☞ Source: Jijia Xiao Zhuge

Holes are important surfaces on boxes, brackets, sleeves, rings, and disc parts, and they are also often encountered in machining. In the case of the same machining accuracy and surface roughness requirements, it is more difficult to process holes than machining outer surfaces, and the productivity is low and the cost is high.

Low productivity and high costs are due to:

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

When the hole is processed 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 machining accuracy of the hole;

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;

The processing methods of holes include drilling, reaming, reaming, boring, drawing, grinding, and finishing of holes. The following will give you a detailed introduction to several hole processing processes to solve the problem of hole processing.

Drilling and reaming

drill

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

The error produced by the above-mentioned two kinds of drilling mode is not the same, in the drilling mode of drill bit rotation, when the drill bit is deviated due to the asymmetry of the cutting edge and the lack of rigidity of the drill bit, the center line of the processed hole can be deflected or not straight, but the hole diameter is basically unchanged; In the case of rotating workpiece drilling, the opposite is true, where the drill bit misalignment causes a change in the hole diameter, while the hole centerline remains straight.

           Two types of drilling

                                                                                                                                a) Drill bit rotation    b) Workpiece rotation

Commonly used drilling knives are: twist drill, center drill, deep hole drill, etc., the most commonly used of which is twist drill, the diameter of which is 0.1~80mm. The structure of the standard twist drill, the shank of which is the clamping part of the drill bit and is used to transmit torque; There are two types of drill shanks, straight shanks and taper shanks, the former is used for small diameter drill bits, and the latter is used for large diameter drill bits.

The structure of a standard twist drill

a) taper shank      b) straight shank

Due to the limitation of structure, the bending stiffness and torsional stiffness of the drill bit are low, coupled with the poor centering, the accuracy of drilling processing is low, and generally can only reach IT13 ~ IT11; The surface roughness is also large, and Ra is generally 50~12.5um; However, the metal removal rate of the drilled hole is large and the cutting efficiency is high.

Drilling is mainly used for machining 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, comparing holes, holes or grinding holes in subsequent processing.

Reaming

Reaming is to use a 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.

A reaming drill is similar to a twist drill, but it has a higher number of teeth and no transverse edge.

Reamer drills

Compared with drilling, reaming has the following characteristics:

(1) The number of reaming and drilling teeth is large (3~8 teeth), the guiding is good, and the cutting is relatively stable:

(2) The reaming drill has no transverse edge, and the cutting conditions are good!

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

The accuracy of reaming is generally IT11~IT10, and the surface roughness Ra is 12.5~6.3um. Reaming is often used to machine holes with a diameter of less than 30mm. When drilling holes with large diameters (D230mm), it is common to pre-drill with a small drill bit (diameter of 0.5~0.7 times of the hole diameter), and then use the corresponding size of reamer to drill and ream, 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 head seat holes and Mingping end faces with a variety of special shapes of reaming drills (also known as silver drills). The front end of the diamond is often equipped with a guide post, which is guided by a machined hole.

Countersinking

Reaming

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

Reamers

Reamers are generally divided into two types: hand reamers and machine reamers. The handle of the hand reamer is a straight handle, the working part is longer, and the guiding effect is better. There are two types of manual hinges: integral (Fig. a) and adjustable outer diameter (Fig. b).

Machine knives can be divided into those with shanks (Fig. C, φ1 ~ 20mm for straight shanks, φ10~32mm for taper shanks) and sleeve (Fig. D). The reamer can be used not only to make round holes, but also to taper holes with taper cutters (Fig. e).

Reamers

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 blunted, it is not easy to obtain a smooth and clean processing surface, and the dimensional tolerance is not easy to guarantee; The allowance is too small to remove the knife marks left by the previous process, and naturally there is no effect on improving the quality of hole processing. Generally, the rough hinge margin is 0.35~0.15mm, and the fine hinge is 0.15~0.05mm.

In order to avoid built-up edge, the reaming is usually machined at a low cutting speed (v< 8 m/min for steel and cast iron with HSS reamers). The value of the feed is related to the diameter of the hole to be processed, the larger the hole diameter, the greater the value of the feed, and the feed is often taken as 0.3~1mm/r when the high-speed steel reamer processes steel and cast iron.

