I. Overview of Mechanical Parts
1. Part classification
According to its structure, parts can be generally divided into six categories: shaft, disk sleeve, bracket box, hexahedron, fuselage base and special category.
When selecting parts, shaft, disk sleeve and bracket are the main parts. (more common)
2. Surface composition of parts
1) Surface composition of parts: three basic surfaces.
① Revolving surface: cylindrical surface, conical surface, revolving forming surface, etc.
② Plane: large plane, end face, torus, etc.
③ Forming surface: involute tooth surface, helicoid, etc.
3. Part surface forming method
(1) Forming method: The contour of the machined workpiece is "copied" by the blade shape (or the projection of the blade shape) of the tool.
(2) Envelope method: The contour of the machined workpiece is the envelope of the continuous position of the cutting edge during the cutting movement.
4. Parts materials
Common materials of parts:
Carbon structural steel Q235A, high-quality carbon structural steel (35, 45), alloy structural steel (40Cr)
Cast steel (ZG570), cast iron (HT 150, HT200)
Nonferrous metals and their alloys, etc.
Annotate in the title bar
5. Heat treatment of parts
Heat treatment methods of common parts: annealing, normalizing, quenching, tempering, tempering, aging, etc.
This is given in the technical requirements.
45 Commonly used: shaft type
Quenching and tempering 220-240 HBS
Surface quenching, hardness HRC 40-50
6. Parts processing quality
The machining quality of parts includes machining accuracy and surface quality.
The machining accuracy of parts refers to the degree of conformity between the actual geometric parameters (size, shape and position) and the ideal geometric parameters after machining. The machining accuracy of parts includes dimensional accuracy, shape accuracy and position accuracy.
Surface quality mainly refers to surface roughness.
Product drawings are represented by dimensional tolerance, shape tolerance, position tolerance and surface roughness respectively.
(1) dimensional accuracy
Refers to how close the actual value of the diameter, length and distance between surfaces of a part is to the ideal value. Dimensional accuracy is controlled by dimensional tolerance. Dimensional tolerance is the allowable deviation of part size when cutting. With the same basic dimensions, the smaller the dimensional tolerance, the higher the dimensional accuracy.
The standard tolerance grade of dimensional accuracy stipulated in national standard GB/T1800.1997 is divided into 20 grades, namely IT0 1, IT0, IT 1, IT2, …, IT1.
The higher the dimensional accuracy, the more complicated the technological process of parts and the higher the processing cost.
Different machining methods can achieve different dimensional tolerance levels.
(2) Shape accuracy
Shape accuracy refers to the degree of conformity between the actual shape of lines and surfaces on the machined parts and the ideal shape.
There are six items (GB/T1182-1996) to evaluate the shape accuracy.
Shape accuracy is controlled by shape tolerance. Except for roundness and cylindricity (0~ 12), all shape tolerances are classified as 12 accuracy grade. 1 highest, 12 lowest.
(3) Position accuracy
Refers to the degree of conformity between the actual position and the ideal position of points, lines and surfaces on the machined parts. There are eight items for evaluating position accuracy: parallelism, verticality, inclination, coaxiality, symmetry, position, circular jump and total jump (GB/T1182-1996). The position accuracy is controlled by the position tolerance. The position tolerance of each item is also divided into 12 accuracy grade.
(4) Surface roughness
In cutting, due to vibration, tool marks and friction between the tool and the workpiece, the machined surface of the workpiece will inevitably leave some tiny peaks and valleys.
The height of these tiny peaks and valleys on the surface of parts is called surface roughness, also called micro-roughness.
Ra evaluation of contour arithmetic mean deviation is commonly used. Gb/t1031-1995 stipulates that the Ra value is 14, and the range is 100, 50, 25, 12.5, 6.3, 3.2,/. . . ,0. 12。 There are also supplementary series values.
Symbol of surface roughness: Ra
Surface roughness unit: micron
Different machining methods can obtain different surface roughness.
(5) Tolerance and cooperation
Dimensional tolerance (tolerance for short): allowable deviation of dimensions. Tolerance is equal to the absolute value of algebraic difference between the maximum limit size and the minimum limit size.
Lowercase letters of the axis, such as h7, js6, g6, m7.
Holes are indicated by capital letters. Such as H7, H6
Fit: the relationship between holes with the same basic size and shaft tolerance zone. Divided into clearance fit, interference fit and transition fit.
Determine the fit relationship, and then consult the manual to determine the tolerance value.
7. Parts design considerations
(1) Mark dimensions and tolerances, shape and position tolerances and surface roughness.
