1. Introduce basic knowledge and terminology of mechanical processing technology, such as procedure, installation, station, work step, etc.
2. Formulate mechanical processing process rules and methods.
3. Design each process in the process route.
Including determining the machining allowance, process size and its tolerance; selecting machine tools and process equipment; determining the cutting amount, calculating the man-hour quota, etc.
Requirements of this chapter: understand and master the basic concepts of machining process, such as procedures, steps, and process regulations, etc. Understand the steps of formulating machining process regulations, and be familiar with the knowledge of machining productivity and economy. Master the main work content of formulating mechanical processing process regulations, master the content of process design, and be able to apply the process size chain to calculate the process size when the benchmarks do not overlap.
3.1 Overview of machining process regulations
3.1.1 Production process and machining process
The production process of mechanical products is the whole process of transforming raw materials into finished products. The production process in the machinery manufacturing plant includes the transportation and storage of raw materials, the technical preparation and production preparation of products, the manufacture of blanks, the machining and heat treatment of parts, the assembly, debugging and inspection of products, as well as the sales and after-sales service of products, etc.
In the production process, the process of directly changing the shape, size, relative position and nature of the production object to make it a finished or semi-finished product is called a process. Such as the manufacture of blanks in the production process, the machining and heat treatment of parts, the assembly, debugging, inspection and other processes of products. the
The machining process refers to the whole process of changing the shape, size, relative position and properties of the blank by machining methods to make it a part.
3.1.2 Composition of machining process
The mechanical processing process can be divided into units of different levels, namely process, installation, station, work step and cutting tool. Among them, the process is the basic unit of the division process, and the mechanical processing process of parts is composed of several processes.
A process refers to the part of the process that is continuously completed by one or a group of workers on the same or several workpieces at the same time. The four elements that maintain a process are the workplace, workers, workpieces, and continuous operations. A change in any one of these elements constitutes a new process.
To complete the process content of a process, sometimes it is necessary to clamp the workpiece multiple times, and the part of the process content completed after the workpiece (or assembly unit) is clamped once is called installation.
When processing on a machine tool with an indexing (or shifting) fixture (or workbench), in one clamping, the workpiece (or tool) has to pass through several positions relative to the machine tool to be processed sequentially. At this time, to complete In a certain process part, after the workpiece is clamped once, each position occupied by the workpiece (or assembly unit) and the movable part of the fixture or equipment relative to the fixed part of the tool or equipment is called a station.
- Work step
A working step is a unit for dividing a process. In a process, a working step is a part of the process that is continuously completed under the condition that the processing surface (or the connecting surface during assembly) and the processing (or assembly) tool remain unchanged. A change in one of the two elements of the machined surface and the machined tool is another process step. For several identical working steps processed continuously in one installation, it can be written as one working step.
- Take the knife
In one working step, if the metal layer to be removed is very thick, the same surface needs to be cut several times. At this time, the part of the feeding movement completed by the tool relative to the workpiece at the feed speed during processing is called Take the knife.
3.1.3 Machining process specification
- Machining Process Regulations
In the production of mechanical products, the process documents used to specify the manufacturing process and operation methods of products or parts are called mechanical processing process regulations. There are a variety of process specification documents used in the production process. The following two commonly used process specification documents are introduced: mechanical processing process card and mechanical processing process card.
(1) Machining process card This card is a process document that describes the machining process of parts in units of procedures. The machining process card outlines the overall picture of the machining process and is the basis for formulating other process documents. However, in single-piece small-batch production, more detailed process documents are usually no longer compiled, and this kind of card is used to directly guide production.
(2) Machining process card This card is a process document compiled according to the process content of each process on the basis of the process card of mechanical processing. The card is generally accompanied by a schematic diagram of the process, and details the processing content, process parameters, operating requirements, and equipment and process equipment used for each step in the process. It is a technical document used to specifically guide workers to operate.
- Process diagram
The process diagram is attached to the mechanical processing process card. The process diagram can clearly and intuitively express the process content of a process. The drawing requirements have the following points:
(1) The schematic diagram of the process can be scaled down and drawn with as few projections as possible, and the secondary structures and lines in the view can be omitted.
(2) The front view of the process diagram should be the position where the workpiece in this process is clamped on the machine tool. For example, the process diagram of shaft parts processed on a horizontal lathe, the center line should be horizontal, the processing end is on the right, and the clamping end of the chuck is on the left.
(3) In the schematic diagram of the process, the surface processed by this process is represented by a thick solid line on the workpiece, and the surface not processed by this process is represented by a thin solid line.
(4) The positioning and clamping of the workpiece are indicated by the specified symbols in the process diagram.
