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1. The condition of the workpiece before machining in this process. For example, castings, forgings or bars, shapes, sizes, allowances, etc.
2. The shape, size of the processed part of the previous process, or the reference surface and reference hole processed in the previous process are required for this process.
3. The part and specific content to be processed in this process.
4. In order to facilitate the preparation of processes and procedures, a rough drawing of the process and a process drawing of the process should be drawn.
2 Determine the mounting method and design fixture of the workpiece Select or design the fixture according to the determined workpiece processing location, positioning reference, and clamping requirements. CNC lathes often use three-jaw self-centering chucks to hold workpieces; shaft-type workpieces can also use tailstock tips to support workpieces. Due to the extremely high spindle speed of CNC lathes, hydraulic high-speed power chucks are used to facilitate the clamping of workpieces. Because it has passed strict balance in the production plant, it has high rotation speed (limit rotation speed up to 4000~6000r/min) and high. Clamping force (maximum push-pull force is 2000~8000N), high precision, convenient claw adjustment, through hole, long service life, etc. It is also possible to use soft jaws to hold the workpiece. The soft jaw arc surface is randomly formulated by the operator to achieve the desired clamping accuracy. By adjusting the cylinder pressure, the clamping force of the chuck can be changed to meet the special needs of clamping various thin-walled and easily deformable parts. In order to reduce the stress deformation of the slender shaft machining, improve the machining accuracy, and when machining the hole of the shaft with a hole, the hydraulic centering center frame can be used, and the centering accuracy can reach 0.03mm.
3 Determine the processing plan
1. The principle of determining the processing plan The processing plan is also called the technological plan. The processing plan of the CNC machine tool includes the formulation of the process, the work step and the route of the knife.
In the processing of CNC machine tools, due to the complex and diverse processing objects, in particular the shape and position of the profile curve, and the influence of various factors such as different materials, different batches, etc., a specific analysis should be performed when formulating a specific part. Treated differently and differently. Only in this way can the rationalized processing plan be established, so as to achieve the objectives of high quality, high efficiency, and low cost.
The general principles for formulating a processing plan are: first coarse after finishing, first near and far, first inside and outside, least program segment, shortest cutting path, and special treatment under special circumstances.
1. After the first rough finish, in order to increase the production efficiency and guarantee the finishing quality of the parts, the rough machining process should be arranged first in the cutting process, and a large amount of machining allowance before finishing in a short period of time (Figure 3). The part indicated by the dashed line in -4) is removed, while satisfying the requirement for the uniformity of the remaining amount of finishing.
After the rough machining process is completed, the semi-finishing and finishing operations performed after the tool change should be arranged. Among them, the purpose of arranging the semi-finishing is to arrange semi-finishing as a transitional process so that the balance of the finishing allowance is small and uniform when the uniformity of the remaining amount after roughing cannot meet the finishing requirements.
When arranging a finishing operation that can be performed with one or more knives, the final contour of the part should be continuously machined from the last knife. At this time, the position of the cutting tool should be properly considered. Try not to arrange cutting and cutting or tool changing and pauses in the continuous contour to avoid elastic deformation caused by abrupt changes in cutting force, resulting in a smooth connecting surface. Scratches, shape mutations, or stabbed knife marks.
2. First and last distances The far and near distances mentioned here are based on the distance of the machined part relative to the cutter point. In general circumstances, especially when roughing, it is usually arranged that the part near the tool cutting point is first machined, and the part far away from the tool point is post-processed so as to shorten the tool movement distance and reduce the idle travel time. For turning processing, the first and the second are far to help maintain the rigidity of the blank or semi-finished part and improve the cutting conditions.
3. First, second, inner and outer parts For the parts that have to process the inner surface (inner shape, cavity) as well as the outer surface, the inner and inner cavities should usually be arranged before the outer surface is processed. This is because it is difficult to control the size and shape of the inner surface, the rigidity of the tool is correspondingly poor, the durability of the cutting edge (blade) is easily affected by the heat of cutting, and it is difficult to remove the cutting chips during processing.
4. The shortest cutting path The work focus of determining the cutting path is mainly used to determine the cutting path for rough machining and idle stroke. The cutting path of the finishing cutting process is basically carried out along the outline of the parts.
The tool path generally refers to the path from the start of tool movement (or the fixed origin of the machine) until the tool returns to this point and ends the machining program, including the cutting path and the non-cutting idle path such as tool introduction and cutting.
Under the premise of guaranteeing the processing quality, making the processing program have the shortest cutting path, not only can save the execution time of the whole processing process, but also can reduce some unnecessary tool consumption and the wear of sliding parts of the machine feed mechanism.
In addition to relying on a great deal of practical experience, the optimized process plan should also be good at analysis, and can be supplemented with some simple calculations if necessary.
The above principles are not static, and in some special cases, flexible and variable solutions are needed. If any of the workpieces must be rough machined after finishing, in order to ensure the accuracy and quality of its processing. These all depend on the constant accumulation and learning of the programmer's actual processing experience.
2. The relationship between the processing route and the machining allowance. Under the condition that the CNC lathe has not reached the universal use, the excess amount on the blank part, especially the forging and casting hard cortex, should be arranged on the ordinary lathe. Processing. If you must use CNC lathe processing, you must pay attention to the flexible arrangement of the program. Arrange some subroutines to perform certain cutting operations on the parts with excess margin.
