shall be coated with thin oil film to prevent rust.
please pay attention to the rotation direction of spindle of CNC external grinder.
it is forbidden to use air gun to clean the work objects and machines.
please pay attention to whether the oil circuit of oil window is smooth.
vacuum box and filter steel, please clean it once a week.
if the suction is weak, please check whether the dust pipe is blocked by dust.
the vacuum pipe of CNC external grinder must be kept clean, otherwise it will cause combustion.

Factors affecting cutting efficiency of CNC lathe

The cutting efficiency of CNC Lathe will be affected by human factors, environmental factors and machine itself. Let’s talk about the influence of cutting quantity and cutting tool on CNC lathe machining.

Reasonable cutting parameters can improve the efficiency of CNC lathe. When the cutting speed is increased by 10 times and the feed speed is increased by 20 times, which is far beyond the traditional cutting forbidden zone, the cutting mechanism changes fundamentally. The results are as follows: the metal removal rate per unit power is increased by 30% ～ 40%, the cutting force is reduced by 30%, the cutting life of the cutting tool is increased by 70%, the cutting heat left on the workpiece is greatly reduced, and the cutting vibration almost disappears; the cutting process has taken an essential leap. According to the current situation of CNC lathe, increase the feed per tooth, improve productivity and tool life.

The effect of cutting tools on CNC lathes is like that of wheels on automobiles. The material used to make the tool must have high temperature hardness and wear resistance, necessary bending strength, impact toughness and chemical inertia, good processability (cutting, forging and heat treatment), and not easy to deform. A good tool can improve the efficiency of machining and the accuracy of the workpiece.

Other factors, such as power supply, familiar with operation, reasonable use of materials and other factors will affect the processing of CNC lathe

Application scope of lathe parts

Lathe parts, as the name implies, are products processed by lathe. Lathe parts can be divided into many kinds according to different types of lathes. The most common ones are automatic lathes, CNC lathes, instrument lathes, etc. The hardware materials used in lathe parts are copper, iron, aluminum, stainless steel, etc.

Lathe parts are widely used, involving electronic appliances, hardware tools, toys, plastics and other industries. Compared with other fasteners, its main feature is precision, the tolerance can reach plus or minus 0.01mm, or even more accurate. Of course, its price is much higher than other hardware.

Automatic lathe parts are parts processed by automatic lathe. The maximum processing diameter is 20 mm and the maximum processing length is 90 mm. Due to the small size and high speed of processing parts, the price is relatively low. The machining precision is high, and the tolerance can be controlled within plus or minus 0.01 mm.

CNC lathe parts are parts processed by CNC CNC lathe. The maximum machining diameter can reach 60 and the maximum processing length is 300 mm. The processed parts are large, complex in shape and high in precision. They can be turned in several times, and the tolerance can be up to plus or minus 0.002 mm. It has more advantages in processing stainless steel products. Due to the expensive machine and low processing efficiency, the processing cost is relatively high.

The reason why lathe parts have higher processing cost than other hardware products is because of the high precision, low speed and low output. After a large part of lathe parts are processed on the machine, they need secondary processing to meet the functional requirements, such as: milling groove, drilling, milling edge, chamfering and so on.

Processing sequence arrangement of lathe parts

The principle of first roughing and then finishing of lathe: the machining sequence of each surface is in accordance with rough machining and semi finishing of leaves_ Finish machining_ The precision machining is carried out in order to improve the machining accuracy and reduce the surface roughness.
The first principle of lathe datum plane: the surface used as precision datum should be processed first, because the more accurate the surface of positioning datum is, the smaller the clamping error is. Since the lathe inverted 40 outer circle is the basis of coaxiality, the surface should be machined first, and then other surfaces.
The principle of the lathe first and then the second: the main working surface and assembly base surface of the parts should be processed first, so as to detect the possible defects on the main surface of the blank as soon as possible. The secondary surface of the lathe can be interleaved, after the main surface is machined to a certain extent and before the final finish machining.
The principle of lathe from near to far: in general, after the workpiece is clamped, the parts near the tool rest are processed first, and the parts far away from the tool rest are processed later, so as to shorten the moving distance of the cutting tool and reduce the idle travel time. Moreover, it is beneficial to maintain the chip of the blank or semi-finished product and improve its cutting conditions. For the inner hole of lathe parts, the inner conical hole should be machined first, then the inner hole of φ 30mm, and finally the inner hole of φ 20mm.