(2) Technical requirements: Shaft parts should have heat treatment (quenching and tempering) requirements, and other parts may not have heat treatment requirements.
(3) Material brand.
(4) Draw the part drawing according to the drawing standard, and the drawing is generally A4 or A3.
Second, the processing technology design
1, basic knowledge of machining process
(1) process
In the production process of products, the process directly related to the transformation of raw materials into finished products is called technological process. Such as casting, forging, welding and machining of parts.
(2) Processing technology
In the process of machining, the shape, size and performance of the blank are directly changed, which is called machining process.
(3) Composition of processing technology
The machining process consists of several continuous processes, through which the blank is processed into qualified parts.
(4) Working procedures
A part of the process in which a worker (or a group of workers) continuously processes one (or several) workpieces on a machine tool (or workplace).
It must be noted here that the main feature of a process is that the processing object, equipment and operators remain unchanged, and the work in the process is completed continuously.
2, mechanical processing program
(1) processing technology specification
Processing technology specification (hereinafter referred to as process specification) is a process document that specifies the processing technology and operation method of parts.
(2) the content of process specification
Technical route;
Contents and requirements of each working procedure;
Machine tools and process equipment used;
Inspection items and methods of workpieces;
Cutting dosage, man-hour quota, etc.
Process route refers to the sequence of products or parts in the production process from blank preparation to finished product packaging and warehousing through relevant departments or processes of the enterprise.
Process equipment (referred to as tooling) is the general name of various tools used in product manufacturing. It includes tools, fixtures, molds, measuring tools, inspection tools and auxiliary tools.
(3) Format of process specification
There are two basic forms of processing technology regulations: processing technology card and processing technology card. Machining process card is a kind of process document, which briefly explains the machining process of parts in the form of working procedure. Generally suitable for single piece and small batch production. (Processing flow direction of parts)
Formulate reasonable parts processing technology;
First of all, we should know what processing methods are available at present and choose them reasonably according to the specific requirements of parts.
Secondly, the processing sequence of each surface and how to arrange heat treatment must be solved.
3, the steps of making process regulations
1) Analyze the part drawings and assembly drawings of the product, analyze the processing requirements and structural manufacturability of the part drawings, and check the integrity of the drawings;
2) Determine the production type according to the production procedure of the parts;
3) selecting a blank;
4) determining the processing method of a single surface;
5) Select the positioning datum and determine the processing route of the parts;
6) Determine the equipment and process equipment used in each process;
7) Calculate machining allowance, process size and tolerance;
8) Determine the cutting amount and estimate the man-hour quota;
9) Fill in the process documents.
4. Production type
When making the machining process specification, parts are generally divided into three production types according to their production procedures.
5, the choice of typical mechanical parts blank
The choice of blank mainly includes the choice of blank material, type and manufacturing method.
(1) Common blank types
All kinds of rolled profiles, castings, forgings, welded parts, stamped parts, powder metallurgy parts and injection molded parts. (combined processing)
(2) Selection of blank of typical mechanical parts
There are three categories: shaft, disk sleeve and box.
For shafts with gears and bearings, forgings or round steel are usually used.
Its journal requires high comprehensive mechanical properties, and medium carbon quenched and tempered steel is commonly used; Such as 45 degrees
Alloy structural steel 40Cr is used for shafts that bear heavy load or impact load and require high wear resistance.
Selection of blank for K-disc sleeve
Common are gears, pulleys, flywheels, handwheels, flanges, couplings, collars, washers, bearing seats, etc.
The gear is made of medium carbon structural steel; Alloy carburized steel is selected as an important gear to bear large impact load; Its blanks are all forged by profiles.
Pulley, flywheel, handwheel and other parts. Gray cast iron blanks or low carbon steel welding blanks are usually used for those who bear little or mainly pressure.
Flanges, collars, washers and other parts can be made of cast iron, forgings or round steel; Small thickness (
Blank selection of L-box parts
Common are the machine body, frame, base, cross beam, workbench, gearbox, box cover, bearing seat, valve body and pump of various mechanical equipment.
Generally, iron castings are used;
For parts with large stress and complexity, cast steel blanks should be used;
Welding blanks can also be used for single piece and small batch production.
High-quality carbon structural steel (35, 45), alloy structural steel (40Cr), cast steel (ZG570) and cast iron (HT 150, HT200).
6. Common surface treatment methods
The machining process of parts is the process of obtaining the surface of parts that meet the requirements through mechanical machining.