(5) The process dimensions and tolerances of this process, the surface roughness of the machined surface, and other technical requirements that should be met in this process are marked in the process diagram.
- The role of machining process regulations
(1) Process regulations are guiding documents for organizing production. Production planning and scheduling, worker operations, and product quality inspections are all based on process regulations. Production personnel must not violate process regulations to ensure the quality of produced products.
(2) Process specification is the basis for production preparation
(3) The process specification is the technical document of the new factory (workshop)
- 3.1.4 Principles and steps for formulating mechanical processing procedures
Under certain production conditions, ensuring processing quality and minimum production cost are the basic principles for formulating process regulations.
The work of formulating the machining process regulations for parts can be roughly divided into the following four stages:
- Preparatory work stage Before drawing up the mechanical processing route of the parts, it is necessary to do the necessary preparatory work, including calculating the production program and determining the production type; analyzing the process of the parts; determining the type of blank.
- Process route drafting stage This is the core of formulating process regulations, and its main contents are: selection of positioning datum; selection of part surface processing method; division of processing stages; arrangement of processing sequence and process integration, etc.
- In the process design stage, after the process route is drawn up, this stage is used to determine the process content of each process in the process route, including determining the machining allowance, process size and tolerance; selecting machine tools and process equipment; determining the cutting amount and calculating the working hours Quota etc.
- Fill in the process documents After the part machining process specification is determined by the above steps, the relevant content should be filled in various cards for implementation. These cards are collectively referred to as craft files. Filling in the process file is the last work in the preparation of the part process specification. There are many types of process documents, and the corresponding process documents can be selected as the process regulations used in production according to the actual needs of production.
3.2 Preparatory work for formulating machining process regulations
The preparatory work for formulating the machining process regulations of parts includes calculating the production program and determining the production type; conducting process analysis on parts; determining the type of blank, etc.
3.2.1 Production program and production type
- Production program
The production program refers to the product output and progress plan that the enterprise should produce within the planning period. The annual production schedule N of parts in the planning period of one year can be calculated according to the following formula:
N=Qn (1+a%) (1+b%) (pieces/year) (3-1)
In the formula, Q—the annual output of the product (unit/year);
n—the number of parts in each product;
a%—the percentage of spare parts;
b%—the percentage of waste.
- production type
- The type of production can reflect the degree of production specialization of the enterprise. According to the characteristics of the products produced by the enterprise (that is, the products are heavy, medium or light parts), the annual production program, the batch size and the continuity of production, it is generally divided into three types of production, namely single-piece production, batch production and mass production.
- Single-piece production means that the number of parts of the same kind produced by the enterprise is small, the product variety of the enterprise is large and seldom repeated, and the processing objects of each workplace in the enterprise are often changed. For example, heavy machinery manufacturing, special equipment manufacturing and new product trial production all belong to single-piece production.
- Mass production refers to the large quantity of the same product produced by the enterprise, and the continuous mass production of the same product. Most workplaces in an enterprise fixedly process a certain process of a certain part. Such as the manufacture of automobiles, bearings, motorcycles and other products.
- Batch production means that enterprises produce the same products in batches on an annual basis, and the production is repeated periodically. For example, general machine tool manufacturing, textile machinery manufacturing, etc. Usually, the enterprise does not put the annual output into the workshop production at one time, but puts it into production in batches according to a certain period of time according to the production cycle of the product, sales and the balance of workshop production. The quantity of the same product or part that is input or output at one time is called production batch, referred to as batch.
- In batch production, according to different batches, it is divided into three types: small batch production, medium batch production and large batch production.
3.2.2 Process analysis of parts
Before formulating the machining process regulations of parts, the manufacturability of parts should be analyzed, mainly in the following two aspects.
- Analyze and review part drawings and assembly drawings of products
When formulating the process specification, by analyzing the part drawing and the assembly drawing of the part, it is mainly to clarify the position and function of the processed part in the product, find out how many main processing surfaces are on the part, and find out the main technical requirements and processing of the part. The key technical issues in the process, understand the basis for the formulation of various tolerances and technical requirements, and solve these problems in a targeted manner during the preparation process.
The specific content includes:
(1) Check whether the views, dimensions, tolerances and technical conditions of the part drawings are complete.
(2) Check whether the technical requirements are reasonable.
(3) Check whether the material of the parts and the selection of heat treatment are appropriate.
