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機械制造技術基礎（試題1） 班級 姓名 學號 成績 一、 填空選擇題（30分） 1．刀具后角是指 。 2．衡量切削變形的方法有 兩種，當切削速度提高時，切削變形（增加、減少）。 3．精車鑄鐵時應選用（YG3、YT10、YG8）；粗車鋼時，應選用（YT5、YG6、YT30）。 4．當進給量增加時，切削力（增加、減少），切削溫度（增加、減少）。 5．粗磨時，應選擇（軟、硬）砂輪，精磨時應選擇（緊密、疏松）組織砂輪。 6．合理的刀具耐用度包括 與 兩種。 7．轉(zhuǎn)位車刀的切削性能比焊接車刀（好，差），粗加工孔時，應選擇（拉刀、麻花鉆）刀具。 8．機床型號由 與 按一定規(guī)律排列組成，其中符號C代表（車床、鉆床）。 9．滾斜齒與滾直齒的區(qū)別在于多了一條（范成運動、附加運動）傳動鏈。滾齒時，刀具與工件之間的相對運動稱（成形運動、輔助運動）。 10．進行精加工時，應選擇（水溶液，切削油），為改善切削加工性，對高碳鋼材料應進行（退火，淬火）處理。 11．定位基準與工序基準不一致引起的定位誤差稱（基準不重合、基準位置）誤差，工件以平面定位時，可以不考慮（基準不重合、基準位置）誤差。 12．機床制造誤差是屬于（系統(tǒng)、隨機）誤差，一般工藝能力系數(shù)Cp應不低于（二級、三級）。 13．在常用三種夾緊機構中，增力特性最好的是 機構，動作最快的是 機構。 14．一個浮動支承可以消除（0、1、2）個自由度，一個長的v型塊可消除（3，4，5）個自由度。 15．工藝過程是指 。 二、 外圓車刀切削部分結構由哪些部分組成？繪圖表示外圓車刀的六個基本角度。（8分） 三、 簡述切削變形的變化規(guī)律，積屑瘤對變形有什么影響？（8分） 四、 CA6140車床主傳動系統(tǒng)如下所示，試列出正向轉(zhuǎn)動時主傳動路線及計算出最高轉(zhuǎn)速與最低轉(zhuǎn)速。（8分） 五、 什么叫剛度？機床剛度曲線有什么特點？（8分） 六、加工下述零件，以B面定位，加工表面A，保證尺寸10＋0.2mm，試畫出尺寸鏈并求出工序尺寸L及公差。（8分） 七、在一圓環(huán)形工件上銑鍵槽，用心軸定位，要求保證尺寸34.8-0.16mm，試計算定位誤差并分析這種定位是否可行。（8分） 八、試制定圖示零件（單件小批生產(chǎn)）的工藝路線。（12分） 九、在六角自動車床上加工一批mm滾子，用抽樣檢驗并計算得到全部工件的平均尺寸為Φ17.979mm，均方根偏差為0.04mm，求尺寸分散范圍與廢品率。 1.20 1.25 1.30 1.5 F 0.3849 0.3944 0.4032 0.4332 試題1參考答案 一． 1．后刀面與切削平面間的夾角。 2. 變形系數(shù)與滑移系數(shù)，減少 3.YG3，YT5 4．增加，增加 5. 軟，緊密 6.Tc，Tp 7. 好，麻花鉆 8．字母，數(shù)字，車床 9.附加運動，成型運動 10. 切削油，退火 11．基準不重合，基準位置 12. 系統(tǒng)，二級 13. 螺旋，圓偏心 14．1，4 15. 用機械加工方法直接改變原材料或毛坯的形狀、尺寸和性能，使之成為合格零件的過程。 二． 外圓車刀的切削部分結構由前刀面、后刀面、付后刀面、主切削刃、付切削刃與刀尖組成。 六個基本角度是：ro、αo、kr、kr’、λs、αo’ 三． 變形規(guī)律：ro↑，Λh↓；Vc↑，Λh↓；f↑, Λh↓; HB↑, Λh↓ 積屑瘤高度Hb↑,引起刀具前角增加，使Λh↓ 四． 最高轉(zhuǎn)速約1400r/min，最低轉(zhuǎn)速約10r/min 五． 剛度是指切削力在加工表面法向分力，F(xiàn)r與法向的變形Y的比值。 機床剛度曲線特點：剛度曲線不是直線；加載與卸載曲線不重合；載荷去除后，變形恢復不到起點。 六． L＝ 七． Δ定＝0.022mm，定位裝置可行。 八． 工序1：粗車各外圓、端面、Φ60圓孔；精車Φ200外圓與端面，精鏜Φ60孔；精車Φ96外圓、端面B與端面C（車床） 工序2：插槽（插床） 工序3：鉆6－Φ20孔（鉆床） 工序4：去毛刺（鉗工臺） 九． 尺寸分散范圍：17.859－18.099mm 廢品率： 17.3% 機械制造技術基礎（試題2） 班級 姓名 學號 成績 一、 填空選擇題（30分） 1．工序是指 。 2．剪切角增大，表明切削變形（增大，減少）；當切削速度提高時，切削變形（增大，減少）。 3．當高速切削時，宜選用（高速鋼，硬質(zhì)合金）刀具；粗車鋼時，應選用（YT5、YG6、YT30）。 4．CA6140車床可加工 、 、 、 等四種螺紋。 5．不經(jīng)修配與調(diào)整即能達到裝配精度的方法稱為（互換法、選配法）。 6．當主偏角增大時，刀具耐用度（增加，減少），當切削溫度提高時，耐用度（增加、減少）。 7．在四種車刀中，轉(zhuǎn)位車刀的切削性能（最好，最差）；粗磨時應選擇（硬，軟）砂輪。 8．機床的基本參數(shù)包括 、 、 。 9．滾齒時，刀具與工件之間的相對運動稱（成形運動、輔助運動）。滾斜齒與滾直齒的區(qū)別在于多了一條（范成運動、附加運動）傳動鏈。 10．衡量已加工表面質(zhì)量的指標有 。 11．定位基準面和定位元件制造誤差引起的定位誤差稱（基準不重合、基準位置）誤差，工件以平面定位時，可以不考慮（基準不重合、基準位置）誤差。 12．測量誤差是屬于（系統(tǒng)、隨機）誤差，對誤差影響最大的方向稱 方向。 13．夾緊力的方向應與切削力方向（相同，相反），夾緊力的作用點應該（靠近，遠離）工件加工表面。 14．輔助支承可以消除（0、1、2）個自由度，限制同一自由度的定位稱（完全定位、過定位）。 15．磨削加工時，應選擇（乳化液，切削油），為改善切削加工性，對不銹鋼進行（退火，淬火）處理。 二、 端面車刀的切削部分的結構由哪些部分組成？繪圖表示表示端面車刀的六個基本角度。（8分） 三、 金屬切削過程的本質(zhì)是什么？如何減少金屬切削變形？（8分） 四、 列出切削英制螺紋的運動方程式，并寫出CA6140車床進給箱中增倍變速組的四種傳動比。（8分） 五、 加工下述零件，要求保證孔的尺寸B＝30＋0.2，試畫出尺寸鏈，并求工序尺寸L。（8分） 六、磨一批d＝mm銷軸，工件尺寸呈正態(tài)分布，工件的平均尺寸X＝11.974，均方根偏差σ＝0.005，請分析改工序的加工質(zhì)量。如何加以改進。（8分） 1.00 2.00 2.50 3.00 F 0.3413 0.4772 0.4938 0.4987 七、滾斜齒時需要多少種運動？列出每條運動鏈的首末端元件及計算位移。（8分） 八、制訂下述零件的機械加工工藝過程，具體條件：45鋼，圓料ф70，單件生產(chǎn)。（12分） 九．在一桿狀形工件銑鍵槽，要求保證尺寸50-0.2mm與34.8-0.16mm，設計該工件的定位方案，并進行定位誤差分析。（10分） 試題2參考答案 一． 