Optimizing Rotary Toolholder Performance: Navigating the Nuances of Balancing Quality and Tolerance6/12/2024  With rotating bodies, imbalance is an omnipresent phenomenon. A typical example are rotating tools on machine tools. Because unbalance creates a centrifugal force, it increases linearly with the unbalance and squares with the number of rounds. The faster the rotor rotates, the more noticeable the unbalance. But how does unbalance arise, how can it be measured and how can it be eliminated by balancing? On the following page we have put together the theoretical fundamentals of balancing, the basis for tool balancing. As a side note to this discussion, there is sometimes confusion over use of the words "imbalance" and "unbalance". Imbalance is the noun meaning the state of being not balanced, while unbalance is the verb meaning to cause the loss of balance. Causes of Unbalance In rotating toolholder, addressing imbalance is crucial for efficiency and longevity. Imbalances arise from uneven mass distribution, design asymmetry, or manufacturing inaccuracies, leading to detrimental centrifugal forces during operation. Balancing, achievable through mass adjustment or component alignment, mitigates these forces. This process involves precise measurement and correction to adhere to industry quality standards, though achieving absolute balance is challenging due to inherent mechanical and environmental factors. Here's a quick overview:
What is Unbalance? Static balance in rotary toolholders refers to the condition where the center of gravity is aligned with the axis of rotation, ensuring no lateral movement when the toolholder is stationary or rotating. Dynamic balance, on the other hand, addresses both the static imbalance and the couple imbalance, ensuring that the toolholder remains stable and does not wobble during high-speed rotation, which involves balancing across two planes. Achieving dynamic balance is crucial for toolholders in high-speed applications to minimize vibration and wear. Let's dig into some details: STATIC UNBALANCE Static unbalance in rotary toolholders manifests when their center of gravity doesn't align with the axis of rotation, leading to centrifugal forces that can cause significant issues when the rotor spins. This kind of imbalance can be detected even in non-moving rotary toolholders and is correctable by adjusting the mass distribution within a single plane. However, correcting static unbalance (a.k.a. Single Plane Balancing) might not address other types of imbalance, like couple unbalance, that can still affect the rotor's operation.
COUPLE UNBALANCE Couple imbalance occurs when a toolholder's center of gravity is aligned with its rotation axis but still experiences a tilting moment during operation due to the distribution of mass. This condition, detectable only when the toolholder is in motion, results from opposing centrifugal forces that create no lateral movement but cause a rotational tilt.
DYNAMIC UNBALANCE Dynamic imbalance in a rotary toolholder refers to a scenario where both static imbalance and couple imbalance are present, causing the toolholder to experience tilting and wobbling during operation. This complex form of imbalance necessitates correction in two planes for the toolholder to rotate smoothly at varying speeds, thereby reducing vibration and wear on associated machinery. In case you were wondering the ideas of dynamic balance was developed by R.S. Berkof, G.G. Lowen in the seminal work A New Method for Completely Force Balancing Simple Linkages. What is "Balancing"?
BALANCING IN A SINGLE PLANE (STATIC BALANCING) Balancing in a single plane corrects the static imbalance by adjusting the toolholder's center of gravity to align with its axis of rotation, minimizing eccentricity. However, this method doesn't correct the couple imbalance associated with dynamic unbalance, which remains unaffected. BALANCING IN TWO PLANES (DYNAMIC or DUAL PLANCE BALANCING) Dynamic or dual plane balancing involves correcting both static and couple imbalances in a toolholder, achieving thorough compensation. The process allows for the selection of any two balancing planes, ideally positioned as far apart as possible, to ensure comprehensive balance throughout the toolholder's operation. Measuring Unbalance The first step in correcting imbalance is to determine where unbalanced mass is located in a single plan or 3 dimensional plane. To do this, the tool holder is inserted into the balancing spindle and made to rotate.
Balancing Quality - G Values The two primary factors to determine permissible unbalance, also called the balancing tolerance, are the mass of the rotating part (G) and the maximum operational speed (RPM). To put a more technically, the grade G values in toolholder balancing are determined based on the permissible residual unbalance for a given rotation speed and the weight of the rotary toolholder, as outlined in standards such as DIN ISO 1940-1 (previously VDI guideline 2060). These values guide the maximum allowable unbalance to ensure operational safety and efficiency, factoring in specific operational conditions. Each grade G value (previously: Q) corresponds to a different level of balancing precision, tailored to the rotor's intended use and operating environment.
Achievable Accuracy Keep in mind that the balancing quality grade is only valid for a specific rotation speed of the the toolholder. e.g. G2.5 at 25,000 RPM. This permissible residual unbalance is calculated from the balancing quality grade, the rotation speed and the weight of the toolholder. Here's the actual formula: Uper = (G•M)/n • 9549
HERE IS AN EXAMPLE
In the above example there is a permissible residual unbalance of 1.3 gmm. To illustrate this value it is useful to convert the unbalance to eccentricity. Uper = M • eper eper = Uper/M =1.3 gmm/800g = 0.0016 mm = 1.6 μm Therefore the centre of gravity of the tool holder can be offset by max. 1.6 μm from the axis of rotation. During balancing the axis of rotation is assumed to be the axis of the taper or HSK. However, in the milling machine the tool rotates about the axis of the spindle. Even new spindles TIR of up to 5 μm (equivalent to eccentricity of e = 2.5 μm). ANOTHER EXAMPLE Balancing quality G = 1 Rotation speed n = 40.000 rpm Tool weight M = 0.8 kg Uper = 0.2 gmm eper = 0.3 μm This permissible eccentricity cannot be achieved in practice. Even good spindles have a repeatability of 1-2 μm when the tool is changed. Even small amounts of dirt, grime or swarf can worsen the result significantly. Complexities of Achieving |
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