- Structural Manufacturability Analysis of Parts
The structural manufacturability of parts refers to the convenience, feasibility and economy of manufacturing the designed parts under the premise of meeting the requirements of use. That is, the structure of the part should be convenient for clamping, tool setting and measurement of the workpiece during processing, and can improve cutting efficiency. Poor structural manufacturability will make processing difficult, waste materials and man-hours, and sometimes even fail to process. Therefore, the technological review of the structure of the parts should be carried out, if the structure of the parts is found to be unreasonable. It should be analyzed together with relevant designers, and necessary modifications and supplements should be made to the drawings according to the prescribed procedures.
- Influence of NC Machining on the Manufacturability of Part Structure
The characteristics of CNC machining are high degree of automation, high machining precision, strong adaptability to the processing object, and can communicate with computer (DNC) to realize the integration of computer-aided design and manufacturing. Therefore, numerical control machining has had a great impact on the traditional measure of the structural manufacturability of parts. In the following cases, numerical control machining is used, and its manufacturability is good:
⑴ Processing of parts produced in small batches, and processing of key processes in batch production.
⑵ High processing precision, processing parts with complex curves or curved surfaces.
(3) Processing of parts that require multiple redesigns.
⑷Workpieces that require multiple steps of drilling, boring, reaming, tapping and milling, such as the processing of box parts.
⑸ high value parts.
⑹The processing of parts that are precisely replicated.
(7) When processing with a general-purpose machine tool, complex special fixtures or parts that require a long adjustment time are required.
3.2.3 Selection of blank
The blank is a production object for further processing made according to the shape and process size required by the part. The types of blanks commonly used in machining are as follows:
- Common blank types
(1) Casting The metal blank obtained by pouring molten metal into the mold and solidifying. It is suitable for parts with complex shapes and castable materials. The casting material can be cast iron, cast steel or non-ferrous metal.
(2) Forgings are blanks obtained by forging and deforming metal materials. It is suitable for parts with high mechanical performance requirements, material (steel) with forgeability, and relatively simple shape. When the production batch is large, die forging can be used instead of free forging. the
(3) Profiles All kinds of hot-rolled and cold-drawn round steel, plates, profiles, etc., suitable for parts with simple shapes and small sizes.
(4) Welding parts are joint parts obtained by welding various metal parts. In single-piece small-batch production, the production cycle can be shortened by using welded parts to make large blanks.
- The shape and size of the blank
One of the trends in the development of modern machinery manufacturing is to refine the blank, so that the shape and size of the blank are as close as possible to the parts, so as to achieve less chip or even chip-free processing.
The steps to determine the shape and size of the blank are as follows: first select the blank machining allowance and blank tolerance, then superimpose the blank machining allowance on the corresponding machining surface of the part to calculate the blank size, and finally mark the blank size and tolerance.
When determining the shape of the blank, it is also necessary to consider the influence of the processing technology on the shape of the blank. For example, sometimes in order to facilitate the clamping of parts during processing, a process boss is made on the blank. The so-called process boss is a boss added to the workpiece to meet the needs of the process, as shown 0Figure 3-1a. After the parts are processed, they should generally be cut off; sometimes the separated parts are made into a blank to make it easy to process and ensure the processing quality. As shown in Figure 3-1b, the split nut of the machine tool screw is made into a blank As a whole, it is cut and separated after processing to a certain stage.
- a) Process boss b) Split nut of lead screw
Figure 3-1 blank shape
3.3 Selection of positioning datum
3.3.1 Types of positioning reference
The positioning reference is the point, line, or surface on the workpiece used to position the workpiece on the machine tool or fixture during processing. According to the surface conditions used for positioning on the workpiece, the positioning datum is divided into rough datum, fine datum, and auxiliary datum.
(1) Rough datum and fine datum In the first process of part processing, only the unprocessed surface on the blank can be used as the positioning datum. This positioning datum is called rough datum. Rough datums are positioned using the unmachined surface on the workpiece. The use of the processed surface on the workpiece as the positioning datum is called the fine datum.
(2) A surface that does not require processing in the design drawing of the auxiliary reference part is sometimes specially processed for positioning for the needs of workpiece clamping; This kind of surface is not the working surface on the part, but the datum plane processed due to the needs of the process, which is called auxiliary datum or process datum. For example, the positioning of the center hole used in the machining process; the process boss of the part shown in Figure 3-1a.
The machining process of the part is to firstly use the rough datum positioning to process the fine datum surface; then use the fine datum positioning to process other surfaces of the part. When selecting the positioning datum, first consider which set of fine datum positioning is used to process the main surface of the workpiece, and then determine what kind of rough datum positioning is used to process the surface of the fine datum.