1．一個工人在一臺機床上對一個（或多個）零件所連續(xù)完成的那部分工藝過程。 2．減少，減少 3.硬質(zhì)合金，YT5 4.公制，英制，模數(shù)和徑節(jié) 5．互換性 6.減少，減少 7.最好，軟 8．尺寸，運動，動力 9.成型運動，附加運動 10.表面層幾何形狀特征，表面層物理機械性能 11.基準位置，基準位置 12.隨機，誤差敏感 13.相同，靠近 14.0，過定位 15.乳化液，調(diào)質(zhì) 二． 端面車刀切削部分結構由前刀面、后刀面、付后刀面、主切削刃，付切削刃與刀尖構成。 端面車刀的六個基本角度：ro、αo、kr、kr’、λs、αo’ 三． 被切金屬在刀刃的擠壓作用下，產(chǎn)生剪切滑移變形，并轉(zhuǎn)變?yōu)榍行嫉倪^程。要減少切削變形，可增大前角，提高速度，增大進給量，適當提高工件材料硬度。 四． u1 ＝ 18/4515/48, u2 ＝28/3515/48 u3 = 18/4535/28 u4 ＝ 28/3535/28 五． L ＝ 六． 廢品率等于2.28％，改進措施：將算術平均值移至公差帶中心，即使砂輪向前移動Δ， Δ＝0.0035mm。 七． 滾齒時需要四種運動： 1．主運動 電動機（n電）－ 滾刀（n刀） 2．范成運動 滾刀（1/k r）－ 工件（1/Z k） 3．進給運動 工件（1 r） － 刀架（f） 4．差動運動 刀架（L） － 工件（1 r） 八． 工序1：粗測各外圓、端面、鉆Φ14孔，精車Φ40外圓及端面；以Φ40為基準面，精鏜Φ30孔，精車Φ60及端面（車床）。 工序2：銑鍵槽（銑床） 工序3：鉆4－Φ6孔（鉆床） 工序4：去毛刺（鉗工臺） 九． 用V型塊及支承釘定位，定位誤差等于0.0034mm，符合要求，定位方案可行。 機械制造技術基礎（試題3） 班級 姓名 學號 成績 一、 填空選擇題（30分） 1．刀具前角是指 ， 用符號 表示。 2．當工件材料硬度提高時，切削力（增加，減少）； 當切削速度提高時，切削變形（增加、減少）。 3．制造復雜刀具宜選用（高速鋼、硬質(zhì)合金）；粗車鋼時，應選用（YT5、YG6、YT30）。 4．為改善切削加工性，對高碳鋼采?。ù慊?，退火）處理；對不銹鋼（淬火，調(diào)質(zhì)）處理。 5．粗加工時應選擇（水溶液、切削油），粗磨時，應選擇（軟、硬）砂輪。 6．精基準的選擇原則是 。 7．進給量越大，表面粗糙度（愈高，愈低）；精加工孔時，應選擇（拉刀、麻花鉆）刀具。 8．機床的運動按其功用可分為 與 兩大類。 9．滾斜齒與滾直齒的區(qū)別在于多了一條（范成運動、附加運動）傳動鏈。滾齒時，刀具與工件之間的相對運動稱（成形運動、輔助運動）。 10．在CA6140車床上加工公制螺紋時，所用掛輪是（63/100100/75, 100/100100/100）。 11．夾緊力作用點應（靠近，遠離）工件的加工表面；工件以平面定位時，可以不考慮（基準不重合、基準位置）誤差。 12．完全互換法適用于（大量，單件）生產(chǎn)，所有零件公差之和應（大于，小于）裝配公差。 13．工藝系統(tǒng)剛度等于（Fy/y，F(xiàn)z/y），工藝系統(tǒng)剛度大，誤差復映（大，小）。 14．一個支承釘可以消除（0、1、2）個自由度，消除六個自由度的定位稱（完全定位、過定位）。 15．生產(chǎn)類型通常分為 、 、 三種。 二、切斷車刀切削部分結構由哪些部分組成？繪圖表示切斷車刀的六個基本角度。（8分） 三、量度切削變形的方法有幾種？各有什么特點？能否全面衡量金屬切削過程的物理本質(zhì)？（8分） 四、 列出切削公制螺紋的運動方程式，并寫出CA6140車床進給箱中基本變速組的八種傳動比。（8分） 五、 在熱平衡條件下，磨一批Φ18-0.035的光軸，工件尺寸呈正態(tài)分布，現(xiàn)測得平均尺寸X＝17.975，標準偏差σ＝0.01，試計算工件的分散尺寸范圍與廢品率。（8分） 1.00 2.00 2.50 3.00 A 0.3413 0.4772 0.4938 0.4987 六、 加工下述零件，以B面定位，加工表面A，保證尺寸10＋0.2mm，試畫出尺寸鏈并求出工序尺寸L及公差。（8分） 七、 指出下列定位方案所消除的自由度。（8分） 八、 根據(jù)所學知識，列出提高零件加工質(zhì)量的措施。（10分） 九、 圖示撥叉零件，材料HT300，單件小批生產(chǎn)（12分） 1． 對零件進行工藝分析。 2． 制訂該零件的加工工藝過程。 試題3參考答案 一． 1．前刀面與基面的夾角（ro） 2.增加，減少 3.高速鋼，YT5 4. 退火，調(diào)質(zhì) 5.水溶液，軟 6.基準重合、統(tǒng)一、自為、互為、可靠原則 7.愈高，拉刀 8.成型運動，輔助運動 9. 附加運動，成型運動 10.63/100100/75 11.靠近，基準位置 12.大量，小于 13.Fy/y, 小 14.1，完全定位 15.單件，成批，大量生產(chǎn) 二． 切斷車刀切削部分由前刀面、后刀面、付后刀面、主切削刃、付切削刃與刀尖組成。 切斷車刀的六個基本角度：ro、αo、kr、kr’、λs、αo’ 三． 量度方法有兩種：1.變形系數(shù)（簡單，直觀但結果近似，從塑性壓縮方面分析）。 2.滑移系數(shù)（復雜，結果較精確，從剪切變形考慮）。 兩種方法都有片面性，不能全面反映金屬切削過程的物理本質(zhì)。 四． u1 = 26/28 u2 = 28/28 u3 = 32/28 u4 = 36/28 u5 = 19/14 u6 = 20/14 u7 = 33/21 u8 = 36/21 五． 分散范圍：Φ17.945－Φ18.005mm，廢品率為6.2％ 六． L＝ 七． ，， ，，； ， ，，； ，，，，， 八． 提高加工精度的措施：減少誤差，誤差補償，誤差分組，誤差轉(zhuǎn)移，就地加工，誤差平均，誤差控制。 提高表面質(zhì)量措施：采用合理的切削用量與刀具參數(shù)，采用冷壓強化與光整工藝，采用良好的切削液，減少積屑瘤、鱗刺和震動。 九． 