Summary Of The Experience Of Cnc Machining, What Is Cnc Machining?


CNC (Numerical Control Machining), also referred to as numerically controlled machining, refers to the practice of using numerically controlled tools to perform machining operations. CNC has several advantages over its traditional counterparts including stable quality machining with high machining accuracy and repeatability as well as the capability of handling complex surfaces efficiently and quickly. However, human factors and operating experience play an integral part in its final quality; let us look at twelve valuable lessons shared by an experienced senior CNC machinist with 10 years’ worth of experience here.

CNC Machining

How to divide the cnc machining process?

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

1. The centralized sorting method of cutting tools is to divide the process according to the cutting tools used, and use the same cutting tool to cnc machine all the parts that can be completed on the part. Use the second and third cutting tools to complete other parts that can be completed. This can reduce the number of tool changes, compress the idle time, and reduce unnecessary positioning errors.

2. For parts with many CNC machining contents, the processing parts can be divided into several parts according to their structural characteristics, such as inner shape, outer shape, curved surface or plane, etc. Generally, the plane and positioning surfaces are processed first, followed by the holes; simple geometric shapes are processed first, followed by complex geometric shapes; lower-precision parts are processed first, followed by higher-precision parts.

3. For parts that are prone to deformation during CNC machining, it is necessary to carry out sizing due to possible deformation after rough machining. Therefore, in general, all processes that require rough and fine machining should be separated. In summary, when dividing processes, it is necessary to flexibly grasp the structure and processability 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 centralization or process decentralization, depending on the actual situation, but must strive to be reasonable.

cnc turning prototype

What principles should be followed in arranging the cnc processing sequence?

The arrangement of the processing sequence should be considered based on the structure of the part and the condition of the blank, as well as the need for positioning and clamping, with the focus on ensuring that the rigidity of the workpiece is not damaged. The sequence should generally be carried out according to the following principles:

The cnc processing of the previous process cannot affect the positioning and clamping of the next process, and the general machine tool processing process interspersed in the middle should also be considered comprehensively.

2. First perform the internal shape and cavity processing sequence, followed by the external shape processing procedure.

3. It is best to connect the processes that are cnc machined with the same positioning, clamping method, or the same tool to reduce the number of repeated positioning, tool changing, and moving of the pressure plate.

4. For multiple processes performed during the same installation, the process that causes the least rigid damage to the workpiece should be arranged first.

cnc machine code

What aspects should be considered when determining the workpiece clamping method?

When determining the positioning benchmark and clamping scheme, the following three points should be noted:

1. Strive to achieve a unified benchmark for design, process, and programming calculations.

2. Minimize the number of clamping times and try to achieve cnc processing of all surfaces to be processed after one positioning.

3. Avoid using manual adjustment plans that take up machine time.

The fixture should be smooth, and its positioning and clamping mechanisms should not affect the cutting process during CNC machining (such as causing collisions). In such cases, it is possible to use a vice or add a base plate to extract the screw for clamping.

stainless steel CNC machining

How to determine a reasonable tool setting point? What is the relationship between the workpiece coordinate system and the programming coordinate system?

The tool setting point can be set on the part being processed, but note that the tool setting point must be the reference position or a part that has been finished. Sometimes the tool setting point is destroyed by CNC processing after the first process, which can lead to difficulties finding the tool setting point for the second and subsequent processes. Therefore, when setting the tool in the first process, it is important to establish a relative tool setting position at a location with a relatively fixed dimensional relationship with the positioning reference, so that the original tool setting point can be found based on their relative positional relationship. This relative tool setting position is usually set on the machine table or fixture. The selection principle is as follows:

1) It is easy to align.

2) Easy programming.

3) Small tool setting error.

4) It is easy to check and inspect during processing.

The initial position for the workpiece coordinate system is determined by an operator after clamping, with its origin position determined by tool settings indicating distance and positional relationship with machine datum. Once established, this coordinate system typically remains static throughout machining; that is to say it must remain consistent between its origin position and programming coordinate system i.e. throughout production run-ups.

stainless steel CNC machining

V. How to choose the cutting path?

The cutting path refers to the trajectory and direction of the tool relative to the workpiece during CNC machining. The reasonable selection of the processing path is very important, as it is closely related to the CNC machining accuracy and surface quality of the part. The following points should be considered when determining the cutting path:

1. Ensure the processing accuracy requirements of parts.

2. Facilitate numerical calculations and reduce programming workload.

Seek the shortest cnc processing route to reduce idle time and improve cnc processing efficiency.

4. Minimize the number of program segments.

5. Ensure the roughness requirements of the workpiece contour surface after CNC processing, and the final contour should be arranged for continuous processing with the last pass.

The tool advance and retreat (cut-in and cut-out) routes should also be carefully considered to minimize tool marks caused by stopping the tool at the contour (sudden changes in cutting force causing elastic deformation), and to avoid scratching the workpiece by cutting perpendicularly on the contour surface.

cnc turning acrylic
How to monitor and adjust during the cnc machining process?

After the workpiece is aligned and the program is debugged, it can enter the automatic processing stage. During the automatic processing process, the operator should monitor the cutting process to prevent abnormal cutting from causing workpiece quality problems and other accidents.

The monitoring of the cutting process mainly considers the following aspects:

The main consideration for monitoring the roughing process during machining is the rapid removal of excess material from the workpiece surface. During automatic machining on a machine tool, the tool automatically cuts according to a predetermined cutting trajectory based on the set cutting parameters. At this point, the operator should pay attention to observing the changes in cutting load during automatic machining through the cutting load gauge, and adjust the cutting parameters based on the tool’s bearing capacity to maximize the efficiency of the machine tool.

