CNC Machining Process

Machining principle

• (1) The Machining of the above step does not affect the positioning and clamping of the next step.
• (2) First, the inside and outside, that is, the inner cavity (inner hole) is processed, and then the outer shape is processed.
• (3) The same tool or the same tool handling method is preferably performed continuously to reduce the resulting rearrangement or tool change errors.
• (4) In the same device, it is first necessary to perform Machining that does not affect the rigidity of the work.

Machining route

The CNC lathe feed Machining route means that the turning tool begins to move from the tool set point (or the fixed origin of the machine tool) until it returns to that point and finishes the machining program. Pass-through (including knife) cut-in, cut-out, non-cut free pass.

The finishing feed path is basically along the contour of the part. The main focus of determining the feed path is to determine the feed path for roughing and empty feed.

In CNC lathe machining, machining routes are generally determined according to the following principles:

• It is necessary to guarantee the accuracy and surface roughness of the object to be processed.
• Minimize the Machining path, shorten the idle time, and improve the Machining efficiency.
• Simplify the burden of numerical calculation as much as possible and simplify the Machining procedure.
• Some reusable programs use subroutines. Cutting feed

Having a machining program with the shortest feed path not only saves the execution time of the entire machining process, but also reduces unnecessary tool consumption and wear of sliding parts of the machine feed mechanism. The shortest path types and implementations are:

(1) Shortest cutting feed path.

The shortest cutting feed path can effectively increase production efficiency and reduce tool wear. Even when arranging the shortest cutting feed path, it is necessary to ensure the rigidity and workability of the workpiece.

(2) The shortest undisconnected route.

Use the tip of a knife. A typical roughing method is generally a rectangular ring. The tool point A setting requires the tool to be replaced during the finishing process, so set it away from the blank so that the starting point and the tool point match.

Skillfully set tool change points. Tool change points may also be set away from blank parts for the convenience and safety of tool change.

At that location, when the second knife is changed, the course of the uncut stroke during finishing is constrained to be longer. You can also reduce the distance of non-cutting strokes by setting the tool change point of the second knife to the midpoint position. Reasonably place a “return to zero” route. Manually compiling complex contouring programs simplifies the calculation process and facilitates verification. The programmer may set the tool endpoint after each tool has been processed. Executing the “zero return” operation instruction returns all of them to the position of the tool cutting point and executes the subsequent program. This increases the distance of the supply path and reduces the production efficiency. Therefore, if the “zero return” route is reasonably arranged, the distance between the end point of the previous knife and the start point of the next knife should be as short as possible. Specify zero to meet the shortest feed route requirement. In addition, if you choose to return to the tool cutting point instruction, the x-axis and z-axis bidirectional simultaneous “zero return” instructions will be used as much as possible without interference, and “return” from 0 to 0 for this function. The route is the shortest.

(3) Large margin ladder cutting supply line.

Two types of large blank cutting feed routes are listed. It is the wrong step cut line, cutting in the order of 1 bucket 5 and the remaining amount of each cut is equal, it is the correct step cut feed route. Because the receding engagement of the cutting edge is the same.

(4) Continuous cutting feed path for contour finishing of parts.

Finishing the contours of a part can be done with one or more knife finishing operations. The finished contour should be machined continuously from the last knife. At this time, it is necessary to properly select the position of the tool. Do not cut and place, or cut or change tools or poses with continuous contours. Avoid sudden changes in cutting force and destroy the balance of the process system. This results in scratches, sudden changes in shape, or stagnant parts on the contours of the parts.

(5) Special supply route.

Under normal circumstances, with CNC lathe. The vertical feed of the tool is performed along the negative direction of the coordinates. However, it is sometimes unreasonable to arrange the feeding route in the usual negative direction. It can even damage the workpiece.

CNC machining has the following advantages.

1. The number of flocs has been significantly reduced, and parts with complex shapes do not require complex flocs. If you want to change the shape or size of a part, you need to modify the component Machining program suitable for developing or modifying a new product.
2. Stable machining quality, high machining accuracy, high repeatability, can meet the machining requirements of aircraft.
3. Due to the high production efficiency of high-mix low-volume production, production preparation time can be shortened, machine tool adjustment and process inspection are performed, and cutting time is shortened by using the optimum cutting amount.
4. It can process complex machinable surfaces that are difficult to machine with traditional methods, and can even process unobservable machined parts.
   The disadvantage of CNC machining is that the machine tools are expensive and require high maintenance personnel.