工序1： 粗精銑尺寸40左右端面（銑床） 工序2： 鉆絞Φ24孔（車床） 工序3： 粗精銑尺寸12左右端面、R25圓弧面（銑床） 工序4： 鉆攻M8螺紋孔（鉆床） 工序5： 去毛刺（鉗工臺） 《Manufacturing Engineering and Technology—Machining》Test Paper (2004-06-17) Name: Student No: Score: 1 Drawing and marking (Close Test. 10 marks) 1.1 Fig 1a) shows a turning tool. Please mark the tool angles (rake angle, cutting edge angle, tool cutting edge inclination angle and minor cutting edge angle) in Fig 1b). (5 marks) 1.2 Pointing the process methods and marking the main motions in Fig 2. (5 marks) 2 Judging. Marking √ for true and for false. (Close Test. 30 marks, 2 marks for each) 2.1 The built-up edge is one of the factors that most adversely affecting surface finish in cutting. (√) 2.2 When continuous chip are formed, the machine tool is easier to vibrate and chatter than the case in which discontinous chips are formed. () 2.3 In turning the highest temperature is usually found in the middle of crater wear area rather than the tool corner. (√) 2.4 When using carbide inserts to hard turning, we should cool the cutting zone with cutting fluid discontinously. () 2.5 In surface grinding we should use oils as grinding fluids and in thread grinding we should use water-based emulsions. () 2.6 Indexable carbide inserts are usually brazed to the tool shank. () 2.7 Diamond tools are usually used to finish-machining cast iron and steels. () 2.8 the most important limitation of high-speed steel is the cutting speeds that can be employed, which are low relative to those of carbide tools. (√) 2.9 In a planer, the workpiece is mounted on a table that travels along a straight path; in a shaper, the tool travels along a straight path, and the workpiece is stationary. (√) 2.10 Three-jaw chucks have jaws that can be moved and adjusted independently of each other; they can be used for square or odd-shaped workpieces. () 2.11 For drilling, reaming and tapping, blind holes must be drilled deeper than subsequent reaming or tapping operations that may be performed. (√) 2.12 In conventional milling (up milling), the feed direction is same as the cutting direction. () 2.13 The broaching operation can be used to machine internal and external surfaces, such as holes of circular, square, keyways, and flat surfaces. (√) 2.14 In gear hobing, the gear blank rotation is main cutting motion, while the hob rotation is feed motion. () 2.15 A hard grinding wheel means its abrasive particles have very high hardness. () 3 Qualitative problems (Close Test. 20 marks, 4 marks for each) 3.1 Why is it not always advisable to increase cutting speed in order to increase production rate? Answer: Because cutting temperature will be obviously raised with the increase of cutting speed, which has a great and adverse influence on tool life so that we have to resharpen or replace tool often. Then the production rate may not be increased finally and a poor machined surface is generated. 3.2 Which tool materials would be suitable for interrupted cutting operations? Why? Answer: The tool materials having good toughness such as HSS, whisker-reinforced nanocrystalline tool materials, would be suitable for interrupted cutting operation. Because in interrupted cutting the tool will suffer impact coming from workpiece, if the tool has no enough impact toughness it will break, generally, chipping will occur in its edges or nose. 3.3 Describe the conditions under which broaching would be the