In order to improve the efficiency of high-speed / low-power processing, when selecting a milling cutter and a hole machining tool, not only should pay attention to high-speed machining (spindle speed), but also make the tool suitable for actual machining. In addition to the safety of high-speed processing, there will be more gains.
As we all know, in the past decade, the main development trend of CNC machining centers used in processing workshops is faster, more intelligent, and the use of spindles with lighter weight and lower power consumption. The current rising energy costs have also accelerated this process. This development trend is completely contrary to the use of powerful machine tools that can achieve large depth of cut in one pass. High-speed machining (HSM) also necessarily means low-power machining (LPH), which requires the use of different tools and a different understanding of the tool.
In recent years, increasing energy costs and concerns about global climate change have given this development a new urgency. Many manufacturers have found that their utility expenditures on electricity and energy are increasing at an annual rate of 25% or more. In response, some companies began to adopt new strategies, promising to implement “green manufacturing” and “low energy manufacturing”. Although more manufacturers did not make big claims, they also silently but consciously sought to change the processing technology within the enterprise to reduce the energy consumption level of metal cutting. Choosing appropriate tools and low-power processing methods can contribute to achieving this goal.
In order to cope with the development trend of high-speed / low-consumption machining, many first-class tool suppliers are developing special tool series for high-speed machining, or increasing the rated spindle speed on their tool products. The pace of some tool suppliers is even more advanced. The reason for this is that although the current status of high-speed machining or rated spindle speed is good, and the safety of high-speed spindles is also necessary, its development is not sufficient. High-speed machining or the rated speed itself only means that the drill or tool has a sufficiently good balance when actually running at a speed of 12000 rpm or 40000 rpm, and the blade is firmly installed in the tool. However, this does not explain the machining efficiency of the tool, and efficiency is the key factor to save energy and protect the structure of light machine tools.
Of course, high-speed machining or rated speed need to be emphasized, but the vision should also look further. You will find that among the various milling cutters and drills currently used for high-speed machining, there are huge differences in machining efficiency and energy efficiency. These differences are particularly important for roughing, one-pass milling and large-diameter hole machining.
Let us first dissect a typical CNC high-speed machining center to see how it differs from traditional machine tools. Of course, it is very fast, has a rated spindle speed of up to 40000r / min, and can reach a very high feed rate; it is also highly intelligent, and its control system can usually implement interpolation processing and tools Path optimization and 3 to 6 axis linkage machining. However, it also has shortcomings. First, the rated power of the spindle motor may be only 20 horsepower (25kW) or less; second, the machine tool structure is very light, so it is more prone to deflection and vibration (this is often overlooked). In fact, it is the structural rigidity rather than the spindle power that limits the material removal rate. More than just a spindle motor, this machine tool is designed for light-loaded and fast multiple-pass cutting (instead of cutting with a large depth of cut and few passes).
From the point of view of the tool, the key to efficient and low-cost machining is to instantaneously heat the cutting zone to soften the metal being cut, and transfer heat to the chips, so that the heat leaves the cutting zone with the chips. Obviously, the softer the metal, the lower the power required to remove the metal. Compared with taking all possible methods to reduce the processing temperature in the past, this is a completely different way of thinking, which requires us to adopt different methods from the past in the tool design stage and in the tool selection stage.
Cutting heat may still be the enemy of the blade, but it can indeed become a favorable factor in the cutting point and chip on the workpiece. The high-speed milling cutter designed today is more advanced. It cooperates with the high-speed spindle (and its high surface cutting speed) to plastically soften the metal at the cutting point. If you can find this kind of tool-it can use the heat generated by the chip deformation to soften the metal to be cut, making it just to achieve a more easy-to-cut state-can achieve faster and more energy-efficient cutting, while extending the tool and machine tool life.
Inserts developed for low-power machining should also have two other important features: one is that the substrate and coating can withstand high temperature and impact resistance; the other is that the design of the cutting edge geometry can fully realize free cutting (high-speed, unsupported cutting) ). The substrate and coating need to withstand the high temperature environment related to the plastic change of the cutting zone material, and also withstand the frequent impact of repeated collisions with the workpiece at a high surface speed. These impact forces increase proportionally with the increase of the spindle speed .
Regarding the rake face geometry of low-power milling tools, the insert should have at least double positive rake angles—both in the radial and axial directions. In this way, a smooth split-cutting effect can be guaranteed in both directions. Compared with the scraping effect produced by a more blunt 0 ° rake tool, the cutting force generated is smaller and the power consumed is also less. However, not all blades have double positive rake angles, so care should be taken when choosing them.
Also needs to find a milling cutter with a helical cutting edge (such milling cutters have very few brands). This milling cutter can significantly reduce power requirements and impact forces. Its curved cutting edge makes it easier for the blade to cut into the workpiece. From a microscopic perspective, it is more like using an angled blade to cut away only a part of the material at a time during sheet metal processing, rather than violently punching the entire metal plate at once. The milling cutter adopts a spiral elevation angle of 20 ° to 45 ° to also reduce the impact of the cutter when cutting in, and can suppress the occurrence of burrs when cutting out.
In the mold processing, a large amount of power is wasted by using a round nose milling cutter, because only a small part of the cutting surface (the area around the mid-latitude line) participates in cutting at the optimal surface speed and efficiency. A better choice is to use a round nose cutter with more straight teeth (such as the Chip-Surfer cutter developed by Ingersoll, which allows more cutting surfaces to participate in cutting).
