切削加工中影响表面粗糙度因素的分析与对策应用科学

作者: admin 日期: 2016-04-22 13:42:41 人气: - 评论: 10740

   各种机床都有其最经济、最适合达到的表面粗糙度范围如果要求达到的粗糙度水平超过其经济水平将导致成本急剧上升如果要求达到的粗糙度水平太低则会造成资源浪费。因此要综合考虑与分析切削加工中影响表面粗糙度的各种因素包括刀具的选择与利用、切削速度和进给量等来达到要求的表面粗糙度。

  一、切削加工时影响表面粗糙度的主要因素分析

  1.刀具几何形状及切削运动的影响

  刀具相对于工件作进给运动时在加工表面留下了切削层残留面积从而产生了表面粗

糙度残留面积的形状是刀具几何形状的复映其高度受刀具的几何角度和切削用量大小的影响。一般使用直线刀刃切削时=f/ctgkr+ctgkr使用圆弧刀刃切削时=2/8ε。从式中可知减少进给量、主偏角、副偏角’以及增大刀尖圆弧半径

均可减小残留面积的高度从而降低表面粗糙度。

  2.积屑瘤的影响

  在切削过程中当刀具前刀面上存在积屑瘤时由于积屑瘤的顶部很不稳定容易破裂一部分连附于切屑低部而排出一部分则残留在加工表面上使表面粗糙度增大。

  3.工件材料性质的影响

  加工塑性材料时由于刀具对加工表面的挤压和摩擦使之产生较大的塑性变形加之

刀具迫使切屑与工件分离时的撕裂作用使表面粗糙度值加大。一般来说材料塑性变形趋

势越大或韧性越大被加工表面粗糙度就越大。切削脆性材料时塑性变形小些控制好切屑崩碎现象就容易达到表面粗糙度的要求。对于同样的材料金相组织越是粗大切削加工后的表面粗糙度值也会越大。

  4.切削用量的影响

  切削用量中切削速度对表面粗糙度的影响比较复杂。在切削塑性材料时一般情况下低速或高速切削时因不会产生积屑瘤加工表面粗糙度值较小。但在中等速度下塑性材料由于容易产生积屑瘤与鳞刺且塑性变形较大因此表面粗糙度值会变大。加工脆性材料时由于塑性变形很小主要形成崩碎切屑切削速度的变化对表面粗糙度影响较小。减少进给可以减少切削残留面积高度使表面粗糙度值减小。但进给量太小刀刃不能切削而形成挤压增大了工件的塑性变形反而会增大表面粗糙度值。一般背吃刀量对表现粗糙度影响不明显。

  5.鳞刺

  鳞刺是指已加工表面上鳞片状的毛刺是用高速钢刀具低速切削时经常见到的一种现象。鳞刺一般是在积屑瘤增长阶段的前期里形成的。甚至在没有积屑瘤的时候以及在更低一些的切削速度范围内也有鳞刺发生。刀具的后角小的时候特别容易产生鳞刺。鳞刺对已加工表面质量有严重的影响它往往使表面粗糙度等级降低2-4级。鳞刺的成因是前刀面上摩擦力的周期变化造成的。

  6.振动

  切削过程中如果有振动表面粗糙度就会显著变大。振动是由于径向切削力太大或工件系统的的刚度小而引起的。

  7.其他因素

  副切削刃对残留面积的挤压使残留面积向与进给相反方向变形使残留面积顶部歪斜而产生毛刺加大了表面粗糙度。过渡刃圆弧部分的切削厚度是变化的近刀尖处的切削厚度很小。当进给量小于一定限度后这部分的切削厚度小于刃口圆弧所能切下的最小厚度时就有部分金属未能切除就会使表面粗糙度增大。切削脆性材料时产生崩碎切屑切屑崩碎时的裂缝深入到已加工表面之下使粗糙度增大。此外排屑状况、机床设备的精度和刚度等也会影响已加工表面的表面粗糙度。

  二、切削加工金属件表面粗糙度的控制措施

  通过对切削加工金属件表面粗糙度形成的理论分析可以看出控制表面粗糙度主要应从合理选择刀具材料、切削用量、刀具几何参数改善加工材料以及控制运动机构本身的精度等几方面入手采取相应的措施。现分析如下

  2.1增加运动机构本身的精度减少振动

  调整车床主轴使轴向窜动和径向跳动小于0.01mm增加跟刀架瓜脚支承面积选用精度比较理想的顶针增加工件装夹刚性调整机床各拖板的塞铁松紧程度消除大拖板的爬行现象。

  2.2改善工件材料加工性能保证加工质量

  在不影响工件性能的条件下适当调整化学成份加入少量的硫、锰、铝等元素降低材料塑性减少刀具粘结磨损抑制积屑瘤的产生。材料加工前进行适当的热处理改善其硬度、塑性。

  2.3合理选用切削液

  切削液的合理选用可减少切屑、刀具、工件接触面间的摩擦减轻粘结现象抑制积屑瘤的产生改善已加工面表面粗糙度。其使用效果与刀具的材料、加工要求、工件材料、

加工方法等因素有关应综合考虑。

  2.4合理选择刀具切削部分材料

  不同的加工材料不同的加工情况对刀具切削部分材料性能要求也不同因而所选用的材料也应不同。目前在车床高速精加工有色金属及非金属材料时一般使用金刚刀。其硬度高、耐磨性好切削刃锋利摩擦系数小切屑与刀具不易产生粘结不产生积屑瘤。

  2.5选择刀具合理的几何参数

  前角一般应取大些以减少振动。但在切削有足够刚度的钢件外圆时也可采用小负前角-2°——15°提高工件加工精度。

主后角大时刀刃锋锐工件表面弹性恢复减少因而减少了恢复层与后刀面的接触长度减少了后刀面与加工表面间的摩擦利于提高表面质量降低表面粗糙度。

  主偏角kr在工艺系统刚性较好精加工时为减少残留高度提高工件表面质量kr应尽量取小值。

  副偏角kr'为减少残留高度条件许可时kr'应尽量小可取5°——10°。有时为提高已加工表面质量还可使用kr=0的带修光刃的车刀此时理论上残留高度不存在。 刀尖形状根据刀具材料选用大的圆弧过渡刃还可以在刀尖处磨有修光刃以减少残留高度改善表面粗糙度。刃倾角选用控制切屑流出方向使其与进给方向一致保护已加工表面。

  2.6合理选择切削用量

  进给量f选用小的进给量f降低残留高度。

  切削速度V避免积屑瘤产生的区域以及自激振动的临界速度。一般推荐或用低速V38m/min之间降低切削温度或用高速V80---100m/min以上消除积屑瘤不利影响并利用因切削热增加而产生的高温进一步减少切屑变形降低表面粗糙度。

  切削厚度ap选用小的切削厚度ap减小工艺系统变形以及振动降低表面粗糙度。

  三、结论

本文论述了切削加工中金属加工件表面粗糙度形成的一系列相关因素以及控制措施。

各措施相互联系、相互影响。企业生产产品时综合考虑各因素选择出符合质量、效益要求的

合理值为制订加工工艺、选择设计刀具提供直接依据。

  参考文献

  [1]周泽华金属切削原理[M].上海上海科学技术出版社.

  [2]王生力重型机器制造工艺学[M].北京冶金出版社.

  [3]陈宏钧实用机械工艺手册[M].北京机械工业出版社.

 作者简介

  苏雯女本科西安理工大学理论与应用力学专业刘海廷男西安理工大学

通信工程专业李雯露女陕西电大。

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