How to accurately adjust the parameters of the machine tool

Vibration analysis is a very important aspect of detecting the performance of a machining center, but it is also a missing aspect. If this analysis is not performed on a high-speed machine, the machine tool builder may not know what the machine can do.

Experienced mechanics and NC programmers respond when milling is performed with harsh, audible sounds due to poor tooling. This is due to the fact that the cutting parameters chosen are not ideal, and in the face of this, they will reduce some parameters. In other words, they will lower certain process parameters.

Some of the specific parameters they reduce may be the depth of cut, or a combination of speed and feed rate, or even the cantilever length of the tool. Regardless of the parameters, “downgrade” is a flexible strategy for achieving better processing performance. Experienced processors are very skilled at adjusting these parameters, and they are able to reduce these parameters to the extent necessary to achieve acceptable levels of cutting. In most processing plants, this method of reducing processing parameters—a compromise—is seen as a correct response from the operator.

However, in high-speed machining, this method of reducing the processing parameters is not the correct countermeasure. In fact, people even found that this practice runs counter to the essence of high-speed machining.

The parameters themselves are important, such as depth of cut, spindle speed and tool length all affect the performance of the process. However, when a high-speed machine tool produces a vibrating sound that reflects poor machine tool performance, the “down” parameter may be counterproductive. There are many workshops that use high-speed machining centers, which wastes their actual production capacity and ultimately fails to meet production targets.

In the field of high-speed machining applications, in order to achieve more stable cutting, many manufacturers may still need to adopt the parameter "down" mode, but there is a more effective method, that is, the parameter "upward" mode. If both the spindle speed and the tool length are involved, then a better answer to the chattering will be to increase one of the parameters. Although this may seem strange, practical experience has shown that higher spindle speeds and lower rigidity tools, or only one of them, allow the machine to handle more aggressive depths of cut because these changes cause system vibration. The tendency is more harmonious. Any processing workshop that recognizes this can understand the true essence of high-speed machining, that is, working within the spindle speed range, which cannot be determined by intuition or experience. Different ideal cutting conditions are available in different situations. There is now a machining shop that knows a lot about this workshop, which is a division of Warner Robins Air Force Base in Georgia.

Milling in the military industry

This fast-responding workshop is a division of the 573th Product Maintenance Squadron of the Air Force's 402th Maintenance Brigade. The task of the shop includes the replacement of spare parts for fighters and transport aircraft, and all replacement parts of the aircraft that fly to the base are processed by the world. Maintenance here is very frequent, as these aircraft need to be quickly returned to the operational state, so the shop often needs to produce some complex and critical parts in small batches according to limited delivery time requirements.

Programmers David Devore and Mike Estes said that when the shop started using high-speed machining centers, they didn't know anything about it, so the machine was very inefficient at the time. When machining with these machines, the tools are often broken, including 3/4 in (1 in = 25.4 mm) end mills. From the reaction situation of this workshop, it can only slow down the cutting speed of some processing projects.

Figure 1 Warner Robins's shop, after purchasing its highest-speed, high-speed machine, began to investigate the use of resonant frequency milling

However, when the shop introduced and installed its MAG 3 high-speed machining center up to 30,000 r/min from Makino, they performed a “knock test” on the machine. That is to say, the workshop uses a sensitive hammer to hit the tool installed in the spindle like a clock to test the vibration characteristics of the machine.

For a specific combination of machining centers, shanks and milling cutters, the “knock test” can be used to find the maximum spindle speed and depth of cut for metal cutting. The purpose of this analysis is to find out the specific machine. The stable cutting speed of each tool or tool holder after commissioning. At Robins, once these stable parameters were found, they were used on new machines to improve the machining quality, productivity and tool life of the shop's parts, all of which exceeded their high-speed milling. The level reached. Because the gap was so large, the shop decided to perform the same analytical tests on its existing high-speed machine tools. On these machines, the shop found that many tools worked more stably, more productively, and more safely when they were running at fairly high speeds and exceeding the speed that the shop had been running before. After starting up, the speed is consistent with these optimal parameters, the workshop immediately begins to work quieter, the cutting amount is increased, the machining quality of the parts is improved, and the tool is not broken.

Figure 3 Warner Robins Air Force Base Air Force Base provides services for cargo aircraft and fighter aircraft.

The machine shop supports the execution of this mission

Now, the shop begins to run high-speed machine tools using parameters obtained from vibration analysis. When a certain speed is exceeded, the workshop recognizes that this analysis is actually critical to achieving a comprehensive benefit for the machine tool.

The reason for this is related to the resonant frequency. Each special combination of machine tool, tool holder and tool has a specific steady speed value, allowing the machine to achieve maximum depth of cut and metal cut. There are more explanations, but for now, it is enough to illustrate the problem, but it is impossible to infer these speed values ​​from experience. For Robins, there is no practical way to find stable parameters by trial cutting, because the shop often uses a lot of tools. Therefore, the shop signed a contract with a partner company that can help the shop quickly measure the vibration of all machine tools, and then determine the best speed according to the combination of each tool or tool holder used in the daily use of the machine. And depth of cut.

