Unit 2 Three-jaw Chuck and Four-jaw Chuck

TEXT

Three-jaw Chuck and Four-jaw Chuck

In lathe work, work-pieces are held either between centers or in a chuck. Sometimes the work-pieces are held in a chuck at one end and supported by a center at the other.

Several types of chucks are available in metal machining. The three-jaw universal and four-jaw independent types are the most commonly used in lathe work. They are designed to hold and revolve the work-pieces.

The three-jaw universal chuck is used to hold cylindrical work. The chucking is done quickly, as the three jaws move in and out together and automatically center the work. This type of chuck is accurate when new. It loses its accuracy as the jaws become worn.

To hold work-pieces of irregular shapes, we often use a four-jaw chuck instead of a three-jaw chuck. Its accuracy is not affected by wear, because each jaw can be adjusted independently. The jaws do not move simultaneously.

★ Translation

三爪卡盘与四爪卡盘

车削加工时，工件可夹持在两顶尖之间或者卡盘内。有时，工件的一端夹持在卡盘内，而另一端则用顶尖来支承。

机加工中，可以使用几种卡盘。三爪万能卡盘与四爪分动卡盘在车削加工中使用最为广泛，它们用来夹持并旋转工件。

三爪万能卡盘用来夹持圆柱形工件。由于三个爪一起张开和收拢且能自动使工件定心，因此夹紧工作可以迅速完成。这种卡盘在新的时候很精密，但是当卡爪受到磨损时，卡盘便会失去其精度。

为夹持不规则形状的工件，我们常用四爪卡盘来代替三爪卡盘。四爪卡盘的精度不受磨损影响，因为每个爪可以分别调整，而且这些爪不是同时移动的。

★ New Words and Expressions

★ Notes

1. In lathe work, work-pieces are held either between centers or in a chuck.

车削加工时，工件可夹持在两顶尖之间或卡盘内。

● “either...or...”是连接词，可译为：或……或……。

● 有些词，如句中的“in”，既可作介词，也可作副词，但用作副词时后面没有受支配的名词。很明显，句中的两个“in”均为介词。例如：

The shafts must rest in bearing.

轴必须装在轴承里。（介词）

The three jaws move in together.

三个爪一起向内移动。（副词）

Come in, please.

请进来。（副词）

2. The three-jaw universal and four-jaw independent types are the most commonly used in lathe work.

三爪万能卡盘与四爪分动卡盘在车削加工中使用最为广泛。

● “most commonly”是副词的最高级形式，意思是“最广泛的”; “are used”是被动语态，意思是“被使用”。

3. This type of chuck is accurate when new.

这种卡盘在新的时候很精密。

● 句中“when”后省略了“it is”，即“...when it is now”。

4. It loses its accuracy as the jaws become worn.

当卡盘受到磨损时，卡盘便会失去其精度。

● 句中“become”是系动词；“worn”是“wear”的过去分词形式，像形容词一样在“become”后作补语。

READING MATERIAL

Plain Center-type Cylindrical Grinder

Plain center-type cylindrical grinder(Fig.2-1) is used to grind the outside of cylindrical parts.

The table that is fastened to the bed of this machine is capable of back-and-forth (reciprocating) movement and can also be swiveled about 10 degrees on either side of the center line. Both the headstock and the footstock are mounted on this table. The work-piece is held between centers and must have center holes. A dog is used to drive the work-piece. The grinding wheel is mounted on a spindle. It can be moved to and from the work-piece.

Both the grinding wheel and the work-piece rotate during the machining operations. The grinding wheel turns rapidly—825 to 1,250 rpm. The work-piece turns at a much slower rate—100 to 400 rpm. Also, the surfaces of the grinding wheel and the work-piece move in opposite directions at their line of contact. The table is used to move the work-piece across the front of the wheel.

★ Translation

顶尖型普通外圆磨床

顶尖型普通外圆磨床（图2-1）用来磨削圆柱形工件的外圆。

装在机床身上的工作台能做往复运动，并能向中心线的任一侧偏转10° 左右。头架和尾架都装在工作台上。由于工件固定在顶尖之间，因此必须有顶尖孔。鸡心夹头用来带动工件。砂轮装在主轴上，能向着工件方向移动，也能做离开工件的运动。

磨削时，砂轮和工件都旋转。砂轮转速快，达到825～1250r/min。工件转速则慢得多，达到100～400r/min。而且，砂轮和工件的表面在其接触线上朝相对的方向旋转。工作台用来移动工件使之做越过砂轮前面的运动。

★ Professional Words and Phrases