Machine tool users often hear that they have to balance the handles they use, but they don't know if this is correct. If the tool holder is not balanced, will it damage the machine or workpiece? The correct question is not " Should the tool and the tool holder be balanced?" but "When should the tool and the tool holder be balanced".
Let us first explain the imbalance. In the machine tool spindle, there are many rotating parts, including the spindle itself. If the motor is not integrated with the spindle, it also includes the rotor, gear or pulley of the motor. In milling, there are shanks, knives, tie rods (pull), collets and a set of Belleville washers. In turning, the workpiece is also a swivel component and may also include gripping devices (such as chuck jaws).
All of these components are made and assembled by humans and are therefore not perfect. The center of mass of the assembled rotor is not at the same center as the center of rotation (as determined by the spindle bearings that support the rotor). The net effect produced is equivalent to the addition of a quality that deviates from the center of the spindle. The size of the unbalanced mass and its distance from the center of rotation play the same role, so they are usually grouped together. The amount of unbalance can be expressed in grams x millimeters (gmm) or ounces x inches (oz. in.).
In most cases, the user does not have the ability to adjust the balance of the spindle. If you have this capability, you need to adjust the balance between the shank and the spindle at the same time. The allowed unbalance amount U is calculated by the following formula:
U(gmm)=(G×9,549×W)/rpm
In the formula, G is the balance level, 9,549 is the parameter consistent with the unit, W is the mass of the rotating object (kg), and rpm is the spindle speed.
The balance level G depends on the processing requirements. For example, in general, the general machine tool has a balance level of G6.3, the high-speed machine tool is G2.5, and the precision machine tool is G1.0. The allowable amount of unbalance U depends mainly on the mass of the object being balanced. For the tool, the tool and the tool holder, and the assembled entire spindle, the U value is calculated differently.
A more important consideration for different machine tools is whether the force generated by the amount of unbalanced rotation accounts for a significant portion of the force generated by the machining process. All forces, whether cutting forces or unbalanced forces, must be absorbed by the bearings and the joint between the tool and the spindle. In roughing, the cutting force can be as high as several hundred Newtons, and the imbalance of a few Newtons does not matter. However, in finishing, the cutting force is only a fraction of a Newton, and the same imbalance is a big problem. So how should the imbalance be calculated? The formula for calculating the imbalance force F is:
F=meω2
In the formula, m is the unbalanced mass, e is the eccentric amount (the distance from the unbalanced mass to the center of rotation), and ω is the spindle speed.
The imbalance force F increases with the square of the spindle rotational speed ω. If the spindle speed is doubled, the imbalance caused by the unbalanced amount of rotation will increase by a factor of four. For example, the unbalance of the tool and the shank is 5 gmm, and when the spindle speed is 5,000 rpm, the resulting imbalance force is 1.37 Newtons. When the spindle speed is 40,000 rpm, the same tool and tool holder produces an unbalanced force of 87.7 Newtons.
It can be beneficial to reduce the imbalance by balancing, but the balancing operation also costs. The shop either needs to buy a balancing machine or pay for the balance service. The balance shank usually has one or more weight rings, and by adjusting its position, the balance can be changed. Compared to conventional shanks, such shanks are more expensive and require extra length on the shank to accommodate the weight ring, while increasing the length of the shank reduces its stiffness and generally reduces its reach. Metal removal rate.
An effective strategy for determining whether a tool holder needs to be balanced is to compare the imbalance force to the cutting force. If the unbalanced force reaches more than 5% of the cutting force, it is worth improving its equilibrium state. In general, for machines with lower spindle speeds, the importance of balance is relatively small, and the balancing problem of high-speed cutting machines is much more important. Even with high-speed machines, many workshops can use pre-balanced shanks and pre-balanced tools to solve the balancing problem. However, for very precise and high speed machining, it is still necessary to balance the assembled tool holders and tools.
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