10 CNC Machining Tips: for Better Machining Quality

CNC machining refers to machining with numerical control machining tools.

Because NC machining is controlled by computer after programming, CNC machining has the advantages of stable machining quality, high machining precision, high repetition precision, machining complex surfaces and high machining efficiency.

In the actual processing process, human factors and operation experience will affect the final processing quality to a great extent.

Next, let’s take a look at the ten valuable machining tips summarized by one CNC machine operator with over ten years of CNC machining experience.

CNC Machining Tips

1. How to divide CNC machining processes?

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

Tool centralized sorting method

The tool centralized sorting method is to divide the processes according to the tools used and uses the same tool to machining all the parts that can be completed.

The second knife and the third knife are used to complete other parts they can complete.

This can reduce the number of tool changes, compress the idle time and reduce unnecessary positioning errors.

Sorting by processing position

For parts with a lot of CNC processing steps, the processing part can be divided into several parts according to its structural characteristics, such as inner shape, outer shape, curved surface or plane, etc.

Generally, the plane and positioning surface are machined first, and then the hole is machined; Machining simple geometry first, and then complex geometry; First process the parts with low precision, and then process the parts with high precision requirements.

Sorting by rough and fine CNC machining

For the parts prone to CNC machining deformation by rough and fine CNC machining sorting method, the shape needs to be calibrated due to the possible deformation after rough machining.

Therefore, generally speaking, the processes should be separated for rough and fine machining.

To sum up, when dividing the process, we must flexibly grasp the structure and manufacturability of the parts, the function of the machine tool, the NC machining quantity of the parts, the installation times and the production organization of the unit.

In addition, it is suggested that the principle of process concentration or process dispersion should be determined according to the actual situation, but it must be reasonable.

2. What principles should be followed in the arrangement of the CNC machining sequence?

The arrangement of the machining sequence shall be considered according to the structure and blank condition of parts and the needs of positioning and clamping, with a focus on the rigidity of the workpiece not being damaged.

The sequence shall generally follow the following principles:

  1. The CNC processing of the previous process shall not affect the positioning and clamping of the next process, and the general machine tool processing process interspersed in the middle shall also be considered comprehensively.
  2. First, carry out the machining process of the inner shape and inner cavity, and then carry out the contour machining process.
  3. CNC machining processes with the same positioning and clamping mode or the same tool should be connected to reduce the times of repeated positioning, tool change and moving the pressing plate.
  4. For multiple processes in the same installation, the process with small damage to the rigidity of the workpiece shall be arranged first.

3. What aspects should be paid attention to in determining the clamping mode of the workpiece?

When determining the positioning datum and clamping scheme, the following four points shall be paid attention to:

  1. Strive to unify the benchmark of design, process and programming calculation.
  2. Reduce the clamping times as much as possible, and try to CNC process all the surfaces to be machined after one positioning.
  3. Avoid using manual adjustment scheme.
  4. The fixture shall be open, and its positioning and clamping mechanism shall not affect the tool walking in CNC processing (such as collision). In such cases, it can be clamped by using vise or adding base plate to extract screws.

CNC Machining Tips

4. How to determine whether the tool setting point is more reasonable? What is the relationship between the workpiece coordinate system and the programming coordinate system?

The tool setting point can be set on the machined part, but note that the tool setting point must be the reference position or the finished part.

Sometimes, after the first process, the tool setting point is damaged by CNC processing, which will make it impossible to find the tool setting points in the second process and after.

Therefore, when setting the tool in the first process, pay attention to setting a relative tool setting position where there is a relatively fixed size relationship with the positioning benchmark, so that the original tool setting point can be found according to the relative position relationship between them.

This relative tool setting position is usually set on the machine tool workbench or fixture. The selection principles are as follows:

  • Alignment is easy.
  • Programming is convenient.
  • Small tool setting error.
  • The inspection during processing is convenient and can be checked.

The origin position of the workpiece coordinate system is set by the operator.

After the workpiece is clamped, it is determined by the tool setting.

It reflects the distance position relationship between the workpiece and the zero point of the machine tool.

Once the workpiece coordinate system is fixed, it is generally not changed.

The workpiece coordinate system and the programming coordinate system must be unified, that is, during machining, the workpiece coordinate system and the programming coordinate system are consistent.

5. How to select the cutting route?

Tool path refers to the motion path and direction of the tool relative to the workpiece in the process of NC machining.

The reasonable selection of the machining route is very important because it is closely related to the CNC machining accuracy and surface quality of parts.

The following points are mainly considered when determining the cutting route:

  • Ensure the machining accuracy requirements of parts.
  • Facilitate numerical calculation and reduce programming workload.
  • Find the shortest CNC processing route and reduce the empty tool time to improve the CNC processing efficiency.
  • Minimize the number of segments.
  • Ensure the requirements of the roughness of the workpiece contour surface after CNC machining, and the final contour shall be processed continuously with the last tool.
  • The forward and backward (cut in and cut out) route of the tool should also be carefully considered to minimize the knife marks caused by stopping the tool at the contour (elastic deformation caused by the sudden change of cutting force) and avoid scratching the workpiece by cutting vertically on the contour surface.

