Hole processing-drilling, reaming, reaming, boring, honing and drawing holes.

Hole is an important surface on box, bracket, sleeve, ring and disk parts, and it is also a surface often encountered in machining. Under the same requirements of machining accuracy and surface roughness, machining holes is more difficult than machining cylindrical surfaces, with low productivity and high cost.


This is because: 1) the size of the tool used in hole processing is limited by the size of the hole to be processed, and its rigidity is poor, which is easy to produce bending deformation and vibration; 2) When machining a hole with a fixed-size cutter, the machining size of the hole often depends directly on the corresponding size of the cutter, and the manufacturing error and wear of the cutter will directly affect the machining accuracy of the hole; 3) When machining holes, the cutting area is inside the workpiece, and the chip removal and heat dissipation conditions are poor, so the machining accuracy and surface quality are not easy to control.

The processing methods of holes include drilling, reaming, reaming, boring, drawing, grinding and finishing. The following is a detailed introduction of several hole processing technologies for you to solve the hole processing problems.

I. Drilling and reaming
1. Drill holes

Drilling is the first process of machining holes on solid materials, and the diameter of drilling holes is generally less than 80 mm. There are two ways of drilling: one is bit rotation; The other is workpiece rotation. The errors produced by the above two drilling methods are different. In the drilling method with rotating drill bit, when the drill bit is deviated due to the asymmetry of cutting edge and insufficient rigidity of the drill bit, the center line of the hole to be processed will be deviated or not straight, but the aperture will remain basically unchanged. However, in the drilling method of workpiece rotation, the opposite is true. The deviation of drill bit will cause the hole diameter change, but the center line of the hole is still straight.

Commonly used drilling tools include twist drill, center drill, deep hole drill, etc. Among them, twist drill is the most commonly used, and its diameter specification is φ 0.1-80 mm.

Due to the structural limitation, the bending stiffness and torsional stiffness of the drill bit are low, and the drilling accuracy is low, which can only reach IT13~IT11;. The surface roughness is also large, Ra is generally 50 ~ 12.5 μ m; However, the metal removal rate of drilling is high and the cutting efficiency is high. Drilling is mainly used to process holes with low quality requirements, such as bolt holes, thread bottom holes and oil holes. For holes with high machining accuracy and surface quality, it should be achieved by reaming, reaming, boring or grinding in the subsequent processing.

2.Reaming

Reaming is to further process the drilled, cast or forged hole with a reamer to enlarge the hole diameter and improve the processing quality of the hole. Reaming can be used as pre-processing before finishing the hole or as the final processing of the hole with low requirements. Reaming drill is similar to twist drill, but it has more teeth and no cross edge.

Compared with drilling, reaming has the following characteristics: (1) The reaming drill has more teeth (3-8 teeth), good guidance and stable cutting; (2) The reamer has no chisel edge and good cutting conditions; (3) The machining allowance is small, the chip groove can be made shallower, the core can be made thicker, and the strength and rigidity of the cutter body are better. The precision of reaming is generally IT11~IT10, and the surface roughness Ra is 12.5 ~ 6.3 μ m.. Reaming is often used to process holes with diameters smaller than. When drilling a hole with a larger diameter (D ≥30mm), a small drill (0.5~0.7 times the diameter of the hole) is often used to pre-drill the hole, and then a reamer with a corresponding size is used to enlarge the hole, which can improve the processing quality and production efficiency of the hole.

In addition to processing cylindrical holes, reaming can also be used to process various countersunk seat holes and countersunk end faces with various special-shaped reaming drills (also known as countersinks). The front end of the countersink drill often has a guide column, which is guided by the processed hole.

II.Reaming

Reaming is one of the finishing methods of holes, which is widely used in production. For smaller holes, reaming is a more economical and practical processing method compared with internal grinding and fine boring.

1. Reamer

Reamers are generally divided into hand reamer and machine reamer. The hand reamer has a straight handle, a long working part and good guiding effect. The hand reamer has two structures: integral and algebraic expression with adjustable outer diameter. Machine reamer has two structures: handle and sleeve. Reamer can not only process circular holes, but also taper holes with taper reamer.

2. Reaming technology and its application

The reaming allowance has a great influence on the reaming quality. If the allowance is too large, the load of the reamer will be large, and the cutting edge will be blunt quickly, so it is difficult to obtain a smooth machined surface and the dimensional tolerance will not be guaranteed. The allowance is too small to remove the knife marks left by the previous procedure, and naturally there is no effect on improving the machining quality of holes. Generally, the allowance for coarse hinge is 0.35~0.15mm, and that for fine hinge is 01.5~0.05mm.

