Holes are important surfaces on parts such as boxes, brackets, sleeves, rings, and disks, and are also surfaces often encountered in machining. Under the same requirements for machining accuracy and surface roughness, machining holes is more difficult than machining cylindrical s

Holes are important surfaces on parts such as boxes, brackets, sleeves, rings, and disks, and are also surfaces often encountered in machining. Under the same requirements for machining accuracy and surface roughness, machining holes is more difficult than machining cylindrical surfaces, resulting in low productivity and high cost.

The productivity is low and the cost is high because:

• The size of the tool used for hole processing is limited by the size of the hole to be processed. It has poor rigidity and is prone to bending deformation and vibration;
• When using fixed-size tools to process holes, the size of the hole processing often directly depends on Depending on the corresponding size of the tool, the manufacturing error and wear of the tool will directly affect the machining accuracy of the hole;
• When processing holes, the cutting area is inside the workpiece, chip removal and heat dissipation conditions are poor, and the machining accuracy and surface quality are difficult to control.

Hole processing methods include drilling, reaming, reaming, boring, drawing, grinding, hole finishing, etc. Below, we will introduce several hole processing techniques in detail to solve the problem of hole processing.

Drilling and reaming

Drilling

Drilling is the first process of processing holes in solid materials. The diameter of the hole is generally less than 80mm. There are two ways of drilling: one is drill bit rotation; the other is workpiece rotation.

The errors produced by the above two drilling methods are different. In the drilling method with drill bit rotation, when the drill bit is deflected due to the asymmetry of the cutting edge and insufficient drill bit rigidity, the center line of the hole to be processed will be deflected. Or not straight, but the hole diameter basically remains unchanged; on the contrary, in the drilling method where the workpiece rotates, the deviation of the drill bit will cause the hole diameter to change, but the hole centerline remains straight.

Two drilling methods

a) Drill bit rotation b) Workpiece rotation

Commonly used drilling tools include: twist drill, center drill, deep hole drill , etc. Among them, the most commonly used twist drill has a diameter specification of φ . 1~80mm. The structure of a standard twist drill is that its shank is the clamping part of the drill bit and is used to transmit torque; the drill bit shank has two types: straight shank and tapered shank, the former is used for small diameter drill bits, and the latter is used for large diameter drill bits.

The structure of the standard twist drill

a) Tapered shank b) Straight shank

Due to structural limitations, the bending stiffness and torsional stiffness of the drill bit are low. In addition, the centering is not good, and the drilling accuracy is low. Generally, only It can reach IT13~IT11; the surface roughness is also 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, oil holes, etc. For holes with higher processing accuracy and surface quality requirements, they should be achieved by enlarging, reaming, boring or grinding in subsequent processing.

Reaming

Reaming is to use reaming drill to further process the hole that has been drilled, cast or forged to expand the hole diameter and improve the processing quality of the hole. reaming processing can be used as a finishing hole Pre-machining can also be used as the final machining of less demanding holes.

reaming

reaming drill is similar to a twist drill, but has more teeth and no chisel edge.

Reaming drill

Compared with drilling, reaming has the following characteristics:

(1) Reaming drill has many teeth (3~8 teeth), good guidance, and relatively stable cutting;

(2) Reaming drill does not have Chisel edge, good cutting conditions;

(3) The machining allowance is small, the chip flute can be made shallower, the drill core can be made thicker, and the cutter body strength and rigidity are better.

The precision of hole expansion processing is generally IT11~IT10 level, and the surface roughness Ra is 12.5~6.3μm. Hole enlarging is often used to process holes with a diameter less than 30mm.When drilling a hole with a larger diameter (D≥30mm), a small drill bit (with a diameter of 0.5~0.7 times the hole diameter) is often used to pre-drill the hole, and then a reamer drill of the corresponding size is used to enlarge the hole. This can improve the hole quality. Processing quality and production efficiency.

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

Countersinking

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 precision boring.

Reamers

Reamers are generally divided into two types: hand reamers and machine reamers . The handle of the hand reamer is straight, the working part is long, and the guiding effect is good. Hand reamers are divided into two types: integral type (Figure a) and adjustable outer diameter type (Figure b).

Machine reamers can be divided into handles (Figure c, φ~20mm are straight handles, φ0~32 are tapered handles) and sleeve type (Figure d). The reamer can not only process circular holes, but also taper reamers can be used to process tapered holes (Figure e).

