With the widespread application of electronic technology in industrial processing, the performance of processing equipment has developed by leaps and bounds. In the metal processing industry, the emergence of CNC machine tools has not only improved the machinability of complex products, but also greatly improved the quality and processing efficiency of products. However, no matter how advanced the performance of the equipment is, the final cutting process is completed by cutting tools. As the saying goes, “If you want to do your work well, you must first sharpen your tools.” The selection of tools plays a great role in the performance and efficiency of CNC equipment. No matter how advanced the equipment used is, if there is no corresponding cutting tool support, the performance of the equipment cannot be demonstrated.
Types and Characteristics of Commonly Used Cutting Tools for CNC Machining
CNC machining cutting tools must adapt to the characteristics of high speed, high efficiency, and a high degree of automation of CNC machine tools. Generally, they should include general-purpose tools, general-purpose connecting tool holders, and a small number of special tool holders. The tool holder should be connected to the tool and installed on the machine tool spindle. It has now been standardized and serialized.
Classification of CNC Cutting Tools
- According to the equipment used, it can be divided into: CNC lathe tools, CNC milling machine tools, and machining center tools;
- According to the materials used to manufacture the tools, it can be divided into high-speed steel tools; carbide tools;
diamond tools; cubic boron nitride tools, ceramic tools, etc.
- According to the cutting process, it can be divided into:
Turning tools, including external circle, internal hole, thread, grooving tools, etc.:
Drilling tools, including drill bits, 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, reaching 30%~40% of the entire CNC tools in terms of quantity, and the amount of metal removal accounts for 80%~90% of the total.
Characteristics of CNC Tools
Compared with tools used on ordinary machine tools, CNC tools have many different requirements, mainly including the following characteristics:
- Good rigidity (especially roughing tools), high precision, low vibration resistance and thermal deformation.
- Good interchangeability, easy to change tools quickly.
- Long life, stable and reliable cutting performance.
- 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.
Selection of CNC Machining Tools
Basic Principles of Tool Selection
There are many issues to consider when selecting cutting tools, and there are many corresponding principles, such as efficiency principle, machining accuracy principle, stability principle, economy principle, etc.
The efficiency principle is actually inseparable from other principles, especially the principle of economy. The main purpose of requiring efficiency is to ensure the economy of the entire processing. But efficiency is particularly important, so it is separated and discussed separately.
The efficiency principle is first of all efficiency under the premise of ensuring that the processing accuracy requirements of the product are met and the stability within a certain period of time. Without this basic condition, efficiency is out of the question. Just like we hope that our means of transportation (such as cars) can bring us faster speed, but safety is often the first priority.
Secondly, we will not emphasize efficiency under all conditions. There are some constraints in the pursuit of efficiency. The improvement of the processing efficiency of a part needs to be adapted to the efficiency of other parts, and the improvement of the efficiency of a process needs to be adapted to the efficiency of other processes. If these constraints are ignored and efficiency is pursued blindly, it will be thankless. For example: In assembly line production, what we need to solve is the “bottleneck” process in the entire production line. As long as the production capacity of this process is improved and the beat is consistent with other equipment, the production capacity of the entire production line can be improved, the production capacity of the entire product can be improved, and the manufacturing cycle can be shortened.
However, the requirements of single machines or flexible manufacturing systems are different. Most of them undertake the production tasks of multi-variety and small batches in production. At the same time, they are subject to fewer constraints, that is, they are less related to other processes. Often, a single or very few devices can complete the processing of the entire component. Due to flexibility, the shortening of the manufacturing cycle of a certain component or a certain process often means that the equipment can be put into the production of other parts or other processes, thereby creating more benefits.
In addition to the principle of processing efficiency, the influence of cutting tools on processing accuracy also needs to be considered, especially in applications such as finishing where processing accuracy and surface quality requirements are relatively high.