preferred method of machining. Answer: The conditions which broaching would be the preferred consideration include: a large numbers of workpiece will be broached; there are high requirements on dimension accuracy and machined surface finish for the workpiece; the the workpiece has a suitable structure so that it can be clamped securely in broaching machine, and it should have sufficient strength and stiffness to withstand cutting forces during broaching. 3.4 Why is the stiffness of machine tool important in machining operations? Answer: Because low stiffness of machine tool will result in larger magnitudes of vibration and chatter in cutting tools and machines, and can have adverse effects on product quality. The possible troubles include poor machined surface finish, loss of dimension accuracy of the workpiece, premature wear, chipping and failure of the cutting tool, damage to the machine components and objectionable noise, etc. 3.5 It is generally recommended that, when grinding hardened steels, the grinding wheel should be of a relatively soft grade. Explain the reason. Answer: The grade of a grinding wheel is a measure of the bond’s strength. A softer wheel has a weaker bond and /or a less amount of bonding material. It is recommended for harder workpiece such as hardened steel, because the dull grains are easily dislodged and other sharp grains can contact the workpiece to remove chips continually, so as to reduce residual stresses and thermal damage to the workpiece. 4 Synthesis and design (Open Test. 30 marks, 15 marks for each) Observing Fig 3. The a) and b) show two cutting tools, and the c) and d) show two parts. Choose one from a), b) and one from c),d) to do your design. 1) Which kind is the cutting tool? What operation can be done with the cutting tool? According with the tool materials and geometries, analyze the applied performance of the tool. 2) The part c) is made from normal medium carbon steel, 5 pieces are required; the part d) is made from wrought aluminum, 10,000 are required. Please choose suitable process methods to machine the part, and arrange the process routing for it. a) b) c) d) Fig. 3 1) a) It is a turning tool (left offset tool). The tool shank is made from medium carbon steel and there is a rhombic carbide insert clamped on the shank by a clamp screw. The insert is indexable and has four cutting edges, when one of its edge becomes dull, another sharp edge can be indexed to use. The tool has a positive working rake angle and clearance angle, and a sharp corner, it can be used for external cylindrical or taper surface turning or facing. The tool has a 90cutting edge angle, so it can be used to turn 90stage surface. The tool is suitable to cutting cast irons or steels at higher cutting speeds and larger back engagements of the cutting edges, but had better to avoid cutting discontinuously; it is suitable for dry cutting or turning with cutting fluids, but should avoid cooling discontinuously. Because carbide insert