- 3.3.2 Selection of coarse datum
The choice of rough datum has two main influences on the workpiece, one is to affect the mutual position of the machined surface and the non-machined surface on the workpiece, and the other is to affect the distribution of machining allowance. The selection principles of rough benchmarks are:
(1) For parts with both machined and unmachined surfaces, when the mutual position between the unmachined surface and the machined surface must be guaranteed, the unmachined surface should be selected as the rough reference. If there are multiple non-machined surfaces on the part, the surface with a higher requirement for the position relative to the machined surface should be selected as the rough datum.
(2) For workpieces with more machined surfaces, the selection of the rough datum should be able to reasonably allocate the machining allowance. Reasonable allocation of machining allowance refers to:
1) If the workpiece must first ensure that the margin of an important surface is uniform, this surface should be selected as the rough reference.
2) The surface with the smallest allowance on the blank should be selected as the rough reference to ensure that each machined surface has sufficient machining allowance.
(3) The surface used as a rough reference should be as flat as possible, and there should be no flash, gate, riser and other defects, which can reduce positioning errors and make the workpiece clamping reliable.
(4) In order to ensure that the margin of the important processing surface is uniform, the important processing surface should be selected as the rough reference.
(5) The repeated use of rough datums should be avoided, and the rough datums can only be used once in the same dimension direction. Because the rough datum is the surface of the blank, the positioning error is large, and there will be a large position error between the surfaces processed under the same rough datum clamping twice.
3.3.3 Selection of fine benchmark
The choice of fine datum should mainly be considered from the two aspects of ensuring the position accuracy of the workpiece and the convenience of clamping. The selection principles of fine benchmarks are:
(1) Principle of datum coincidence The design datum of the machined surface should be selected as the positioning datum as much as possible. This principle is called the principle of datum coincidence.
(2) The principle of unified datum When parts need to be processed in multiple processes, the same set of precise datum positioning should be selected in most processes as much as possible, which is called the principle of unified datum.
(3) The principle of self-based datum Sometimes the finishing or finishing process requires a small and uniform allowance, so the processing surface itself should be used as the positioning datum, which is called the self-based datum principle. Such as hole pulling, reaming, grinding, centerless grinding, etc.
(4) The principle of mutual reference. There are two surfaces on a workpiece that require high mutual position accuracy. The two surfaces on the workpiece are used as positioning references to each other, and the other surface is repeatedly processed, which is called mutual reference.
(5) The selected fine datum should be able to ensure accurate positioning of the workpiece, convenient clamping, simple and applicable fixture structure.
3.3.4 Selection example of positioning datum
3-2 shaft seat parts
3.4 Drafting of machining process route
The machining process route refers to the process of parts in the production process, that is, simply using the sequence of procedures to indicate the parts. Drafting the machining process route is a key link in the process of formulating the machining process. When drawing up the process route, in addition to choosing a reasonable positioning datum, the following problems need to be solved:
3.4.1 Selection of part surface processing method
- Machining economical precision and machining economical surface roughness
The processing accuracy that can be guaranteed by a processing method has a considerable range, but if the processing accuracy guaranteed by it is required to be too high, some special technological measures need to be taken, and the processing cost will increase accordingly. The processing economic precision of a processing method refers to the processing accuracy that can be guaranteed under normal processing conditions (using equipment, process equipment and workers with standard technical grades that meet quality standards, without extending the processing time). The processing economic precision and processing economic surface roughness achieved by various processing methods can be found in various metal cutting process manuals.
- Processing route of typical surface
Mechanical parts are composed of some simple geometric surfaces such as outer cylinders, holes, planes, etc., so the process route of the parts is an appropriate combination of these surface processing routes, Table 3-3, Table 3-4 and Table 3-5 They are the typical processing routes of outer cylinder, hole and plane respectively, for reference when selecting. the
3.4.2 Determination of process sequence
After selecting the surface processing method of the part and the positioning reference during processing, the processing of the part should be distributed to each process to complete, and the content and sequence of each process in the process route should be determined. At this time, the following two must be considered question:
- Division of processing stages
When processing a workpiece with higher precision, if there are many processes, the rough machining processes on each surface of the workpiece can be concentrated. When arranging the sequence of processes, the first processing is called the rough machining stage; and then the semi-finishing of each surface is concentrated. The process is called the semi-finishing stage; the final intensive finishing process of each surface is called the finishing stage. That is, the process route is divided into several processing stages, and the functions of each processing stage are:
(1) Rough machining stage: Efficiently remove most of the allowance on each machined surface, and provide precision preparation and surface roughness preparation for semi-finishing. The precision that can be achieved in the rough machining stage is low, and the surface roughness is large, which requires high productivity in rough machining.
(2) Semi-finishing stage The purpose is to eliminate the machining error left after rough machining on the main surface, so that it can reach a certain accuracy, prepare for further finishing, and complete the processing of some secondary surfaces at the same time.