In order to avoid chip buildup, reaming is usually processed at a low cutting speed (V < 8m/min when high-speed steel reamer processes steel and cast iron). The feed value is related to the hole diameter to be processed. The larger the hole diameter, the greater the feed value. When the high-speed steel reamer processes steel and cast iron, the feed value is usually 0.3 ~ 1 mm/r..

When reaming, proper cutting fluid must be used for cooling, lubrication and cleaning, so as to prevent chip accumulation and remove chips in time. Compared with grinding and boring, reaming has high productivity and is easy to ensure the accuracy of the hole; However, reaming can’t correct the position error of the hole axis, and the position accuracy of the hole should be guaranteed by the previous process. Reaming is not suitable for processing stepped holes and blind holes.

The dimensional accuracy of reaming is generally IT9~IT7, and the surface roughness Ra is generally 3.2~0.8 μm m.. For holes with medium size and high precision requirements (such as IT7 precision holes), drilling-reaming technology is a typical processing scheme commonly used in production.

III. Boring

Boring is a processing method to enlarge a prefabricated hole with a cutting tool. Boring can be carried out on both a boring machine and a lathe.

1. Boring method

There are three different machining methods for boring.

(1) When the workpiece rotates, the cutter moves in feed.
Boring on lathe mostly belongs to this boring method. The technological characteristics are: the axis of the hole after machining is consistent with the axis of rotation of the workpiece, the roundness of the hole mainly depends on the rotation accuracy of the machine tool spindle, and the axial geometric error of the hole mainly depends on the position accuracy of the tool feed direction relative to the axis of rotation of the workpiece. This boring method is suitable for machining holes with coaxial requirements with the cylindrical surface.

(2) When the cutter rotates, the workpiece moves in feed.
The spindle of the boring machine drives the boring cutter to rotate, and the workbench drives the workpiece to make feed movement.

(3) the cutter rotates and makes feeding motion.

When boring with this boring method, the cantilever length of the boring bar is changed, and the stress deformation of the boring bar is also changed. The aperture near the headstock is large, and the aperture away from the headstock is small, forming a cone hole. In addition, the cantilever length of the boring bar increases, and the bending deformation of the spindle due to its own weight also increases, and the axis of the machined hole will bend accordingly. This boring method is only suitable for machining shorter holes.

Step 2: Diamond boring

Compared with ordinary boring, diamond boring is characterized by small back-feed, small feed and high cutting speed. It can obtain high machining accuracy (IT7~IT6) and smooth surface (Ra is 0.4~0.05 μm). Diamond boring was originally processed with diamond boring tools, and now it is generally processed with cemented carbide, CBN and synthetic diamond tools. It is mainly used for processing non-ferrous metal workpieces, and can also be used for processing iron castings and steel parts.

The commonly used cutting parameters for diamond boring are: back-feed pre-boring is 0.2~0.6mm, and final boring is 0.1 mm; The feed rate is 0.01 ~ 0.14 mm/r; The cutting speed is 100~250m/min when processing cast iron, 150~300m/min when processing steel and 300~2000m/min when processing nonferrous metals.

In order to ensure that diamond boring can achieve high machining accuracy and surface quality, the machine tool (diamond boring machine) used must have high geometric accuracy and stiffness, the spindle of the machine tool is usually supported by precise angular contact ball bearings or hydrostatic sliding bearings, and the high-speed rotating parts must be accurately balanced; In addition, the movement of the feeding mechanism must be very stable, so as to ensure that the workbench can do stable low-speed feeding movement.

Diamond boring has good machining quality and high production efficiency, and is widely used in the final machining of precision holes in mass production, such as engine cylinder holes, piston pin holes and spindle holes on machine tool headstock. However, it should be noted that when machining ferrous metal products with diamond, only boring tools made of cemented carbide and CBN can be used, but not diamond, because the carbon atoms in diamond have great affinity with iron group elements and the tool life is short.

Iv. Honing hole

1. Honing principle and honing head

Honing is a method of finishing holes with a honing head with grinding bars (whetstone). When honing, the workpiece is fixed, and the honing head is driven by the spindle of the machine tool to rotate and make reciprocating linear motion. In honing process, the grinding bar acts on the surface of the workpiece with a certain pressure, and a very thin layer of material is cut off from the surface of the workpiece, and its cutting trajectory is cross-reticulate. In order to make the trajectory of abrasive grains of sand bar not repeat, the revolutions per minute of the honing head’s rotary motion and the number of reciprocating strokes per minute of the honing head should be prime numbers.

