The selection of CNC cutting tools and the determination of cutting parameters are important aspects of CNC machining technology. They not only affect the machining efficiency of CNC machine tools, but also directly affect the machining quality.
Types and Characteristics of Commonly Used CNC Machining Tools
CNC machining tools must adapt to the characteristics of high speed, high efficiency and high degree of automation of CNC machine tools. Generally, they should include general tools, general connection tool holders and a small number of special tool holders. The tool holder should be connected to the tool and installed on the power head of the machine tool, so it has gradually been standardized and serialized. There are many ways to classify CNC tools.
According to the tool structure, it can be divided into:
- Integral type.
- Inlaid type. Welding or machine clamp connection is adopted, and the machine clamp type can be divided into non-rotating and rotating types. Special types, such as composite tools, shock-absorbing tools, etc.
According to the materials used to manufacture the tools, it can be divided into:
- High-speed steel tools.
- Carbide tools.
- Diamond tools.
- Other material tools, such as cubic boron nitride tools and ceramic tools.
From the cutting process, it can be divided into:
- Turning tools, which are divided into external circles, internal holes, threads, cutting tools, etc.
- Drilling tools, including drills, reamers, taps, etc.
- Boring tools.
- Milling tools, etc.
In order to meet the requirements of CNC machine tools for durable, stable, easy to adjust and replaceable tools, machine-clamped indexable tools have been widely used in recent years, accounting for 30%~40% of the entire CNC tools in quantity, and the metal removal volume accounts for 80%~90% of the total.
Compared with the tools used on ordinary machine tools, CNC tools have many different requirements, mainly with the following characteristics:
- Good rigidity (especially roughing tools), high precision, low vibration resistance and thermal deformation.
- Good interchangeability, convenient and fast tool change.
- Long life, stable cutting performance, and reliable.
- The size of the tool is easy to adjust to reduce the tool change adjustment time.
- The tool should be able to reliably break or roll chips to facilitate chip removal.
- Serialization and standardization to facilitate programming and tool management.
CNC Machining Tool Selection
Although CNC machining tools have many advantages that ordinary tools cannot match, only reasonable selection in machining can give full play to their effectiveness. Faced with a wide variety of tools and complex tool samples, many operators feel at a loss and do not know how to choose. After several years of teaching accumulation and consulting with operators and technicians, the author believes that when selecting tools, inserts and tool holders should be correctly selected according to the processing capacity of the machine tool, the performance of the workpiece material, the processing procedure, the cutting amount and other related factors.
Generally, when selecting, you should first select the insert, and then select the tool bar or tool holder according to the insert and processing conditions. Therefore, the selection of insert is the key and prerequisite for tool selection, and the selection of insert has two aspects: material selection and shape selection. The material selection is mainly based on the material characteristics of the machined parts and the processing conditions to select the material of the blade. If you want to process stainless steel parts, you must first select a blade made of coated cemented carbide (GC). The processing conditions are fine processing and require a higher surface quality.
The key to insert selection is the selection of blade shape and groove type. The shape, angle and tip radius of the blade are selected according to the process type and processing part. When selecting, the main deflection angle, accessibility and versatility requirements required by the combined tool county should be considered, such as whether the tool’s secondary cutting edge interferes with the processed surface, the strength of the tool tip, and the surface roughness that can be achieved after processing. Now let’s introduce the selection method and key points of the tool (blade) shape.
Selection of Tools for CNC Turning
During the machining process, CNC lathes mainly use programs to control the various interpolation movements of the tool to achieve the machining of various profiles. Therefore, the shape of the tool cutting edge is not the main factor to ensure the machining accuracy. In order to facilitate selection and management, the types of commonly used CNC turning tools (inserts) are relatively simple. Generally, there are three types of external turning tools and three types of internal hole turning tools. Different materials of blades are used according to the different materials of the workpiece during machining, forming turning tools that adapt to different machining conditions. Therefore, the selection of turning tools is mainly the selection of these three types of turning tools.
CNC turning mainly processes some formed surfaces. At this time, the main consideration is whether the tool angle will interfere with the workpiece machining part. The relative position relationship between the tool and the workpiece when machining the external circle and the inner hole.
The application characteristics of the three commonly used external turning tool inserts are introduced as follows. For the convenience of introduction, I classify them according to the angles and characteristics of the tool components.
