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钳工画线及注意事项 一、划线的作用及种类划线是根据图样的尺寸要求,用划针工具在毛坯或半成品上划出待加工部位的轮廊线(或称加工界限)或作为基准的点、线的一种操作方法。划线的精度一般为0.25~0.5mm。1.划线的作用—即毛坯或半成品为什么要进行划线呢?(1)所划的轮廊线即为毛坯或半成品的加工界限和依据,所划的基准点或线是工件安装时的标记或校正线。(2)在单件或小批量生产中,用划线来检查毛坯或半成品的形状和尺寸,合理地分配各加工表面的余量,及早发现不合格品,避免造成后续加工工时的浪费。(3)在板料上划线下料,可做到正确排料,使材料合理作用。划线是一项复杂、细致的重要工作,如果将划线划错,就会造成加工工件的报废。所以划线直接关系到产品的质量。对划线的要求是:尺寸准确、位置正确、线条清晰、冲眼均匀。2.划线的种类(1)平面划线—即在工件的一个平面上划线后即能明确表示加工界限,它与平面作图法类似。(2)立体划线—是平面划线的复台,是在工件的几相互成不同角度的表面(通常是相互垂直的表面)上都划线,即在长、宽、高三个方向上划线。二、划线的工具及其用法按用途不同划线工具分为基准工具、支承装夹工具、直接绘划工具和量具等。1.基准工具—划线平板划线平板由铸铁制成,基个平面是划线的基准平面,要求非常平直和光洁。使用时要注意:(1)安放时要平稳牢固、上平面应保持水平;(2)平板不准碰撞和用锤敲击,以免使其精度降低;(3)长期不用时,应涂油防锈,并加盖保护罩。2.夹持工具—方箱、千斤顶、V形铁等(1)方箱 方箱是铸铁制成的空心立方体、各相邻的两个面均互相垂直。方箱用于夹持、支承尺寸较小而加工面较多的工件。通过翻转方箱,便可在工件的表面上划出互相垂直的线条。(2)千斤顶 千斤顶是在平板上支承较大及不规划工件时使用,其高度可以调整。通常用三个千斤顶支承工件。(3)V形铁 V形铁用于支承圆柱形工件,使工件轴线与底板平行。3.直接绘划工具—划针、划规、划卡、划针盘和样冲等(1)划针 是在工件表面划线用的工具,常用的划针用工具钢或弹簧钢制成(有的划针在其尖端部位焊有硬质合金),直径φ3~6mm。(2)划规 是划圆或弧线、等分线段及量取尺寸等用的工具。它的用法与制图的圆规相似。(3)划卡 或称单脚划规,主要用于确定轴和孔的中心位置。(4)划针盘 主要用于立体划线和校正工件的位置。它由底座、立杆、划针和锁紧装置等组成。(5)样冲 用于在工件划线点上打出样冲眼,以备所划线模糊后仍能找到原划线的位置;在划圆和钻孔前了应在其中心打样冲眼,以便定心。4.量具—钢尺、直角尺、高度尺(普通高度尺和高度游标尺)等高度游标尺除用来测量工件的高度外,还可用来作半成品划线用,其读数精度一般为0.02mm。它只能用于半成品划线,不允许用于毛坯。三、划线基准用划线盘划各种水平线时,应选定某一基准作为依据,并以此来调节每次划针的高度,这个基准称为划线基准。一般划线基准与设计基准应一致。常选用重要孔的中心线为划线基准,或零件上尺寸标注基准线为划线基准。若工件上个别平面已加工过,则以加工过的平面为划线基准。常见的划线基准有三种类型:1.以两个相互垂直的平面(或线)为基准;2.以一个平面与对称平面(和线)为基准;3.以两个互相垂直的中心平面(或线)为基准。四、划线操作要点1.划线前的准备工作(1)工件准备 包括工件的清理、检查和表面涂色;(2)工具准备 按工件图样的要求,选择所需工具,并检查和校验工具。2.操作时的注意事项(1)看懂图样,了解零件的作用,分析零件的加工顺序和加工方法;(2)工件夹持或支承要稳妥,以防滑倒或移动;(3)在一次支承中应将要划出的平行线全部划全,以免再次支承补划,造成误差;(4)正确使用划线工具,划出的线条要准确、清晰;(5)划线完成后,要反复核对尺寸,才能进行机械加工。
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