In the field of CNC programming, problems such as tool collision, tool deflection, over-cutting, missed machining, redundant machining, frequent air cutting, excessive tool lifting times, and chaotic tool paths are often encountered. This article will focus on these common CNC programming issues and explore corresponding solutions.

Tool Collision Problem

Tool collision refers to a situation where, during the CNC cutting process, the cutting amount exceeds a reasonable range, causing the tool shank (other than the cutting edge) to collide with the workpiece. The main causes of tool collision include inappropriate or even lack of safe height settings; unsuitable selection of machining methods; incorrect use of tools; and in the case of secondary roughing, the set allowance being smaller than that set for the first roughing, etc.

Situation of Excessive CNC Cutting Depth

Solutions:

Reduce the CNC cutting depth. The smaller the tool diameter, the smaller the corresponding cutting depth should be. Generally speaking, in the roughing stage of mold processing, the CNC cutting depth per pass should not exceed 0.5mm, and the cutting depth for semi-finishing and finishing should be even smaller.

 

For Inappropriately Selected Machining Methods

Solutions:

Adjust the machining method from contour parallel milling to pocket milling. If the machining allowance exceeds the tool diameter, the contour parallel milling method is not suitable.

3. Unreasonable Setting of Safe Height

Solutions:

Adjust the machining method from contour parallel milling to pocket milling. If the machining allowance exceeds the tool diameter, the contour parallel milling method is not suitable.

 

Unreasonable Setting of Safe Height

Solutions:

The allowance left for secondary roughing should be slightly larger than that of the first roughing, usually by 0.05mm. For example, if the allowance for the first roughing is 0.3mm, then the allowance for the secondary roughing should be set to 0.35mm. Otherwise, the tool shank is prone to colliding with the upper sidewall.

Comments:

In addition to the above factors, modifying the tool path can sometimes also cause tool collision, so try not to trim the tool path. The most direct consequence of tool collision is damage to the tool and the workpiece; in more serious cases, it may even damage the machine tool spindle.

 

Tool Chatter

Tool chatter refers to relatively large amplitude vibrations of the tool due to excessive force applied to it. The harm caused by tool chatter is over-cutting of the workpiece and damage to the tool. Tool chatter is likely to occur when the tool diameter is small and the tool shank is too long, or when the tool is subjected to excessive force.

1. Small tool diameter with excessively long tool shank

Solutions:

Replace with a ball nose cutter of slightly larger diameter for corner cleaning, or use electrical discharge machining (EDM) to process deeper corners.

 

Excessive force (i.e., the CNC cutting depth exceeds a reasonable range)

Solutions:

Reduce the CNC cutting depth (i.e., the global cutting depth per pass). When the machining depth exceeds 120mm, it is necessary to mount the tool in two steps: first, use a shorter tool shank to machine up to a depth of 100mm, then replace it with an extended tool shank to machine the part below 100mm, with a smaller CNC cutting depth set.

Comments:

Tool chatter is easily overlooked by beginners in programming, so sufficient attention must be paid to it. During programming, the cutting depth, maximum machining depth, and whether electrical discharge machining is needed for overly deep parts should be determined based on the properties of the CNC cutting material, as well as the diameter and length of the tool.

 

Over-Cutting

Over-cutting refers to the situation where the tool cuts into areas that should not be cut, resulting in damage to the workpiece. There are various causes of over-cutting, mainly including poor machine tool accuracy, tool collision, tool chatter, using a small tool in programming but mistakenly using a large tool in actual machining, etc. In addition, if the machine operator fails to set the tool accurately, it may also cause over-cutting.

The situation shown in the following figure is an example of over-cutting caused by unreasonable setting of the safe height.

Comments:

During the programming process, it is essential to maintain a rigorous and careful attitude. After completing the program, the tool path must be carefully checked to prevent over-cutting and other issues; otherwise, the mold may be scrapped, or even the machine tool may be damaged.

 

Missed Usinagem

Missed machining refers to the situation where there are areas in the mold that can be reached by the tool but are not machined. The corners in the plane are the most prone to missed machining, as shown in the following figure.

To improve machining efficiency, a larger-sized flat-end mill or bull nose cutter is usually used for facing. However, when the corner radius is smaller than the tool radius, there will be residual allowance at the corner, as shown in the following figure.

To eliminate the residual allowance at the corner, it is necessary to use a ball nose cutter to add a tool path at the corner.

 

Redundant Machining

Redundant machining refers to performing machining operations on areas that cannot be reached by the tool or on parts that require electrical discharge machining (EDM). This situation mostly occurs in the finishing or semi-finishing stages.

Some key parts of the mold, or parts that cannot be completed by ordinary CNC machining, need to be processed by EDM. Therefore, after roughing or semi-finishing, there is no need to perform finishing with tools on these parts; otherwise, it will not only waste time but also may cause over-cutting. For example, the mold parts shown below do not require finishing.

(1) Parts that do not require finishing

Comments:

It is necessary to define the machining range by selecting the machining surfaces, and do not check the surfaces that are not involved in machining.

 

Excessive Air Cuts

An air cut refers to a situation where the tool does not cut the workpiece during the machining process. Excessive air cuts will waste machining time. The main causes of air cuts include inappropriate selection of machining methods, unreasonable setting of machining parameters, unclear remaining allowance in machined areas, and large-area machining, among which the selection of large-area machining is most likely to cause air cuts.

To avoid excessive air cuts, the machining model should be carefully analyzed before programming, and divided into multiple machining areas. The overall programming idea is as follows: use pocket milling tool paths for roughing, face milling tool paths for semi-finishing or finishing planes, contour parallel milling tool paths for steep areas, and fixed-axis contour milling tool paths for flat areas.

For the model shown in the following figure, it is not advisable to select all curved surfaces for contour parallel milling during semi-finishing, otherwise excessive air cuts will occur.

Comments:

The way to reduce excessive air cuts is to refine the tool path, and divide large machining areas into multiple small ones by means of selecting machining surfaces or trimming boundaries.

 

Excessive Tool Lifts and Chaotic Tool Paths

In programming and machining, tool lifts are unavoidable, but excessive tool lifts will consume time, significantly reduce machining efficiency, and increase processing costs. In addition, excessive tool lifts will make the tool path chaotic, which is not only unsightly but also makes it difficult to check whether the tool path is correct.

The causes of excessive tool lifts include the complexity of the model itself, unreasonable setting of machining parameters, inappropriate selection of CNC cutting modes, and failure to set reasonable tool entry points.