Holes are critical surfaces on parts such as housings, brackets, sleeves, rings, and discs, and are frequently encountered during machining. Given the same precision and surface finish requirements, hole machining is more difficult than machining external cylindrical surfaces, resulting in lower productivity and higher costs.
This is because: 1) The size of the tool used for hole machining is limited by the size of the hole being machined, resulting in poor rigidity and prone to bending, deformation, and vibration; 2) When using a fixed-size tool for hole machining, the hole size is often directly determined by the tool’s corresponding dimensions, and tool manufacturing errors and wear directly affect hole machining accuracy; 3) During hole machining, the cutting zone is located within the workpiece, resulting in poor chip removal and heat dissipation, making it difficult to control both machining accuracy and surface quality.
Hole machining methods include drilling, reaming, boring, broaching, grinding, and finishing. Below, our metalworking team will provide a detailed introduction to several hole machining techniques to help you overcome hole machining challenges.
1. Drilling and Reaming
1. Drilling
Drilling is the first step in machining a hole in solid material, and the diameter is generally less than 80mm. There are two drilling methods: one with drill bit rotation and the other with workpiece rotation. The errors generated by these two methods differ. In the rotary drilling method, due to cutting edge asymmetry and insufficient drill bit rigidity, the drill bit deflects, causing the centerline of the processed hole to deviate or become crooked, but the hole diameter remains essentially unchanged. In the workpiece rotation method, the opposite occurs: drill bit deflection causes the hole diameter to change, while the centerline remains straight.
Common drilling tools include twist drills, center drills, and deep-hole drills. Twist drills are the most commonly used, with diameters ranging from 0.1 to 80 mm.
Due to structural limitations, drill bits have low bending and torsional stiffness, coupled with poor centering. This results in low drilling accuracy, generally reaching only IT13 to IT11. Surface roughness is also high, with an Ra of 50 to 12.5 μm. However, drilling provides a high metal removal rate and cutting efficiency. Drilling is mainly used for processing holes with low quality requirements, such as bolt holes, threaded bottom holes, oil holes, etc. For holes with high processing accuracy and surface quality requirements, they should be achieved through reaming, boring, boring or grinding in subsequent processing.
2. Reaming
Reaming is to use a reaming drill to further process the holes that have been drilled, cast or forged to enlarge the hole diameter and improve the processing quality of the hole. Reaming can be used as a pre-processing before finishing the hole or as the final processing of holes with low requirements. Reaming drills are similar to twist drills, but have more teeth and no chisel edge.
Compared with drilling, reaming has the following characteristics: (1) Reaming drills have more teeth (3 to 8 teeth), good guidance, and more stable cutting; (2) Reaming drills have no chisel edge, and the cutting conditions are good; (3) The processing allowance is small, the chip groove can be made shallower, the drill core can be made thicker, and the tool body strength and rigidity are better. The precision of hole reaming is generally IT11-IT10, with a surface roughness Ra of 12.5-6.3μm. Hole reaming is commonly used for holes with diameters smaller than . When drilling larger holes (D ≥ 30mm), a small drill (diameter 0.5-0.7 times the hole diameter) is often used to pre-drill the hole, followed by a correspondingly sized reamer. This improves hole quality and production efficiency.
In addition to producing cylindrical holes, reamer drills (also known as countersinks) of various special shapes can also be used to produce various countersunk holes and countersink end faces. Countersinks often have a guide pin at the front end to guide the drilled hole.
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2. Reaming
Reaming is a finishing method for holes and is widely used in production. For smaller holes, reaming is a more economical and practical method compared to internal grinding and fine boring.
1. Reamers
Reamers are generally divided into hand reamers and machine reamers. Hand reamers have straight shanks and longer working parts, providing better guiding. Hand reamers are available in both integral and adjustable OD configurations. Machine reamers are available in both shank-mounted and sleeve-mount configurations. Reamers can be used not only for round holes but also for tapered holes using tapered reamers.
