Owing to this difference in the manner of guiding and supporting the cutting edges, and the advantages of an axial support for tools in boring, it becomes an operation by which the most accurate dimensions are attainable4, while drilling is a comparatively imperfect operation; yet the ordinary conditions of machine fitting are such that nearly all small holes can be drilled with sufficient accuracy.
Boring may be called internal turning, differing from external turning, because of the tools performing the cutting movement, and in the cut being made on concave instead of convex surfaces; otherwise there is a close analogy between the operations of turning and boring. Boring is to some extent performed on lathes5, either with boring bars or by what is termed chuck-boring, in the latter the material is revolved7 and the tools are stationary8.
Boring may be divided into three operations as follows: chuck-boring on lathes; bar-boring, when a boring bar runs on points or centres, and is supported at the ends only; and bar-boring when a bar is supported in and fed through fixed9 bearings. The principles are different in these operations, each one being applicable to certain kinds of work. A workman who can distinguish between these plans of boring, and can always determine from the nature of a certain work which is the best to adopt, has acquired considerable knowledge of fitting operations.
Chuck-boring is employed in three cases; for holes of shallow depth, taper10 holes, and holes that are screw-threaded. As pieces are overhung in lathe6-boring there is not sufficient rigidity11 neither of the lathe spindle nor of the tools to admit of deep boring. The tools being guided in a straight line, and capable of acting13 at any angle to the axis14 of rotation15, the facilities for making tapered16 holes are complete; and as the tools are stationary, and may be instantly adjusted, the same conditions answer for cutting internal screw-threads; an operation corresponding to cutting external screws, except that the cross motions of the tool slide are reversed.
The second plan of boring by means of a bar mounted on points or centres is one by which the greatest accuracy is attainable; it is like chuck-boring a lathe operation, and [138] one for which no better machine than a lathe has been devised, at least for the smaller kinds of work. It is a problem whether in ordinary machine fitting there is not a gain by performing all boring in this manner whenever the rigidity of boring bars is sufficient without auxiliary17 supports, and when the bars can pass through the work. Machines arranged for this kind of boring can be employed in turning or boring as occasion may require.
When a tool is guided by turning on points, the movement is perfect, and the straightness or parallelism of holes bored in this manner is dependent only on the truth of the carriage movement. This plan of boring is employed for small steam cylinders18, cylindrical19 valve seats, and in cases where accuracy is essential.
The third plan of boring with bars resting in bearings is more extensively practised, and has the largest range of adaptation. A feature of this plan of boring is that the form of the boring-bar, or any imperfection in its bearings, is communicated to the work; a want of straightness in the bar makes tapering20 holes. This, of course, applies to cases where a bar is fed through fixed bearings placed at one or both ends of a hole to be bored. If a boring-bar is bent21, or out of truth between its bearings, the diameter of the hole being governed by the extreme sweep of the cutters is untrue to the same extent, because as the cutters move along and come nearer to the bearings, the bar runs with more truth, forming a tapering hole diminishing toward the rests or bearings. The same rule applies to some extent in chuck-boring, the form of the lathe spindle being communicated to holes bored; but lathe spindles are presumed to be quite perfect compared with boring bars.
The prevailing22 custom of casting machine frames in one piece, or in as few pieces as possible, leads to a great deal of bar-boring, most of which can be performed accurately23 enough by boring bars supported in and fed through bearings. By setting up temporary bearings to support boring-bars, and improvising24 means of driving and feeding, most of the boring on machine frames can be performed on floors or sole plates and independent of boring machines and lathes. There are but few cases in which the importance of studying the principles of tool action is more clearly demonstrated than in this matter of boring; even long practical experience seldom leads to a thorough understanding of the [139] various problems which it involves.
Drilling differs in principle from almost every other operation in metal cutting. The tools, instead of being held and directed by guides or spindles, are supported mainly by the bearing of the cutting edges against the material.
A common angular-pointed drill is capable of withstanding a greater amount of strain upon its edges, and rougher use than any other cutting implement25 employed in machine fitting. The rigid12 support which the edges receive, and the tendency to press them to the centre, instead of to tear them away as with other tools, allows drills to be used when they are imperfectly shaped, improperly26 tempered, and even when the cutting edges are of unequal length.
