What are the ways in which holes are machined?

Compared with the cylindrical surface processing, the conditions of hole processing are much worse, and it is more difficult to process holes than to process the cylindrical surface. This is because:

1) The size of the tool used in hole processing is limited by the size of the hole being processed, and the rigidity is poor, which is easy to produce bending deformation and vibration;

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

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

Drilling and reaming

(1) Drill holes

Drilling is the first process of machining holes on solid materials, and the diameter of the drilling hole is generally less than 80mm. There are two ways of drilling: one is the bit rotation; The other is workpiece rotation. The error generated by the above two drilling methods is not the same, in the drilling method of the bit rotation, due to the asymmetry of the cutting edge and the insufficient rigidity of the bit and the bit deflection, the center line of the hole will be skewed or not straight, but the aperture is basically unchanged; On the contrary, in the drilling method of workpiece rotation, the bit deflection will cause the aperture to change, but the center line of the hole is still straight.

Commonly used drilling knives have: twist drill, center drill, deep hole drill, etc., of which the most commonly used is twist drill, its diameter specification is Φ0.1-80mm.

Due to structural limitations, the bending stiffness and torsional rigidity of the drill bit are low, coupled with poor centering, the drilling accuracy is low, generally only IT13 ~ IT11; The surface roughness is also large, Ra is generally 50~12.5μm; However, the metal removal rate of drilling is large and the cutting efficiency is high. Drilling is mainly used for processing holes with low quality requirements, such as bolt holes, thread bottom holes, oil holes, etc. For holes with high machining accuracy and surface quality requirements, they should be achieved by reaming, reaming, boring or grinding in subsequent processing.

(2) Reaming

Reaming is to further process the hole that has been drilled, cast or forged with a reaming drill to enlarge the aperture and improve the processing quality of the hole. Reaming can be used either as a 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 has more teeth and no cross edge.

Compared with drilling, reaming has the following characteristics:

1) The number of reaming drill teeth (3~8 teeth), good directivity, cutting is relatively stable;

2) Reaming drill without transverse edge, cutting conditions are good;

3) The processing allowance is small, the chip sink can be made shallower, the drill core can be made thicker, and the tool body strength and rigidity 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 process holes with smaller diameters. When drilling a large diameter hole (D ≥30mm), often use a small drill bit (diameter of 0.5 to 0.7 times of the aperture) to pre-drill, and then use the corresponding size of the hole reaming drill, which can improve the processing quality and production efficiency of the hole.

In addition to processing cylindrical holes, reaming drills of various special shapes (also known as countersinks) can be used to process various countersunk seat holes and countersinks. The front face of the countersink is often equipped with a guide post, guided by a machined hole.

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 machining method than internal grinding and fine boring.

(1) Reamer

Reamer is generally divided into two kinds of hand reamer and machine reamer. The handle part of the hand reamer is straight handle, the working part is longer, and the guiding function is better. The hand reamer has two kinds of structures: integral and adjustable outside diameter. The machine reamer has two kinds of structure with handle and sleeve. The reamer can not only process round holes, but also taper reamer can process taper holes.

(2) Reaming process and its application

Reaming allowance has a great influence on the quality of reaming, the allowance is too large, the load of the reamer is large, the cutting edge is soon blunted, it is not easy to obtain a smooth machining surface, and the dimensional tolerance is not easy to guarantee; The margin is too small to remove the knife marks left by the previous process, and naturally there is no role in improving the quality of hole processing. Generally, the margin of coarse hinge is 0.35~0.15mm, and that of fine hinge is 0.15~0.05mm.

To avoid chip nodules, reaming is usually processed at a lower cutting speed (v < 8m/min for steel and cast iron with HSS reamers). The value of feed is related to the aperture to be machined, the larger the aperture, the larger the feed value, the feed rate of high-speed steel reamer processing steel and cast iron is usually 0.3~1mm/r.