80°Diamond Insert External Turning Tool
The main rake angle of the tool holder used for this insert is mostly 93°, and the secondary rake angle of the tool after being combined into a tool is 7°. The tool has very good rigidity and is mainly suitable for machining stepped shaft parts such as cylindrical step surfaces. It can achieve large cutting depth and is mostly used for rough machining. Blades of different materials can also be used for semi-finishing and finishing.
35° Diamond Insert External Turning Tool
The tool holder used for this type of insert is mostly 93°, and the tool’s secondary rake angle after being combined into a tool is 52°. The tool has poor rigidity, but a large secondary rake angle can avoid interference with the processed surface. It is mainly suitable for processing some formed surfaces and concave contours. The cutting depth of each cut is small, and the feed rate should also be selected to be smaller, otherwise the surface roughness is large. Different blades can be used for roughing, semi-finishing and finishing, but are generally used for finishing.
55° Diamond Insert External Turning Tool
The main rake angle of the tool holder selected for this type of insert is 93°, and the secondary rake angle of the tool after being combined into a tool is 32°. The rigidity of the tool is average. A larger secondary rake angle can avoid interference with the processed surface. It can process some formed surfaces and contours with a small degree of concave, and can also maintain good rigidity to achieve fast cutting. Its cutting performance and application range are between 80° diamond blade external cylindrical turning tools and 35° diamond blade external cylindrical turning tools. Different blades can be used for roughing, semi-finishing and finishing. It is generally used for semi-finishing and finishing.
The selection method and requirements of internal turning tools are basically the same as those of external turning tools. It is also necessary to pay attention to whether the secondary cutting edge of the tool interferes with the processing surface and whether the strength of the tool tip meets the requirements. In addition, when selecting, try to choose a tool with a large tool tip angle, so that the tool has good rigidity. Generally, the tool secondary deflection angle can be about 2° larger than the maximum cutting angle of the workpiece contour.
Generally, the tool handle model corresponding to the blade is fixed, and it cannot be interchanged even if the shape is the same, otherwise the center height of the tool will not meet the requirements. After the shape of the blade is selected, the selection of the blade groove type should be considered. Common groove types of blades.
Reasonable selection of blade groove type can improve production efficiency. The three blades in Figure 3 are innovative high-productivity blades for semi-finishing and finishing, and have gradually been widely used in various industries. Through a slight change in the radius of the tool tip, the feed rate can be doubled while ensuring the surface quality. It is recommended to refer to the recommended value of the tool sample when selecting.
Selection of Tools for CNC Machining Centers
The general principle for selecting tools for machining centers is: easy installation and adjustment, good rigidity, high durability and precision. On the premise of meeting the machining requirements, try to choose a thicker and shorter tool handle to improve the rigidity of tool processing.
Commonly used tools for machining centers are mainly milling tools and hole processing tools. Milling tools include face milling cutters, end mills, etc. Hole processing tools mainly include center drill bits, twist drills, boring cutters, taps, countersinks, etc. Now briefly introduce the key points for selecting these tools.
End mill Selection
Solid carbide end mills have bottom blades and side blades. They are mainly used for processing step surfaces, grooves, profiling, and concave contours and formed surfaces with limited tool size. The bottom blade and side blade of the tool can be used for cutting at the same time. The processing range is wide and the processing method is flexible. They are widely used in production.
There are many types of commonly used solid carbide end mills, and the selection is relatively simple. When selecting, try to choose a tool with a larger diameter, so that the tool has good rigidity and high metal removal rate. When selecting, be sure to pay attention to the tool radius being smaller than the radius of the concave contour of the part. CNC machine tools have the function of circular interpolation. For the processing of concave circular contours, try to use a small diameter tool and use circular instructions to process. Different end mills have different numbers of teeth. Usually, milling cutters with more teeth are used for finishing. General end mills cannot be used for drilling if the blade does not pass the center. Other methods should be used when feeding. If drilling is required, a milling cutter or a keyway milling cutter with a blade passing the center should be used.
When selecting a tool for milling, the size of the tool should be adapted to the surface size of the workpiece being processed. In production, end mills are often used to process the peripheral contours of flat parts; when milling planes, carbide blade milling cutters should be selected; when processing bosses and grooves, high-speed steel end mills should be selected; when processing blank surfaces or rough processing holes, corn milling cutters with carbide blades can be selected; for processing some three-dimensional surfaces and variable angle contour shapes, ball head milling cutters, ring milling cutters, tapered milling cutters and disc milling cutters are often used.
