How to determine the machining plan for CNC lathe programming

(1) Principles for determining the processing plan

The processing plan is also called the process plan. The machining plan of the CNC machine tool includes the formulation of the process, the work step and the route of the cutter.

In the machining process of CNC machine tools, due to the complex and diverse processing objects, especially the shape and position of the contour curve, and the influence of various factors such as different materials and batches, specific analysis should be carried out when formulating the processing plan for specific parts. And treat differently and handle it flexibly. Only in this way can the processing plan formulated be reasonable, so as to achieve the objectives of superior quality, high efficiency and low cost.

The general principles for formulating a processing plan are: first coarse and then fine, first and then far, first inside and outside, the least program, the shortest route and special treatment.

(1) first coarse and fine

In order to improve the production efficiency and ensure the finishing quality of the parts, the roughing process should be arranged first during the cutting process, and a large amount of machining allowance before the finishing is removed in a short time, and the remaining allowance is satisfied as much as possible. Uniformity requirements.

When the roughing process is completed, the semi-finishing and finishing after the tool change should be arranged. Among them, the purpose of arranging the semi-finishing is to arrange the semi-finishing as a transitional process when the uniformity of the remaining amount after the rough machining can not meet the finishing requirements, so that the finishing allowance is small and uniform.

When arranging a finishing process that can be performed with one or more knives, the final contour of the part should be continuously machined from the last knive. At this time, the position of the advance and retraction of the machining tool should be considered properly. Try not to arrange the cutting and cutting or changing and stopping in the continuous contour to avoid the elastic deformation caused by the sudden change of the cutting force, resulting in the surface of the smooth connecting contour. Sickness, sudden shape change, or stagnant knife marks.

(2) First and then far

The far and near here is based on the distance of the processing part relative to the tool point. Under normal circumstances, especially in roughing, it is usually arranged to process the part close to the tool point and to process it away from the tool point to shorten the tool moving distance and reduce the idle travel time. For turning machining, it is advantageous to maintain the rigidity of the blank or semi-finished parts and improve the cutting conditions.

(3) First inside and outside

For parts that need to be machined inside (internal, cavity) and outer surface, when formulating the machining plan, it is usually necessary to arrange the inner and inner cavities and the outer surface. 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 reduced by the heat of cutting, and it is difficult to remove the chips during processing.

(4) The shortest route

The work focus of the pass route is determined, which is mainly used to determine the roughing and idle travel path, because the cutting path of the finishing cutting process is basically carried out along the order of its parts.

The path of the tool generally refers to the path that the tool starts from the point of the tool point (or the fixed point of the machine tool) until it returns to the point and ends the machining process, including the path of the cutting process and the non-cutting idle path such as tool introduction and cutting.

Under the premise of ensuring the processing quality, the machining program has the shortest path, which not only saves the execution time of the whole machining process, but also reduces unnecessary tool consumption and wear of the sliding parts of the machine feed mechanism.

In addition to relying on a large amount of practical experience, the optimized process plan should be good at analysis and can be supplemented with some simple calculations if necessary.

The above principles are not static, and for some special cases, flexible and flexible solutions are needed. If any workpiece is finished, it must be roughed and finished to ensure its processing accuracy and quality. These depend on the continuous accumulation and learning of the actual processing experience of the programmer.

(2) Relationship between processing route and machining allowance

Under the condition that the numerical control lathe has not yet reached universal use, it is generally necessary to arrange excessive margin on the blank, especially the margin containing the forged and cast hard skin layer on the ordinary lathe. If you must use CNC lathes, you should pay attention to the flexible arrangement of the program. Arrange some subroutines to perform certain cutting operations on the parts with excessive margin.

(1) Processing route for step cutting of large margin blank

(2) End position of the tool during layer cutting

(3) Spindle speed when the thread is threaded

When machining a thread on a CNC lathe, due to the change of its transmission chain, in principle, as long as the rotation speed of the spindle can be ensured, the tool can be displaced by one pitch along the main feed axis (mostly the Z-axis), and should not be restricted. However, when machining a thread on a CNC lathe, it will be affected by the following aspects:

(1) The pitch (lead) value of the command in the thread machining block is equivalent to the feedrate F expressed by the feed amount (mm/r). If the spindle speed of the machine tool is selected too high, the converted The speed (mm/min) must be significantly higher than the normal value;

(2) The tool will be constrained by the servo drive system up/down frequency and the numerical control device interpolation operation speed at the beginning/end of its displacement. Since the rising/lowering frequency characteristics cannot meet the processing requirements, etc., it may be due to the main progress. The "advance" and "lag" generated by the movement cause the pitch of some of the screws to meet the requirements;

(3) The turning thread must be realized by the synchronous running function of the spindle, that is, the turning thread requires a spindle pulse generator (encoder). When the spindle speed is selected too high, the positioning pulse sent by the encoder (ie a reference pulse signal sent every revolution of the spindle) may be due to "overshoot" (especially when the quality of the encoder is unstable). This causes the workpiece threads to be smashed.

Therefore, when the thread is threaded, the spindle speed should be determined according to the following principles:

(1) In the case of ensuring production efficiency and normal cutting, it is advisable to select a lower spindle speed;

(2) When the lead-in length δ1 and the cut-out length δ2 (as shown in the figure) in the threading program block are considered to be sufficient, that is, if the thread feed distance exceeds the length of the specified thread on the pattern, the height may be selected to be higher. Spindle 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) Under normal circumstances, the spindle speed (n screw) at the time of threading should be determined according to the calculation formula specified in the machine tool or numerical control system manual. The calculation formula is:

n 螺 ≤ n 允 / L (r / min) where n allows - the maximum operating speed (r / min) allowed by the encoder; L - the pitch of the workpiece thread (or lead, mm).