Compared with the surface machining of outer circle,the condition of hole machining is much worse,and hole machining is more difficult than that of outer circle.This is due to:
The size of the tool used for hole machining is limited by the size of the hole being machined,poor rigidity is easy to bending deformation and vibration.
The size of the hole is often directly dependent on the corresponding size of the tool,and the manufacturing error and wear of the tool will directly effect the machining accuracy of the hole.
When machining the hole, 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 be controlled.
Drilling hole and reaming hole
Drilling is the first step when machining the hole in a solid material,drilling diameter is generally less than 80mm.There are two ways of drilling:one is the bit rotation;The other is the workpiece rotation.The errors generated by the above two drilling methods are not the same.In the drilling method of bit rotation,when the bit deviates due to asymmetric cutting edge and insufficient rigidity of the bit,the center line of the machined hole will skew or be not straight,but the aperture is basically unchanged.In the workpiece rotating drilling mode, on the contrary,the deviation of the bit will cause the aperture change,but the hole center line is still straight.
The commonly used drill tool has:twist drill,center drill,deep hole drill, etc.,among which the most commonly used is twist drill,its diameter specification is φ0.1~φ80mm.
Due to the limitation of the structure,the bending stiffness and torsional stiffness of the drill bit are low,coupled with poor centering, and the machining accuracy of the drill bit is low,which can only reach IT13 ~ IT11 in general.The surface roughness is also greater,Ra is generally 50~12.5μm.The metal removal rate of drilling hole is large and the cutting efficiency is high.Drilling is mainly used for machining 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 through reaming, reaming, boring or grinding in subsequent machining.
Reaming refers to the further processing of the holes that have been drilled,cast or forged by the reaming drill,so as to enlarge the aperture and improve the processing quality of the hole.The reaming process can be used as the pre-processing before finishing the hole,or as the final processing of the hole with low requirements.The reaming drill is similar to the twist drill,but it has more teeth and no horizontal blade.
Compared with drilling hole,reaming has the following characteristics:(1) reaming drill more teeth (3~8 teeth),good conductivity,relatively stable cutting;(2) Reaming drill has no cross edge and good cutting conditions;(3) The machining allowance is small,the chip holding groove can be made shallow,the drilling core can be made thicker,the strength and rigidity of the knife body is good.The precision of reaming is generally IT11~IT10,and the surface roughness Ra is 12.5~6.3.Reaming is often used to process holes that are smaller in diameter.When drilling a hole with a larger diameter (D ≥30mm),a small bit (0.5~0.7 times the diameter) is often used for pre-drilling,and then the corresponding size of the hole drilling and reaming,which can improve the processing quality and production efficiency of the hole.
In addition to machining cylindrical holes,various countersunk head seat holes and facing spot facing can also be machined by various special shape counterboring drills (also known as counter bit).The front end of the shaving drill is usually equipped with a guide column and guided by the machined hole.
Reaming is one of the finishing methods of hole,which is widely used in production. For smaller holes,reaming is a more economical and practical machining method compared with inner circle grinding and fine boring.
Reamer is generally divided into two kinds:hand reamer and machine reamer.The handle of the hand reamer is straight,the working part is longer,the guiding function is better,the hand reamer has two kinds of structure:integral type and adjustable outer diameter type.The machine reamer has two kinds of structure,one with handle and the other with sleeve.Not only circular hole can be machined by reamer,but also taper hole can be machined by reamer.
2.Reaming process and its application
The reaming allowance has a great influence on the reaming quality. The allowance is too large, the reamer’s load is too large, the cutting edge is quickly blunt, and it is not easy to obtain smooth processing surface, and the dimensional tolerance is not easy to ensure; The margin is too small to remove the cut marks left by the upper process, which naturally does not improve the quality of hole processing. Generally, the coarse hinge is 0.35~0.15mm, and the fine hinge is 01.5~0.05mm.
In order to avoid built-up edge,reaming usually uses lower cutting speeds (high speed steel reamer when machining steel and cast iron,V < 8m/min) for processing.The value of feed is related to the aperture of the machine.The larger the aperture is,the larger the feed value will be.When high-speed steel reamer is used to process steel and cast iron, the feed value is usually 0.3~1mm/r.
