Summary and explanation of basic knowledge points related to hole processing

Compared with the surface processing of the outer circle, the conditions of hole processing are much worse, and the processing of the hole is more difficult than the processing of the outer circle. This is because:

 

1) The size of the tool used for hole processing is limited by the size of the hole to be processed, and the rigidity is poor, which is prone to bending deformation and vibration;

 

2) When machining holes with fixed-size tools, the size of the hole processing is often directly dependent on the corresponding size of the tool, and the manufacturing error and wear of the tool will directly affect the machining accuracy of the hole;

 

3) When machining holes, the cutting area is inside the workpiece, so the chip removal and heat dissipation conditions are poor, and the machining accuracy and surface quality are not easy to control.

 

1. Drilling and reaming 

(1)Drilling 

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

 

Commonly used drilling tools include twist drills, center drills, deep hole drills, etc. The most commonly used one is twist drills with a diameter of Φ0.1-80mm.

 

Due to structural limitations, the bending rigidity and torsional rigidity of the drill bit are low, and the centering is not good, the drilling accuracy is low, generally only IT13 ~ IT11; the surface roughness is also relatively large, Ra is generally 50 ~12.5μm; but the metal removal rate of drilling is large, and the cutting efficiency is high. Drilling is mainly used to process holes with low quality requirements, such as bolt holes, threaded bottom holes, and oil holes. For holes that require high machining accuracy and surface quality, they should be achieved by reaming, reaming, boring or grinding in the subsequent processing.

 

(2)Reaming

Reaming is to use a reaming drill to further process a hole that has been drilled, cast or forged to enlarge the aperture and improve the processing quality of the hole. The reaming process can be used as a pre-processing before finishing the hole, or as a Final machining of less demanding holes. Reamers are similar to twist drills, but they have more teeth and no chisel.

 

Compared with drilling, reaming has the following characteristics: (1) The reaming drill has a large number of teeth (3~8 teeth), good guidance, and the cutting is relatively stable; (2) The reaming drill has no chisel edge, and the cutting conditions are good; (3) The machining allowance is small, the chip pocket can be made shallower, the drill 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 12.5~6.3μm. Reaming is often used to machine holes with diameters less than Ø. When drilling a hole with a larger diameter (D ≥30mm), a small drill bit (0.5~0.7 times the diameter of the hole diameter) is often used to pre-drill the hole, and then a reaming drill of the corresponding size is used to ream the hole, which can improve the hole Processing quality and production efficiency.

 

In addition to processing cylindrical holes, reaming can also use various special-shaped reaming drills (also known as countersink drills) to process various countersunk seat holes and countersinking. The front end of the countersink often has a guide post, which is guided by a machined hole.

 

 

2. Reaming

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

 

(1)Reamers 

Reamers are generally divided into two types: hand reamer and machine reamer. The handle of the hand reamer is a straight handle, the working part is longer, and the guiding effect is better. The hand reamer has two structures: an integral type and an adjustable outer diameter. The machine reamer has two kinds of structures: a shank and a sleeve type. The reamer can not only process circular holes, but also taper holes can be processed by taper reamer.

 

(2) Reaming process and its application

The reaming allowance has a great influence on the quality of the reaming. The allowance is too large, the load of the reamer is large, the cutting edge is quickly blunt, it is not easy to obtain a smooth processing surface, and the dimensional tolerance is not easy to guarantee; the allowance is too small, The tool marks left by the previous process cannot be removed, and naturally there is no effect of improving the quality of hole processing. Generally, the margin of rough hinge is 0.35~0.15mm, and the margin of fine hinge is 01.5~0.05mm.

 

In order to avoid the build-up edge, the reaming is usually processed at a lower cutting speed (v ＜8m/min when processing steel and cast iron with high-speed steel reamer). The value of the feed is related to the aperture to be machined. The larger the aperture, the greater the value of the feed. The feed of high-speed steel reamer is usually 0.3~1mm/r when processing steel and cast iron.