Reamer

Reaming process and its application

The reaming allowance has a great influence on the reaming quality. If the allowance is too large, the reamer will have a heavy load, and the cutting edge will be blunted quickly, making it difficult to obtain smooth processing. On the surface, it is not easy to guarantee the dimensional tolerance of and ; the margin is too small and the knife marks left by the previous process cannot be removed, which naturally has no effect on improving the quality of hole processing. Generally, the rough reaming allowance is 0.35~0.15mm, and the fine reaming allowance is 0.15~0.05mm.

In order to avoid built-up edge , reaming is usually performed at a lower cutting speed (when high-speed steel reamers process steel and cast iron, v<8m/min).>

When reaming, appropriate cutting fluid must be used for cooling, lubrication and cleaning to prevent the occurrence of built-up edge 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 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 processing step holes and blind holes.

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

Boring

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

Boring method

There are three different processing methods for boring.

(1) The workpiece rotates and the tool makes feed motion

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 of the hole The shape error 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 require coaxiality with the outer circular surface.

The boring method of rotating the workpiece and feeding the tool

(2) The tool rotates and the workpiece makes a feed motion

The spindle of the boring machine drives the boring tool to rotate, and the workbench drives the workpiece to make a feed motion.

(3) The tool rotates and makes feed motion

uses this boring method to bore holes. The overhang length of the boring bar changes, and the force deformation of the boring bar also changes. The hole diameter at near the spindle box is large. , 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.

Boring method in which the tool rotates and feeds

- Boring bar 2 - Boring tool 3 - Workpiece 4 - Workbench 5 - Spindle

High-speed fine boring (diamond boring)

Compared with general boring, diamond boring The characteristics of boring are The back cutting amount is small, the feed amount is small, and the cutting speed is high. It can achieve high machining accuracy (IT7~IT6) and a very smooth surface (Ra is 0.4~0.05μm). Diamond boring was initially processed with diamond boring tools. Now it is commonly processed with carbide , CBN and artificial diamond tools. It is mainly used to process non-ferrous metal workpieces, and can also be used to process cast iron parts and steel parts.

Commonly used cutting amounts for diamond boring are:

Back cutting amount for pre-boring is 0.2~0.6mm;

final boring is 0.1mm;

feed rate is 0.01~0.14mm/r;

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

is 150~300m/min when processing steel;

is 300~2000m/min when processing non-ferrous metals;

In order to ensure that diamond boring can achieve higher processing accuracy and surface quality, the machine tool (diamond boring machine) used must With high geometric accuracy and stiffness, the main shaft of machine tools commonly supports precision angular contact ball bearings or hydrostatic sliding bearings. High-speed rotating parts must be accurately balanced; in addition, the movement of the feed mechanism must be very smooth to ensure that the worktable can Make a smooth and low-speed feed motion.

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, spindle holes on machine tool spindle boxes, etc. However, it should be noted that when processing ferrous metal products with diamond boring, only boring tools made of carbide and CBN can be used, and boring tools made of diamond cannot be used, because the carbon atoms in diamond have a strong affinity with iron group elements. , tool life is low.

boring tools

boring tools can be divided into single-edged boring tools and double-edged boring tools. The structure of the single-edged boring tool (pictured) is similar to the turning tool , with only one main cutting edge. When boring a hole with a single-edged boring tool, the size of the hole is guaranteed by the operator adjusting the position of the boring head.

Single-edge boring tool

a) Through-hole single-edge boring tool b) Blind hole single-edge boring tool

Double-edge boring tool has two symmetrical cutting edges, which is equivalent to two symmetrically installed turning tools participating in cutting at the same time; hole size The accuracy is guaranteed by the size of the boring tool itself. The floating boring tool shown in the picture is a type of double-edged boring tool. The boring blade is inserted into the slot of the boring bar and automatically balances its position by relying on the back force acting on the two cutting edges, which can eliminate the installation error of the boring tool. Or the error caused by the deflection of the boring bar; but it is similar to reaming and can only ensure dimensional accuracy and cannot correct the position error of the hole axis before reaming.

Floating boring tool

Boring process characteristics and application scope

Compared with drilling-expanding-reaming process, the hole diameter size is not limited by the tool size, and boring has strong error correction ability, and can be passed through multiple passes The tool can be used to correct the deviation error of the original hole axis, and can maintain high positional accuracy between the boring hole and the positioning surface.

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

From the above analysis, it can be seen that boring has a wide processing range and can process holes of various sizes and different precision levels. For holes and hole systems with larger diameters and higher requirements for size and position accuracy, boring is almost the only method. processing method. The machining accuracy of boring holes is IT9~IT7 level. Boring can be performed on boring machines, lathes, milling machines and other machine tools. It has the advantage of mobility and flexibility and is widely used in production. In mass production, in order to improve boring efficiency, boring mold is often used.