(3) Finishing stage In this stage, the machining allowance and cutting amount are very small, and its main task is to ensure the size, shape, position accuracy and surface roughness of the main surface of the workpiece.
(4) The finishing processing stage includes honing, superfinishing, mirror grinding and other finishing processing methods. The processing allowance is extremely small. The main purpose is to further improve the dimensional accuracy and reduce the surface roughness. Generally, it cannot be used to correct the position error.
The reasons for dividing the processing stages are:
(1) Guarantee processing quality
(2) Rational use of machine tools and equipment
(3) Blank defects can be found in time during the rough machining stage
(4) Easy to arrange heat treatment process
Dividing the process route into several processing stages will increase the number of processes, thereby increasing the processing cost. Therefore, when the rigidity of the workpiece is high and the processing accuracy can be guaranteed without dividing the process route, the processing stage should not be divided, that is, the rough, semi-finishing and finishing steps of a certain surface are continuously completed in one process. For example, in the processing of heavy parts, in order to reduce the transportation and clamping of the workpiece, some surface processing is often completed in one clamping. Due to the high rigidity, high power and high precision of the equipment in CNC machining, the processing stages are often not divided. Usually, the machining center completes the rough machining, semi-finishing and finishing steps of multiple surfaces of the workpiece under one clamping to achieve Part design dimension requirements.
- Arrangement of machining sequence
The machining sequence should follow the following principles:
(1) Process the datum surface first, then process other surfaces. That is, use the rough datum positioning to process the fine datum surface first, provide a reliable positioning datum for the processing of other surfaces, and then use the fine datum positioning to process other surfaces.
(2) Process the plane first and then process the hole. Box parts generally first process the plane with the main hole as the rough reference, and then process the hole system with the plane as the fine reference.
(3) Arrange the rough machining process first, and then arrange the finishing process.
(4) Process the main surface first, and then process the secondary surface. The main surface of the part is a surface with high processing accuracy and surface quality requirements. It has many processes, and its processing quality has a great impact on the quality of the part, so it is processed first.
3.4.3 Combination of processes
That is to arrange multiple work steps in one process. Therefore, after determining the processing sequence, it is necessary to properly combine the sequence of steps to form a process with the process as the unit. In the combination of processes, the following two aspects should be considered.
- Determine the content of the process
To determine a number of steps included in a process, it is necessary to consider whether these steps can be processed on the same machine tool; whether they need to be processed in one installation to ensure mutual position accuracy. The fact that several work steps can be carried out on the same machine is a prerequisite for them to be combined into one process. In addition, a set of surfaces of a part is machined in one setup, which guarantees relative positional accuracy between these surfaces. Therefore, for a group of surfaces with high position accuracy requirements, they should be processed in one process.
- Centralization and decentralization of processes
How to determine the number of processes in the part process is the problem of concentration and decentralization of processes. If the processing of a part is concentrated in a few processes, and each process has a lot of processing content, it is called process concentration. On the contrary, it is called process dispersion.
The process concentration makes the process route short and reduces the number of workpiece clamping, which can not only improve productivity, but also help ensure the position accuracy of the processed surface and reduce production costs. Process dispersion facilitates the use of simple processing equipment and process equipment, easy processing adjustment, the most reasonable cutting amount can be used, and it is easy to divide the processing stages.
When drawing up the process route, usually single-piece small-batch production mostly adopts process concentration.
3.4.4 Arrangement of heat treatment process
Heat treatment is used to improve the mechanical properties of materials, eliminate residual internal stress, and improve the processing properties of metals. According to the purpose of heat treatment, it can be divided into: preliminary heat treatment, final heat treatment and aging treatment.
(1) Preliminary heat treatment The treatment process includes: annealing, normalizing, quenching and tempering. Its purpose is to improve the cutting performance of the material and eliminate the internal stress generated during blank manufacturing. Annealing and normalizing are usually arranged before rough machining, and quenching and tempering are arranged after rough machining and before semi-finishing. Due to quenching and tempering, the comprehensive mechanical properties of the material are better, and it can also be used as the final heat treatment process for some parts that do not require high hardness and wear resistance.
(2) Aging treatment is divided into artificial aging and natural aging. The purpose is to eliminate the internal stress generated in blank manufacturing and machining. It is generally arranged after rough machining to eliminate the internal stress generated by casting and rough machining at the same time. . Sometimes in order to reduce the workload of transportation, it can also be carried out before rough machining. Parts with high precision requirements should be arranged for the second or even multiple aging after semi-finishing.
(3) Final heat treatment including quenching, carburizing and quenching, nitriding, etc. It is often arranged after semi-finishing and before grinding, the purpose of which is to improve the mechanical properties of the material such as hardness, wear resistance and strength.