Reaming must be cooled,lubricated,and cleaned with appropriate cutting fluid to prevent the formation of chip deposits and timely chip removal.Compared with grinding hole and boring hole,reaming productivity is high and the precision of hole can be easily guaranteed.However,the reaming hole cannot correct the position error of the hole axis,and the position accuracy of the hole should be guaranteed by the previous procedure.Reaming hole is not suitable for machining step hole and blind hole.
Reaming accuracy is generally IT9 ~ IT7,and surface roughness Ra is generally 3.2~0.8. For holes with medium size and high precision requirements (such as IT7 precision holes),drilling-reaming and reaming process is a typical processing scheme commonly used in production.
Boring is a machining method in which prefabricated holes are enlarged by cutting tools.The boring can be done either on a boring machine or on a lathe.
Boring can be done in three different ways.
Workpiece rotation,cutting tool feed movement in the lathe boring mostly belongs to this boring way.The process features are as follows:the axis line of the hole after processing is consistent with the axis of rotation of the workpiece,the roundness of the hole mainly depends on the rotary precision of the machine tool spindle,and the axial geometric shape error of the hole mainly depends on the precision of the tool feed direction relative to the axis of rotation of the workpiece.This boring method is suitable for machining holes with coaxiality required on the surface of the outer circle.
Tool rotation,the workpiece for feed motion boring machine spindle drive boring cutter rotation,table drive the workpiece for feed motion.
Tool rotation and feed motion using this boring way boring,the length of the hanging boring bar is changing,the boring bar deformation is also changing,close to the spindle box of the aperture,far from the spindle box of the aperture is small,forming a taper hole. In addition,with the increase of the hanging length of the boring bar,the bending deformation of the spindle due to its dead weight will also increase,and the axis of the processed hole will be bent accordingly.This boring method is suitable only for shorter holes.
2.King Kim Boring
Compared with general boring,diamond boring is characterized by small back feed, small feed,high cutting speed, it can obtain high machining accuracy (IT7 ~ IT6) and very clean surface (Ra 0.4~0.05).Diamond boring was originally processed with diamond boring tool,but now it is commonly processed with hard alloy,CBN and artificial diamond tool.Mainly used for processing non-ferrous metal workpiece,also can be used for processing cast iron and steel pieces.
The commonly used cutting parameters of the diamond boring are as follows:the pre-boring of back cutting tool is 0.2~0.6mm,and the final boring is 0.1mm;The feed quantity is 0.01~0.14mm/r;The cutting speed is 100~250m/min for cast iron,150~300m/min for steel and 300~2000m/min for non-ferrous metal.
In order to ensure that the diamond boring can achieve higher machining accuracy and surface quality,the machine tool (diamond boring machine) must have a higher geometric accuracy and stiffness,machine spindle support commonly used precision angular contact ball bearings or static sliding bearings,high-speed rotating parts must be precisely balanced;In addition, the movement of the feeding mechanism must be very smooth, to ensure that the table can do smooth low-speed feeding movement.
Diamond boring has good machining quality and high production efficiency.It is widely used in the final machining of precision holes in a large number of production,such as engine cylinder hole,piston pin hole, main axle hole on machine tool spindle box,etc. However,it must be noted that:when machining black metal products with diamond boring,only hard alloy and CBN can be used to make the boring tool,can not be used to make the boring tool diamond,because of the affinity of carbon atoms and iron group elements in diamond, tool life is low.
Boring tool can be divided into single-edge boring tool and double-edge boring tool.
4.Technological characteristics and application range of boring
Compared with drilling-Reaming process,bore size is not limited by tool size,and boring has a strong error correction ability,the original hole axis deviation error can be corrected through multiple tool moves,and can make the boring and positioning surface maintain a higher position accuracy.
Compared with the boring and turning round,due to the poor rigidity and deformation of the tool rod system,the heat dissipation and chip removal condition is not good,the thermal deformation of the workpiece and the tool is relatively large,the machining quality and production efficiency of the boring hole is not as high as the turning round.
In conclusion,it can be seen from the above analysis that boring can be used for a wide range of processing and can be used for holes of different sizes and accuracy levels. For holes and systems with large aperture and higher requirements for size and position accuracy,boring is almost the only processing method.The machining accuracy of boring is IT9 ~ IT7,and the surface roughness Ra is. Boring can be used in boring machine,lathe,milling machine and other machine tools,with the advantages of flexibility,the production is very widely used.In order to improve the efficiency of boring, boring die is often used in mass production.