The intersection angle of the honing trajectory is related to the reciprocating speed and circumferential speed of the honing head, and the angle affects the machining quality and efficiency of honing. Generally, it is for rough honing and-0 for fine honing. In order to facilitate the discharge of broken abrasive particles and chips, reduce the cutting temperature and improve the machining quality, sufficient cutting fluid should be used during honing.

In order to make the wall of the hole to be processed evenly, the stroke of the sand bar should exceed a certain amount at both ends of the hole. In order to ensure uniform honing allowance and reduce the influence of machine tool spindle rotation error on machining accuracy, floating connection is mostly adopted between honing head and machine tool spindle.

The radial expansion and contraction adjustment of honing head grinding bar has many structural forms such as manual, pneumatic and hydraulic.

2. Technological characteristics and application scope of honing.

1) Honing can obtain high dimensional accuracy and shape accuracy, and the machining accuracy is IT7~IT6, and the roundness and cylindricity errors of the hole can be controlled within the range, but honing cannot improve the position accuracy of the machined hole.

2) Honing can obtain high surface quality, the surface roughness Ra is 0.2~0.25μm, and the depth of metamorphic defect layer of surface metal is extremely small 2.5 ~ 25 μ m..

3) Compared with the grinding speed, the circumferential speed of honing head is not high (vc=16~60m/min), but the contact area between the sand bar and the workpiece is large and the reciprocating speed is relatively high (va=8~20m/min), so honing still has high productivity.

Honing is widely used in the machining of engine cylinder holes and precision holes in various hydraulic devices in mass production, and deep holes with aspect ratio greater than 10 can be machined. However, honing is not suitable for machining holes in non-ferrous metal workpieces with large plasticity, nor can it process holes with keyways and spline holes.

V. Pulling holes

1. Broaching and broaching

Broaching is a high-productivity finishing method, which is carried out on a broaching machine with a special broach. Broaching machines are divided into horizontal broaching machines and vertical broaching machines, with horizontal broaching machines being the most common.

When broaching, the broach only makes low-speed linear motion (main motion). Generally, the number of teeth of a broach working at the same time should be no less than 3, otherwise the broach will work unsteadily, and it is easy to produce annular ripples on the surface of the workpiece. In order to avoid excessive broaching force to break the broach, when the broach works, the number of teeth of the working cutter should not exceed 6~8.

There are three different broaching methods for broaching holes, which are described as follows:
1) Layered broaching

The characteristic of this broaching method is that the broach cuts off the machining allowance of the workpiece layer by layer. In order to facilitate chip breaking, the cutter teeth are ground with staggered chip separation grooves. Broach designed according to hierarchical broaching method is called ordinary broach.

2) Block broaching

The characteristic of this broaching method is that each layer of metal on the machined surface is cut by a group of cutter teeth with basically the same size but staggered cutter teeth (usually each group consists of 2-3 cutter teeth). Each cutter tooth only cuts off a part of a layer of metal. Broach designed according to block broaching method is called wheel cutting broach.

3) Integrated broaching

This method concentrates the advantages of layered broaching and block broaching, with block broaching for rough cutting teeth and layered broaching for fine cutting teeth. This can not only shorten the length of broach, improve productivity, but also obtain better surface quality. Broach designed according to comprehensive broaching method is called comprehensive broach.

2. Process characteristics and application scope of hole drawing.

1) Broach is a multi-edge tool, which can finish rough machining, finish machining and finishing machining of holes in one broaching stroke, with high production efficiency.

2) The precision of hole drawing mainly depends on the precision of broach. Under normal conditions, the precision of hole drawing can reach IT9~IT7, and the surface roughness Ra can reach 6.3~1.6 μm m..

3) During hole drawing, the workpiece is positioned by the hole itself (the leading part of the broach is the positioning element of the workpiece), so it is difficult to ensure the mutual position accuracy between the hole and other surfaces during hole drawing; For those revolving parts whose inner and outer circular surfaces require coaxiality, holes are often drawn first, and then other surfaces are machined based on the holes.

4) Broach can not only process round holes, but also process forming holes and spline holes.

5) Broach is a fixed-size tool with complex shape and expensive price, which is not suitable for machining large holes.

Broaching is commonly used in mass production to process through holes in small and medium-sized parts with a hole diameter of Ф10~80mm and a hole depth of no more than 5 times the hole diameter.

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