Tool selection

1. Selection principle of CNC tool

Tool life is closely related to the amount of cutting. When setting the amount of cutting, you must first select a reasonable tool life and then determine a reasonable tool life based on your optimization goals. It is typically divided into the most productive tool life and the lowest cost tool life. The former is determined based on the goal, the working time per unit is the lowest, and the latter is determined based on the goal of the lowest process cost.

Tool life depends on tool complexity, manufacturing and sharpening costs.

Complex and precision tool life should be chosen to be longer than single-edged tools. Due to the short tool change time, a lower tool life can be selected to maximize cutting performance and improve production efficiency, typically taking 15-30 minutes.

Multi-knife, machine tools, and automated machining tools make tool loading, tool replacement, and tool adjustment more complex. You need to choose a longer tool life, especially to ensure tool reliability

If the productivity of a process in a workshop limits the productivity of the entire workshop, the tool life of that process is chosen to be lower. Tool life should also be chosen low if the overall plant expenditure shared by a particular process unit hour is relatively high.

For large finishes, tool life should be determined based on part accuracy and surface roughness to ensure at least one pass to avoid tool changes during cutting.

Compared to ordinary machine tool machining methods, CNC machining not only imposes high demands on tools, but also requires high precision, as well as dimensional stability and high durability.

Cutting and line performance also requires easy installation and adjustment to meet the high efficiency requirements of CNC machine tools. The tools selected for CNC machines are commonly used for the use of high speed cutting tool materials (high speed steel, ultrafine carbides, etc.) and machinable inserts.

2.Select CNC turning tool

CNC turning tools CNC lathe tools generally use the general points: lathe tools, sharp turning tools, arc turning tools, 5 types.

Molding turning tools are also known as model turning tools. The contour shape of the machined part is completely determined by the shape and size of the turning edge of the turning tool. In CNC turning, common turning tools include small diameter arc cutters, non-rectangular slot cutters, and thread cutters. In CNC machining, use as few or as few molding tools as possible.

A sharp turning tool is a turning tool characterized by a straight cutting edge. The tip of this type of turning tool consists of a straight main blade and an auxiliary blade. Internal and external turning tools 900, left and right end face turning, grooving (cutting) tools and various small chamfered tip and cylindrical hole tools. The method of selecting the geometric parameters (mainly geometric angles) of a sharp turning tool is basically the same as for normal turning.

However, it is necessary to comprehensively consider the characteristics of CNC machining (machining route, machining interference, etc.) and consider the strength of the tooltip itself.

3.Select a tool for CNC milling

In numerical control machining, the inner and outer contours of the milling plane and milling surface are commonly used for flat bottom mills. The empirical data of the related parameters of the tool are as follows.

One is that the cutter radius RD must be less than the minimum radius of curvature Rmin of the inner contour surface of the part, generally RD = (0.8-0.9) Rmin.

The second is the machining height of the H <(1 / 4-1 / 6) RD portion, which ensures that the knife has sufficient rigidity.

Third, when grinding the bottom of the inner groove with a flat-bottomed end mill, the bottom edges of the groove must overlap by two passages, the radius of the cutting edge of the tool Re = Rr or diameter d = 2Re = 2 (Rr). ). The radius of the tool is Re = 0.95 (Rr) at the time of programming.

Spherical milling cutters, ring milling cutters, drum milling cutters, conical milling cutters and disc milling cutters are often used for 3D surface and variable angle profiling.

CNC milling tool

Most CNC machine tools use serialized standardization tools. In addition, international standard and serialized models can be used with tool holders and cutter heads for turning tools such as external lathe tools and face turning tools. For machining centers and machine tools with automatic tool changers, the tool holders are serialized and standardized. For example, the standard code for taper shank tooling systems is TSG-JT and the standard code for straight shank tooling systems is DSG-JZ.

In addition, the selected tool requires rigorous measurement of the tool size to obtain accurate data before use. The operator enters this data into the data system and calls a program to complete the machining process, thereby producing qualified workpieces.