(1) Common treatment methods
Different types and requirements of parts surface need different processing methods. (five types of machine tools)
Turning: all kinds of revolving surfaces. Such as external circle, internal circle, thread
Drilling: hole
Milling: plane and groove (keyway, spiral groove)
Planing: plane and V-groove
Grinding: external circle, internal circle, conical surface and plane.
There are also boring, broaching, finishing and special processing.
(2) Division of processing stages
According to the different surface quality requirements of parts, surface processing is usually divided into the following stages.
1) The main task of rough machining stage is to remove most of the allowance on each machined surface.
2) The task of the semi-finishing stage is to reduce the errors left by finishing, prepare for the finishing of the main surface, and complete the processing of some sub-surfaces.
3) The task of finishing stage is to ensure that all main surfaces meet the requirements specified in the drawings.
(3) Parts processing should follow the following principles:
(1) Roughing and finishing are carried out separately. In order to ensure the processing quality of parts, improve production efficiency and economic benefits, and achieve their different purposes and requirements.
(2) Machining of parts is generally not done by one process method on a machine tool, but often requires several processing methods to cooperate with each other, and the surface machining of parts can be gradually completed through certain processes.
(3) If a surface can be processed by different processing methods, then in terms of specific production conditions, one of the processing methods must be the most suitable.
(4) Common machining schemes for cylindrical surfaces
The excircle is the main surface or auxiliary surface of shaft, disk sleeve and other parts.
The most commonly used methods for machining cylindrical surfaces are turning and grinding.
(1) turning: used for machining the cylindrical surface of medium-precision disks, sleeves and short-axis pins; External circle of non-ferrous metal parts; The outer surface of a part structure (such as the outer surface of a notch) that is not suitable for grinding.
Rough turning [it12 ~ it11,ra25 ~ 12.5um] → tempering (as required) → semi-finished turning [it 10 ~ it9, ra6.3 ~ 3.2 um].
(4) Common machining schemes for cylindrical surfaces
(2) Turning and grinding: it is used to process the cylindrical surfaces of various parts with high precision and suitable structure and shape except non-ferrous metal parts, especially the parts that need quenching treatment.
Rough turning [it12 ~ it11,ra25 ~ 12.5um] → quenching (as required) → semi-finish turning [it 10 ~ it9, ra6.3 ~ 3.2 um].
(5) Common machining schemes for inner circular surfaces (holes)
Inner circle (hole) surface is the basic surface of mechanical parts, especially parts such as disk sleeve and bracket box, and hole is one of the important surfaces.
Commonly used hole processing methods are:
Drilling, reaming, reaming, turning, boring, drawing and grinding.
(1) Lathe (boring machine) type: used for machining the aperture except hardened steel parts D >; 15 holes in various metal parts.
Drilling/rough turning or rough boring [it12 ~ it11,ra25 ~ 12.5 um] → tempering (as required) → semi-finish turning or semi-finish boring [it 10 ~ it9, ra6.
(2) Turning (boring) and grinding: used for machining holes of hardened steel parts and non-quenched steel parts, as well as high-precision holes on shaft and disk sleeve metal parts except non-ferrous metal parts.
Drilling/rough turning or rough boring [it12 ~ it11,ra25 ~ 12.5 um] → tempering (as required) → semi-finish turning or semi-finish boring [it 10 ~ it9, ra6.
(6) Plane machining scheme
Plane is the main surface of disk, plate and box parts.
Common plane processing methods are:
Milling, planing, turning and grinding. Among them, milling, planing and grinding are the main processing methods.
① Milling (planing) category: used for machining medium-precision planes of various parts except quenched hardware. Milling is suitable for all kinds of batches, and planing is suitable for single piece and small batch production and maintenance.
Rough milling or rough planing [it13 ~ it11,ra25 ~ 12.5um] → tempering (as required) → semi-finish milling or semi-finish planing [it 10 ~ it9, ra6.3 ~.
(6) Plane machining scheme
(2) Milling (planing) and grinding: it is used to process planes with high precision and low Ra value on various parts except non-ferrous metal parts.
Rough milling or rough planing [it13 ~ it11,ra25 ~ 12.5 um] → tempering (as required) → semi-finish milling or semi-finish planing [it 10 ~ it9, ra6.3 ~.
(3) Turning: it is mostly used to process the end faces and step faces of shafts, disks, sleeves and other parts.
Rough turning [it13 ~ it11,ra25 ~ 12.5um] → tempering (as required) → semi-finished turning [it 10 ~ it9, ra6.3 ~ 3.2 um].
