摘要
刀尖点动力学特性直接影响切削稳定性和加工表面的质量。现阶段对主轴运行状态下刀尖点动力学特性的理论研究和实验研究分别存在着建模复杂和设备昂贵等局限性。为此,提出一种基于半理论法的主轴运行状态下刀尖点动力学行为分异特征辨识方法。该方法将分异特征辨识转化为一类优化设计问题,即以不同转速下刀尖点动力学特性参数为变量、以极限切深和颤振频率的实验标定值与理论预测值的偏差之和最小为目标构建优化模型,并借助粒子群退火优化算法进行求解,从而获得在不同转速下刀尖点动力学行为的分异特征及规律。以某型立式加工中心为平台,通过变切深铣削实验,对所提出的辨识方法进行验证,结果显示极限切深预测值与标定值吻合度较高。在不需复杂建模和昂贵实验设备的条件下,利用所提出的方法能够准确预测运行状态下刀尖点动力学行为分异特征,实现切削稳定性的精准预测,为进一步提高铣削加工质量和效率提供理论基础和数据支撑。
切削颤振严重制约着切削效率和表面加工质量,亦是产生机床故障、刀具磨损等问题的主要原
针对刀尖点动力学特性及其影响因素,国内外学者进行了较为深入的研究。Iglesias
不难看出,在现阶段针对主轴运行状态下刀尖点动力学行为的研究主要借助实验检测或数字化分析。实验研究可以避免复杂的理论建
为此,本文以某立式加工中心为研究对象,以频域零阶近似法为切削稳定性预测理论基础,考虑在主轴运行状态下刀尖点刚度的变化,提出一种基于半理论法的主轴运行状态下刀尖点动力学行为分异特征辨识方法。该方法将分异特征辨识转化为一类优化设计问题,即以不同转速下刀尖点动力学特性参数为变量、以极限切深和颤振频率的实验标定值与理论预测值的偏差之和最小为目标构建优化模型,借助粒子群退火优化算法对其进行求解,从而获取在不同转速下刀尖点动力学行为的分异特征及规律。
Budak
(1) |
(2) |
式中:N为铣刀齿数;Kt为铣削力系数;T为齿通周期;,其中ΛR和ΛI分别为闭环铣削系统特征值的实部和虚部,其须借助刀尖点频响函数进行求解。
以刀尖点x向的频响函数Gxx为例,利用固有频率ω、阻尼比ζ和刚度k可将其表示为:
(3) |
其中:
(4) |
式中:j为模态阶数,j=1,2,…,m;i为虚数单位。
刀尖点动力学特性参数共有6个。由1.1节可知,极限切深ɑlim和颤振频率ωcha均为刀尖点非对称动力学特性参数的函数,即:
(5) |
式中:和分别为刀尖点x和y方向的固有频率、阻尼比和刚度。
利用主轴静止状态下(即n=0 r/min)刀尖点动力学特性参数,可对其极限切深alim_pre(0)和颤振频率ωcha_pre(0)进行预测,但所得结果并未考虑刀尖点动力学行为随转速变化的分异特征;利用主轴运行状态下(n≠0 r/min)刀尖点动力学特性参数,则可以预测考虑分异特征的极限切深alim_pre(n)和颤振频率ωcha_pre(n)。此外,通过变切深切削噪声测试,可以对不同转速下实际极限切深alim_cal(n)和颤振频率ωcha_cal(n)进行标定。所得标定结果是对不同转速下切削过程的真实反映。
因此,为准确辨识刀尖点动力学行为随转速变化的分异特征,基于半理论法,结合理论预测与实验标定结果,将不同转速下刀尖点动力学特性参数的辨识问题转化为如
(6) |
式中:Θn为待求优化变量,即在转速n下的刀尖点动力学特性参数:
(7) |
式中:分别为极限切深、颤振频率的实验标定值与理论预测值之间的偏差,即:
(8) |
利用粒子群退火优化算
由上所述,可建立在不同主轴转速下刀尖点动力学行为分异特征辨识方法,如

图1 刀尖点动力学行为分异特征辨识方法
Fig.1 Identification method of dynamics behavior differential characteristic of tool tip
在VMC850立式加工中心上进行刀尖点锤击模态实验,以获取静态刀尖点动力学特性参数。刀尖点锤击模态实验平台如

图2 刀尖点锤击模态实验平台
Fig.2 Tool tip hammering modal experimental platform
在VMC850立式加工中心上搭建切削噪声测试平台,如

图3 切削噪声测试平台
Fig.3 Cutting noise test platform
当n=3 000 r/min 时变切深铣削实验结果如

图4 n=3 000 r/min 时变切深切削实验结果
Fig.4 Experimental results of variable cutting depth milling when n=3 000 r/min
类似地,当n=3 300, 4 000 r/min时对噪声信号和加工表面进行分析,可得各转速下切削稳定性标定值,如
根据1.2节所提出的方法,辨识得到不同转速下刀尖点各向模态参数,如
刀尖点各向模态参数随转速的分异规律如

图5 刀尖点各向模态参数随转速的分异规律
Fig.5 Variation law of tool tip modal parameters with rotating speed
利用
① 括号内数值为机床实际极限切深与预测极限切深的相对误差。
根据
此外,分析

图6 刀尖刚度对极限切深的影响
Fig.6 Influence of tool tip stiffness on limit cutting depth
1)提出了在主轴运行状态下刀尖点动力学行为分异特征的辨识方法。利用该方法能够准确获取在主轴运行状态下刀尖点的动力学特性及其分异规律,实现切削稳定性的精准预测。
2)利用所提出的辨识方法对所得到的各转速下刀尖点动力学参数进行对比分析,结果表明,刀尖点各向模态固有频率和模态刚度均随转速的增大而减小,刀尖点各向模态阻尼比随转速变化而产生分异,但未呈明显的分异规律。
3)通过对比分析考虑及未考虑刚度分异时切削稳定性的预测精度,结果表明,考虑刚度分异时切削稳定性的预测精度相比于未考虑刚度分异时的预测精度有明显提升,说明了在辨识过程中考虑刚度分异的必要性。
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