The reaming must be cooled, lubricated and cleaned with the appropriate cutting fluid to prevent built-up edge and remove chips in a timely manner. Compared with grinding holes and holes, reaming has high productivity and is easy to ensure the accuracy of holes; However, the 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 should not be processed for stepped holes and blind holes.

The dimensional accuracy of the reaming hole is generally IT9 ~ IT7 level, and the surface coarse accuracy Ra is generally 3.2 ~ 0.8, um. For medium-sized holes with high precision requirements (e.g., IT7 precision holes), the drill-and-expand-hinge process is a typical machining solution commonly used in production.

      Boring

Boring is a processing method that enlarges the prefabricated hole with a cutting tool, and the boring work can be carried out both on the boring machine and on the lathe.

Boring method

There are three different ways to process boring.

(1) The workpiece rotates and the tool moves for feeding

The characteristics of the process are: the axis line of the hole after machining is consistent with the rotary 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 mantis hole method is suitable for machining holes that require coaxiality with the outer surface.

Boring method for workpiece rotation and tool feed

(2) The tool rotates and the workpiece moves in feed

The boring machine spindle drives the knife to rotate, and the workbench drives the workpiece to make feed movements.

(3) The tool rotates and makes a feed movement

Adopt this boring mode boring, the overhang length of the rod is changed, and the stress deformation of the boring bar is also changed, and the aperture near the headstock is large, and the aperture at the headstock is small, and the hole that is far away from the headstock is small, and who hole is formed. 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 produce corresponding bending. This boring method is only suitable for adding short holes.

 The hole method in which the tool is both rotated and fed

   1-Boring bar, 2-Boring cutter, 3-Workpiece, 4-Worktable, 5-Spindle

High-speed fine boring (diamond boring)

Compared with the general hole, the characteristics of the diamond is that the back of the knife is small, the feed is small, the cutting speed is high, it can obtain high machining accuracy (IT7 ~ IT6) and a very smooth surface (Ra is 0.4 ~ 0.05um). Originally machined with diamond knives, diamond tools are now commonly used for cemented carbide, CBN and synthetic diamond tools. It is mainly used for processing non-ferrous metal workpieces, and can also be used for processing rusted iron parts and steel parts.

The commonly used cutting amount of diamond boring is:

The amount of back knife pre-tent is 0.2~0.6mm:

The final boring is 0.1mm;

The feed is 0.01~0.14mm/r;

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

150~300m/min when processing steel:

When processing non-ferrous metals, it is 300~2000m/min:

In order to ensure that diamond boring can achieve high machining accuracy and surface quality, the machine tool (diamond bed) used must have high geometric accuracy and rigidity, the machine tool spindle support commonly used precision angular contact ball bearings or hydrostatic sliding bearings, high-speed rotation elements must be accurately balanced; In addition, the movement of the feed mechanism must be very stable to ensure that the workbench can do a smooth and 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 mass production, such as engine cylinder holes, piston pin holes, spindle holes on the spindle box of machine tools, etc. 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, and the tool life is low.

Boring knives

Boring cutters can be divided into single-edged boring cutters and double-edged boring cutters. A single-edge boring tool (pictured) has a similar structure to a 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 bit.

     Single-edged mantis

                a) Through-hole single-edged mantis b) Blind single-edged mantis

The double-edged boring cutter has two symmetrical cutting edges, which is equivalent to two symmetrically mounted turning tools participating in cutting at the same time; The dimensional accuracy of the hole is guaranteed by the size of the boring tool itself. The floating boring cutter shown in the figure is a kind of double-edged boring cutter, the boring blade is inserted in the groove of the rod, and its position is automatically balanced by the back force acting on the two cutting edges, which can eliminate the error caused by the boring tool installation error or the boring bar yaw; However, it is similar to the reaming in that it can only ensure the dimensional accuracy, and cannot correct the position error of the axis of the front hole of the reaming.