is hard and brittle, special attention should be given to proper mounting and using the tool in order to prevent from impact and vibrating as well heat stress. b) It is an end milling cutter and made from HSS. It has a solid structure and flat end, four helical cutting edges. It can be holden by a collet or special chuck, cut workpiece with its circumferential cutting edges. It can be used to mill plane, side surface and curved surface as well slot, but is not suitable to feed in axial direction. It is suitable to cutting nonferrous alloys, also ferrous alloys such as cast irons and mild or medium carbon steels. Because its hot hardness is low relatively, it is suitable to cut at lower cutting speeds and needs cooling continuously. However, it possesses good impact toughness, so, it can be used to cut discontinuously. 2) c) The part has a stepped cylindrical shape, so it can be produced by turning. Only 5 pieces are needed, we can choose an energy lathe to make it with P-type carbide insert or a HSS solid tool, and choose a rolled stock as the blank. Because the part has a severe shape and position tolerance, so the left end face and cylindrical surface should be completed in one setup of the part. The through hole has a higher finish requirement, after drilling a finish turning is needed. In addition, screw threading and chamfer are needed. Machining routing: Sawing the blank (longer than 5 times of the part)——holding the blank with a three chuck in lathe, let the overhang be little longer than the final length of the part——facing one end——rough-turning external surfaces——medium finish-turning theФ50 external cylindrical surface to the dimension for threading M50——finish turning the Ф44 segment to dimension accuracy and the step face with a left offset tool——coring——drilling the through hole——internal turning the hole——chamfering——grooving in theФ50 external surface——screw threading——parting——turning around the workpiece and chamfering another end. d) The part is made from wrought aluminum alloy, a quantity of 10,000 pieces and a roughness of Ra 1.6 is required, so some economic processes such as extrusion should be used to produce the blank and a semi-finish-machining, finish-machining process should be applied. The through hole should be per-formed in the blank, then, the final dimension including the keyway may be broached optimally. There is no finish requirement for its external cylindrical surface and side surfaces, so these surfaces do not need to be machined. The three end faces marked Ra3.2 can be are produced with end milling cuter (the left end face and right cylindrical end face also can be produced with face milling). In order to obtain a high machining efficiency, a special fixture should be designed for fixing the workpiece on milling machine. Machining routing: fixing the workpiece on a vertical milling machine, milling the left end face——turning around the workpiece, milling the right plane and cylindrical end face——mounting the workpiece on a broaching machine, broaching the through hole and the keyway. 