What balance grade is required for high-speed spindles?

G2.5 is the most common grade for machine tool spindles up to about 20,000 RPM. For spindles running above 20,000 RPM (HSC machining), G1.0 or G0.4 is recommended. The tighter grade reduces centrifugal force and improves surface finish.

How does spindle unbalance affect surface finish?

Unbalance creates a centrifugal force that deflects the tool. This deflection appears as waviness on the workpiece surface. At high spindle speeds, even a few g·mm of unbalance can cause micron-level tool deflection, degrading surface finish by several Ra grades.

Should I balance the toolholder with the tool installed?

Yes, for best results the complete assembly (toolholder + tool + retention knob + collet + cutter) should be balanced together. Each component adds its own unbalance contribution, and the total determines the actual vibration during cutting.

What is the difference between HSK and BT toolholders for balance?

HSK toolholders have a hollow taper shank with face contact, providing better radial stiffness and repeatability than BT (7/24 taper). HSK is preferred for high-speed applications because its dual contact reduces runout and makes balance more repeatable between tool changes.

How do I measure spindle unbalance?

Spindle unbalance is measured using vibration sensors mounted near the spindle bearings while the spindle runs. A portable balancing instrument (like Vibromera Balanset) measures the 1× vibration amplitude and phase, then calculates the required correction mass and angle.

免費工程工具

機床主軸不平衡計算器

依據 ISO 21940 計算主軸與刀柄允許的不平衡量、偏心量（nm）、離心力及等效刀具撓度。

ISO 21940-11 HSK / BT / CAT G0.4 – G2.5

結果

許用不平衡量

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偏心

—

離心力

—

等效刀具撓度估算

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角速度 ω

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允許之主軸不平衡量（ISO 21940）

G — 平衡品質等級 (mm/s)
m — 主軸與刀柄組件質量（kg）
ω = 2π·n/60 — 角速度 (rad/s)

偏心量（單位不平衡量）

對於高速主軸，偏心量通常以奈米表示（1 μm = 1000 nm）。

離心力與刀具撓度

不平衡產生的離心力作用於主軸軸承，導致刀具尖端產生撓度。撓度估算採用簡化的主軸剛性模型（HSK-63 典型值約為 20 N/μm）。

機床平衡等級

等級	應用	典型轉速
G0.4	超高精度研磨、光學	> 60,000 RPM
G1.0	研磨機、高速銑削	15,000–40,000 RPM
G2.5	一般加工機床、車床	5,000–24,000 RPM

實務範例

範例 — 24,000 RPM 下的 HSK-63 刀柄

已知： 質量 = 2 kg，轉速 = 24,000 RPM，等級 = G2.5

ω = 2π × 24000 / 60 = 2,513.3 rad/s

U_每 = 2.5 × 2 × 1000 / 2513.3 = 1.99 g·mm

e_每 = 2.5 × 1000 / 2513.3 = 0.995 μm = 995 nm

F = 1.99 / 1e6 × 2513.3² = 12.6 N

⚠️ 注意： 刀具撓度估算採用簡化的剛性模型。實際撓度取決於軸承預壓、主軸設計、刀具懸伸量及夾持方式。僅供粗略參考。