3.4.5 Arrangement of auxiliary processes
Auxiliary processes include deburring, chamfering, cleaning, rust prevention, inspection and other processes. Among them, the inspection process is one of the effective measures to ensure product quality. The inspection process can generally be arranged: before and after key processes; before and after parts are transferred from one workshop to another; after the rough machining stage; after all parts are processed. It should be noted that when there is no deburring process after a certain process, the burrs generated in this process should be removed by this process.
3.4.6 Design and implementation of machine tool processing procedures
After drawing up the process route of the parts, it is necessary to design each process and determine its process content. The main tasks of process design are as follows.
- Determine machining allowance
Machining allowance refers to the difference in size before and after machining of the machined surface. That is, the thickness of the metal layer removed to achieve the required precision and surface quality of the surface. The machining allowance is divided into process allowance and total machining allowance.
In each process, the processing technical requirements of this process must be given. The process size is the size that the processed surface of the workpiece should reach after processing, that is, the process size is the size requirement that the workpiece should reach after a certain process.
(1) Process margin The difference between the process dimensions of two adjacent processes is called the process margin. The process margin is the thickness of the metal layer removed in one process.
(2) The total machining allowance is also called the blank allowance, which refers to the difference between the blank size of the part and the design size of the part drawing.
The tolerance of the process size is generally marked with the “in-body principle”. The so-called “in-body principle” means that when the limit deviation of the process size is selected, the upper deviation of the process size of the contained surface (axis) is taken as zero; for the containment surface ( Hole) process size removal deviation is zero. The tolerance of the blank is generally marked with a two-way symmetrical deviation.
The method of determining the machining allowance
(1) Calculation method It is the most economical and accurate to determine the machining allowance with the above calculation formula, but it is generally less used because it is difficult to obtain complete and reliable data.
(2) Empirical estimation method: Estimate the size of the machining allowance based on previous processing experience. In order to avoid waste products due to insufficient processing allowance, the estimated allowance is generally too large, which is only applicable to single-piece and small-batch production.
(3) The table look-up correction method can be based on the “process manual” or the technical data on machining allowance formulated by each factory according to its own production practice characteristics, directly find the machining allowance, and at the same time make corrections based on the actual processing situation to determine the processing margin. This method is widely used in production.
- Determination of Process Dimensions and Tolerances When Datums Are Overlapped
The process size is the size that a certain process should achieve. Obviously, after a surface of a part is processed by the last process, it should meet its design requirements, so the process size and tolerance of the last process of a certain surface of a part should be the design size and tolerance of the surface on the part. The process size of the intermediate process needs to be determined by calculation.
When each process of machining a certain surface adopts the same positioning datum and coincides with the design datum, the calculation of the process size only needs to consider the process allowance. The operation steps are: ①Determine the value of the allowance of each process. ②The process size of the last process is equal to the design size on the part drawing, and the process size of each process is calculated from the last process to the previous process. ③The process dimensional tolerance of the last process is equal to the design dimensional tolerance on the part drawing, and the dimensional tolerance of the intermediate process is taken as the processing economic precision. The surface roughness that each process should achieve is determined in the same way. ④ The upper and lower deviations of the dimensions of each process are determined according to the “in-body principle”. That is, for the hole, the lower deviation is zero, and the upper deviation is positive; for the axis, the upper deviation is zero, and the lower deviation is negative. the
- Process size chain
(1) Definition of dimensional chain
A dimension chain is composed of closed dimensions that are interconnected and arranged in a certain order. The process dimension chain is a dimension chain composed of various related process dimensions in the process of part processing. As shown in Figure 3-3a, the size and size have been marked in the part drawing. After the upper and lower surfaces are processed, if you want to use 1 side to position and process 3 sides, you need to give the process size so that the tool can be set according to the size. The size and The dimensions marked in the part drawing are related to each other, forming a dimension chain, as shown in Figure b.
- a) b) Figure 3-3 Processing size chain
(2) Composition of dimensional chain
Each dimension included in the dimensional chain, as in Fig. 3-3b, is called a ring of the dimensional chain. There are two types of rings, closed rings and constituent rings.
A closed loop is a loop that is naturally formed during part processing or assembly. That is, the closed ring is the size obtained indirectly in the process of processing, denoted as . The ring in Figure 3-3b.