       Floating boring cutters

Characteristics of boring process and application range

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

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

To sum up, it can be seen that the processing range of boring is wide, and a variety of holes of different sizes and different accuracy levels can be processed, and boring is almost the only processing method for holes and hole systems with large hole diameter, high size and position accuracy requirements. The machining accuracy of boring is IT9 ~ IT7 level. Boring can be carried out on machine tools such as mantises, lathes, milling machines, etc., which has the advantages of mobility and flexibility, and is widely used in production. In mass production, in order to improve the efficiency of boring, boring dies are often used.

     Honing holes

Honing principle and honing head

Honing is a method of smoothing holes using a honing head with a grinding rod (oil stone). During 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, the grinding strip acts on the surface of the workpiece under a certain pressure, and a very thin layer of material is removed from the surface of the workpiece, and the cutting trajectory is a crossed reticulation. In order to make the movement trajectory of the abrasive bar not repeat, the number of 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.

   Honing principle

        a) Forming motion b) Expansion of the grinding trajectory of the grinding strip c) Resultant speed

The intersection angle of the honing trajectory is related to the reciprocating speed and circumferential speed of the honing head, and the size of the angle affects the processing quality and efficiency of the grinding 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 when grinding.

In order to make the wall of the hole to be machined evenly, the stroke of the sand bar must exceed a certain amount of overpass at both ends of the hole. In order to ensure the uniformity of the honing allowance and reduce the influence of the spindle rotation error of the machine tool on the machining accuracy, the floating connection between the honing head and the machine tool spindle is mostly adopted. The radial telescopic adjustment of the grinding rod of the honing head is available in manual, pneumatic and hydraulic forms.

Honing process characteristics and application range

(1) Honing can obtain high dimensional accuracy and shape accuracy, the machining accuracy is IT7~IT6 level, and the roundness and cylindricity error of the hole can be controlled within the range of 3~5um, but honing can not improve the position accuracy of the machined hole

(2) Honing can obtain high surface quality, the surface roughness Ra is 0.2 ~ 0.025um, and the depth of the metamorphic defect layer of the surface metal is very small (2.5 ~ 25um).

(3) Compared with the grinding speed, although the circumferential speed of the grinding 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 the grinding still has a high productivity

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

     Pull holes

Broaching and broaches

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

When broaching, the broach only moves in a linear motion at low speed (main motion). The number of teeth of the broach working at the same time should generally not be less than 3, otherwise the broach will not work smoothly and will easily produce annular ripples on the surface of the workpiece. In order to avoid excessive broaching force and broth breakage, the number of working teeth should generally not exceed 6~8 when the broach is working.

There are two different types of broaching, which are described below:

1) Layered broaching

This broaching method is characterized by the fact that the broach removes the machining allowance of the workpiece in sequence, 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 pattern b) Cutting part tooth profile c) Chip

2) Block broaching

This broaching method is characterized by the removal of each layer of metal on the machined surface by a set of teeth that are essentially the same size, but with staggered teeth (usually each group consists of 2-3 teeth). Each tooth only cuts off a portion of a layer of metal. Broaches designed according to the block broaching method are called wheel cut broaches.

3) Integrated broaching

This method concentrates the advantages of layered and block broaching, the rough tooth part adopts block broaching, and the fine tooth part adopts layered broaching. This shortens the length of the broach, increases productivity, and achieves better surface quality. Broaches designed according to the integrated broaching method are called integrated broaches.

Characteristics of the hole drawing process and the scope of application

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

2) The basking 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 6.3~1.6um.

3) when the hole is drawn, the workpiece is positioned by the processed hole itself (the leading part of the broach is the positioning element of the workpiece), and the hole is not easy to ensure the mutual position accuracy of the hole and other surfaces, and the processing of the rotary body parts with the coaxiality requirements on the inner and outer circular surfaces is often the first to draw the hole, and then the hole is used as the positioning reference to process other surfaces.

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

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

Hole drawing is commonly used in mass production to process through holes on small and medium-sized parts with a hole diameter of φ10~80mm and a hole depth of no more than 5 times the hole diameter.