2. Reaming Process and Applications
The reaming allowance significantly impacts the quality of the hole. Excessive allowance places a heavy load on the reamer, quickly dulling the cutting edge and making it difficult to achieve a smooth surface and maintain dimensional tolerances. Too little allowance prevents the removal of tool marks left by the previous process, thus failing to improve the hole quality. Generally, the rough reaming allowance is 0.35-0.15mm, and the fine reaming allowance is 0.15-0.05mm.
To avoid built-up edge, reaming is typically performed at a lower cutting speed (v < 8m/min when using a high-speed steel reamer on steel and cast iron). The feed rate is related to the diameter of the hole being machined. The larger the hole diameter, the higher the feed rate. When using a high-speed steel reamer on steel and cast iron, the feed rate is typically 0.3-1mm/r.
When reaming, appropriate cutting fluid must be used for cooling, lubrication, and cleaning to prevent built-up edge and ensure prompt chip removal. Compared to grinding and boring, reaming offers higher productivity and easier hole accuracy. However, reaming cannot correct for positional errors in the hole axis; hole positional accuracy should be ensured by the preceding process. Reaming is not suitable for step holes or blind holes.
The dimensional accuracy of reamed holes is generally IT9-IT7, with a surface roughness Ra of 3.2-0.8 μm. For medium-sized holes with high precision requirements (such as IT7-level precision holes), the drilling-reaming-reaming process is a typical processing solution commonly used in production.
III. Boring
Boring is a processing method that uses a cutting tool to enlarge a prefabricated hole. Boring can be performed on a boring machine or a lathe.
1. Boring method
There are three different boring processing methods.
(1) The workpiece rotates and the tool feeds
Most boring on a lathe belongs to this boring method. The process characteristics are: the axis of the hole after processing is consistent with the rotation axis of the workpiece, the roundness of the hole mainly depends on the rotation accuracy of the machine tool spindle, and the axial geometric shape error of the hole mainly depends on the position accuracy of the tool feed direction relative to the workpiece rotation axis. This boring method is suitable for processing holes that have coaxiality requirements with the outer cylindrical surface.
(2) The tool rotates and the workpiece is fed. The boring machine spindle drives the boring tool to rotate, and the worktable drives the workpiece to feed. (3) The tool rotates and feeds. When boring with this boring method, the overhang length of the boring bar varies, and the stress and deformation of the boring bar also vary. The hole diameter near the spindle box is large, and the hole diameter away from the spindle box is small, forming a tapered hole. In addition, as the overhang length of the boring bar increases, the bending deformation of the spindle due to its own weight also increases, and the axis of the processed hole will bend accordingly. This boring method is only suitable for processing shorter holes. 2. Diamond boring Compared with general boring, diamond boring is characterized by small back cutting amount, small feed rate, and high cutting speed. It can obtain very high processing accuracy (IT7~IT6) and very smooth surface (Ra is 0.4~0.05 μm). Diamond boring was originally performed with diamond boring tools, but carbide, CBN, and synthetic diamond tools are now commonly used. It is primarily used for machining non-ferrous metal workpieces, but can also be used for machining cast iron and steel.
Common cutting parameters for diamond boring are: pre-boring depth of 0.2-0.6mm, final boring depth of 0.1mm; feed rate of 0.01-0.14mm/r; cutting speed of 100-250m/min for cast iron, 150-300m/min for steel, and 300-2000m/min for non-ferrous metals.
To ensure high machining accuracy and surface quality when using diamond boring, the machine tool (diamond boring machine) must possess high geometric accuracy and rigidity. Precision angular contact ball bearings or hydrostatic plain bearings are often used for spindle support, and high-speed rotating parts must be precisely balanced. Furthermore, the feed mechanism must operate very smoothly to ensure smooth, low-speed feed motion of the worktable.
Diamond boring offers excellent machining quality and high production efficiency, making it widely used in large-scale mass production for the final machining of precision holes, such as engine cylinder bores, piston pin holes, and spindle holes in machine tool spindle boxes. However, it is important to note that when using diamond boring for ferrous metals, only boring tools made of carbide and CBN should be used; boring tools made of diamond should not be used. This is because the carbon atoms in diamond have a strong affinity for iron group elements, resulting in a shorter tool life.