When processing free-form surfaces (molds), since the cutting speed of the end of the ball head tool is zero, in order to ensure the processing accuracy, the cutting line spacing generally adopts the top end close spacing, so the ball head is often used for the finishing of the surface. Flat head tools are superior to ball head tools in terms of surface processing quality and cutting efficiency. Therefore, as long as it is guaranteed not to overcut, flat head tools should be preferred for both rough processing and finishing of curved surfaces. In addition, the durability and accuracy of the tool are closely related to the price of the tool. In most cases, although the selection of a good tool increases the tool cost, the resulting improvement in processing quality and processing efficiency can greatly reduce the overall processing cost.
Selection of Hole Processing Tools
Hole processing on a machining center is usually divided into three steps: first, use a center drill to drill the center hole, then use a drill bit to drill the hole, and finally use a boring tool and a reamer to finish the hole. Center drills are usually only φ1.5 and φ3. Try to choose a larger one. When choosing a drill bit, pay attention to leaving a margin of 0.4~0.6mm for finishing. The reamer is a fixed-size tool and should be selected according to the mean difference of the part. The focus of hole processing is the selection of boring tools. Commonly used boring tools are divided into two categories: rough boring tools and fine boring tools. Commonly used rough boring tools are mostly boring tools with two blades. They are generally used for boring through holes and step holes. Single-edged boring can also be used. Fine boring tools are equipped with a boring hole diameter adjustment device, and the tool head can be adjusted within a certain range. When using it, it should also be adjusted to the mean difference size of the boring hole.
Determination of Cutting Amount During Processing
The selection of cutting amount is the most important factor in determining processing efficiency and surface processing quality. It should be selected reasonably during processing. The main principle for selecting cutting amount is that during rough processing, it is generally based on improving productivity, but also, economic and processing costs should be considered; during semi-finishing and finishing, cutting efficiency, economy and processing costs should be taken into account while ensuring processing quality. The specific value should be determined according to the machine tool manual, cutting amount manual, and combined with experience. The following factors should be considered specifically.
Cutting depth ap. When the rigidity of the machine tool, workpiece and tool allows, ap is equal to the machining allowance, which is an effective measure to improve productivity. In order to ensure the machining accuracy and surface roughness of the parts, a certain allowance should generally be left for finishing. The finishing allowance of CNC machine tools can be slightly smaller than that of ordinary machine tools.
Cutting width L. Generally, L is proportional to the tool diameter d and inversely proportional to the cutting depth. In the processing of economical CNC machine tools, the general value range of L is: L=(0.6~0.9)d.
Cutting speed v. Increasing v is also a measure to improve productivity, but v is closely related to the durability of the tool. As v increases, the durability of the tool drops sharply. Therefore, the choice of v mainly depends on the durability of the tool. In addition, the cutting speed is also closely related to the processing material. For example, when milling alloy 30Cr Ni2MO VA with an end mill, v can be about 8m/min; when milling aluminum alloy with the same end mill, v can be selected to be more than 200m/min. Generally, the recommended cutting speed value for the selected blade is provided in the tool sample. The user can select it according to the material of the workpiece to be processed. The cutting speed values recommended in the machine tool manual are relatively large, and it is recommended to reduce it appropriately when selecting.
Spindle speed n (r/min). The spindle speed is generally selected according to the cutting speed v. The calculation formula is: v=πdn/1000. The control panel of the CNC machine tool is generally equipped with a spindle speed adjustment (multiplier) switch, which can adjust the spindle speed by an integer multiple during the processing.
Feed speed vf. vf should be selected according to the machining accuracy and surface roughness requirements of the workpiece, as well as the knife county and workpiece material. The increase of vf can also improve production efficiency. However, the surface roughness will be greatly reduced, so when the rough machining and machining surface roughness requirements are low, vf can be selected to be larger. Finishing is generally selected based on the surface roughness value. Now CNC machine tools are equipped with feed rate adjustment knobs, which can be set larger during programming. During the processing, manual adjustments are made through the adjustment switch on the machine control panel according to the specific processing status. However, the maximum feed speed is subject to limitations such as equipment stiffness and feed system performance.
With the widespread application of CNC machine tools in actual production, more and more parts are processed on CNC machine tools. The materials and processing requirements of workpieces are also different. In order to ensure processing quality and improve processing efficiency, operators and programmers must be familiar with the selection method of tools and the principle of determining cutting amount, give full play to the advantages of CNC machine tools, and improve the economic benefits and production level of enterprises.