Tool dashboard

The programmer says that the cutting performance can now remain the same. In particular, the environment in the workshop has changed a lot. Due to the reduced chatter vibration, the processing environment in the workshop is very quiet. Moreover, because high-speed machine tools can perform deep and powerful cutting at higher speeds without tool breakage, the machine tool has higher productivity. Moreover, it is now possible to provide information on the optimum processing parameters, so that the productivity of the programmer itself is further improved. One of the resources now available to them is the Tool Dashboard, which allows these programmers to accurately predict the characteristics and productivity of any cutting parameters selected and set for a particular milling pass.

From here you can see that the technician is installing a newly machined part on the wing of the aircraft. Now, when the tool and the tool holder used in one of the high-speed machining centers are selected, they can choose the most according to their situation. Good parameters. In fact, the parameters obtained from the vibration analysis will be automatically entered into Catia. When these cutting conditions require adjustment settings, such as the depth of cut may not meet the cutting requirements, the relevant parameters can be manipulated through the tool dashboard to find another set of effective cutting conditions to eliminate the source of the chatter. Even the feed rate can be adjusted in such a way that it is tested according to the bending moment of the spindle.

On the “Speed” table of the tool dashboard, the green area represents the stable cutting area for this particular tool.

The tool dashboard allows the programmer to easily manipulate speed, depth of cut and cutting load, and predicts the effects of these changes based on dynamic changes in the machine.

Warner Robins and BlueSwarf have signed a contract for "strike test analysis" and the provision of tool dashboards. BlueSwarf is a company specializing in machine vibration analysis and testing. The company sells the “Metalmax” analyzer to some of the shop for internal analysis of the shop, but the company also conducts on-site analysis and measurement for those workshops that contract the analysis to the outside unit. The actual test simply hits the tool running in the machining center, which means that the tool is hit with a sensitive hammer and the feedback is captured by the sensor on the tool tip. Since the different combinations of tools and holders need to be tested separately, the entire analysis test may take hours or days. The details of some tools, such as the fillet radius, have nothing to do with the test results, so not every actual data for each tool must be tested. Despite this, Warner Robins's shop still needs to test 45 to 50 tool settings on each high-speed machine.

Specific speed

So why do machining centers need to do this? Why does the machining center work better at a specific speed or with a specific length of tool? This answer is related to the fact that each mechanical system carries several sets of inherent natural vibration frequencies, and machining centers, tool holders and tooling systems are no exception. The system has these natural frequencies, which, to some extent, vibrate at these frequencies as it is being machined. At the same time, a microwave effect occurs on the processed surface. These fluctuations may have a slight effect on the machining pass. Moreover, this may also affect the feedback mechanism and cause damage to the cutting process.

This stability legend shows that it is possible to increase the depth of the cut over a narrow range of spindle speeds over the entire speed range. The purpose of the tap test was to find these stable speeds when the machine encountered such surface fluctuations during machining. This fluctuation causes a change in the depth of cut, and as a result, the load on the tool also fluctuates. This kind of fluctuation is often negligible, so the machine response is also small. However, in the process of strong cutting, the degree of vibration becomes more apparent, and in fact the fluctuation itself generates a driving force. The deflection of the tool causes more ripples, which will increase the degree of vibration in the depth of cut. This self-excited vibration is actually the so-called "vibration". The service life of the tool and the quality of the part are affected, and the sound is even more unbearable.

A different possibility is set where the spindle speed exceeds approximately 10000 r/min. At higher speeds, it is possible to align a cutting speed when the blade is in contact with the workpiece with the natural frequency of the system.

When this happens, the machine is still vibrating, but the tip moves essentially with the ripples. At this time, the cutting load is uniformly balanced and the cutting is very smooth. For many workshops, this result is a very strange phenomenon. Some high speed spindle speeds allow for higher depths of cut at higher speeds or lower speeds.

This can explain this problem. From the entire range of spindle speeds seen here, the peak value indicates the higher production cutting efficiency that this particular machine and tool setting can achieve. Areas with cross-hatching indicate where chattering occurs. Therefore, when the rotation speed is 17792r/min, it is possible to perform a stable cutting process with a very large depth of cut. If the running speed of the machine tool is slightly higher or lower than this value, it is possible even if the same level of powerful milling is not achieved. . In fact, any processing shop that does not understand this magical speed will not know what the machining center can do.

In the above photo, the computer shows the test of two tools. The test was originally carried out in the Robins workshop. After only a few minutes of testing, the different properties of the two tools can be seen. The red peak indicates the depth of cut that can be achieved with a shorter tool, while the blue peak behind it indicates the larger depth of cut allowed with a longer tool. In fact, the shop may not know exactly what the tool can do. What are you doing?

The magical character of the tool

The difference in tool performance reveals a more potentially surprising fact in high-speed machining. When BlueSwarf conducted a tap test on Warner Robins' oldest high-speed machining center, it found the phenomenon shown in Figure 7. In general, shorter tools are more stable for milling, but this is not always the case. On this particular machine, when two longer tools were used, the system was found to be able to work more stably and achieve a greater depth of cut. The tool settings for the tool are shown in Figure 7. On this particular machine, the higher the flexibility of the tool, the higher the speed of metal cutting.

This workshop has been using the machine for many years, the machine works well, and of course the information is of great help. The performance of the machine has been good for many years. If the operation of the machine tool does not have such valuable information, it will not be possible to achieve such a multiplier effect, or the potential production and quality of the machine tool will be reduced.