3. Boring Tools
Boring tools can be divided into single-edge and double-edge boring tools.
4. Boring Process Characteristics and Applications
Compared to the drilling-reaming-reaming process, boring is not limited by tool size in terms of hole diameter. Boring also offers strong error correction capabilities, allowing correction of deviations in the original hole axis through multiple passes. It also maintains a high degree of positional accuracy between the bored hole and the positioning surface.
Compared to external turning, boring suffers from lower toolholder rigidity, greater deformation, poor heat dissipation, and significant thermal deformation of the workpiece and tool. Consequently, boring’s machining quality and production efficiency are inferior to those of external turning.
From the above analysis, it can be seen that boring has a wide range of machining applications, capable of producing holes of various sizes and accuracy levels. For larger holes and hole systems requiring high dimensional and positional accuracy, boring is virtually the only machining method. Boring achieves machining accuracy levels of IT9 to IT7. Boring can be performed on various machine tools, including boring machines, lathes, and milling machines. Its flexibility and maneuverability make it widely used in production. In large-scale mass production, boring dies are often used to improve boring efficiency.
IV. Honing Holes
1. Honing Principles and Honing Heads
Honing is a method of finishing holes using a honing head with a grinding wheel (oilstone). During honing, the workpiece is stationary while the honing head is rotated and reciprocated by the machine tool spindle. During honing, the grinding wheel applies pressure to the workpiece surface, removing a very thin layer of material from the surface. The cutting path forms a cross-shaped pattern. To ensure that the paths of the abrasive grains in the grinding wheel are not repeated, the rotational speed of the honing head and the number of reciprocating strokes per minute of the honing head must be prime numbers.
The intersection angle of the honing paths is related to the reciprocating speed and peripheral speed of the honing head. The size of this angle affects the honing process quality and efficiency. It is generally set at 0° for rough honing and 0° for fine honing. To facilitate the removal of broken abrasive particles and chips, reduce cutting temperatures, and improve machining quality, sufficient cutting fluid should be used during honing.
To ensure uniform machining of the hole wall, the abrasive strip must extend beyond the hole by a certain amount at both ends. To ensure uniform honing allowance and minimize the impact of spindle rotation errors on machining accuracy, a floating connection is typically used between the honing head and the machine spindle.
Radial adjustment of the honing head strips can be manual, pneumatic, or hydraulic.
2. Honing Process Characteristics and Applications
1) Honing can achieve high dimensional and shape accuracy, with machining accuracy reaching IT7-IT6 levels. Hole roundness and cylindricity errors can be controlled within a range of . However, honing does not improve the positional accuracy of the machined hole.
2) Honing can achieve high surface quality, with a surface roughness Ra of 0.2-0.25μm and a minimal depth of 2.5-25μm for the surface metal’s deteriorated defect layer.
3) Although the circumferential speed of the honing head is not as high as that of grinding (vc = 16-60m/min), honing still offers high productivity due to the large contact area between the abrasive strip and the workpiece and the relatively high reciprocating speed (va = 8-20m/min).
Honing is widely used in mass production for machining precision holes in engine cylinder bores and various hydraulic systems, and can produce deep holes with aspect ratios greater than 10. However, honing is not suitable for machining holes in non-ferrous metal workpieces with high plasticity, nor for machining holes with keyways or splines.
V. Hole Broaching
1. Broaching and Broaching Tools
Hole broaching is a highly productive finishing method performed on a broaching machine using a specially designed broaching tool. Broaching machines are classified into two types: horizontal and vertical, with horizontal broaching machines being the most common.
During broaching, the broach only moves in a low-speed linear motion (the main motion). The number of teeth in the broach operating simultaneously should generally be no less than three. Otherwise, the broach will not operate smoothly, and annular ripples may easily form on the workpiece surface. To avoid excessive broaching forces that could break the broach, the number of teeth in operation should generally not exceed six to eight.
