High-speed machining (HSM) has many advantages, but it is necessary to reduce the cutting costs, but also to maximize the advantages of high-speed processing, in the actual production also need to overcome many difficulties. In high-speed machining, when the cutting speed is increased, the cutting force will be reduced. Using this feature, parts with smaller dimensional tolerances and surface roughness can be machined by increasing the spindle speed.
Although there is no clear definition for high-speed machining, the spindle speed can reach 500000r/min in some tests, and it can reach 100,000r/min in actual production. As the spindle speed exceeds 8000r/min, the rotating parts need to be balanced, so the high-speed machining can be defined as the speed is not less than 8000r/min. When the spindle speed is lower than 8000 r/min, the centrifugal force will only adversely affect the machining if the mass distribution of the tool bar is very unbalanced.
With the increasing number of machine tools with spindle speeds above 8000 r/min, the balancing problem of combined tools is particularly important. It involves tools and tool holders. When the theoretical central axis of the rotating arbor is not concentric with the actual axis of rotation, an imbalance occurs.
Many problems should be considered when deciding whether or not to balance the tool, which is often overlooked by many users. One of the most critical issues is the damage to the spindle bearings of the machine tool caused by tool imbalance during high-speed machining. Unbalanced tools add an extra load to the spindle, causing premature wear and failure of the spindle bearings. This not only wastes the cost of repairing and replacing the spindle bearings, but also causes loss of production time due to downtime.
Unbalanced tools can degrade tool performance and life, and can also cause chatter and degrade the quality of the machined surface.
There are many potential imbalances in cutting tools and toolholders, especially the 7:24 taper. When the spindle speed increases, centrifugal force will cause the spindle hole to swell slightly, and the taper shank of the arbor may not expand with the spindle, which will reduce the contact area between the arbor and the spindle, thereby reducing the firmness of the connection.
By changing the design and carefully selecting the tool holders and tools, some unbalanced factors can be corrected. This means that symmetrical designs can avoid imbalances to some extent. However, for adjustable arbors with moving parts and asymmetric arbors, a tool balance adjustment system is required. When assembling tools for high-speed machining, careful consideration should be given to the design factors of the taper shank, the balance adjustable factors, and the choice of tool accuracy and symmetry.
Taper shank taper size tolerance has a great impact on the performance of high-speed machining tools. The degree of accuracy of the taper can usually be referred to the international standard ISO 1947. The taper tolerance is used to check the tolerance of the spindle locking hole and the taper taper that affect the balance and runout of the tool bar. The spindle taper tolerance grade of the machine tool is generally AT2, but AT3 is better.
In addition to taper tolerances, other factors need to be considered in tool assembly. Each of these tolerances adds up to cause the tool to decenter. These factors include the round runout of the tool, the length of the tool holder system, and the symmetry of other parts (such as chucks, snap rings, etc.).
In high-speed machining, we must be clear about the causes of imbalances and measures to reduce imbalances. One of the most influential is the tool holder. In high-speed machining, a balanced design tool holder or adjustable balance tool holder should be used preferentially. The balanced design tool holder is designed to guarantee the balance of the tool holder, but it cannot compensate the imbalance caused by other parts during the assembly process. At this time, only assuming that the imbalance caused by other parts is negligible, it is often not feasible in actual production. Therefore, we recommend measuring the amount of unbalance when using this type of tool holder and ensuring that it is within the allowable range of the standard.
In addition to the design guarantee, the adjustable balance tool holder also compensates for the imbalance caused by other parts during the assembly process. Manufacturers have used many methods to balance the tool holder, such as the use of screws, steel balls, adjusting rings, and adjusting blocks.
The ability of the adjustable balance toolholder to compensate for unbalance is limited. In some cases, the unbalanced mass is so large that the tool cannot be rebalanced. As a result, the machine tool cannot be operated at the set rotation speed. In this case, the machine tool must be reduced. Speed ​​and reassemble the tool.
Other factors constraining high-speed machining include workpiece material, workpiece overhang, fixtures, tool profile, depth of cut, etc.
The balancing process can improve the mass distribution of the tool and the tool holder so that the two produce the smallest unbalanced centrifugal force when they rotate. However, the balance process cannot involve all the unbalanced factors.
Any degree of unbalanced quality will cause vibration. Tool holders and their installation, machine structure, etc. will affect the characteristics of vibration.
ISO 1940/1 and ANSI 2.19-1989 are standards that define the degree of balance. Both standards are on a practical basis and the "G" rating system is used to define the balance requirements. For example, a balance of G2.5 means that the circumferential vibration speed does not exceed 2.5 mm/s. These levels continue to increase with the emergence of high-quality machine tools, tools and tool holders. When the specified balance cannot be achieved, the spindle speed should be reduced.
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