 

When reaming, it is necessary to use appropriate cutting fluid for cooling, lubrication and cleaning to prevent the buildup of edge and remove the chips in time. Compared with hole grinding and boring, reaming has high productivity and easy to ensure the accuracy of the hole; but reaming cannot 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 machining stepped holes and blind holes.

 

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

 

3.Boring

Boring is a processing method to enlarge the prefabricated hole with a cutting tool. The boring work can be carried out on a boring machine or a lathe.

 

(1)Boring method 

There are three different processing methods for boring.

1) The workpiece rotates and the tool performs feed motion. Most boring on a lathe belongs to this boring method. The characteristic of the process is: the axis line of the hole after machining 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 feed direction of the tool relative to the rotation axis of the workpiece The location accuracy. This boring method is suitable for processing holes that require coaxiality with the outer circular surface.

 

2) The tool rotates and the workpiece moves in feed motion. The spindle of the boring machine drives the boring tool to rotate, and the worktable drives the workpiece in feed motion.

 

3)The tool rotates and makes the feed movement. This boring method is used for boring. The overhang length of the boring bar is changed, and the deformation of the boring bar is also changed. The hole near the headstock is large, and the hole is far away from the headstock. The hole diameter is small, forming a tapered hole. In addition, the overhang 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 be bent accordingly. This boring method is only suitable for processing shorter holes.

 

(2) Diamond Boring

Compared with general boring, diamond boring is characterized by a small amount of back tooling, a small feed rate, and a high cutting speed. It can obtain high machining accuracy (IT7～IT6) and a very 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. Mainly used for processing non-ferrous metal workpieces, but also for processing cast iron and steel parts.

 

The commonly used cutting parameters for diamond boring are: the amount of back-cutting is 0.2~0.6mm for pre-boring, and the final boring is 0.1mm; the feed is 0.01~0.14mm/r; the cutting speed is 100~250m/min when machining cast iron. 150~300m/min for steel and 300~2000m/min for non-ferrous 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 rigidity. The spindle of the machine tool supports commonly used precision angular contact ball bearings or hydrostatic sliding bearings, and high-speed rotating parts. It must be accurately balanced; in addition, the movement of the feed mechanism must be very stable to ensure that the worktable can do a smooth low-speed feed movement.

 

Diamond boring has good processing quality and high production efficiency. It is widely used in the final processing of precision holes in mass production, such as engine cylinder holes, piston pin holes, and spindle holes on machine tool spindle boxes. However, it should be noted that when using diamond boring to process ferrous metal products, only boring tools made of cemented carbide and CBN can be used, and boring tools made of diamond cannot be used, because the carbon atoms in diamond have a large affinity with iron group elements. , Tool life is low.

 

(3)Boring tool

Boring tools can be divided into single-edged boring tools and double-edged boring tools.

 

(4) Process characteristics and application scope of boring

Compared with the drilling-expanding-reaming process, the aperture size is not limited by the tool size, and the boring has a strong error correction ability. The deviation of the original hole axis can be corrected through multiple passes, and it can make The boring hole and the positioning surface maintain a high position accuracy.

 

Compared with the outer circle of the car, due to the poor rigidity and large deformation of the tool holder system, the heat dissipation and chip removal conditions are not good, the thermal deformation of the workpiece and the tool is relatively large, and the processing quality and production efficiency of the boring are not as high as the outer circle of the car. .

 

From the above analysis, it can be seen that boring has a wide processing range, and can process holes of various sizes and different accuracy levels. For holes and hole systems with large diameters, high size and position accuracy requirements, boring is almost the only processing method. Method. The machining accuracy of boring is IT9～IT7. Boring can be carried out on machine tools such as boring machines, lathes, milling machines, etc. It has the advantages of mobility and flexibility, and is widely used in production. In mass production, in order to improve boring efficiency, boring dies are often used.