Most of the difficulties which formerly27 pertained28 to drilling are now removed by machine-made drills which are manufactured and sold as an article of trade. Such drills do not require dressing29 and tempering or fitting to size after they are in use, make true holes, are more rigid than common solid shank drills, and will drill to a considerable depth without clogging30.
A drilling machine, adapted to the usual requirements of a machine fitting establishment, consists essentially31 of a spindle arranged to be driven at various speeds, with a movement for feeding the drills; a firm table set at right angles to the spindle, and arranged with a vertical32 adjustment to or from the spindle, and a compound adjustment in a horizontal plane. The simplicity33 of the mechanism34 required to operate drilling tools is such that it has permitted various modifications35, such as column drills, radial drills, suspended drills, horizontal drills, bracket drills, multiple drills, and others.
Drilling, more than any other operation in metal cutting, requires the sense of feeling, and is farther from such conditions as admit of power feeding. The speed at which a drill may cut without heating or breaking is dependent upon the manner in which it is ground and the nature of the material drilled, the working conditions may change at any moment as the drilling progresses; so that hand feed is most suitable. Drilling machines arranged with power feed for boring should have some means of permanently36 disengaging the feeding mechanism to prevent its use in ordinary drilling.
I am well aware how far this opinion is at variance37 with practice, especially in England; yet careful observation in a workshop [140] will prove that power feed in ordinary drilling effects no saving of time or expense.
(1.) What is the difference between boring and drilling?—(2.) Why will drills endure more severe use than other tools?—(3.) Why is hand feeding best suited for drills?—(4.) What is the difference between boring with a bar supported on centres and one fed through journal bearings?
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1
distinguished
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| adj.卓越的，杰出的，著名的 | |
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annular
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| adj.环状的 | |
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applied
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| adj.应用的；v.应用，适用 | |
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attainable
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| a.可达到的，可获得的 | |
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lathes
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| 车床( lathe的名词复数 ) | |
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lathe
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| n.车床，陶器，镟床 | |
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7
revolved
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| v.（使）旋转( revolve的过去式和过去分词 )；细想 | |
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8
stationary
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| adj.固定的，静止不动的 | |
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9
fixed
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| adj.固定的，不变的，准备好的；(计算机)固定的 | |
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taper
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| n.小蜡烛，尖细，渐弱；adj.尖细的；v.逐渐变小 | |
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rigidity
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| adj.钢性，坚硬 | |
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rigid
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| adj.严格的，死板的；刚硬的，僵硬的 | |
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13
acting
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| n.演戏，行为，假装；adj.代理的，临时的，演出用的 | |
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14
axis
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| n.轴，轴线，中心线；坐标轴，基准线 | |
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15
rotation
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| n.旋转；循环，轮流 | |
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16
tapered
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| adj. 锥形的，尖削的，楔形的，渐缩的，斜的 动词taper的过去式和过去分词 | |
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17
auxiliary
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| adj.辅助的，备用的 | |
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18
cylinders
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| n.圆筒( cylinder的名词复数 )；圆柱；汽缸；（尤指用作容器的）圆筒状物 | |
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19
cylindrical
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| adj.圆筒形的 | |
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tapering
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| adj.尖端细的 | |
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21
bent
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| n.爱好，癖好；adj.弯的；决心的，一心的 | |
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22
prevailing
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| adj.盛行的；占优势的；主要的 | |
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accurately
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| adv.准确地，精确地 | |
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24
improvising
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| 即兴创作（improvise的现在分词形式） | |
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25
implement
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| n.(pl.)工具，器具；vt.实行，实施，执行 | |
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improperly
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| 不正确地，不适当地 | |
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formerly
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| adv.从前，以前 | |
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pertained
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| 关于( pertain的过去式和过去分词 )； 有关； 存在； 适用 | |
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dressing
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| n.(食物)调料；包扎伤口的用品，敷料 | |
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30
clogging
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| 堵塞，闭合 | |
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31
essentially
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| adv.本质上，实质上，基本上 | |
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32
vertical
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| adj.垂直的，顶点的，纵向的；n.垂直物，垂直的位置 | |
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33
simplicity
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| n.简单，简易；朴素；直率，单纯 | |
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34
mechanism
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| n.机械装置；机构，结构 | |
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35
modifications
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| n.缓和( modification的名词复数 )；限制；更改；改变 | |
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36
permanently
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| adv.永恒地，永久地，固定不变地 | |
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variance
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| n.矛盾，不同 | |
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