(7) Thread processing
Machining method of thread
Cutting-turning, milling, grinding, tapping and tapping;
Non-cutting processing-thread rolling and rolling;
① Thread turning: the roughness can reach 3.2 ~ 0.8; Grade 9-4
② tapping: the roughness can reach 6.3 ~1.6; Grade 8-6
③ set of threads: the roughness can reach 3.2 ~1.6; Grade 8-6
Examples of marking: internal thread M 12-6h and external thread M 12-6g.
7. Selection of positioning datum
Datum is used to determine the points, lines and surfaces on which geometric relations between geometric features on production objects are based.
According to different functions, benchmarks can be divided into design benchmarks and process benchmarks.
Process benchmarks are benchmarks used in the process.
Process benchmarks can be divided into:
1) Positioning datum 2) Measurement datum
3) Assembly benchmark 4) Process benchmark
Design benchmark
Design datum is the datum adopted in design drawings, and it is the starting point for marking design dimension or position tolerance.
Positioning datum
Used as positioning reference in machining.
In the process of machining parts, each working procedure has a problem of selecting positioning datum. It plays a decisive role in ensuring the machining accuracy of parts and reasonably arranging the machining sequence, so it is an important issue in the process of making technology.
Principles of rough benchmark selection
In the machining process, the benchmark used in the first process is always a rough benchmark. Affect the distribution of machining allowance on each machining surface in the future; The size and mutual position between the unprocessed surface and the machined surface.
Select the important surface as the rough datum.
Choose the raw surface as a rough benchmark.
Select the surface with the smallest machining allowance as the rough benchmark.
The smooth surface with large machining area is selected as the rough benchmark.
Coarse datum can only be used once in the same machining dimension direction.
2) the principle of fine benchmark selection
When selecting precision datum, it should be considered that the selected precision datum should be conducive to ensuring machining accuracy and making the machining process easy to operate.
(1) principle of datum coincidence
That is, try to choose the design datum of the machined surface as the fine datum to avoid the error caused by the dislocation of the datum.
(2) the principle of benchmark unification
That is to say, choose a unified precision datum as far as possible to process multiple surfaces on the workpiece.
For shaft parts, all cylindrical surfaces are often machined with the central hole as the unified benchmark, which can ensure high coaxiality between surfaces;
Generally, a large plane and two far holes are used as the fine benchmark of the box;
8. Formulation of process route
Main tasks:
Selection of surface treatment methods,
Arrangement of processing sequence,
The number of processes in the whole process.
1) Selection of surface treatment methods
The machining of parts is essentially the combination of these simple geometric surfaces (outer cylindrical surface, hole, plane or molding surface). Therefore, when drawing up the processing route of parts, we must first determine the processing scheme of each surface of parts.
(1) Select the machining method to ensure the dimensional accuracy and surface roughness of the machined surface.
(2) The selected machining method should ensure the geometric shape accuracy of the machined surface and the mutual position accuracy of the surfaces.
(3) The choice of processing method should be compatible with the machinability and heat treatment of parts materials.
(4) The selection of processing method should be suitable for the production type (batch).
⑤ The selected processing method should be suitable for the existing production conditions of our factory.
2) Arrangement of processing sequence
The arrangement of processing sequence plays an important role in ensuring processing quality, improving production efficiency and reducing costs, and is one of the keys to formulating process routes.
Arrangement of cutting sequence
Arrangement of heat treatment process
Arrangement of auxiliary processes
(1) Arrangement of cutting sequence
(1) first coarse and then fine.
Arrange rough machining first, semi-finish machining in the middle, and finish machining and finish machining at last.
(2) Arrangement of heat treatment process
The division of processing stages is usually based on heat treatment.
(3) Arrangement of auxiliary processes
Inspection process is one of the necessary measures to ensure product quality.
Usually, after the rough machining is completely completed,
Before and after the processing of important processes,
When parts are transferred between workshops,
After all parts are processed.
Sometimes, after some processes, some auxiliary processes such as deburring, cleaning, demagnetization and antirust oil are arranged.
(4) Concentration and dispersion of processes (process-quantity determination principle)
After arranging the processing sequence, it is necessary to combine each step of processing the surface into several processes according to different processing stages and processing sequences, so as to draw up the whole processing route.
When combined into processes, the principle of centralized or decentralized processes can be adopted.
Process concentration is to concentrate the processing of parts on several processes, and each process has a lot of processing content.
Process dispersion means that the processing of parts is dispersed into many processes, and the processing content of each process is relatively small.
