What are the methods of machining holes?

What is Machined hole

Machining is the process of creating a part by cutting away material from a solid block of material using an abrasive tool. Machining can also refer to the finished product produced by this process, called a machined part.

Machining holes is one of the most common types of manufacturing processes. It can be used to cut circular, square, or rectangular holes in metal. Machined holes are used for various purposes like forming, molding, and machining.

Machined holes are made by cutting a round or square hole in a solid block of metal and then removing the waste material. This process can be done using either manual tools or automated machines, it is often done by turning a piece of metal or other material into a cylinder, and then drilling it through with an end mill. The end mill is then removed, leaving a hole in its place.

Machined holes have been used for centuries to create various products, ranging from jewelry to furniture.

What are the methods of machining holes?

There are many methods of machining holes in metal. The most common methods are drilling, reaming, boring, and honing. These five main methods are the most common ways of machining holes in metal.

01: Drilling hole

Drilling is the first process of machining holes in solid materials, and the diameter of the holes is generally less than 80mm. There are two ways of drilling: one is the rotation of the drill; the other is the rotation of the workpiece. The errors generated by the above two drilling methods are different. In the drilling method with the drill bit rotating, when the drill bit is deflected due to the asymmetry of the cutting edge and the insufficient rigidity of the drill bit, the center line of the machined hole will be skewed or distorted. It is not straight, but the hole diameter is basically unchanged; on the contrary, in the drilling method in which the workpiece is rotated, the deviation of the drill bit will cause the hole diameter to change, while the hole center line is still straight.

Due to structural limitations, the bending rigidity and torsional rigidity of the drill bit are both low, coupled with poor centering, the drilling accuracy is low, generally only reaching IT13 ~ IT11; the surface roughness is also large, and 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, 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 machining.

02: Reaming hole

Reaming is the further processing of holes that have been drilled, cast or forged with a reaming drill to expand the aperture and improve the processing quality of the holes. Final machining of less demanding holes. A reaming drill is similar to a twist drill, but with more teeth and no chisel edge.

Compared with drilling, reaming has the following characteristics:

(1) The reaming drill has a large number of teeth (3~8 teeth), good guidance, and relatively stable cutting;
(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 hole reaming is generally IT11~IT10, and the surface roughness Ra is 12.5~6.3μm. Reaming is often used to machine holes with a diameter smaller than . When drilling a hole with a larger diameter (D ≥ 30mm), a small drill bit (diameter is 0.5~0.7 times the diameter of the hole) is often used to pre-drill the hole, and then the corresponding size of the reaming drill is used to ream the hole, which can improve the hole’s quality. Process quality and production efficiency.

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

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 method than internal grinding and fine boring.

03: Boring hole

Boring is a processing method that uses cutting tools to enlarge prefabricated holes. Boring work can be carried out on a boring machine or a lathe.

There are three different machining methods for boring holes.

(1) The workpiece rotates and the tool feeds. Most of the boring on the lathe belongs to this boring method. The process characteristics are: 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 geometry error of the hole mainly depends on the feed direction of the tool relative to the rotation axis of the workpiece. position accuracy. This boring method is suitable for processing holes that have coaxiality requirements with the outer surface.

(2) The tool rotates and the workpiece makes a feeding motion. The spindle of the boring machine drives the boring tool to rotate, and the worktable drives the workpiece to make a feeding motion.

(3) When the tool rotates and makes a feeding motion, the boring method is used for boring. The overhang length of the boring bar is changed, and the force deformation of the boring bar is also changed. 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 main shaft 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 short holes.

Diamond boring

Compared with ordinary boring, diamond boring is characterized by small back-feeding, small feed, and high cutting speed. It can obtain high machining accuracy (IT7~IT6) and 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.

The commonly used cutting quantities for diamond boring are: the back-cut amount of pre-boring is 0.2~0.6mm, and the final boring is 0.1mm; the feed rate is 0.01~0.14mm/r; the cutting speed is 100~250m/min when machining cast iron. 150~300m/min for steel, 300~2000m/min for processing 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 main shaft of the machine tool is usually supported by precision angular contact ball bearings or hydrostatic sliding bearings, and high-speed rotating parts. It must be precisely balanced; in addition, the movement of the feeding mechanism must be very stable to ensure that the worktable can perform stable and low-speed feeding movement.

Diamond boring has good processing 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 spindle boxes. However, it should be noted that when machining ferrous metal products with diamond boring, 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. , the tool life is low.

Boring tool

Boring tools can be divided into single edge boring tools and double edge boring tools.

Technological characteristics and application range of boring

Compared with the drilling-expanding-reaming process, the diameter of the hole is not limited by the size of the tool, and the boring has a strong error correction ability. The boring and positioning surfaces maintain high positional accuracy.

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

Based on 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 and high dimensional and positional accuracy requirements, boring is almost the only processing. 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 flexibility and is widely used in production. In mass production, in order to improve the boring efficiency, boring dies are often used.

04: Honing holes

1. Honing principle and honing head

Honing is a method of finishing a hole with a honing head with a grinding stick (whitstone). During honing, the workpiece is fixed, and the honing head is driven by the spindle of the machine to rotate and make a reciprocating linear motion. In the honing process, the grinding bar acts on the surface of the workpiece with a certain pressure, and cuts a very thin layer of material from the surface of the workpiece, and the cutting trajectory is a crossed mesh. In order to make the movement trajectory of the abrasive grains of the sand bar not repeat, the revolutions per minute of the rotary motion of the honing head and the number of reciprocating strokes per minute of the honing head should be prime numbers of each other.

The intersection angle of the honing track is related to the reciprocating speed and peripheral speed of the honing head, and the size of the angle affects the processing quality and efficiency of 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 hole wall to be processed uniformly, the stroke of the sand bar should exceed an overrun 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, most of the honing heads and machine tool spindles are connected by floating.

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

2. The process characteristics and application range of honing

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

2) Honing can obtain high 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, although the peripheral 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). 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. However, honing is not suitable for processing holes on non-ferrous metal workpieces with large plasticity, nor can it process holes with key grooves, spline holes, etc.

05: Broaching hole

1. Broaching and broaching

Hole broaching is a highly productive finishing method that is performed on a broaching machine with a special broach. There are two types of broaching bed: horizontal broaching bed and vertical broaching bed, with horizontal broaching bed being the most common.

When broaching, the broach only makes 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 annular ripples on the surface of the workpiece. In order to prevent the broach from breaking due to excessive broaching force, when the broach is working, the number of working teeth 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 sequentially. In order to facilitate chip breaking, the cutter teeth are ground with staggered chip separation grooves. The broach designed according to the layered 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 consists of a group of teeth with basically the same size but staggered teeth (usually each group consists of 2-3 teeth) excised. Each tooth only cuts off part of a layer of metal. The broach designed according to the block broaching method is called a wheel-cut broach.

3) Comprehensive broaching This method concentrates the advantages of layered and segmented broaching. The rough tooth part adopts segmented broaching, and the fine tooth part adopts layered broaching. In this way, the length of the broach can be shortened, the 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 hole pulling

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

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

3) When pulling a hole, the workpiece is positioned by the machined hole itself (the leading part of the broach is the positioning element of the workpiece), and it is not easy to ensure the mutual positional accuracy of the hole and other surfaces; In the processing of body parts, holes are often drawn first, and then other surfaces are machined using the holes as the positioning reference.

4) The broach can not only process round holes, but also form 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.

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

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

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

2) When machining a hole with a fixed-size tool, the size of the hole is often directly determined by 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, the chip removal and heat dissipation conditions are poor, and the machining accuracy and surface quality are not easy to control.