Selection of Tools in CNC Machining

1. Classification of CNC tools


CNC machining tools can be divided into two categories: conventional tools and modular tools. Modular cutting tools are the direction of development.


Tools in CNC Machining


The main advantages of developing modular tools


Reduce tool change downtime and increase production and processing time; speed up tool change and installation time, improve the economy of small batch production; improve the standardization and rationalization of tools; improve tool management and flexible processing levels; expand tool utilization, give full play to the performance of the tool; effectively eliminate the interruption of tool measurement work, and can use off-line pre-setting.

In fact, due to the development of modular tools, CNC tools have formed three major systems, namely turning tool systems, drilling tool systems and boring and milling tool systems.


1) Structurally it can be divided into


(1) Integral style


(2) Mosaic type


Can be divided into welding type and machine clamp type. The machine-clamp type is divided into indexable and non-indexable according to the different structure of the cutter body.


(3) Vibration-absorbing type


When the ratio between the working arm length and the diameter of the tool is large, this type of tool is often used in order to reduce the vibration of the tool and improve the machining accuracy.


2) Based on the materials used in manufacturing


(1) High-speed steel cutting tools


High-speed steel is usually a parison material with better toughness than cemented carbide, but worse hardness, wear resistance and red hardness than cemented carbide. It is not suitable for cutting materials with higher hardness, nor is it suitable for high-speed cutting. High-speed steel knives need to be sharpened by the manufacturer before use, and they are easy to sharpen and are suitable for various non-standard knives with special needs.

(2) Carbide cutting tools


Carbide inserts have excellent cutting performance and are widely used in CNC turning. Carbide inserts are available in a series of standard specifications. Specific technical parameters and cutting performance are provided by the tool manufacturer.

Carbide inserts are divided into three categories according to international standards: P-type, M-type, and K-type.

Category P – suitable for processing steel and long-chip malleable cast iron (equivalent to my country’s YT category).

M type – suitable for processing austenitic stainless steel, cast iron, high manganese steel, alloy cast iron, etc. (equivalent to my country’s YW type).

M-S type – suitable for processing heat-resistant alloys and titanium alloys.

Class K – suitable for processing cast iron, chilled cast iron, short-chip malleable cast iron, and non-titanium alloys (equivalent to my country’s YG class).

K-N category – suitable for processing aluminium and non-ferrous alloys. Class K-H – suitable for processing hardened materials.


(3) Diamond cutting tools


Various types of diamond (brazed polycrystalline, single crystal) cutting tools have been rapidly used in high-hardness, high-strength, difficult-to-machine and non-ferrous metal cutting industries.


3) From the cutting process, it can be divided into


Turning tools are divided into outer circles, inner holes, external threads, internal threads, grooving, end face cutting, incisal face ring grooves, cutting off, etc.

CNC lathes generally use standard machine clamps for indexable tools. The inserts and cutter bodies of machine-clamped indexable tools are standard, and the insert materials are carbide, coated carbide and high-speed steel. The types of indexable tools for CNC lathe machine clamps include external cylindrical tools, external thread tools, internal cylindrical tools, internal thread tools, cutting tools, and hole processing tools (including centre hole drills, boring tools, taps, etc.). Machine-clamped indexable tools usually use structures such as screws, screw pressure plates, lever pins or wedges to clamp non-grindable blades.

The tools used by CNC lathes are divided into three categories in terms of cutting methods: circular surface cutting tools, end face cutting tools and centre hole tools.


2. Characteristics of CNC machining tools


There is actually not much difference between CNC tools and the tools used on ordinary machine tools. In order to achieve the goals of high efficiency, versatility, quick change, and economy, compared with ordinary metal cutting tools, CNC machining tools should be easy to install and adjust. , good rigidity, high precision, good durability and other requirements, they need to have the following characteristics:


1) The durability of the blade or tool and the reasonableness of the economic life index.


At present, tool durability is usually used to evaluate the quality of tools in production. The greater the tool’s durability, the better the cutting performance of the tool. However, when cutting a batch of identical parts, since the tool materials and workpiece materials used cannot be exactly the same, and there are some factors such as sharpening quality that cannot be completely strictly controlled, the tool durability will still vary randomly even under the same conditions.

Therefore, in CNC, in addition to the average index of tool durability, the reliability index Tp of the tool should also be given. It has become a key indicator for tool selection. It is usually specified that the reliability P≥0.9, that is, the tool cutting time is T0.9 at 9%. Research shows that when the random variable of durability is close to a normal distribution, such as using the average value of durability T as the standard, the reliability of the tool is only 50%.