9, the determination of machining allowance
1) machining allowance
Machining allowance refers to the thickness of the metal layer cut from the machined surface during machining.
Machining allowance can be divided into total machining allowance and working procedure machining allowance (working procedure allowance). Working procedure allowance can be divided into unilateral allowance and bilateral allowance.
(1) On the plane, the machining allowance is asymmetric unilateral allowance.
(2) The machining allowance of the revolving surface (excircle and hole) is symmetrical bilateral allowance, and the thickness of the actually cut metal layer is half of the machining allowance.
2) Determination of machining allowance
(1) analysis and calculation method
(2) Look-up table correction method (widely used)
(3) Empirical estimation method
When a single piece is produced in small batches, the machining allowance of common processes for small and medium-sized parts is:
Roughing allowance is about1-1.5 mm;
Semi-finishing allowance is about 0.5-lmm；;
The allowance of high-speed finishing is about 0.4-0.5 mm; The allowance of low-speed finishing is about 0.1-0.3 mm; The grinding allowance is about 0.15-0.25mm. ..
10, determination of cutting dosage and man-hour quota
Cutting parameters: cutting speed, feed speed and reverse feed speed-cutting.
Man-hour quota: the time it takes to process a part.
In single piece and small batch production
The man-hour quota is generally decided by craftsmen.
Cutting parameters are usually determined by the experience of the processor.
1 1. Selection of machine tools and process equipment
1) Selection of machine tools
Forming requirements, specifications and dimensions, accuracy and productivity of machine tools.
2) Selection of process equipment
(1) fixture selection: single piece and small batch production, try to choose universal fixture.
(2) The selection of cutting tools generally adopts general cutting tools or standard cutting tools, and when necessary, cutting tools with high productivity can also be used. The types, specifications and accuracy of cutting tools should meet the processing requirements of parts.
(3) Selection of measuring tools General measuring tools should be adopted for single piece and small batch production.
III. Process Analysis of Typical Parts-Examples
(A) the processing technology of shaft parts
As shown in the figure, the transmission shaft is the most commonly used shaft part and the most typical stepped shaft. Now take it as an example to introduce the technology of general stepped shaft.
The material of the transmission shaft is 40Cr, and the technical requirements of the transmission shaft are: quenched and tempered HBS220 ~ 240；; The production quantity is 5 pieces.
1, main surfaces of transmission shaft parts and their technical requirements
Part drawing and assembly drawing analysis;
From the transmission shaft drawing and its assembly drawing, it can be seen that the journal M and N of the transmission shaft are the support journals for installing bearings, and are also the installation benchmarks for installing the shaft into the box. A worm wheel is installed on the excircle P in the middle of the shaft, and the movement can be transmitted to the worm wheel through the worm. After deceleration, the motion will be transmitted through the gear on the excircle Q at the left end of the shaft. Therefore, the dimensional accuracy of journals M, N, excircles P and Q is high, and the tolerance level is IT6. The surface roughness Ra of the shaft shoulder g, h, 1 is 0. 8um, and the mutual position is required to be accurate.
2, processing process analysis
(1) Select the blank type.
The shaft blank is a forging.
(2) Processing method of main surface
Most of the shaft is a revolving surface, and turning should be the main method. The tolerance level of surface M, N, P and Q is high, and the surface roughness Ra is small, so it needs grinding after turning. Therefore, the processing sequence of these surfaces should be: rough turning, tempering, semi-finish turning and grinding.
(3) determine the positioning datum.
Several main mating surfaces and step surfaces of the shaft have the requirements of radial circular runout and end circular runout, and the central holes at both ends are used as the positioning accuracy benchmark.
Transmission shaft process card
(4) Draw up the technological process
In the process of making shaft technology, besides the processing of primary surface, the processing and heat treatment requirements of secondary surface should also be considered. The excircle with low requirements can be machined to the specified size in semi-finish turning, and unloading grooves, overtravel grooves, chamfers and threads should be machined in semi-finish turning. Keyway should be milled after semi-finish turning, and quenched and tempered after rough turning. After quenching and tempering, the central hole must be repaired to eliminate heat treatment deformation and scale. Before grinding, the center hole should be repaired once to improve the positioning accuracy.
To sum up, the process card of this part is shown in the table.
(2) Processing technology of disk sleeve parts
As shown in the figure, the connecting plate is a sleeve-like part. Here, take it as an example to introduce the technological process of connecting plate parts.
The material of receiving plate is 45 steel, and the technical requirements of receiving plate are: quenched and tempered HBS220 ~ 240；; A special program in which all the actors participate in the performance.