There are three different broaching methods for hole broaching, described below:
1) Layered Broaching
This broaching method is characterized by the broach removing the workpiece stock layer by layer. To facilitate chip breaking, the teeth are ground with staggered chip grooves. Broaches designed for layered broaching are called standard broaches.
2) Block Broaching
This broaching method is characterized by the broach removing each layer of metal from the workpiece surface using a group of teeth (usually consisting of two to three teeth per group) of essentially identical dimensions but staggered. Each tooth removes only a portion of a layer of metal. A broach designed for segmented broaching is called a wheel-cut broach.
3) Combined Broaching
This method combines the advantages of both segmented and segmented broaching: segmented broaching is used for roughing the tooth, while segmented broaching is used for finishing. This shortens the broach length, improves productivity, and achieves better surface quality. A broach designed for combined broaching is called a combined broach.
2. Hole Broaching Process Characteristics and Applications
1) A broach is a multi-edge tool that can sequentially complete roughing, finishing, and finishing of a hole in a single broaching stroke, resulting in high production efficiency.
2) Hole broaching accuracy primarily depends on the accuracy of the broach. Under normal conditions, hole broaching accuracy can reach IT9 to IT7, and surface roughness Ra can reach 6.3 to 1.6 μm.
3) When broaching, the workpiece is positioned relative to the hole being machined (the broaching tool guide acts as the workpiece positioning element). This makes it difficult to ensure the relative positional accuracy of the hole and other surfaces. For machining rotating parts with coaxiality requirements for internal and external circular surfaces, the hole is often broached first, and then other surfaces are machined using the hole as a positioning reference.
