[讨论]曲面为何不能转为实体？

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我用三维图板制作的椭圆球不能转换为实体，下面是我的制作步骤，请各位共同分析一下原因。

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第一步，先画出如图所示的三个相互垂直的椭圆，并把水平面内的椭圆在A、B、C三点打断。
<img src="attachments/dvbbs/2003-11/2003113014231273749.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2003-11/2003113014231273749.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

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第二步，用双导动线、单截面、变高方式生成曲面，生成曲面时拾取第一步打断的水平面内的两条曲线作为导动线，垂直面内的另一个椭圆为截面线。<img src="attachments/dvbbs/2003-11/2003113014351743919.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2003-11/2003113014351743919.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

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第三步，用同样的方法或者镜像生成另一半曲面，即可得到如图所示的椭圆球。
用这种方法制作的椭圆球与三个椭圆吻合的相当于好。
<img src="attachments/dvbbs/2003-11/2003113014441338369.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2003-11/2003113014441338369.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

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