Reaming must be cooled, lubricated and cleaned with appropriate cutting fluid to prevent chip buildup and remove chips in time. Compared with grinding and boring, the reaming productivity is higher and the accuracy of the hole is easily guaranteed. However, reaming can not 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 step holes and blind holes.

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

Bore a hole



Boring is a machining method in which the prefabricated hole is enlarged with a cutting tool. The boring work can be carried out either on the boring machine or on the lathe.

(1) Boring method

There are three different machining methods for boring.

1) Workpiece rotation, cutting tool for feed movement on the lathe boring mostly belongs to this boring mode. The characteristics of the process are: the axis line 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 geometry error of the hole mainly depends on the position accuracy of the tool feed direction relative to the rotation axis of the workpiece. This boring method is suitable for machining holes with coaxial requirements on the surface of the outer circle.

2) Tool rotation, workpiece feed movement boring machine spindle drive boring tool rotation, table drive workpiece feed movement.

3) The tool rotates and makes the feed movement using this boring method to boring, the overhang length of the boring bar is changed, the force deformation of the boring bar is also changed, the aperture near the headstock is large, the aperture away from the headstock is small, forming a cone hole. In addition, with the increase of the overhang length of the boring bar, the bending deformation of the main shaft caused by its own weight also increases, and the axis of the machined hole will have a corresponding bending. This boring method is only suitable for machining short holes.

(2) Diamond boring

Compared with general boring, diamond boring is characterized by a small amount of back cutting, small feed, high cutting speed, it can obtain a high processing accuracy (IT7 ~ IT6) and a very smooth surface (Ra is 0.4~ 0.05μm). Diamond boring was originally processed with diamond boring tools, and is now commonly processed with cemented carbide, CBN and artificial diamond tools. Mainly used for processing non-ferrous metal workpieces, can also be used for processing cast iron and steel parts.

The commonly used cutting parameters of diamond boring are:

The amount of back cutting tool pre-boring is 0.2~0.6mm,

The final boring is 0.1mm;

The feed rate is 0.01~0.14mm/r;

The cutting speed is 100~250m/min when processing cast iron,

Steel processing is 150~300m/min,

300~2000m/min for processing non-ferrous metals.

In order to ensure that the diamond boring machine can achieve high machining accuracy and surface quality, the machine tool (diamond boring machine) must have high geometric accuracy and stiffness, the main shaft of the machine tool supports the commonly used precision angular contact ball bearing or static pressure plain bearing, and the high-speed rotating parts must be accurately balanced; In addition, the movement of the feed mechanism must be very smooth to ensure that the table can do smooth low-speed feed movement.

The machining quality of diamond boring is good, the production efficiency is high, and it is widely used in the final processing of precision holes in a large number of mass production, such as the engine cylinder hole, the piston pin hole, the main shaft hole on the spindle box of the machine tool. However, it should be noted that when machining ferrous metal products with diamond boring, only the boring tool made of cemented carbide and CBN can be used, and the boring tool made of diamond can not be used, because the carbon atoms in diamond have a large affinity with the iron group elements, and the tool life is low.

(3) Boring tool

Boring tool can be divided into single-edge boring tool and double-edge boring tool.

(4) Boring process characteristics and application range

Compared with the drilling, expanding and reaming process, the bore size is not limited by the tool size, and the boring has a strong error correction ability, and the deviation error of the original hole axis can be corrected by multiple cutting, and the boring can maintain a higher position accuracy with the positioning surface.

Compared with the outer circle of the boring, due to the poor rigidity of the tool bar system, large deformation, poor heat dissipation and chip removal conditions, the hot 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.

In summary, it can be seen that the processing range of boring is wide, and holes of different sizes and different precision levels can be processed. For holes and hole systems with large aperture, high size and position accuracy requirements, boring is almost the only processing method. The machining accuracy of boring is IT9 ~ IT7. Boring can be carried out on the boring machine, lathe, milling machine and other machine tools, which has the advantages of flexibility and flexibility, and is widely used in production. In mass production, boring die is often used to improve boring efficiency.