Key technologies of processing:

The key technology of precision machining of lathe parts is the overall comprehensive design technology of lathe parts and machine tool system. Conventional machine tool design and manufacturing, each link technology has a great tolerance. Each link of ultra precision machine tool is basically in a state of technical limit or critical application, which link is slightly considered or not handled properly will lead to the overall failure. Therefore, the design needs to have a very comprehensive and profound understanding of the whole machine tool system and the technology of each part. According to the feasibility and from the overall optimization, it is necessary to carry out the related comprehensive design in great detail. High rigidity, high stability machine body structure design, manufacturing technology. In particular, lodtm machine tool, because of its large body, its own weight, load-bearing workpiece weight changes greatly, any small deformation will affect the machining accuracy. In addition to meeting the requirements of material, structure form and technology, the operability of the machine tool must be taken into account in the structural design.

Spindle technology of ultra precision lathe parts. The scheme of Aerostatic Spindle is often used in medium and small machine tools. Aerostatic Spindle has small damping and is suitable for high-speed rotary machining applications, but its bearing capacity is small. The rotary accuracy of Aerostatic Spindle can reach 0.05 μ M. The spindle of lodtm machine tool bears large size and weight of workpiece, so hydrostatic spindle should be used generally. The hydrostatic spindle has large damping, good vibration resistance and large bearing capacity, but the high-speed heating of hydrostatic spindle is large, so liquid cooling and constant temperature measures should be taken. The rotation accuracy of hydrostatic spindle can reach 0.1 μ M. In order to ensure the accuracy and stability of the spindle, both pneumatic and hydraulic sources need constant temperature, filtration and pressure precision control.

High precision air, liquid, temperature, vibration and other working environment control technology of lathe parts. Vibration isolation and horizontal attitude control of machine tool. The influence of vibration on ultra precision machining is very obvious, which is affected by far driving cars. Special measures should be taken to isolate the vibration of the machine tool. The air floating vibration isolation system of machine tool should also have the function of automatic leveling to prevent the influence of horizontal state change on machining. For the machine tool with high lodtm vibration isolation requirements, the natural frequency of the vibration isolation system should be less than 1Hz.

Speed control of CNC machining

Flexible acceleration and deceleration control

Flexible automatic acceleration control

The acceleration curve, analytic curve and non analytical curve are set and stored in the acceleration and deceleration curve library as templates in the form of data table.

Flexible automatic deceleration control

The concept of machining precision of computer gongs

The whole computer network can ensure 24-hour uninterrupted operation. At the same time, the advanced power-saving system can minimize the cost of human and material resources, shorten the production cycle by 50%, with high capacity and faster delivery time.

Online integrated production, computer simulation of the production process, error reporting device to ensure product accuracy. Professional precision automation equipment can ensure the error is within 0.01mm, which is 500 times of traditional equipment.

The deviation of the actual geometric parameters from the ideal geometric parameters is called machining error. The accuracy is measured by the size of machining error. If the error is large, the precision will be low, otherwise it will be high.

Machining accuracy includes three aspects: dimensional accuracy, shape accuracy and position accuracy.

① Dimensional accuracy the degree of correspondence between the actual size of the machined part surface itself or between the surfaces and the ideal part size. The ideal part size refers to the middle value of the dimension marked on the part.

② The degree to which the actual shape of the machined part surface is consistent with the ideal part surface shape. The ideal surface shape of parts refers to the absolutely accurate surface geometry.

④ The degree of correspondence between the actual position of each surface of the part and the position between the ideal parts after position precision machining. The position between the surfaces of an ideal part refers to the absolutely accurate position between the surfaces.