Question 2: What is the concept of machining allowance?
Machining allowance refers to the thickness of metal layer cut off during machining. There are total machining allowance and process allowance. In the process of changing from a blank to a part, the total thickness of the metal layer cut off on a machined surface is called the total machining allowance of the surface (also called the blank allowance); Generally, the total machining allowance is not removed at one time, but gradually removed in each working procedure, so the thickness of the metal layer removed in each working procedure is called the machining allowance of the working procedure. The process allowance is the difference between the process sizes of two adjacent processes, and the blank allowance is the difference between the blank size and the design size of the part drawing.
Due to the tolerance of process size, the actual removal allowance is different.
The following figure shows the relationship between process allowance and process size.
As can be seen from the figure, the basic size of process allowance (called basic allowance or nominal allowance) z can be calculated according to the following formula.
For included surfaces: Z= basic dimensions of the previous operation-basic dimensions of this operation.
For the inclusion surface: Z= the basic size of this operation-the basic size of the previous operation.
The following figure shows the relationship between process allowance and process size and tolerance.
(a) Covering surface (shaft) (b) Covering surface (hole)
In order to facilitate machining, the process size is marked with the limit deviation according to the principle of "into the body", that is, the process size of the inclusion surface is taken as zero; The process dimension deviation of the containment surface is zero. The blank size is arranged in two directions: up and down deviation.
Formulas for calculating process allowance, process size and tolerance;
Z=Zmin+Ta ( 1- 1)
Zmax=Z+Tb= Zmin+Ta +Tb ( 1-2)
Where zmin—— is the minimum process allowance;
Zmax- maximum machining allowance;
Ta—— the dimensional tolerance of the previous process;
T B- dimensional tolerance in this process.
Because there are errors in blank size, part size and process size of each process, the total machining allowance and process machining allowance are both variable values, and there are maximum and minimum machining allowances. Their relationship with process dimensions and their tolerances can be illustrated in figure 1-9.
As can be seen from the figure, the nominal machining allowance is the difference between the dimensions of the previous process and this process, and the minimum machining allowance is the difference between the minimum dimension of the previous process and the maximum dimension of this process; The maximum machining allowance is the difference between the maximum size of the previous process and the minimum size of this process. The variation range of working procedure machining allowance (the difference between the maximum machining allowance and the minimum machining allowance) is equal to the sum of the working procedure dimension tolerances of the previous working procedure and this working procedure.
Question 3: What is the machining allowance? It should be the gap in lathe processing. For example, the size of the parts you want to process is different, such as 20, or 10.
Question 4: What does machining allowance mean? Machining allowance refers to the part that exceeds the actual size after rough machining and is reserved for fine cutting.
Question 5: What is machining allowance? The size of machining allowance is equal to the sum of machining allowance of each intermediate process.
In machining, in order to ensure the machining quality of parts, the thickness of metal layer removed from a certain surface is called machining allowance. It can be divided into total allowance and process allowance. The total allowance is equal to the difference between the blank size and the design size of the part. The process allowance is equal to the difference between the process sizes of two adjacent processes, as shown in Figure 5-7.
Process allowance can be divided into unilateral allowance and bilateral allowance. For asymmetric surfaces such as planes, the machining allowance is generally unilateral.
For the outer surface (Figure 5-7a): ZB = AB.
For the inner surface (Figure 5-7b): zb = ba
Where zb-the process allowance of this process;
B- the basic scope of this procedure;
A- the basic dimensions of the previous procedure.
The machining allowance of the surfaces of revolving bodies such as inner and outer cylindrical surfaces is bilateral allowance.
For cylindrical surface (Figure 5-7c): 2zb = dadb.
For the inner circular surface (Figure 5-7d): 2zb = dbda
Where 2zb- machining allowance on diameter;
DB- the diameter of the machined surface in this process;
Da- the diameter of the surface processed in the previous procedure.
Figure 5-7 machining allowance
The relationship between total machining allowance and process allowance is: Z0=
Due to the deviation of process size in machining, the actual cutting allowance value will inevitably change. Therefore, machining allowance can be divided into basic (or nominal) allowance Z, maximum allowance Zmax and minimum allowance Zmin. For the inclusion surfaces shown in Figures 5-8:
Z = LaLb
Zmin = LaminLbmax
Zmax=LamaxLbmin .
Figure 5-8 machining allowance and its tolerance
Where la- the basic size of the upper process;
Lb-the basic size of the process;
Lamax and lamin- the maximum and minimum dimensions of the previous process;
Lbmax and lb min- the maximum and minimum dimensions of the process.