Monitoring the sound of cutting during the cutting process In the automatic cutting process, the sound of the tool cutting the workpiece is generally stable, continuous, and light when cutting starts. At this time, the movement of the machine tool is smooth. As the cutting process progresses, when there are hard spots on the workpiece or tool wear or tool feeding and other reasons, the cutting process becomes unstable. The unstable performance is that the cutting sound changes, there will be mutual impact sound between the tool and the workpiece, and the machine tool will vibrate. At this time, it is necessary to adjust the cutting amount and cutting conditions in a timely manner. When the adjustment effect is not obvious, the machine tool should be paused to check the condition of the tool and workpiece.

3. Monitoring the finishing process During the finishing process, the main focus is to ensure the processing dimensions and surface quality of the workpiece, with high cutting speed and large feed rate. At this time, it is important to pay attention to the impact of built-up edge on the processed surface. For cavity processing, it is also important to pay attention to overcutting and cutting allowance at corners. To solve the above problems, firstly, it is necessary to adjust the spray position of cutting fluid to keep the processed surface in cooling conditions at all times; secondly, it is important to observe the quality of the processed surface of the workpiece and adjust the cutting amount as much as possible to avoid changes in quality. If the adjustment still has no obvious effect, it is necessary to stop the machine and check whether the original program is reasonable. It is particularly important to pay attention to the position of the tool when pausing or stopping inspection. If the tool stops during cutting and the spindle suddenly stops rotating, it will cause tool marks on the surface of the workpiece. Generally, it is considered to stop when the tool leaves the cutting state.

4. Tool monitoring The quality of the tool largely determines the quality of the workpiece processing. During the automatic machining and cutting process, it is necessary to judge the normal wear condition and abnormal damage condition of the tool through sound monitoring, cutting time control, pausing inspection during the cutting process, workpiece surface analysis, etc. According to the processing requirements, the tool should be processed in a timely manner to prevent processing quality problems caused by delayed processing of the tool.

cnc milling stainless steel

How to choose the processing tool reasonably? What are the main factors of cutting parameters? What kinds of materials do you have? How to determine the speed, cutting speed, and cutting width of the tool?

When milling in a plane, it is recommended to use a non-regrind carbide end mill or end mill. Generally, during milling, it is best to use a secondary cutting process. The first pass is best done with an end mill, and the cutting should be continuous along the surface of the workpiece. The recommended width of each pass is 60% to 75% of the tool diameter.

End mills with end milling cutters and carbide inserts are mainly used for machining bosses, grooves, and box openings.

3. Ball and circular tools (also known as round nose tools) are commonly used for machining curved surfaces and varying bevel profiles. Ball tools are mostly used for semi-finishing and finishing. Cemented carbide-inlaid circular tools are mostly used for roughing.

What is the function of the processing procedure sheet? What should be included in the processing procedure sheet?

1.The processing procedure sheet is one of the contents of the numerical control processing process design, and it is also a procedure that requires the operator to comply with and implement. It is a detailed description of the processing procedure, with the aim of allowing the operator to clarify the content of the procedure, the clamping and positioning methods, and the issues that should be noted for the tools selected for each processing procedure.

2. In the processing procedure sheet, it should include: drawing and programming file names, workpiece names, clamping sketches, program names, tools used for each program, maximum cutting depth, processing properties (such as roughing or finishing), theoretical processing time, etc.

cnc measure machine

What should I do before programming?

After determining the processing technology, it is necessary to understand before programming:

1. Workpiece clamping method;

2. The size of the workpiece blank – to determine the scope of processing or whether multiple clamping is required;

3. The material of the workpiece – in order to choose the type of tool used for processing;

4. What kinds of tools are in stock? Avoid having to modify the program during processing because there is no such tool. If it is necessary to use this tool, it can be prepared in advance.

What are the principles for setting a safe height in programming?

The principle of setting the safe height is generally higher than the highest surface of the island. Alternatively, the programming zero point can be set at the highest surface, which can also minimize the risk of tool collision.

After the tool path is programmed, why do we need post-processing?

Because different machine tools recognize different address codes and NC program formats, it is necessary to select the correct post-processing format for the machine tool being used to ensure that the program can be run.

cnc boring tools

What is DNC communication?

The method of program delivery can be divided into CNC and DNC. CNC refers to the process of transferring programs through media (such as floppy disks, tape readers, communication lines, etc.) to the memory of the machine tool for storage, and then retrieving the program from the memory for processing during machining. Due to the limited capacity of the memory, when the program is large, DNC can be used for processing. Since DNC processing involves the machine tool directly reading the program from the control computer (i.e., sending and processing simultaneously), it is not limited by the size of the memory.

There are three main factors in cutting amount: cutting depth, spindle speed, and feed rate. The general principle for selecting cutting amount is: less cutting, fast feed (i.e. small cutting depth, fast feed rate)

2. According to material classification, cutting tools are generally divided into ordinary hard white steel knives (made of high-speed steel), coated cutting tools (such as titanium plating), and alloy cutting tools (such as tungsten steel and boron nitride cutting tools).

If you want to switch from a CNC machining operator to a programmer, the above are the things you must know. In addition to the above, what else do you think is important to know? Is it also important to improve efficiency and avoid collisions and empty cuts?

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