6. How to monitor and adjust during CNC machining?

After the workpiece is aligned and the program is debugged, it can enter the automatic machining stage.

In the process of automatic machining, the operator shall monitor the cutting process to prevent workpiece quality problems and other accidents caused by abnormal cutting.

Monitoring the cutting process mainly considers the following aspects:

1) Machining process monitoring rough machining mainly considers the rapid removal of excess allowance on the workpiece surface.

In the automatic machining process of the machine tool, the tool automatically cuts according to the predetermined cutting path according to the set cutting parameters.

At this time, the operator shall observe the change of cutting load in the automatic machining process through the cutting load table, and adjust the cutting parameters according to the bearing force of the tool to give full play to the maximum efficiency of the machine tool.

2) Monitoring of cutting sound in the process of cutting

In the process of automatic cutting, when cutting is generally started, the sound of the tool cutting the workpiece is stable, continuous and light.

At this time, the movement of the machine tool is stable. With the progress of the cutting process, when there are hard spots on the workpiece, tool wear or tool clamping, the cutting process is unstable.

The unstable performance is that the cutting sound changes, the tool and workpiece will collide with each other, and the machine tool will vibrate.

At this time, the cutting parameters and cutting conditions shall be adjusted in time.

When the adjustment effect is not obvious, the machine tool shall be suspended to check the condition of tools and workpieces.

3) The monitoring of the finishing process is mainly to ensure the machining size and surface quality of the workpiece.

The cutting speed is high and the feed rate is large.

At this time, attention should be paid to the influence of chip accumulation on the machining surface.

For cavity machining, attention should also be paid to over-cutting and tool yield at the corner.

To solve the above problems, first, pay attention to adjusting the spraying position of cutting fluid to keep the machining surface in cooling conditions at all times;

Second, pay attention to the quality of the machined surface of the workpiece, and avoid the change of quality as much as possible by adjusting the cutting parameters.

If the adjustment still has no obvious effect, stop the machine to check whether the original program is reasonable.

In particular, pay attention to the position of the tool when pausing the inspection or stopping the inspection.

If the cutting tool stops in the cutting process, the sudden spindle stop will produce tool marks on the workpiece surface.

In general, shutdown should be considered when the tool leaves the cutting state.

4) Tool monitoring tool quality largely determines the machining quality of the workpiece.

In the process of automatic machining and cutting, the normal wear and abnormal damage of the tool should be judged by means of sound monitoring, cutting time control, pause inspection in the cutting process, workpiece surface analysis and so on.

According to the processing requirements, the cutting tools shall be handled in time to prevent the processing quality problems caused by the untimely handling of the cutting tools.

CNC Machining Tips

7. How to reasonably select machining tools? How many factors are there in cutting parameters? How many kinds of cutting tools are there? How to determine the tool speed, cutting speed and cutting width?

1) Non-regrinding carbide end milling cutter or end milling cutter shall be selected for plane milling.

In general milling, the second tool feeding shall be used as far as possible.

For the first tool feeding, it is best to rough mill with an end milling cutter and continuously feed along the workpiece surface.

The recommended width of each tool feeding is 60% ~ 75% of the tool diameter.

2) End mills and end mills with carbide inserts are mainly used to process bosses, grooves and box mouth surfaces.

3) Ball cutter and round cutter (also known as round nose cutter) are often used to process curved surfaces and variable angle contours.

Ball cutters are mostly used for semi-finishing and finishing.

The circular cutter with cemented carbide cutter is mostly used for roughing.

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

1) The machining program list is one of the contents of the NC machining process design, and it is also a procedure that needs to be observed and executed by the operator.

It is a specific description of the machining program.

The purpose is to let the operator clarify the content of the program, the clamping and positioning mode, the problems that should be paid attention to by the tools selected for each machining program, etc.

2) In the machining program sheet, it shall include: the drawing and programming file name, workpiece name, clamping sketch, program name, the tool used in each program, maximum cutting depth, machining nature (such as rough machining or finish machining), theoretical machining time, etc.

9. What preparations should be made before programming?

After determining the processing technology, understand the following before programming:

  • Workpiece clamping mode;
  • Size of workpiece rough embryo ——in order to determine the processing range or whether multiple clamping is required;
  • Material of workpiece – in order to select which tool to use for machining;
  • What are the tools in stock – avoid modifying the program because there is no such tool during machining. If you must use this tool, you can prepare it in advance.

10. What are the principles for setting the safety height in programming?

Setting principle of safety height:

Generally higher than the highest surface of the island, or set the programming zero point to the highest surface, which can also avoid the risk of knife collision to the greatest extent.

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