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在激光修整凹圆弧轮廓金刚石砂轮的过程中,激光光斑辐射在轮廓表面的分散性将影响激光功率密度的分布,从而极大地影响轮廓效率和精度,本文作者分析了激光修整凹形面金刚石砂轮斜面误差的来源,提出了激光遮蔽效应和激光斜面分散效应。
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使用功率100 W、波长1064 nm的光纤纳秒激光修整金刚石砂轮的主要机制是利用高能量汽化材料和气体的压力喷出砂轮内部的熔融材料。有效激光能量密度的大小决定了能否有效去除砂轮内的材料。只有当激光有效能量大于材料的去除阈值时,才能去除材料。激光的有效能量密度与激光照射在材料被加工部位的能量密度和吸收率有关,如果工艺位置保持不变,则认为材料对激光的吸收率是恒定的。
在激光加工过程中,等离子体和被蒸发的物质形成一个遮挡层[18]。这部分遮挡层阻挡了部分激光能量。但是,可以通过吹辅助气体来避免这种激光能量的损失。
如图 1所示,h表示加工深度;hi表示第i道工序的加工深度; Li表示第i道工序的剩余材料;rhi表示第i道工序激光束在最外层旋转表面的光斑半径;r0表示在激光束在最内层的光斑半径。
当激光用于加工一定深度h1、且激光光斑处于第1道工序时,照射在加工表面的激光能量密度为I1,如图 2所示。图中, R为平行砂轮半径,rs表示激光光束扩张半径。最外层旋转面上的激光束半径将大于加工平面中焦点处的激光束半径,然后最外层旋转面上的材料将阻挡部分激光束,从而改变落在聚焦面上的激光能量。由图 2可知,激光能量在切向上的变化大于径向,即在相同的修整深度下,切向发生的能量损失更多。金刚石砂轮半径越大,这种现象就越明显。在第1道工序,由于遮蔽导致加工面的能量减少形成一定的剩余材料。
图 2 激光焦距变化H和最外侧旋转面的激光光束扩张半径rs与加工深度h的关系
Figure 2. Laser focal length change H and laser radius rs of the expanding laser beam at the outermost rotary surface vs. processing depth h
当激光修整达到一定深度时,即处于第2道工序。一部分激光能量被最外层的旋转面阻挡,一部分激光能量被第1个位置的剩余材料阻挡。未被加工的存留材料会对激光进行遮挡,遮挡效应加剧,造成能量损失。当激光在第i道工序中不断移动时,激光被最外层的旋转面和前一道堆叠位置i-1处的剩余材料遮蔽。
最外层旋转面激光光斑半径变化越大,激光的能量被最外层旋转面阻挡得越多,导致残留物质积聚得越多。上述分析中采样间隔将足够小,使剩余材料处于边缘, 在连续堆积状态下,逐渐形成堆积的坡度。这种现象被称为激光遮蔽效应。
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凹形成形砂轮的修整精度还受到激光束与砂轮表面位置关系的影响。在激光整形斜边轮廓砂轮过程中,激光束在砂轮表面的分散程度会影响激光束的能量密度,从而会极大影响砂轮的修整效率和修整精度。由于聚焦的激光束具有一定的瑞利长度,因此平行金刚石轮表面上的激光光斑面积可以近似描述为一个小圆柱与另一个较大圆柱的相交面积。当砂轮表面为斜边轮廓时,激光光斑面积可近似描述为小圆柱形与圆锥形的相交面积。
如图 3所示,x, y, z代表坐标位置,当激光修整平面平行于旋转轴时,照射在砂轮上的光束可以等效于照射在圆柱形表面上的光束,如WANG等人[18-21]所述,焦激光在平行砂轮表面的辐照光斑面积S1。如图 4所示,当激光对斜面进行修整时,照射在砂轮上的光束可以等效于照射在圆锥平面上的光束,激光辐照在斜面轮廓砂轮表面的光斑面积S2。S1和S2计算公示如下所示:
$ S_1=\int_{A-r}^{A+r} \mathrm{~d} x \int_{-\sqrt{r^2-(x-A)^2}}^{\sqrt{r^2-(x-A)^2}} \frac{R}{\sqrt{\left(R^2-x^2\right)}} \mathrm{d} y $
(1) $ \begin{gathered} S_2= \\ \int_{A-r}^{A+r} \mathrm{~d} x \int_{-\sqrt{r^2-(x-A)^2}}^{\sqrt{r^2-(x-A)^2}} \frac{\left(1+\cot ^2 \alpha\right)(R-y \cot \alpha)}{\sqrt{(R-y \cot \alpha)^2-x^2}} \mathrm{~d} y \end{gathered} $
(2) 式中: r为激光焦斑半径; α为砂轮斜边角; A为激光辐照位置。
当激光照射圆柱面时,激光束会被圆柱面沿周向分散,因此需要一定的激光切深量,以保证激光能量大部分集中,且激光能量足以去除材料。如果入射光束不偏转,再照射锥形表面,那么激光束不仅会沿周向分散,还会沿轴向分散。色散程度可以用激光光斑沿圆锥表面的面积来表示,S2可以表示为:
$ S_2 \approx S_1\left(1+\cot \alpha+\cot ^2 \alpha\right) $
(3) 在激光切向修整复杂轮廓的过程中,激光遮蔽效应和激光斜面分散效应都将被引入。上述两种误差造成的剩余材料面积称为残余面积。随着加工深度的增大,激光遮蔽效应对轮廓的影响更大,对于斜面斜边较陡的斜面,激光斜面分散效应更为突出。激光遮蔽效应和激光斜面分散效应综合导致廓形精度较差,因此在金刚石砂轮切向修整时必须抑制上述误差。
偏转激光切向修整凹形面金刚石倒角砂轮的研究
Research on diamond chamfer grinding wheel for tangentially dressing concave surface with deflection laser
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摘要: 为了减小凹形面金刚石砂轮的激光修整误差,建立激光遮蔽效应和激光斜面分散效应误差模型,分析了激光切向修整凹形面金刚石砂轮的误差来源,提出了偏转激光修整凹形面成形砂轮法,并通过理论分析和实验验证,探索了修整参数对凹形面砂轮轮廓精度和圆弧半径的影响。结果表明,在1°~1.5°的偏转角范围内,斜边轮廓修整精度为8 μm;在合适的偏转角度下,圆弧半径误差小于10 μm;在修整凹弧半径为0.2 mm和0.5 mm、补偿半径分别为0.03 mm和0.06 mm时,凹弧半径误差分别缩小0.02 mm和0.03 mm。偏转激光修整法有效地提高了修整精度,减小了修整误差,为凹形面金刚石修整提供了新思路。Abstract: In order to reduce the laser dressing error of concave-faced diamond wheels, the error models of laser masking effect and laser bevel dispersion effect were established, the error sources of laser tangential dressing of concave-faced diamond wheels were analyzed, and the deflection laser dressing method of concave-faced forming wheels was proposed. Through theoretical analysis and experimental verification, the influence of dressing parameters on the contour accuracy and circular radius of concave-faced wheels was explored. The results show that in the deflection angle range of 1°~1.5°, the beveled edge profile dressing accuracy is 8 μm; under the suitable deflection angle, the circular arc radius error is less than 10 μm; when dressing the concave arc radius of 0.2 mm and 0.5 mm, the compensation radius is 0.03 mm and 0.06 mm respectively, the concave arc radius error is reduced by 0.02 mm and 0.03 mm respectively. The deflection laser dressing method effectively improves dressing accuracy, reduces the dressing error, and provides a new idea for the dressing of concave diamonds.
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