4) Broaches can not only machine round holes, but also form holes and spline holes.
5) Broaches are fixed-size tools with complex shapes and high cost, making them unsuitable for machining large holes.
Broaching is commonly used in mass production to machine through holes in small and medium-sized parts with diameters ranging from 10 to 80 mm and depths no more than five times the hole diameter.
二、铰孔
铰孔是孔的精加工方法之一，在生产中应用很广。对于较小的孔，相对于内圆磨削及精镗而言，铰孔是一种较为经济实用的加工方法。
1. 铰刀
铰刀一般分为手用铰刀及机用铰刀两种。手用铰刀柄部为直柄，工作部分较长，导向作用较好，手用铰刀有整体式和外径可调整式两种结构。机用铰刀有带柄的和套式的两种结构。铰刀不仅可加工圆形孔，也可用锥度铰刀加工锥孔。
2. 铰孔工艺及其应用
铰孔余量对铰孔质量的影响很大，余量太大，铰刀的负荷大，切削刃很快被磨钝，不易获得光洁的加工表面，尺寸公差也不易保证；余量太小，不能去掉上工序留下的刀痕，自然也就没有改善孔加工质量的作用。一般粗铰余量取为0.35~0.15mm，精铰取为 01.5~0.05mm。
为避免产生积屑瘤，铰孔通常采用较低的切削速度（高速钢铰刀加工钢和铸铁时，v ＜8m/min）进行加工。进给量的取值与被加工孔径有关，孔径越大，进给量取值越大，高速钢铰刀加工钢和铸铁时进给量常取为 0.3~1mm/r。
铰孔时必须用适当的切削液进行冷却、润滑和清洗，以防止产生积屑瘤并及时清除切屑。与磨孔和镗孔相比，铰孔生产率高，容易保证孔的精度；但铰孔不能校正孔轴线的位置误差，孔的位置精度应由前工序保证。铰孔不宜加工阶梯孔和盲孔。
铰孔尺寸精度一般为 IT9～IT7级，表面粗糙度Ra一般为 3.2~0.8 μm。对于中等尺寸、精度要求较高的孔（例如IT7级精度孔），钻—扩—铰工艺是生产中常用的典型加工方案。
三、镗孔
镗孔是在预制孔上用切削刀具使之扩大的一种加工方法，镗孔工作既可以在镗床上进行，也可以在车床上进行。
1. 镗孔方式
镗孔有三种不同的加工方式。
（1）工件旋转，刀具作进给运动
在车床上镗孔大都属于这种镗孔方式。工艺特点是：加工后孔的轴心线与工件的回转轴线一致，孔的圆度主要取决于机床主轴的回转精度，孔的轴向几何形状误差主要取决于刀具进给方向相对于工件回转轴线的位置精度。这种镗孔方式适于加工与外圆表面有同轴度要求的孔。
（2）刀具旋转，工件作进给运动
镗床主轴带动镗刀旋转，工作台带动工件作进给运动。
（3）刀具旋转并作进给运动
采用这种镗孔方式镗孔，镗杆的悬伸长度是变化的，镗杆的受力 变形也是变化的，靠近主轴箱处的孔径大，远离主轴箱处的孔径小，形成锥孔。此外，镗杆悬伸长度增大，主轴因自重引起的弯曲变形也增大，被加工孔轴线将产生相应的弯曲。这种镗孔方式只适于加工较短的孔。
2. 金刚镗
与一般镗孔相比，金刚镗的特点是背吃刀量小，进给量小，切削速度高，它可以获得很高的加工精度（IT7～IT6）和很光洁的表面（Ra为 0.4~0.05 μm）。金刚镗最初用金刚石镗刀加工，现在普遍采用硬质合金、CBN和人造金刚石刀具加工。主要用于加工有色金属工件，也可用于加工铸铁件和钢件。
金刚镗常用的切削用量为：背吃刀量预镗为 0.2~0.6mm，终镗为0.1mm ；进给量为 0.01~0.14mm/r ；切削速度加工铸铁时为100~250m/min ，加工钢时为150~300m/min ，加工有色金属时为 300~2000m/min。
为了保证金刚镗能达到较高的加工精度和表面质量，所用机床（金刚镗床）须具有较高的几何精度和刚度，机床主轴支承常用精密的角接触球轴承或静压滑动轴承，高速旋转零件须经精确平衡；此外，进给机构的运动必须十分平稳，保证工作台能做平稳低速进给运动。
金刚镗的加工质量好，生产效率高，在大批大量生产中被广泛用于精密孔的最终加工，如发动机气缸孔、活塞销孔、机床主轴箱上的主轴孔等。但须引起注意的是：用金刚镗加工黑色金属制品时，只能使用硬质合金和CBN制作的镗刀，不能使用金刚石制作的镗刀，因金刚石中的碳原子与铁族元素的亲和力大，刀具寿命低。
