I. Overview of mechanical parts
1. Classification of parts
According to their structures, parts can be generally divided into six categories: shaft, sleeve, bracket box, hexahedron, fuselage frame and special category
When selecting parts, shaft, sleeve and bracket are the main ones. (relatively common)
2. Part surface composition
1) Part surface composition: three basic surfaces
① Revolving surface: cylindrical surface, conical surface, revolving forming surface, etc
② Plane: large plane, end surface, torus, etc
③ Forming surface: involute tooth surface, helicoid surface, etc
3. Part surface.
(2) Envelope method: The profile of the machined workpiece is the envelope of the continuous position of the cutting edge during the cutting movement.
4. Materials of parts
Common materials of parts:
carbon structural steel Q235A, high-quality carbon structural steel (35, 45), alloy structural steel (4Cr)
cast steel (ZG57), cast iron (HT15, HT2)
nonferrous metals and their alloys, etc.
marked in the title bar
5. Heat treatment of parts
Common heat treatment methods of parts: annealing, normalizing, quenching, tempering, tempering, aging, etc.
in the technical requirements,
45 common uses are given: shaft
tempering 22-24 HBS
surface quenching, hardness HRC 4-5
6. Processing quality of parts
Processing quality of parts includes two aspects: processing accuracy and surface quality.
the machining accuracy of a part refers to the degree of conformity between the actual geometric parameters (size, shape and position) and the ideal geometric parameters of the part 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 values of the dimensions such as the diameter, length and distance between surfaces of parts are to the ideal values. Dimensional accuracy is controlled by dimensional tolerance. Dimensional tolerance is the allowable variation of part size in cutting. With the same basic dimensions, the smaller the dimensional tolerance, the higher the dimensional accuracy.
the standard tolerance grade of dimensional accuracy specified in GB/T18.1-1997 is divided into 2 grades, namely IT1, IT, IT1, IT2, …, IT18, in which IT1 has the smallest tolerance and the highest dimensional accuracy.
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 to which the actual shapes of lines and surfaces on the machined parts conform to the ideal shapes.
There are six items to evaluate the shape accuracy, such as straightness, flatness, roundness, cylindricity, line profile and surface profile (GB/T1182-1996).
shape accuracy is controlled by shape tolerance. All shape tolerances are divided into 12 precision grades except roundness and cylindricity, which are divided into 13 precision grades (~12). Level 1 is the highest and level 12 is the lowest.
(3) Position accuracy
refers to the degree to which the actual positions of points, lines and surfaces on the machined parts conform to the ideal positions. There are eight items for evaluating position accuracy, such as parallelism, verticality, inclination, coaxiality, symmetry, position, circular jump and total jump (GB/T1182-1996). Position accuracy is controlled by position tolerance. The position tolerance of each project is also divided into 12 precision grades.
(4) Surface roughness
In cutting, some tiny peaks and valleys are inevitably left on the machined surface of the workpiece due to vibration, tool marks and friction between the tool and the workpiece.
the height of these tiny peaks and valleys on the surface of parts is called surface roughness, also called micro-roughness.
commonly used is the evaluation of contour arithmetic mean deviation Ra. According to GB/T131-1995, the Ra value is 14 grades, ranging from 1, 5, 25, 12.5, 6.3, 3.2, 1.6, .8. . . ，.12。 There are also supplementary series values.
symbol of surface roughness: Ra
unit of surface roughness: μm
different machining methods can achieve different surface roughness.
(5) tolerance and fit
dimensional tolerance (tolerance for short): allowable size variation. Tolerance is equal to the absolute value of algebraic difference between the maximum limit size and the minimum limit size.
use small letters for the shaft, such as capital letters for holes h7, js6, g6 and m7
. For example, H7 and H6
fit: the relationship between the combined holes with the same basic size and the 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. Precautions for parts design
(1) Mark the dimensions and tolerances, form and position tolerances and surface roughness.
(2) technical requirements: shaft parts should have heat treatment (quenching and tempering) requirements, and other types may not have heat treatment requirements.
(3) material grade.
(4) Draw the part drawing according to the drawing standard, and the drawing is generally A4 or A3.
second, machining process design
1. Basic knowledge of machining process
(1) Technological process
In the production process of products, the process directly related to the transformation of raw materials into finished products is called technological process. For example, casting, forging, welding and mechanical processing of parts.
(2) machining process
in the process of machining, the shape, size and performance of the blank are directly changed into finished products, which is called machining process.
(3) Composition of machining process
The machining process is composed of several sequential processes, and the blank is processed into qualified parts through these different processes.
(4) Process
The part of the mechanical processing process that a worker (or a group of workers) continuously processes one (or several) workpieces on a machine tool (or a work place).
it must be noted here that the main feature of a working procedure is that the object, equipment and operator are not changed, and the work in the working procedure is completed continuously.
2. machining process specification
(1) machining process specification
machining process specification (hereinafter referred to as process specification) is a technical document that specifies the machining process and operation methods of parts.
(2) the content of the process specification
the process route;
processing 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 from blank preparation to finished product packaging and warehousing through relevant departments or processes of the enterprise.
process equipment (tooling for short) is the general name of various tools used in the product manufacturing process. It includes tools, fixtures, molds, measuring tools, inspection tools and auxiliary tools.