All the rings in the dimensional chain except the closed ring are called constituent rings, and the constituent rings are the dimensions obtained directly in the process of processing. According to the nature of the influence of the constituent rings on the closed ring, the constituent rings are divided into increasing rings and reducing rings. In a dimensional chain, the remaining rings that make up the ring remain unchanged, and when the ring increases, the closed ring also increases, which is called an increasing ring. For the dimensional chain with a large number of rings, it is easy to make mistakes in judging the increase and decrease of rings by definition. In order to quickly judge the increase and decrease of rings, when drawing the size chain diagram, the single arrows connected end to end can be used to represent each ring in sequence. Among the rings, the ring in the same direction as the closed ring arrow is a decreasing ring, and the ring in the opposite direction to the closed ring arrow is an increasing ring.
(3) The basic calculation formula of the extreme value method to solve the size chain
Common methods for calculating the process size chain are extreme value method and probability method, and the extreme value method is introduced here.
1) The basic size of the closed ring The basic size of the closed ring is equal to the sum of all the basic sizes of the rings minus the sum of the sizes of the ring bases, that is:
Where – the basic size of the closed ring;
i—the basic size of the augmented ring;
j—the basic size of the ring reduction;
m—the ring number of ring augmentation;
n—total number of rings (not including closed rings).
2) The limit size of the closed loop The maximum limit size of the closed loop is equal to the sum of the maximum limit sizes of all the rings, minus the sum of the minimum limit sizes of all the reduction rings; and the minimum limit size of the closed loop is equal to the sum of the minimum limit sizes of all the rings , minus the sum of the maximum limit sizes of all subtracting rings
3) Limit deviation of the closed loop The upper deviation of the closed loop is equal to the sum of the upper deviations of all the increasing rings, minus the sum of the lower deviations of all the reducing rings; the lower deviation of the closed loop is equal to the sum of the lower deviations of all the increasing rings, minus all the reducing rings The sum of the upper deviations.
4) Tolerance of the closed loop The tolerance of the closed loop is equal to the sum of the tolerances of the constituent rings, where, are the tolerances of the closed loop and the constituent rings, respectively.
- Machine tool selection
The selection of ordinary machine tools should consider the following aspects:
(1) The main specifications and dimensions of the machine tool should be compatible with the outline size of the workpiece, that is, small workpieces should be processed by small machine tools, large workpieces should be processed by large machine tools, and equipment should be used reasonably.
(2) The accuracy of the machine tool should be compatible with the machining accuracy required by the process.
(3) The productivity of the machine tool should be compatible with the production type of the parts. Make use of the existing machine tool equipment in the factory as much as possible.
CNC machine tool selection
Choosing CNC machine tools as the processing equipment in the process is called CNC machining. The CNC machining method is to compile a processing program according to the drawings and process requirements of the parts to be processed, and the processing program controls the CNC machine tool and automatically processes the workpiece. Compared with ordinary machine tools, CNC machine tools have many advantages, and its application range is still expanding. However, the initial investment cost of CNC machine tools is relatively large, and its economic benefits should be fully considered when selecting CNC machine tools for processing. Generally speaking, CNC machine tools are suitable for occasions with complex processing parts, high precision requirements, fast product updates, and short production cycle requirements.
- Selection of process equipment
Process equipment in machining refers to the general term for various tools used in the manufacturing process of parts, including fixtures, knives, measuring tools and auxiliary tools.
Selection of fixtures: The fixtures used should be compatible with the type of production. For single-piece small batch production, general-purpose fixtures should be preferred. Such as various general chucks, flat vises, dividing heads, rotary tables, etc. Combination clamps are also available. For mid-batch production, general fixtures, special fixtures, adjustable fixtures, and combined fixtures can be selected. Mass production should try to use high-efficiency special fixtures, such as pneumatic, hydraulic, and electric fixtures. In addition, the accuracy of the fixture should be able to meet the requirements of machining accuracy.
Selection of fixtures and auxiliary tools: Generally, standard tools should be preferred, and high-efficiency composite tools and special tools can also be used if necessary. The type, specification and precision of the tools used should be able to meet the processing requirements. Machine tool accessories are tools used to connect the tool and the machine tool, such as tool handles, adapters, chucks, etc. Generally, auxiliary tools should be selected according to the tool and machine tool structure, and standard auxiliary tools should be selected as much as possible.
Selection of measuring tools: General measuring tools should be used for single-piece small batch production, such as vernier calipers, dial gauges, etc. In mass production, limit gauges and high-efficiency special inspection tools should be used as much as possible.
3.5 Productivity of the machining process
When formulating process regulations, it is necessary to improve labor productivity and reduce costs under the premise of ensuring product quality. Machining labor productivity refers to the quantity of qualified products produced by workers per unit time.
3.5.1 Time quota
One of the contents of process design is to determine the time quota, which is the time consumed to produce a product or complete a process under certain production conditions. Time quota is one of the important basis for arranging production plan and calculating product cost. For new factories (or workshops), it is also the basis for calculating the number of equipment, number of workers, workshop layout, and production organization.