Under rough processing conditions, we generally adopt the principle of efficiency first. At this stage, quickly removing the processing allowance on the workpiece blank and quickly approaching the “net size” state of the finished size of the workpiece is the first factor we consider in tool selection and processing parameters. But under the conditions of finishing, the situation will be very different. When finishing, we should adopt the principle of accuracy first, that is, first ensure the dimensional accuracy, surface roughness and surface quality of the processing.
Specific Cutting Tool Selection
Specific tool selection should be based on the processing capacity of the machine tool, the performance of the workpiece material, the processing procedure, the cutting amount and other related factors to correctly select the tool and tool holder. The general principle of tool selection is: easy installation and adjustment, good rigidity, high durability and precision. Under the premise of meeting the processing requirements, try to choose a shorter tool holder to improve the rigidity of tool processing. The following is an explanation of some basic principles for specific tool selection according to the cutting process classification.
Turning Tools
The turning tools are divided into the choice of turning tool bars and carbide inserts in the specific selection. Turning tools are the most standardized of all CNC tools. Among them, ordinary external circle, internal hole, end face, and profile processing tool carbide inserts are almost completely universal. Therefore, tool bars should be selected from some large companies with good quality, complete specifications and varieties, and fast delivery.
At the same time, the tool accessories of large companies are guaranteed. You must know that a lot of accessories are needed during the entire use cycle of the tool bar. Some small companies’ products often change. If you can’t buy turning tool accessories, you can only scrap them. Inserts are based on the characteristics of the processed products. After actual cutting comparisons, choose products with higher cost performance, and when new material blades appear, they can be used without changing the tool bar.
The versatility of grooving tools is very low, and the products of each company are almost not universal. Therefore, you must do a cutting experiment before selecting, and then make a choice after comparison.
Drilling Tools
The spindle speed and processing accuracy of CNC equipment are very high, so when drilling, try to use carbide tools to improve the cutting efficiency of drilling. In general, when U drills can be used, do not use integral alloy drill bits, which can not only improve processing efficiency, but also save the grinding process of the drill bit and simplify tool change. Use carbide inserts when the machine tool is restricted.
Try to use coated taps in thread processing, which can increase the processing speed by 20%~30%. The quality of domestic taps is generally poor, so imported taps can be appropriately selected or non-standard customization can be made in China using high-performance materials.
Boring Tools
Boring tools are divided into rough boring tools and fine boring tools.
During rough boring, if the product batch is large and the hole structure is complex, such as step holes, you can choose to use non-standard boring tools. Let the manufacturer design a tool with multiple cutting edges according to the structure of the processed hole, so that the rough processing of complex holes can be completed in one time during processing.
Fine boring tools are the most complex and precise tools among all CNC tools, and of course, they are also the most valuable. Three points must be noted in the selection: First, the size adjustment of the boring tool must be accurate, and the size after the tool adjustment must be accurately reflected on the processed hole, otherwise, it is impossible to process precision holes. Second, the accuracy of the boring tool must be maintained for a long time. Third, the specifications of the boring tool must be complete. For example, a small hole boring tool must be equipped with boring bars of various sizes and various boring depths.
Milling Cutter
Carbide end mills are the most expensive tools in processing. At the same time, they are also the tools with the highest metal removal rate. Try to choose small diameter, dense tooth tools to achieve maximum production efficiency. At the same time, the alloy inserts used are required to have multiple cutting edges to minimize processing costs. Before using in large quantities, it is necessary to conduct multiple processing comparisons and field processing experiments, and select the most cost-effective milling cutter based on the processing results.
Tool Holders
In machining centers, various tools are installed on tool magazines, and tool selection and tool change actions can be performed at any time according to program regulations. Therefore, standard toolholders must be used so that standard tools used in drilling, boring, expanding, milling and other processes can be quickly and accurately installed on the machine tool spindle or tool magazine. Programmers should understand the structural dimensions, adjustment methods and adjustment range of the toolholders used on machine tools so as to determine the radial and axial dimensions of the tool during programming.