1.Honing principle and honing head
Honing is the polishing of holes by means of a honing head with a honing rod (whetstone).When honing,the workpiece is fixed.The honing head is driven by the main shaft of the machine tool to rotate and reciprocate in a straight line.During honing, the honing rod is applied to the workpiece surface under certain pressure.A very thin layer of material is cut off from the workpiece surface. In order to avoid repeating the movement of the abrasive grains,the revolutions per minute of the honing head and the reciprocating journey per minute of the honing head should be prime.
The crossing Angle of the honing track is related to the reciprocating speed and circular speed of the honing head.The Angle affects the processing quality and efficiency of the honing process.Generally, the honing time is set at while the honing time is set at fine.To facilitate the removal of broken abrasive particles and chips,reduce cutting temperature,and improve machining quality, sufficient cutting fluid should be used in honing.
In order to make the machined hole wall can be machined evenly,the stroke of sand strip should exceed a certain amount at both ends of the hole.In order to ensure the uniformity of honing allowance and reduce the influence of rotation error on machining accuracy of machine tool spindle,floating connection is mostly used between honing head and machine tool spindle.
The radial expansion and adjustment of honing head grinding strip can be done in manual,pneumatic and hydraulic structures.
2.The technological characteristics and application range of honing.
Honing can achieve higher dimensional and shape accuracy,and the machining accuracy is IT7~IT6.The roundness and cylindricity error of the holes can be controlled within the range,but the honing cannot improve the position accuracy of the holes to be machined.
Honing results can got higher surface quality,Surface roughness Ra is0.2~0.025um,the metamorphic defect layer depth of the surface metal is extremely small 2.5~25um.
The circumferential speed of the honing head is not high compared with the grinding speed（vc=16~60m/min）,but because the sand strip and the workpiece contact area is large and reciprocating velocity is relatively high(va=8~20m/min),so honing still has higher productivity.
1.Broaching and Scraping out cutter
The broach is a high productivity finishing method, which is carried out on the broach bed with a special broach.Broaching bed is divided into horizontal broaching bed and vertical broaching bed two,horizontal broaching bed is the most common.
The broach moves only in a low speed straight line (main motion) during cutting.The number of teeth that broach works at the same time should not be less than 3 in general,otherwise the broach will not work smoothly,it’s easy to produce annular ripples on the surface of workpiece.To avoid excessive broaching force to break the broach,the number of working knife teeth should not exceed 6~8 when broach at work.
There are three different broaching methods, which are described as follows:
Layered broaching:This broaching method is characterized by the broach cutting the machining allowance layer by layer.In order to facilitate chip breaking,there are interleaving chip dividing slots on the knife teeth.The broach designed according to the layered broach is called the common broach.
Skip-stepping:The feature of this broaching method is that each metal layer on the machined surface is cut by a set of cutter teeth (usually composed of 2-3 of each cutter teeth) with the same size but interleaving each other.Only one layer of metal is cut off from each tooth.Broach designed by block broach is called wheel cutting broach.
Composite broaching:This method concentrates the advantages of stratified and segmented broaching.For the rough part of the teeth,the segmented broaching is adopted,while for the fine part of the teeth, the layered broaching is adopted.In this way, the broach length can be shortened, productivity can be increased and better surface quality can be obtained.The broach designed according to the integrated broach is called the integrated broach.
- Technological characteristics and application scope of hole drawing.
Broach is a multi-blade tool.In one broaching stroke,the rough machining,finishing and finishing of holes can be completed sequentially,with high production efficiency.
The drawing precision mainly depends on the broach precision. Under normal conditions, the drawing precision can reach IT9~IT7, and the surface roughness Ra can reach 6.3~1.6 um.
When pulling holes,the workpiece is positioned by the processed hole itself (the leading part of broach is the positioning element of the workpiece).It is not easy to ensure the mutual position accuracy of the hole and other surfaces; For those inside and outside the circular surface has the coaxiality request the rotary body part’s processing,usually is first draws the hole,then takes the hole as the localization datum processing other surface.
Broach can not only process round hole,but also can process forming hole,spline hole.
Broach is a fixed size cutter with complex shape and high price,which is not suitable for machining large hole.Draw hole are commonly used in the large number of mass production processing aperture is Ф 10 to 80 mm,the depth of hole is not more than five times the aperture of the hole on the small and medium-sized parts.
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