I. Overview
(A), the function and structural characteristics of shaft parts
1. Function: Support transmission parts (gears, pulleys, etc.). ), transfer torque, bear load, and ensure that the workpiece or tool mounted on the spindle has certain rotary accuracy.
2. Classification: According to the characteristics of its structure and shape, shaft parts can be divided into four categories: optical axis, stepped shaft, hollow shaft and special-shaped shaft (including crankshaft, camshaft and eccentric shaft).
Type of chart axis
A) optical axis b) hollow shaft c) half shaft d) stepped shaft e) spline shaft f) cross shaft g) eccentric shaft
H) crankshaft i) camshaft
According to the ratio of shaft length to diameter, it can be divided into rigid shaft (L/D < 12 =) and flexible shaft (L/D > 12).
3. Surface features: outer circle, inner hole, cone, thread, spline and transverse hole.
(2) Main technical requirements:
1, dimensional accuracy
Journal is the main surface of shaft parts, which affects the rotary accuracy and working state of shaft. The diameter accuracy of journal is usually IT6 ~ 9 according to its use requirements, and the accuracy of journal can reach IT5.
2, geometric shape accuracy
Generally speaking, the geometric accuracy (roundness and cylindricity) of journal should be limited to the common point of diameter. When the accuracy of geometric shape is required, the allowable tolerance can be specified separately on the part drawing.
3. Position accuracy
Mainly refers to the coaxiality of the matching journal of the assembly transmission parts relative to the supporting journal of the assembly bearing, which is usually expressed by the radial circular runout of the matching journal to the supporting journal; According to the application requirements, the high precision axis is 0.00 1 ~ 0.005 mm, and the general precision axis is 0.01~ 0.03 mm.
In addition, there are also requirements for the coaxiality of the inner and outer cylindrical surfaces and the verticality between the axial positioning end face and the axis.
4. Surface smoothness
There are different surface roughness values according to the different working parts on the surface of the parts. For example, the surface roughness of the spindle support journal of a common machine tool is Ra 0. 16 ~ 0.63 um, and the surface roughness of the supporting journal is Ra 0.63 ~ 2.5 um. With the improvement of the running speed and accuracy of the machine, the requirements for the surface roughness of shaft parts will become smaller and smaller.
(3) Materials and blanks of shaft parts
The reasonable selection of materials and the technical requirements of heat treatment are of great significance to improve the strength and service life of shaft parts, and also have great influence on the processing technology of shaft.
1, material of shaft parts
45 steel is often used for shaft parts with different heat treatment specifications (such as normalizing, quenching and tempering). ) to obtain a certain strength, toughness and wear resistance.
For shaft parts with medium precision and high speed, alloy steel such as 40Cr can be selected. This kind of steel has high comprehensive mechanical properties after quenching and tempering and surface quenching. For high precision shafts, materials such as bearing steel GCrls and spring steel 65Mn are sometimes used. After quenching and tempering, they have higher wear resistance and fatigue resistance.
For shafts working under high-speed and heavy-load conditions, low-carbon gold-bearing steel can be selected, such as 20CrMnTi, 20MnZB, 20Cr or 38CrMoAIA nitrided steel. After carburizing and quenching, low-carbon alloy steel has high surface hardness, impact toughness and core strength, but the heat treatment deformation is very small.
2. Shaft parts blank
The blanks of shaft parts are mostly round bars and forgings, and only some large shafts with complex structures are castings.
(4) Pretreatment of shaft parts
The wheel parts should be pretreated before cutting. Pretreatment includes correcting, cutting and cutting the end face and drilling the center hole.
1. correction: correct the bending deformation of the rod blank during manufacturing, transportation and storage to ensure uniform machining allowance and reliable feeding and clamping. Calibration can be performed on various printing machines.
2. Cutting: When the bar is used, it should be cut according to the required length, and then the excircle should be turned. Cutting is carried out on a bow saw, and cutting of high-hardness bars can be carried out on a cutting machine with a thin grinding wheel.
3. Cut the end face and drill the center hole: the center hole is the most commonly used positioning benchmark for shaft parts processing. In order to ensure that the drilled center hole is not skewed, the end face should be cut before drilling the center hole.
4. Blanking: If the shaft blank is a radial forging or a large casting, it needs to be turned over to reduce the shape error of the outer surface of the blank and make the processing scene of the subsequent process unified.
Second, the typical spindle parts processing technology analysis
The purpose, structural shape, technical requirements and output of shaft parts are different, and their processing technology is also different. The process planning of shaft is the most common process work in production.
(A) the main problems of shaft parts processing
The main problem in machining shaft parts is how to ensure the dimensional accuracy, surface roughness and mutual position accuracy of main surfaces.
Typical processing routes of shaft parts are as follows:
Blank and its heat treatment → pretreatment → turning excircle → milling keyway → heat treatment → grinding.