 

4. honing the hole

(1)Honing principle and honing head

Honing is a method that uses a honing head with a grinding rod (whetstone) to smooth the hole. During honing, the workpiece is fixed, and the honing head is driven by the main shaft of the machine tool to rotate and make a reciprocating linear motion. In the honing process, the grinding rod acts on the surface of the workpiece with a certain pressure, and a very thin layer of material is removed from the surface of the workpiece, and the cutting track is a cross net. In order to make the trajectory of the abrasive grains not repeatable, the number of revolutions per minute of the honing head's rotary motion and the number of reciprocating strokes per minute of the honing head should be mutually prime.

 

The cross angle picture of the honing track is related to the reciprocating speed picture and the circumferential speed picture of the honing head. The size of the picture angle affects the processing quality and efficiency of the honing. Generally, the picture is taken for rough honing and the picture is taken for fine honing. In order to facilitate the discharge of broken abrasive particles and chips, reduce the cutting temperature, and improve the processing quality, sufficient cutting fluid should be used during honing.

 

In order to make the wall of the hole to be processed evenly processed, the stroke of the sand bar must exceed a certain amount of overtravel 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, the honing head and the machine tool spindle are mostly connected by floating connections.

 

The radial expansion and contraction adjustment of the honing head grinding rod has various structures such as manual, pneumatic and hydraulic.

 

(2)Process characteristics and application scope of honing

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

 

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

 

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

 

Honing is widely used in the machining of precision holes in engine cylinder bores and various hydraulic devices in mass production. The range of diameters is generally or larger, and deep holes with a length-to-diameter ratio greater than 10 can be machined. But honing is not suitable for processing holes on non-ferrous metal workpieces with greater plasticity, nor can it machine holes with keyways, spline holes, etc.

 

5.Lacon 

(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 two types, horizontal broaching machines and vertical broaching machines, and horizontal broaching machines are the most common.

 

When broaching, the broach only makes a low-speed linear motion (main motion). The number of teeth of the broach working at the same time should generally be no less than 3, otherwise the broach will not work smoothly and it is easy to produce ring-shaped waves on the surface of the workpiece. In order to prevent the broach from being broken due to excessive broaching force, when the broach is working, the number of working teeth at the same time should generally not exceed 6 to 8.

 

There are three different broaching methods for broaching, which are described as follows:

1) Layered broaching The characteristic of this broaching method is that the broach cuts the workpiece machining allowance layer by layer. In order to facilitate chip breaking, the cutter teeth are ground with interlaced chip dividing grooves. Broaches designed according to the layered broaching method are called ordinary broaches.

 

2) Block broaching. The feature of this broaching method is that each layer of metal on the processed surface is composed of a group of teeth with the same size but interlaced teeth (usually each group is composed of 2-3 teeth) Excised. Each tooth cuts only part of a layer of metal. Broaches designed according to the block broaching method are called wheel-cutting broaches.

 

3) Comprehensive broaching This method concentrates the advantages of layered and divided broaching. The rough cutting part adopts divided broaching, and the fine cutting part adopts layered broaching. In this way, the length of the broach can be shortened, productivity can be improved, and better surface quality can be obtained. The broach designed according to the comprehensive broaching method is called the comprehensive broach.

 

(2) Process characteristics and application scope of broaching

1) The broach is a multi-edged tool, which can complete the roughing, finishing and smoothing of the hole sequentially in one broaching stroke, with high production efficiency.

 

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

 

3) When the hole is drawn, the workpiece is positioned by the machined hole itself (the leading part of the broach is the positioning element of the workpiece). It is difficult for the hole to ensure the mutual position accuracy of the hole and other surfaces; for those rotations that require coaxiality on the inner and outer circular surfaces In the processing of body parts, holes are often drawn first, and then other surfaces are processed using the hole as a positioning reference.

 

4) The broach can not only process round holes, but also process shaped holes and spline holes.

 

5) The broach is a fixed-size tool with complex shape and high price, which is not suitable for processing large holes.

 

Pull holes are commonly used in mass production to process through holes on small and medium-sized parts with an aperture of Ф10~80mm and a hole depth not exceeding 5 times the aperture.