3. 镗刀
镗刀可分为单刃镗刀和双刃镗刀。
4. 镗孔的工艺特点及应用范围
镗孔和钻—扩—铰工艺相比，孔径尺寸不受刀具尺寸的限制，且镗孔具有较强的误差修正能力，可通过多次走刀来修正原孔轴线偏斜误差，而且能使所镗孔与定位表面保持较高的位置精度。
镗孔和车外圆相比，由于刀杆系统的刚性差、变形大，散热排屑条件不好，工件和刀具的热变形比较大，镗孔的加工质量和生产效率都不如车外圆高。
综上分析可知， 镗孔的加工范围广，可加工各种不同尺寸和不同精度等级的孔，对于孔径较大、尺寸和位置精度要求较高的孔和孔系，镗孔几乎是唯一的加工方法。镗孔的加工精度为 IT9～IT7级。镗孔可以在镗床、车床、铣床等机床上进行，具有机动灵活的优点，生产中应用十分广泛。在大批大量生产中，为提高镗孔效率，常使用镗模。
四、珩磨孔
1. 珩磨原理及珩磨头
珩磨是利用带有磨条（油石）的珩磨头对孔进行光整加工的方法。珩磨时，工件固定不动，珩磨头由机床主轴带动旋转并作往复直线运动。珩磨加工中，磨条以一定压力作用于工件表面，从 工件表面上切除一层极薄的材料，其切削轨迹是交叉的网纹。为使砂条磨粒的运动轨迹不重复，珩磨头回转运动的每分钟转数与珩磨头每分钟往复行程数应互成质数。
珩磨轨迹的交叉角
与珩磨头的往复速度
及圆周速度
有关， 角的大小影响珩磨的加工质量及效率，一般粗珩时取
°，精珩时取。为了便于排出破碎的磨粒和切屑，降低切削温度，提高加工质量，珩磨时应使用充足的切削液。
为使被加工孔壁都能得到均匀的加工，砂条的行程在孔的两端都要超出一段越程量。为保证珩磨余量均匀，减少机床主轴回转误差对加工精度的影响，珩磨头和机床主轴之间大都采用浮动连接。
珩磨头磨条的径向伸缩调整有手动、气动和液压等多种结构形式。
2. 珩磨的工艺特点及应用范围
1）珩磨能获得较高的尺寸精度和形状精度，加工精度为 IT7~IT6 级，孔的圆度和圆柱度误差可控制在 的范围之内，但珩磨不能提高被加工孔的位置精度。
2）珩磨能获得较高的表面质量，表面粗糙度Ra为 0.2~0.25μm ，表层金属的变质缺陷层深度极微2.5~25μm。
3）与磨削速度相比，珩磨头的圆周速度虽不高（vc=16~60m/min），但由于砂条与工件的接触面积大，往复速度相对较高(va=8~20m/min)，所以珩磨仍有较高的生产率。
珩磨在大批大量生产中广泛用于发动机缸孔及各种液压装置中精密孔的加工，并可加工长径比大于10的深孔。但珩磨不适用于加工塑性较大的有色金属工件上的孔，也不能加工带键槽的孔、花键孔等。
五、拉孔
1. 拉削与拉刀
拉孔是一种高生产率的精加工方法，它是用特制的拉刀在拉床上进行的。拉床分卧式拉床和立式拉床两种，以卧式拉床最为常见。
拉削时拉刀只作低速直线运动（主运动）。拉刀同时工作的齿数一般应不少于3个，否则拉刀工作不平稳，容易在工件表面产生环状波纹。为了避免产生过大的拉削力而使拉刀断裂，拉刀工作时，同时工作刀齿数一般不应超过6~8个。
拉孔有三种不同的拉削方式，分述如下：
1）分层式拉削
这种拉削方式的特点是拉刀将工件加工余量一层一层顺序地切除。为了便于断屑，刀齿上磨有相互交错的分屑槽。按分层式拉削方式设计的的拉刀称为普通拉刀。
2）分块式拉削
这种拉削方式的特点是加工表面的每一层金属是由一组尺寸基本相同但刀齿相互交错的刀齿（通常每组由2-3个刀齿组成）切除的。每个刀齿仅切去一层金属的一部分。按分块拉削方式设计的拉刀称为轮切式拉刀。
3）综合式拉削
这种方式集中了分层及分块式拉削的优点，粗切齿部分采用分块式拉削，精切齿部分采用分层式拉削。这样既可缩短拉刀长度，提高生产率，又能获得较好的表面质量。按综合拉削方式设计的拉刀称为综合式拉刀。
2. 拉孔的工艺特征及应用范围
1）拉刀是多刃刀具，在一次拉削行程中就能顺序完成孔的粗加工、精加工和光整加工工作，生产效率高。
2）拉孔精度主要取决于拉刀的精度，在通常条件下，拉孔精度可达 IT9~IT7，表面粗糙度Ra可达 6.3~1.6 μm。
3）拉孔时，工件以被加工孔自身定位（拉刀前导部就是工件的定位元件），拉孔不易保证孔与其它表面的相互位置精度；对于那些内外圆表面具有同轴度要求的回转体零件的加工，往往都是先拉孔，然后以孔为定位基准加工其它表面。
4）拉刀不仅能加工圆孔，而且还可以加工成形孔，花键孔。
5）拉刀是定尺寸刀具，形状复杂，价格昂贵，不适合于加工大孔。
拉孔常用在大批大量生产中加工孔径为 Ф10~80mm 、孔深不超过孔径5倍的中小零件上的通孔。
Post time: Jul-29-2025