The time quota in the process file is the time for a single piece. The time specified for a process in the process of machining a part in machining is called the time for a single piece Td, which includes the following components:
(1) The basic time Tj refers to the time consumed by the process of directly changing the size, shape, mutual position, surface state or material properties of the production object. For cutting processing, it is the time consumed directly for the cutting allowance (including the cutting out and cutting time of the tool), which can be determined by calculation.
(2) Auxiliary time Tf refers to the time consumed by various auxiliary actions necessary to realize the process. It includes loading and unloading the workpiece on the machine tool, starting and stopping the machine tool, feeding and retracting the tool, measuring the workpiece, etc. The sum of the basic time and auxiliary time is called the operating time Tz. Obviously operating time is the time spent directly in making the part.
(3) The time Tb for arranging the work place refers to the time it takes for the workers to take care of the work place (such as changing tools, lubricating machine tools, cleaning chips, cleaning tools, etc.) in order to make the processing proceed normally. Generally, it can be calculated according to 2% to 7% of the working time.
(4) Rest and physiological needs time Tx refers to the time spent by workers in the work shift to restore physical strength and meet physiological needs. Generally, it can be calculated according to 2% to 4% of the working time.
To sum up, the single piece time Td is expressed as:
(5) The preparation and termination time Te refers to the time it takes for a worker to prepare and complete a batch of workpieces for batch production. For example, be familiar with process documents, receive blanks, borrow and install tools and fixtures, adjust machine tools, return process equipment, and deliver finished products. The preparation and finalization time is only consumed once for a batch of workpieces. If the number of workpieces in each batch (batch) is recorded as N, the preparation and finalization time allocated to each workpiece is “Te/N”. Therefore, the unit time in batch production is:
3.5.2 Technological approaches to improve machining labor productivity
Improving labor productivity involves many factors such as product design, manufacturing process, and production management. As far as mechanical processing is concerned, the technological approach to improving labor productivity is: shortening the working hours of a single piece and adopting modern production methods such as automated processing.
- Shorter piece time
Taking reasonable technological measures to shorten the unit time of each process is one of the effective measures to improve labor productivity. The following is an analysis from the composition of the unit time.
⑴ shorten the basic time
Increase the amount of cutting Increasing the amount of cutting is an effective way to shorten the basic time. At present, high-speed turning and high-speed grinding are widely used. In high-speed cutting, the cutting speed of cemented carbide turning tools generally reaches 200m/min, and the cutting speed of ceramic cutting tools reaches 500m/min. The cutting speed reaches 900m/min, and when cutting hardened steel above HRC60, the cutting speed reaches 90m/min. The cutting speed of the high-speed hobbing machine can reach 65-75 m/min. In terms of grinding, high-speed grinding reaches more than 60m/s. In addition, the grinding depth of powerful grinding can reach 6-12mm, and the metal removal rate is several times higher than that of ordinary grinding.
Reducing the working stroke In the cutting process, methods such as multi-tool cutting, multi-piece processing, and merging steps can be used to reduce the working stroke.
⑵ Shorten the auxiliary time First, directly shorten the auxiliary time, using high-efficiency fixtures, such as pneumatic, hydraulic, electric and multi-piece clamping fixtures, can reduce the time for clamping workpieces; adopt active measuring devices to reduce downtime measurement time during processing. The second is to shorten the auxiliary time indirectly, and overlap the auxiliary time with the basic time in whole or in part. For example, by adopting measures such as multi-station fixtures and double workbenches, the loading and unloading time of the workpiece can completely coincide with the basic time, which can indirectly reduce the auxiliary time.
(3) The main measures to shorten the time for arranging the work site are: to improve the durability of the tool or grinding wheel to reduce the number of tool changes; to use the tool fine-tuning device, special tool setting template, etc. to reduce the tool adjustment time; CNC machine tools can also use external tool adjustment instrument Adjusting the tool outside the machine saves the time of tool setting on the CNC machine tool; using non-regrinding blades, when the blade wears and needs to be replaced, just use the elastic screw to replace the standard blade or the blade can be repositioned, and the tool change time is reduced. shorten.
⑷Shorten the preparation and termination time. During batch production, the batch size of workpieces should be expanded as much as possible, and the preparation and termination time allocated to each workpiece should be reduced. Such as the use of group technology.
- Automated production methods
Adopt modern production technology; in mass production and mass production, use combined machine tools and automatic line processing; in single-piece small batch and medium batch production, use numerical control processing and group processing, which can effectively improve productivity.