The notion that a balanced toolholder assembly is essential for chatter-free machining at high spindle speeds is an easy concept to grasp. An unbalanced rotating assembly creates centrifugal forces that increase by the square of the machine’s spindle speed. These centrifugal forces create greater vibration at high spindle speeds, which causes poor surface finish and decreases tool and spindle life.
Similarly, machine tools that operate at relatively low spindle speeds can benefit from using balanced toolholder assemblies. In fact, the reason why a machine operates at a low spindle speed may be because the toolholder assembly is unbalanced. In these circumstances, it can be difficult to improve cycle times because any increase in spindle speed likely would drastically reduce cutting tool life.
There is value in using pre-balanced toolholders on both high- and low-speed machines. That said, Brendt Holden, president of Haimer USA (Villa Park, Illinois), believes shops should consider the importance of balancing all toolholders after the cutting tool and other related components have been installed. Depending on the application, it may be necessary to re-balance even a balanced toolholder after adding pull studs, collets, clamping nuts, data chips and so on. This is often the case when using inherently unbalanced asymmetrical tools, such as a tool that has a flat on its shank.
In addition to improved cutting tool life and surface finish, Mr. Holden offers the following reasons why shops should consider an in-house balancing system for toolholder assemblies used on low-rpm machine tools.
- Spindle life can be improved in production environments. Spindle problems tend to occur more frequently when using unbalanced toolholder assemblies in high-production machining applications. The centrifugal forces of an unbalanced assembly can cause more frequent spindle repairs, leading to additional service cost and production loss because of extended machine downtime. A study of 12,000-rpm machines at a Haimer production facility showed balanced toolholder assemblies could save $20,000 per spindle each year by increasing spindle life, cutting tool life and spindle speed.
- Spindle speeds can be increased. The purchase of a high speed machine tool sometimes spurs shops to add a toolholder balancing system. Shops that also use the balancing system for toolholders on older, slower machines typically notice improved performance, Mr. Holden notes. For example, an 8,000-rpm machine might operate at only 4,000 rpm for a particular application because of vibration problems (hence a slower feedrate and material removal rate). When using a balanced toolholder assembly, the machine can run effectively at higher speeds, yielding greater production capabilities.
- Tooling mix-ups can be eliminated. Using balanced toolholder assemblies on all shop machines eliminates the chance that an unbalanced toolholder assembly is mistakenly used in a high speed machine, which can quickly cause damage to the machine’s spindle.