(2) Analysis of processing technology of CA 6140 spindle.
Technical analysis of 1 and CA6 140 spindles
(1), technical requirements for bearing journal.
The tolerance of roundness and radial runout of the two bearing journals A and B of the spindle is 0.005 mm, the contact rate of the conical surfaces of the two bearing journals 1: 12 is more than 70%, the surface roughness Ra is Ra0.4um, and the bearing journal diameter is manufactured according to the accuracy of IT5-7.
General precision machine tools require the roundness of the spindle excircle to be within 50% of the dimensional tolerance, improved precision machine tools to be within 25% of the dimensional tolerance, and high-precision machine tools to be within 5 ~ 10% of the dimensional tolerance.
(2), the technical requirements of the cone hole
The runout from the spindle taper hole (Morse No.6) to the supporting journals A and B is allowed to be 0.005mm near the shaft end and 0.065438+300mm near the shaft end. The contact ratio of conical surface is more than 70%, the surface roughness Ra is Ra0.4um, and the hardness requirement is HRC48.
(3) Technical requirements of short cone
The radial runout tolerance of short conical surfaces of journal A and journal B of main shaft bearing is 0.008 mm, and the radial runout tolerance of end surfaces D of journal A and journal B is 0.008 mm.. The roughness of the conical surface and the end surface is ra 0.8 micron.
(4) Technical requirements of idler axle journal
The radial runout tolerance from the idler journal to the support journals A and B is 0.015mm.
(5) Technical requirements of threads
This is a necessary requirement to limit the end-face runout of the matching compression nut. Therefore, when machining the spindle thread, it is necessary to control the coaxiality between the thread surface axis and the supporting journal axis, which is generally less than 0.025mm..
From the above analysis, it can be seen that the main machining surfaces of the spindle are two supporting journals, conical holes, short conical surfaces at the front end and its end face, and various journals used for gear installation. The key of spindle machining is to ensure dimensional accuracy, geometric accuracy, coaxiality between two bearing journals, mutual position accuracy and surface roughness between bearing journals and other surfaces.
(3), CA6 140 spindle processing technology 4), spindle processing technology analysis
1. Manufacturing method and heat treatment of spindle blank
Batch: large batch; Material: 45 steel; Blank: die forging
(1) material
In single piece and small batch production, the blank of shaft parts usually adopts hot rolled bar.
For stepped shafts with large diameter difference, forgings are often used in order to save materials and reduce the labor of machining. Generally, the stepped shafts produced in small batch are all forged freely, and die forging is used in large batch production.
(2) heat treatment
The local hardness of 45 steel can reach HRC 62 ~ 65(235 HBS) after quenching and tempering, and can be reduced to the required hardness after proper tempering (for example, the main shaft of CA6 140 is defined as HRC52).
The tool steel 9Mn2V with about 0.9% carbon content is superior to 45 steel in hardenability, mechanical strength and hardness. After proper heat treatment, it is suitable for the dimensional accuracy and stability requirements of high-precision machine tool spindle. For example, the headstock and grinding wheel spindle of the universal cylindrical grinder M 1432A are made of this material.
38CrMoAl is a medium carbon alloy nitrided steel. Because the nitriding temperature is 540-550℃ lower than the general quenching temperature, it has smaller deformation, higher hardness (HRC > 65, center hardness HRC > 28) and excellent fatigue resistance, so the spindle box shaft and grinding wheel shaft of MBG 1432 high-precision semi-automatic cylindrical grinder are all made of this steel.
In addition, for shaft parts with medium precision and high speed, alloy structural steel such as 40Cr is mostly selected. After quenching and tempering and high frequency quenching, this kind of steel has high comprehensive mechanical properties and can meet the application requirements. Some shaft parts are also made of ball bearing steel such as GCr 15 and spring steel such as 66Mn. After quenching and tempering, these steels have extremely high wear resistance and fatigue resistance. When shaft parts are required to work at high speed and heavy load, low-carbon gold-bearing steels such as 18CrMnTi and 20Mn2B can be selected. These steels have high surface hardness, impact toughness and core strength after carburizing and quenching, but the deformation caused by heat treatment is greater than 38CrMoAl.
For the spindle that needs local high-frequency quenching, quenching and tempering treatment should be arranged in the previous process (some steels should be normalized). When the blank allowance is large (such as forgings), quenching and tempering should be placed after rough turning and before semi-finish turning, so that the internal stress generated by rough turning can be eliminated during quenching and tempering; When the blank allowance is small (such as bar), it can be quenched and tempered before rough turning (equivalent to semi-finish turning of forgings). High-frequency quenching treatment is generally placed after semi-finishing turning. Because the spindle only needs local quenching, there is a certain requirement for accuracy, and the processing of hardened parts such as turning threads and milling keyways is arranged after local quenching and rough grinding. For the spindle with high precision, it is necessary to carry out low-temperature aging treatment after local quenching and rough grinding to keep the metallographic structure and stress state of the spindle stable.