Where did the tool start moving to the specified position?

Therefore, it is necessary to determine the position of the tool in the work coordinate system at the beginning of program execution. This position is the starting point of the tool for the workpiece during program execution. It is called the starting point of the program or the point of the knife.

This point is also called the knife point because this starting point is generally determined by the knife. When programming, you need to choose the correct toolpoint position. The principle of setting toolpoints is to facilitate numerical Machining and simplify programming. It’s easy to fix and easy to check during Machining. Machining error is reduced. Tool cutting points can be set for machined parts or fixtures or machine tools. To improve the machining accuracy of a part, it is necessary to set the tool cutting point as much as possible in the part or process-based design standard.

When the machine tool is actually operated, the knife position of the tool can be placed at the tool setting point by manually setting the tool. That is, the “knife position” and the “cut point” match.

The so-called “knife position” refers to the positioning reference point of the tool. The tool position of the turning tool is the tool tip or the center of the tool tip arc.

The flat-end milling cutter is the intersection of the tool shaft and the bottom of the tool. The ball end milling cutter is the ball center of the ball head and the drill bit is the drill point.

With manual knife operation, the accuracy of the tool is low and the efficiency is low. Some factories use optical alignment tools, tool aligners, and automatic tool setup devices to reduce tool setup time and improve tool setup accuracy.

If tool change is required during machining, a tool change point must be specified. The so-called “tool change point” refers to the tool change position when the holder is rotated, the tool change is on the outside of the workpiece or fixture, the workpiece is not touched, and other tool change parts are used. Prioritize.

Cutting Amount

In NC programming, the programmer needs to determine the amount of cutting for each process and write the program in the form of instructions. Cutting speeds include spindle speeds, cutting depths, and feed rates. For different Machining methods, it is necessary to select different cutting amounts. The principle of selecting the cutting amount is to ensure the accuracy and surface roughness of the part and to fully utilize the cutting performance of the tool. Ensure reasonable tool life, maximize machine performance, maximize productivity and reduce costs.

1.Determine the spindle speed

The spindle speed should be selected based on the allowable cutting speed and the diameter of the workpiece (or tool). The formula is as follows: n = 1000 v / 7 1D type: v? Cutting speed, unit is determined by m / m movement, tool durability. n 1 Spindle speed (in r / min), D is the workpiece diameter or tool diameter (in mm). Calculate the spindle speed n and finally select the machine’s own speed or a closer speed.

2.Determine the feed rate

Feed rate is an important parameter in the amount of cutting of CNC machine tools. It is mainly selected based on the machining accuracy and surface roughness requirements of the part and the material properties of the tool and workpiece. The maximum feed rate is limited by the stiffness of the machine and the performance of the feed system.

Feed Speed ​​Determining Principle: If you can guarantee the quality requirements of your workpiece, you can choose a higher feed rate to increase production efficiency. Generally within the range of 100-200 mm / min;

For deep hole machining and machining with high speed steel tools, it is generally necessary to choose a low feed rate in the range of 20-50 mm / min.

If machining accuracy and surface roughness requirements are high, feed rates should generally be chosen to be small, in the range of 20-50 mm / min.

You can set the maximum feed rate set by the machine’s numerical control system when the tool is idle, especially when “returning to zero” at long distances.

3.Determine the depth of the cut

The cutting depth is determined by the stiffness of the machine, workpiece and tool. If stiffness is acceptable, the cutting depth should be as equal as possible to the machining allowance of the workpiece, which reduces the number of passes and improves production efficiency. A small finishing allowance, generally 0.2-0.5 mm, may be left to ensure the quality of the machined surface. In short, the specific value of the amount of cutting should be determined by analogy based on machine tool performance, relevant manuals and hands-on experience.

At the same time, the spindle speed, cutting depth and feed rate can be adapted to each other to form the optimum amount of cutting.

The amount of cutting is not only an important parameter that must be determined before adjusting the machine tool, but whether it is reasonable or not has a very important effect on Machining quality, Machining efficiency, and manufacturing cost. Whether or not is also important.

The so-called “reasonable” cutting amount is the complete use of tool cutting performance and machine output performance (force, torque) on the premise of ensuring quality in order to obtain high productivity and low Machining cost of cutting amount. Point to.