At present, most of the toolholders of imported or domestic machining centers we use adopt the Japanese MAS403BT standard, but some imported equipment also adopts the ANSIB5.50CAT standard. The taper shank sizes are 30, 35, 40, 45, 50, and 60. The more commonly used domestic ones are BT40 and BT50 shanks. The production samples of various manufacturers include almost all toolholder forms, but most manufacturers only have BT40 and BT50 shanks in stock. Therefore, it is very important to determine the form of the toolholder when ordering machine tools. Try to use universal tool holders, and within a company, all machine tools should use a standard tool holder. In this way, it is relatively simple to equip the tool holder of the machine tool, borrow tools between machine tools, and backup tools.
In the processing of economical CNC machine tools, since the sharpening, measurement, and replacement of tools are mostly done manually, which takes a long time, the order of tool arrangement must be arranged reasonably. Generally, the following principles should be followed:
- Minimize the number of tools.
- After a tool is clamped, it should complete all the processing steps it can perform.
- Tools for rough and fine processing should be used separately, even if they are tools of the same size.
- Mill first and then drill.
The durability and accuracy of the tool are closely related to the price of the tool. It must be noted that in most cases, although the selection of good tools increases the cost of tools, the improvement of processing quality and processing efficiency brought about by this can greatly reduce the overall processing cost. The development of tools is changing with each passing day. Many new tools and new processing methods appear every year. We must boldly try them. The improvement of technology will greatly improve the processing efficiency.
Improvement of Machining Efficiency
In the machining process, the reasonable selection of cutting amount is an important factor in improving machining efficiency. The principle of reasonable selection of cutting amount is. In rough machining, the main focus is generally on improving productivity, but economy and machining costs should also be considered. In semi-finishing and finishing, cutting efficiency, economy, and machining cost should be taken into account under the premise of ensuring machining quality. The county value should be determined according to the machine tool manual, and 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 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 machining process of economical CNC machine tools, the general range of L is L=(0.6~0.9) d.
Cutting speed V. Increasing cutting speed is also an effective measure to improve productivity, but the relationship between cutting speed and tool durability is relatively close. As the cutting speed increases, tool durability drops sharply, so the choice of cutting speed mainly depends on tool durability. In addition, the cutting speed is also closely related to the processing material. For example, when using an end mill to mill alloy 30Cr Ni2MO VA, the cutting speed is about 8m/min. When using the same end mill to mill aluminum alloy, the cutting speed can be selected to be more than 200m/min.
The cutting speeds of tools provided by different tool manufacturers vary greatly. Tools with high cutting speeds should be selected as much as possible to give full play to the performance of CNC equipment and improve processing efficiency. For example, in drilling processing, there are two U drill bits to choose from. Their feed is the same, but one uses a speed of 1000 rpm and the other uses a speed of 3000 rpm. Obviously, the ratio of their drilling efficiency is 1:3, which is very different. Experiments show that increasing the cutting speed by 20% reduces the total cost per part by 15%; if the cutting speed is reduced and the tool life is increased by 50%, the total cost per part processing is only reduced by 1%.
Spindle speed n (r/min). The spindle speed is generally selected according to the cutting speed v. 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 integer multiples during the processing process.
Feed speed Vf. Vf should be selected according to the processing accuracy and surface roughness requirements of the part as well as the tool and workpiece material. Increasing V can also improve production efficiency. When the surface roughness requirement is low, V should be larger. During the processing, Vf can be manually adjusted through the adjustment mechanism on the machine control panel, but the maximum feed speed is limited by the equipment rigidity and feed system performance.
The selection of CNC milling tools is the result of a comprehensive consideration of various influencing factors. Under the premise of following the basic principles, select the appropriate tool according to the specific situation. At the same time, combined with the actual conditions of the selected tools and equipment, reasonably select the processing parameters to maximize the processing efficiency and reduce the total processing cost. At the same time, any choice that is considered to be the best at the time is not always unchanged, and should be adjusted in time according to the problems encountered in the processing and the development of new processing methods and tools.