The pitch radius of the tool and the related surface cutting speed are quite consistent across the entire cutting surface. At both ends of the cutting surface, the surface speed does not approach zero, because it must be close to the nose of the bulbous cutter. Secondly, in linear cutting, a large scraping radius is used, and the chip thinning effect can be used to achieve faster material removal. The combination of the large radius of the tool tip arc and the inverse taper makes it easier to clean the corners and minimize cutting forces. All cutting surfaces use a large positive rake angle to reduce cutting forces and reduce power consumption.
Once a suitable cutter is selected, it must be fully utilized. The rules for cutting most steels are: fast cutting, hot cutting, dry cutting. Increasing the spindle speed and feed rate can cause the plasticizing effect of the material, while also improving productivity. The cutting parameters (feedrate and cutting speed) recommended by the tool manufacturer are only used as a starting point, and further improved on this basis. The most important point is that no coolant can be used. In addition to protecting the tool from thermal shock, the tool also needs to generate the heat required to soften the workpiece material. In machining, it may not be necessary to use cutting fluid for flushing chips. High-speed machining with positive rake tools can evacuate chips well. It will be beneficial to add a jet device with dehumidification function.
The following are some guidelines, some of which can usually be applied to rough milling, but for low power milling, all these principles are important.
Try to use down milling if possible, it can make the cutting edge cut into the workpiece more smoothly, which can protect the structure of the machine tool with lighter weight and prolong the life of the tool.
By studying the color of chips, you can find clues related to cutting efficiency. When milling steel parts, there is no need to worry about dark blue chips, because it means good milling status and material softening effect, and the cutting heat is being taken away by the chips in the correct way. When milling stainless steel, light yellow grass chips are also a sign of good milling status.
Narrow shoulder cutting is more energy efficient than wide shoulder cutting, and the contact width of each cutting should not exceed 75% of the tool. For the same reason, do not use more than two inserts to participate in cutting at the same time, otherwise it will only produce more friction and consume more power, which is not worth the loss. If chatter occurs during machining, you can change the geometric parameters of the tool (such as rake angle, mounting angle, or lead angle), increase the chip load, and / or reduce (rather than increase) the blade inclination.
Hole machining is generally considered to be the most energy-consuming machining per unit of material removed. Even with the new twist drill, only a small part of the cutting surface can be cut at the ideal surface speed; even under the best processing environment conditions, the friction between the chip and the chip flute will consume cutting energy. In addition, the liquid pump that transports the cutting fluid to the cutting surface also consumes energy. The larger and deeper the hole being processed, the greater the energy consumption.
For large holes with diameters greater than 25.4mm, a better alternative is to use the spiral milling method. Of course, this requires the machine tool to have an interpolation control function. Using this dry, energy-saving cutting method to replace the wet and energy-consuming traditional machining process has a good effect. Using a single-tooth or multi-tooth end mill to process large-diameter holes is more powerful than the power and system rigidity of any drill. smaller.
According to the report of the application of spiral milling technology, the processing cycle time of the positioning pin hole has been shortened by 3/4, and the power consumption of milling is only 40%. If flat drills are used to process such large-diameter holes, most modern CNC machine tools will not be able to provide the required power. For this reason, many mold makers have to move tooling to coordinate boring machines or heavy-duty drilling machines just to process the dowel pin holes. With the help of helical milling technology, they can complete the processing of all positioning pin holes on a small power milling machine used for cavity processing with one clamping. Believe it or not, using spiral milling technology can directly process large holes on the workpiece without pre-holes, without the need to waste a lot of time and energy for the increasing number of large hole drilling processes.
When using spiral milling to process holes or blind holes of various depths, care must be taken to remove chips. The geometry of the milling cutter can generate small chips, but it is not necessary to remove the chips by the tool itself. In vertical milling and some horizontal milling, air blowing may be required.
Interchangeable crown drill bits (such as Ingersoll Quick-Twist drill bits) can also be used for large hole processing, and the power required is less than slotted drill bits. Due to its unique geometry, the cutting efficiency of the tooth crown is very high, and the diameter of the drill rod is smaller than the diameter of the tooth crown, which can more easily remove chips and reduce friction. In addition, the alloy steel drill pipe with good toughness can withstand the vibration and tool deformation common on light machine tools, while the solid carbide drill bit is easily damaged by vibration and deformation.
Interchangeable crown drills are mainly used for mass production to avoid readjusting the large inventory of machine tools and various solid carbide drills. Due to their more efficient cutting edge geometry and tougher drill rods, low power is improved The added value of consumption drilling.
In order to improve the efficiency of high-speed / low-power machining, when choosing a milling cutter and a hole machining tool, not only should pay attention to high-speed machining (spindle speed), but also make the tool suitable for actual machining. In addition to the safety of high-speed processing, there will be more gains. When milling, choose the tool geometry of “completely free cutting” (double positive rake angle cutter with spiral cutting edge), and use a tool with good thermal hardness. When drilling, spiral milling can be considered to process large holes. For ordinary drilling, you can try a replaceable crown drill bit with a high-toughness alloy steel drill rod to avoid breaking the solid carbide drill bit under unstable installation conditions. Interchangeable crown drills can increase the metal removal rate, protect light machine tools and cutting tools, and save processing power.