Honing holes

Honing principle and honing head

Honing is a method of finishing the hole using a honing head with a grinding bar ( oilstone ). When honing, the workpiece is fixed and the honing head is driven by the machine tool spindle to rotate and make reciprocating linear motion. During the honing process, the grinding bar 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 its cutting trajectory is a cross pattern. In order to prevent the movement trajectories of the abrasive grains from being repeated, the number of revolutions per minute of the honing head's rotary motion and the number of reciprocating strokes of the honing head per minute should be prime numbers to each other.

Honing principle

a) Forming motion b) Sand bar grinding trajectory expansion diagram c) Resultant speed

The intersection angle of the honing trajectory is related to the reciprocating speed and circumferential speed of the honing head. The size of the angle affects the processing quality and efficiency of honing. In order to facilitate the discharge of broken abrasive grains and chips, reduce the cutting temperature and improve the processing quality, sufficient cutting fluid should be used during honing.

In order to ensure that the wall of the hole to be processed can be processed evenly, the stroke of the sand bar must exceed an overtravel amount at both ends of the hole. In order to ensure uniform honing allowance and reduce the impact of machine tool spindle rotation error on machining accuracy, floating connections are mostly used between the honing head and the machine tool spindle. The radial telescopic adjustment of the honing head grinding bar has various structural forms such as manual, pneumatic and hydraulic.

Honing process characteristics and application scope

① Honing can obtain high dimensional accuracy and shape accuracy. The processing accuracy is IT7~IT6 level. The roundness and cylindricity error of the hole can be controlled within the range of 3~5 μm, but Honing cannot improve the positional accuracy of the machined hole.

② Honing can obtain higher surface quality, the surface roughness Ra is 0.2~0.025μm, 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), 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 high productivity.

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

broaching

broaching and broaching

broaching is a high-productivity finishing method, which is performed on a broaching machine using a special broach . broaching machine is divided into two types: horizontal broaching machine and vertical broaching machine, with horizontal broaching machine being the most common.

During 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 avoid excessive broaching force causing the broach to break, when the broach is working, the number of working teeth should generally not exceed 6 to 8 at the same time.

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

) Layered broaching

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

Layered broaching

a) Broaching graphics b) Cutting part tooth shape c) Chips

) Block broaching

The characteristic of this broaching method is that each layer of metal on the machined surface is composed of a group of basically the same size But the cutter teeth are cut with staggered teeth (usually each group consists of 2-3 teeth). Each tooth cuts only a portion of a layer of metal. Broaches designed according to the block broaching method are called wheel-cut broaches.

) Comprehensive broaching

This method combines the advantages of layered and block broaching. The rough cutting tooth part adopts block broaching, and the fine cutting tooth part adopts layered broaching. This can not only shorten the length of the broach, improve productivity, but also obtain better surface quality. Broaches designed according to the comprehensive broaching method are called comprehensive broaches.

broaching process characteristics and application scope

) The broach is a multi-edged tool that can sequentially complete the rough machining, finishing and finishing of the hole in one broaching stroke, with high production efficiency.

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

) When drawing holes, the workpiece is positioned by the hole to be processed (the leading part of the broaching tool is the positioning element of the workpiece). It is not easy to ensure the mutual position accuracy of the hole and other surfaces when drawing holes; for those rotations with coaxiality requirements on the inner and outer circular surfaces In the processing of solid parts, the hole is often drawn first, and then other surfaces are processed using the hole as the positioning reference.

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

) Broaching tools are fixed-size tools with complex shapes and expensive prices, and are not suitable for processing large holes.

broaching is commonly used in mass production to process through holes on small and medium-sized parts with apertures of φ0~80mm and hole depth not exceeding 5 times of the aperture.

Summary

Drilling characteristics: poor tool rigidity, difficulty in chip removal, and difficult to discharge cutting heat. Features of

hole expansion: ① The cutting edge does not need to extend from the outer circle to the center, avoiding the undue influence of the chisel edge and its hardness; ② Because ap is small, the cutting is narrow and easy to remove; at the same time, the chip flute can be made smaller and shallower , increase tool stiffness; ③ High productivity, good guidance, and smooth cutting; ④ The processing quality of enlarging holes is higher than drilling.

reaming characteristics: good tool rigidity, good guidance, small reaming allowance, small cutting force, Vc low, less cutting heat, which means the heating and deformation of the workpiece are reduced, and it can be used for finishing.

In addition, drilling, expanding, and reaming can only ensure the accuracy of the holes themselves, but not the dimensional accuracy of the distance between the holes. At this time, a clamp or boring clamp can be used to ensure.