(3) Format of process planning
There are two basic forms of machining process planning: machining process planning card and machining process planning card. The machining process card is a kind of process document that briefly explains the machining process of parts in terms of working procedure. Generally suitable for single piece and small batch production. (the flow direction of parts processing)
Formulating a reasonable part processing process:
First of all, we must master what processing methods are available at present and be able to choose them reasonably according to the specific requirements of parts.
Secondly, the processing sequence of each surface and how to arrange the heat treatment must be solved.
3. Steps for formulating process regulations
1) Analyze the part drawings and assembly drawings of products, 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 program of the parts;
3) selecting a blank;
4) determining the processing method of a single surface;
5) Select the positioning datum to determine the processing route of the parts;
6) determine the equipment and process equipment used in each process;
7) calculate the machining allowance, process size and tolerance;
8) determine the cutting amount and estimate the working hour quota;
9) fill in the process documents.
4. Production types
When making the machining process regulations, the parts are generally divided into three production types according to the production program of the parts.
5. Selection of typical machine parts blanks
The selection of blanks mainly includes the selection of blank materials, types and production methods.
(1) Common blank types
Various rolled profiles, castings, forgings, weldments, stamping parts, powder metallurgy parts and injection molded parts. (combined machining)
(2) Selection of typical machine parts blanks
There are three categories: shaft, disk sleeve and frame box
j For shafts with gears and bearings, forgings or round steel are mostly used
The journal requires high comprehensive mechanical properties, and medium carbon quenched and tempered steel is often selected; For example, 45
shafts that bear heavy load or impact load and require high wear resistance are mostly made of alloy structural steel, and 4Cr
k-type sleeve blanks are commonly selected as gears, pulleys, flywheels, handwheels, flanges, couplings, collars, washers and bearing seats.
the gear is made of medium carbon structural steel; Alloy carburized steel is selected as the important gear bearing large impact load; Its blanks are all made of profiles by forging.
The parts with little stress or mainly compression, such as pulleys, flywheels, handwheels, etc., usually use gray cast iron blanks, or low-carbon steel welded blanks.
flanges, collars, washers and other parts can be made of cast iron, forgings or round steel; Small thickness (<: 4) Steel plates can also be directly cut as blanks.
the blank selection of l-frame box parts
There are usually the fuselage, frame, base, beam, workbench, reducer box, box cover, bearing seat, valve body and pump body of various mechanical equipment.
generally, iron castings are mostly used;
for the parts with large stress and complex stress, the blank of steel castings should be adopted;
weldment blanks can also be used in small batch production.
high-quality carbon structural steel (35, 45), alloy structural steel (4Cr), cast steel (ZG57), cast iron (HT15, HT2)
6. Processing methods of common surfaces
The processing process of parts is the process of obtaining parts surfaces that meet the requirements through processing.
(1) Common processing methods
Different types and requirements of parts' surfaces require different processing methods. (Five kinds of machine tools)
Turning: all kinds of revolving surfaces. Such as excircle, inner circle and thread
drilling processing: hole
milling processing: plane and groove (keyway and spiral groove)
planing processing: plane and V-groove
grinding processing: excircle, inner circle, conical surface and plane
There are also boring processing, broaching processing and finishing processing here.
1) The main task in the rough machining stage is to remove most of the allowance on each machined surface.
2) The task of semi-finishing stage is to reduce the errors left by finishing, prepare for the finishing of the main surface, and finish the machining of some secondary surfaces.
3) The task in the finishing stage is to ensure that all major surfaces meet the requirements specified in the drawings.
(3) The machining of parts follows the following principles:
① Rough machining and finish machining are separated. In order to ensure the processing quality of parts, improve production efficiency and economic benefits, and achieve their different purposes and requirements.
(2) The processing of parts is generally not completed by one process method on one machine tool, but often requires several processing methods to cooperate with each other, and the surface of parts can be processed step by step after a certain process.
③ if a surface can be processed by different processing methods, one of the processing methods must be the most suitable in terms of specific production conditions.
(4) Common machining scheme for excircle
The excircle is the main surface or auxiliary surface of parts such as shaft and disk sleeve.
The most commonly used methods for machining the cylindrical surface are: turning and grinding
① Turning: used for machining the cylindrical surface of medium-precision disc, sleeve and short-axis pin parts; External circle of non-ferrous metal parts; The excircle surface (e.g. the excircle of the notch) of the part structure that is not suitable for grinding
rough turning [it12 ~ it11, ra25 ~ 12.5 um] → quenching and tempering (as required) → semi-finish turning [it1 ~ it9, ra6.3 ~ 3.2 um] → finish turning [it8 ~ it7, ra1.6 ~ .8].
roughing [it12 ~ it11, ra25 ~ 12.5 um] → tempering (as required) → semi-finishing [it1 ~ it9, ra6.3 ~ 3.2 um] → quenching (as required) → rough grinding [it8 ~ it7, ra1.6 ~ .8 um] → fine grinding [
The commonly used methods of hole processing are:
drilling, reaming, turning, boring, pulling and grinding
① Turning (boring) type: used to process the hole diameter except hardened steel parts. 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 [it1 ~ it9, ra6.3 ~ 3.2 um] → finish turning or finish boring [it8 ~ it7, ra1.6 ~ .8 um] <
drilling/rough turning or rough boring [it12 ~ it11, ra25 ~ 12.5 um] → tempering (as required) → semi-finish turning or semi-finish boring [it1 ~ it9, ra6.3 ~ 3.2 um] → quenching (as required) → rough grinding [it8 ~ it7, ra1.6 ~ .
the common machining methods of plane are:
milling and planing.