2. Selection of positioning datum
For solid shaft parts, the precision datum plane is the center hole, which meets the requirements of datum coincidence and datum unification. For a hollow spindle such as CA6 140A, there are not only the central hole but also the cylindrical surface of the journal, which are used alternately and are mutual benchmarks.
3. Division of processing stages
Machining errors and stresses will occur in different degrees in each machining process and heat treatment process of spindle machining, so machining stages should be divided. Spindle processing is basically divided into the following three stages.
(1), rough machining stage
1) blank processing blank preparation, forging and normalizing
2) Roughing saw off redundant parts, milling end faces, drilling center holes, turning excircles, etc.
(2) Semi-finishing stage
1) heat treatment before semi-finishing 45 steel is generally tempered to 220 ~ 240 HBS.
2) Semi-finish turning process cone (positioning cone hole) semi-finish turning cylindrical end face and drilling deep hole, etc.
(3) the final stage
1) heat treatment before finishing and local high frequency quenching.
2) All kinds of processing before finishing include rough grinding of locating cone, rough grinding of excircle, milling of keyway and spline groove, turning of thread, etc.
3) Finishing the excircle and inner and outer conical surfaces to ensure the accuracy of the most important surface of the spindle.
4. Arrangement of processing sequence and determination of working procedure
When considering the processing sequence of main surfaces such as bearing journal, common journal and inner cone, shaft parts with hollow and inner cone characteristics can have the following schemes.
① Rough machining of outer surface → deep hole drilling → finish machining of outer surface → rough machining of taper hole → finish machining of taper hole;
(2) rough machining of outer surface → deep hole drilling → rough machining of taper hole → finish machining of taper hole → finish machining of outer surface;
③ Rough machining of outer surface → deep hole drilling → rough machining of taper hole → finish machining of outer surface → finish machining of taper hole.
According to the machining sequence of CA6 140 lathe spindle, the following analysis and comparison can be made:
The first scheme: In the rough machining of the taper hole, the precision and roughness of the cylindrical surface will be destroyed because the finished cylindrical surface will be used as the precision benchmark, so this scheme is not suitable.
Scheme 2: When finishing the cylindrical surface, the taper hole plug should be reinserted, which will destroy the accuracy of the taper hole. In addition, there will inevitably be machining errors when machining the conical hole (the grinding conditions of the conical hole are worse than that of the outer circle, and the error of the conical plug itself will cause the outer circle surface and the inner conical surface to be different axes, so the scheme is not suitable.
Scheme 3: When finishing the tapered hole, the finished cylindrical surface should also be used as the precision benchmark; However, due to the small machining allowance of conical surface finishing, the grinding force is not large; At the same time, the finish machining of taper hole is in the final stage of shaft machining, which has little influence on the accuracy of cylindrical surface; Combined with the machining sequence of this scheme, the coaxiality can be gradually improved by taking the cylindrical surface and conical hole alternately as the benchmark.
Through this comparison, it can be seen that the third scheme is the best for the processing sequence of CA6 140 spindle and other axial parts.
Through the analysis and comparison of the schemes, it can also be seen that the processing sequence of each surface of shaft parts is largely related to the conversion of positioning datum. When the coarse and fine benchmarks of machined parts are selected, the machining sequence can be roughly determined. Because the positioning datum is always processed at the beginning of each stage, that is to say, the first process must prepare the positioning datum for the following processes. For example, when machining the CA6 140 spindle, the end face is milled and the center hole is drilled at the beginning. This is the positioning reference of the excircle of rough turning and semi-finish turning; Semi-finish turning the excircle provides a positioning reference for deep hole machining; Semi-finish turning the excircle also prepares a positioning benchmark for the processing of front and rear taper holes. On the other hand, the front and rear taper holes are consistent with the top hole behind the taper plug, which prepares the positioning benchmark for the semi-finishing and finishing of the excircle. The positioning standard of the final grinding cone hole is the journal surface ground in the previous process.
According to the processing sequence to determine the process, we should master two principles:
1) The positioning datum in the process shall be processed before this process. For example, deep hole machining is arranged after rough turning of the cylindrical surface, so as to have a more accurate journal as a positioning benchmark and ensure the uniform wall thickness during deep hole machining.
2) The machining of each surface should be divided into rough machining and fine machining, with rough machining before fine machining and repeated machining to gradually improve its accuracy and roughness. The finishing of the main surface should be arranged at the end.
In order to improve the metal structure and machinability, heat treatment procedures, such as annealing and normalizing, should generally be arranged before mechanical processing.
In order to improve the mechanical properties of parts and eliminate internal stress, heat treatment processes, such as quenching, tempering, aging treatment, etc. , generally should be arranged after rough machining, before finishing.
5. Comparison between mass production and small batch production process