1. Machining route for step cutting of large margin blanks 2. End position of cutting tool during layered cutting 3. Spindle rotation speed at the time of threading When a CNC lathe processes a thread, in principle, its rotation speed is only due to the change of the transmission chain. It can ensure that when the spindle rotates one revolution, the tool can be shifted by one pitch in the direction of the main feed axis (mostly Z axis) and should not be limited. However, when CNC lathes machine threads, they will be affected by the following aspects:
1. The pitch (lead) value of the command in the thread machining block corresponds to the feedrate F in terms of feedrate (mm/r). If the spindle speed of the machine tool is selected too high, the converted feedrate The speed (mm/min) must be significantly greater than the normal value;
2. At the beginning and end of the displacement of the tool, the tool will be constrained by the up/down frequency of the servo drive system and the interpolation speed of the numerical control device. Since the up/down frequency characteristics cannot meet the machining needs, etc., it may be due to the main feed. The “lead†and “lag†produced by the movement result in the thread pitch of some threads not meeting the requirements;
3. The turning thread must be realized by the synchronous operation function of the spindle, that is, a spindle pulse generator (encoder) is required for the turning thread. When the spindle speed is selected too high, the positioning pulse sent by the encoder (that is, a reference pulse signal emitted by the spindle once per revolution) may be due to "overshoot" (especially when the quality of the encoder is unstable). Lead to a random buckle in the workpiece thread.
Therefore, when threading, the determination of spindle speed should follow the following principles:
1. In the case of ensuring production efficiency and normal cutting conditions, a lower spindle speed should be selected;
2. When the length of the imported length d1 and the length of the cut-out d2 (as shown in the figure) in the thread cutting block are considered sufficient, that is, the thread feed distance exceeds the length of the specified thread on the pattern, the appropriately higher spindle can be selected. Rotating speed;
3. When the allowable working speed specified by the encoder exceeds the maximum speed of the spindle specified by the machine tool, the spindle speed can be selected as high as possible;
4. Normally, the spindle speed (n screw) at the time of threading should be determined according to the calculation formula defined in its machine tool or CNC system specification. Its calculation formula is mostly:
n screw ≤ n allowable/L(r/min)
In the formula: n allow - the maximum allowable operating speed encoder (r / min);
L - pitch (or lead, mm) of the workpiece thread.
4 Determine the amount of cutting and the amount of feed In programming, the programmer must determine the amount of cutting for each process. When selecting the cutting amount, it is necessary to fully consider various factors that affect the cutting, correctly select the cutting conditions, and reasonably determine the cutting amount, which can effectively improve the machining quality and output. Factors affecting cutting conditions include: rigidity of machine tools, tools, tools and workpieces; cutting speed, cutting depth, cutting feed rate; workpiece accuracy and surface roughness; tool life expectancy and maximum productivity; type of cutting fluid, cooling method; Workpiece material hardness and heat treatment conditions; number of workpieces; machine tool life.
Among the above factors, cutting speed, cutting depth, and cutting feed rate are the main factors.
The speed of cutting directly affects the cutting efficiency. If the cutting speed is too small, the cutting time will be longer and the tool will not be able to perform its function. If the cutting speed is too fast, although the cutting time can be shortened, the tool will easily generate high heat and the tool life will be affected. There are many factors that determine cutting speed. To sum up, there are:
1. Tool material. Different tool materials, the maximum allowable cutting speed is also different. High-speed steel cutting tool cutting speed is less than 50m/min, carbide cutting tool high-temperature cutting speed of up to 100m/min, ceramic cutting tool high-temperature cutting speed up to 1000m/min.
2. Workpiece material. The hardness of the workpiece material will affect the cutting speed of the tool. When the same tool is used to process hard materials, the cutting speed should be reduced. When machining softer materials, the cutting speed can be increased.
3. Tool life. If the tool usage time (life) is long, a lower cutting speed should be used. Conversely, higher cutting speeds can be used.
4. Cutting depth and feed rate. Depth of cutting and the amount of feed, cutting resistance is also large, cutting heat will increase, so the cutting speed should be reduced.
5. The shape of the tool. The shape of the cutter, the size of the angle, and the sharpness of the cutting edge all affect the selection of the cutting speed.
6. Use coolant. A machine with good rigidity and high precision can increase the cutting speed; otherwise, it needs to reduce the cutting speed.
Among the above factors that affect cutting speed, the influence of the tool material is the most important.
The cutting depth is mainly conditioned by the rigidity of the machine tool. If the rigidity of the machine tool allows, the cutting depth should be as large as possible. If it is not limited by the machining accuracy, the cutting depth can be equal to the machining allowance of the part. This will reduce the number of passes.
The spindle speed is determined based on the allowable cutting speed of the machine and the tool. It can be selected by calculation method or look-up table method.
Feed rate f (mm/r) or feed speed F (mm/min) is selected according to the part's machining accuracy, surface roughness, tool and workpiece material. The maximum feed rate is limited by the stiffness of the machine and the feed drive and numerical control system.
When selecting the cutting amount, the programmer must select the cutting amount suitable for the characteristics of the machine tool and the best durability of the tool according to the requirements of the machine tool specification and the tool durability. Of course, analogy can also be used to determine the amount of cutting. Regardless of the method used to select the cutting amount, the durability of the tool must be ensured to complete the machining of one part, or the durability of the tool is not less than one work shift, and the minimum can not be less than half the shift time.
1 Determine the machining part and the specific content of the workpiece . Determine the content of the workpiece that needs to be processed on the machine and its connection with the previous and following processes.