Tool balance during high-speed machining

High-speed machining (HSM) has many advantages, but in order to reduce cutting costs and maximize the advantages of high-speed machining, many difficulties need to be overcome in actual production. In high-speed machining, when the cutting speed increases, the cutting force will decrease. 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 of high-speed machining, the spindle speed can reach 500000r/min in some tests, and it can reach 100000r/min in actual production. Because when the spindle speed exceeds 8000r/min, the rotating parts need to be balanced, so high-speed machining can be defined as the speed not less than 8000r/min. When the spindle speed is lower than 8000r/min, only when the mass distribution of the tool bar is very unbalanced, the centrifugal force will have an adverse effect on processing.

As the number of machine tools with spindle speeds higher than 8000r/min continues to increase, the balance of combined tools becomes extremely important, which involves tools and tool holders. When the theoretical center axis of the rotating tool bar is not concentric with the actual axis of rotation, an imbalance will occur.

There are many issues that should be considered when deciding whether to balance the tool, and under normal circumstances many users tend to ignore these issues. One of the most critical problems is the damage to the spindle bearings of the machine tool due to tool imbalance during high-speed machining. An unbalanced tool will add an extra load to the spindle, causing premature wear and failure of the spindle bearing. This not only results in waste of repair and replacement of spindle bearings, but also loss of production time due to downtime. Guide: (The advantages of high-speed milling mold CNC machining center are beginning to show)

Unbalanced tool will reduce tool performance and life, and also cause chatter, which will deteriorate the quality of the machined surface.

Cutting tools and tool chucks (especially 7:24 taper shanks) have many potential unbalance factors. When the spindle speed increases, the centrifugal force will cause the spindle hole to expand slightly, and the taper shank of the tool bar cannot expand with the spindle, which will reduce the contact area between the tool bar and the spindle, thereby reducing the firmness of the connection.

Some unbalance factors can be corrected by changing the design and carefully selecting the tool holder and tool. This means that symmetrical design can avoid imbalance to a certain extent. However, for adjustable toolholders with moving parts and asymmetric toolholders, a tool balance adjustment system is required. When the assembled tool is used for high-speed machining, careful consideration should be given to the design factors of the taper shank toolholder, the balance adjustable factors, and the selection of tool accuracy and symmetry.

The taper size tolerance of the taper shank toolholder has a great influence on the performance of high-speed machining tools. The accuracy grade of taper can usually refer to the international standard ISO1947. The taper tolerance is used to check the tolerance of the spindle lock hole and the taper of the tool bar that affects the balance and runout of the tool bar. The machine tool spindle taper tolerance grade is generally AT2, but AT3 is better.

In addition to the taper tolerance, other factors need to be considered in the tool assembly. Each tolerance will add up to cause the eccentricity of the tool. These factors include the circular 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 aware of the factors that cause imbalance and the measures to reduce the imbalance. The most influential one is the tool holder. In high-speed machining, the balanced design toolholder or the adjustable balanced toolholder should be used first. The balanced design of the tool holder is designed to ensure the balance of the tool holder, but it cannot compensate for the imbalance caused by other parts during the assembly process. At this time, it is only assumed that the imbalance caused by other parts is negligible, but it is often not feasible in actual production. Therefore, we recommend measuring the unbalance when using this tool holder, and as far as possible to ensure that it is within the range allowed by the standard.

In addition to the design guarantee, the adjustable balance tool holder can also compensate the imbalance caused by other parts during the assembly process. Manufacturers have adopted many methods to balance tool holders, such as using 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 too large to restore the balance of the tool, resulting in the machine tool not being able to run at the set speed. Need to reduce the speed of the machine tool and reassemble the tool.

Other factors restricting high-speed machining include workpiece material, workpiece overhang, fixture, tool profile, cutting depth, etc.

The balance process can improve the mass distribution of the tool and the tool holder, so that the smallest unbalanced centrifugal force is generated when the two rotate. However, the balancing process cannot involve all unbalanced factors.

Any degree of unbalanced mass will cause vibration. Tool holder and its installation, machine tool structure, etc. will affect the characteristics of vibration.

ISO1940/1 and ANSI2.191989 are the standards that define the degree of balance. Both standards are on a practical basis and apply the G-level system to stipulate balance requirements. For example, the degree of balance G2.5 means that the circumferential vibration speed does not exceed 2.5mm/s. These levels continue to increase with the advent of high-quality machine tools, tools and toolholders. When the specified balance cannot be achieved, the spindle speed should be reduced.

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