2) Tools or blades facilitate cutting control


The tool must be able to reliably break or roll chips, that is, cutting control. At each station equipment of CNC machine tools. There are many cutting tools installed, with a large cutting volume and a lot of chips. Therefore, when cutting plastic metal, it is necessary to control the chips not to wrap around the cutting tools, workpieces and process equipment, to control the chips not to splash, to ensure the safety of the operator, and not to affect the cutting fluid injection. , does not affect the positioning and transportation of parts, does not scratch the machined surface, and makes the chips easy to clean. To this end, use tools with chip-rolling grooves or chip-breaking blocks, or use gap-cutting or vibration-cutting measures to improve the chip-breaking effect.


3) Standardization and typification of tool or insert geometric parameters and cutting parameters.


The tool should have high precision, including the shape accuracy of the tool, the relative position accuracy of the blade and tool holder to the machine tool spindle, the indexing of the blade and tool holder, and the repeatability of disassembly and assembly. The geometric changes in the cutting part of the tool should be small, and the accuracy should be stable even if the cutter body, arbour and insert are repeatedly loaded and unloaded.


4)The positioning reference of the blade and tool holder and the automatic tool changing system should be optimized.


The blades and tool handles are highly generalized, standardized, and serialized, so that the tools can be quickly or automatically replaced, and they need to have the function of controlling and adjusting the size or having an automatic compensation device for tool wear to reduce the downtime of tool change adjustments.


3. Selection of CNC tools


The selection of cutting tools is one of the important aspects in CNC machining technology. It not only affects the processing efficiency of the machine tool but also directly affects the processing quality of the parts. The selection of cutting tools is carried out in the human-computer interaction state of CNC programming. The cutting tools and tool holders should be selected correctly based on the processing capabilities of the machine tool, the performance of the workpiece material, the processing procedures, the cutting amount and other related factors.

Since the spindle speed and range of CNC machine tools are much higher than that of ordinary machine tools, and the spindle output power is large, compared with traditional machining methods, higher requirements are put forward for CNC machining tools, including high precision, high strength, and rigidity. It is good, has high durability, and requires stable dimensions and easy installation and adjustment.

This requires the tool to have a reasonable structure, standardization and serialization of geometric parameters. CNC cutting tools are one of the prerequisites for improving processing efficiency. Their selection depends on the geometry of the parts to be processed, the material state, the rigidity of the clamps and the cutting tools used in the machine tool. The following aspects should be considered:


1) Select the tool according to the cutting performance of the part material.


Comprehensively consider the hardness, stiffness, plasticity, toughness and wear resistance of metals and non-metals. For example, when turning or milling high-strength steel, titanium alloy, or stainless steel parts, it is recommended to choose indexable carbide tools with good wear resistance.


2) Select the tool according to the processing stage of the part.


That is to say, the rough machining stage is mainly to remove the allowance, and tools with better rigidity and lower precision should be selected. The semi-finishing and finishing stages are mainly to ensure the machining accuracy and product quality of the parts, and tools with high durability and high precision should be selected. For high-precision tools, the tools used in the roughing stage have the lowest accuracy, while the tools used in the finishing stage have the highest accuracy. If you choose the same tool for roughing and finishing, it is recommended to use the tool that has been eliminated from finishing for roughing. This is because the wear of the tool eliminated from finishing is mostly slight wear on the edge and smoothing of the coating. Continued use will affect finishing. Processing quality, but has less impact on rough machining.


3) Select tools and geometric parameters according to the characteristics of the processing area.


When the part structure allows, a tool with a large diameter and a small aspect ratio should be selected; when selecting a tool, the size of the tool should be adapted to the surface size of the workpiece being processed.


4) Select tools based on the geometry of the workpiece, machining allowance, and technical and economic indicators of the part.


Positive rake angle inserts: For inner contour processing, small machine tool processing, poor process system rigidity and complex workpiece structure and shape, positive inserts should be given priority.

Negative rake angle inserts: For cylindrical machining, when the metal removal rate is high and the processing conditions are poor, negative inserts should be preferred.

Generally, 80° convex triangle, square and 80° rhombus inserts are commonly used for cylindrical turning; 55°, 35° rhombus and circular inserts are commonly used for profiling processing; taking into account the cutting amount that the tool can withstand, the machine tool rigidity, power, etc. Conditions, for machining large allowances and rough machining, you should choose a blade with a larger tip angle, otherwise choose a blade with a smaller tip angle.

Reasonable and scientific selection of CNC tools has a great impact on CNC machining production efficiency, processing quality and processing costs. Therefore, we should attach great importance to the correct selection and rational use of CNC tools, so that they can better serve CNC machining and give full play to the advantages of CNC machining.

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