5 Comprehension and discussion (10 marks) As you can appreciate, the cost of machining centers is considerably higher than more traditional machine tools. In view of the fact that many operations performed by these centers can also be done on conventional machines, how would you go about justifying the high cost of these centers? Do you think under what condition we should purchase a precise and expensive machining center, and when we should use an economic machining center, even traditional machine tools only? Answer outline: Machinine efficiency Machining accuracy and roughness Automation Multiple functions Machining time, labor, workshop area 《Manufacturing Engineering and Technology—Machining》Test Paper (2005-06-14) Name: Student No: Score: 1. Drawing and marking (Close Test. 15 markings) 1.1 Fig 1a) shows a turning tool. Please mark its major cutting edge and minor cutting edge, rake face and major flank, cutting edge angle, rake angle and clearance angle. ( 3.5 markings) 1.2 Give the process method name, mark the main motion and feed motion(s) for each illustration in Fig 2. (6.5 markings) 1.3 There is a poor design consideration for each of the following workpieces, mark it and give your correction (5 markings). (a) to be threaded (b) to be shaped (c) gear to be generated (d) to be drilled (e) cavity to be milled (by end milling cutter) 2. Judging. Mark √ for true and for false. (Close Test. 30 markings, 2 markings for each) 2.1 It is always advisable to increase cutting speed in order to increase production rate. () 2.2 Long, continuous chips are undesirable because they tend to become entangled and interfere with cutting operations. (√) 2.3 Tool with high brittleness and hardness, or large positive rake angle is easy to chipping. (√) 2.4 When using carbide inserts to hard turning, we should cool the cutting zone with cutting fluid discontinously. () 2.5 In surface grinding we should use oils as grinding fluids and in thread grinding we should use water-based emulsions. () 2.6 Coated tools have unique properties, such as lower friction and higher resistance to cracks and wear, they can be used at high cutting speeds, reducing required machining time and costs. (√) 2.7 Diamond tools are usually used to finish-machining cast iron and steels. () 2.8 In machining, workpieces made from aluminum and magnesium alloys have less tendency to chatter than do workpieces made from high carbon steel and cast iron. (√) 2.9 In a planer, the workpiece is mounted on a table that travels along a straight path; in a shaper, the tool travels along a straight path, and the workpiece is stationary. (√) 2.10 Collet is a longitudinally-spit tapered bushing, it usually used for holding round tools or workpieces having a small diameter. (√) 2.11 If an existing hole has centerline deflection, we can correct it by reaming, while boring can only make an existing hole dimensionally more accurate. ()