The variation range (allowance tolerance) Tz of the nominal allowance is equal to the sum of the process dimension tolerance Tb of this process and the process dimension tolerance Ta of the previous process, namely
Tz=ZmaxZmin=Tb+Ta
(2) Factors affecting machining allowance
1) surface roughness RZ and surface defect layer Ha left by previous process. This part of the thickness should be removed in this process.
2) The dimensional tolerance Ta of the previous procedure. Without considering other errors, the machining allowance of this process should not be less than Ta.
3) The space error ρa left by the previous procedure that needs to be considered separately.
4) Installation error εb in this process.
Due to the directionality of space error and installation error, the calculation formula of machining allowance is:
For unilateral allowance: Zmin=Ta+Rz+Ha+
For bilateral allowance: Zmin=Ta/2+Rz+Ha+
(3) Method of determining machining allowance
1) calculation method; (2) Empirical estimation method; (3) Look-up table method.
Question 6: What are the machining allowance, process allowance and total allowance in the basis of mechanical manufacturing? Machining allowance is the remaining machining amount before the parts are processed into finished products, which can also be called total allowance. Inter-process allowance refers to dividing the total allowance into various processes and determining the processing quantity according to the processing characteristics.
Question 7: The correct determination of the process allowance is related to the correct determination of the process size of each process when the specific content of each process is further arranged after the part processing allowance is determined. When determining the process size, the processing allowance should be determined first.
Machining allowance Because the blank can't reach the required precision and surface roughness of the parts, it is necessary to leave machining allowance in order to meet these requirements through machining. Machining allowance refers to the thickness of the metal layer removed from the machined surface during machining.
① Total machining allowance and working procedure machining allowance. In order to obtain the required precision and surface quality of a surface on a part, all the excess metal layers removed from the surface of the blank are called the total machining allowance of the surface; The metal layer removed from the surface after completing a process is called process allowance. The relationship between them is that the total machining allowance is equal to the sum of each working procedure allowance.
② Nominal machining allowance, maximum machining allowance and minimum machining allowance. When making the process plan, the machining allowance of each process should be determined according to the nature of each process, and then the size of each process can be calculated. Because the size of each working procedure has tolerance in the machining process, the actual removal allowance also changes. Therefore, machining allowance can be divided into nominal machining allowance, maximum machining allowance and minimum machining allowance. Generally speaking, the machining allowance refers to the nominal machining allowance, and its value is equal to the difference between the basic dimensions of the previous process.
Question 8: The concept of machining allowance and how to determine 1. The concept of machining allowance refers to the thickness of the metal layer cut off during machining. There are total machining allowance and process allowance. Total machining allowance (also known as blank allowance): The total thickness of the metal layer cut off on a machined surface in the process of converting a blank into a part is called the total machining allowance of this surface (also known as blank allowance). Generally, the total machining allowance is not removed at one time, but gradually removed in each working procedure, so the thickness of the metal layer removed in each working procedure is called the machining allowance of the working procedure. Process allowance: it is the difference between the process sizes of two adjacent processes, and the blank allowance is the difference between the blank size and the design size of the part drawing. Due to the tolerance of process size, the actual removal allowance is different. As can be seen from the figure, the basic size of process allowance (referred to as basic allowance or nominal allowance for short) z can be calculated according to the following formula: Z= basic size of upper process-basic size of this process for inclusion surface: Z= basic size of this process-basic size of upper process. In order to facilitate processing, the process size is marked with limit deviation according to the principle of "integrity". The upper deviation of the process size of the containment surface is zero; The process dimension deviation of the containment surface is zero. The blank size is arranged in two directions: up and down deviation. (1) The formula for calculating the process allowance, process size and tolerance of the upper (axis) and lower (hole) containment surface: Z=Zmin+Ta Zmax=Z+Tb= Zmin+Ta +Tb, where: Zmin- the minimum process allowance; Zmax- maximum machining allowance; Ta—— the dimensional tolerance of the previous process; T B- dimensional tolerance in this process. It can be seen from the figure that: process machining allowance and its tolerance nominal machining allowance = previous process size-minimum machining allowance of this process size = minimum machining allowance of this process size-maximum machining allowance of this process size = minimum machining allowance of this process size; The variation range of working procedure machining allowance (the difference between the maximum machining allowance and the minimum machining allowance) = the dimensional tolerance of the previous working procedure+the dimensional tolerance of the current working procedure II. Affect the machining allowance. The main influencing factors are: (1) surface roughness and defect layer depth (Ra and da) formed in the previous process; (2) Shape error and position error (△x and △w) formed in the previous procedure. To sum up, the factors that affect the work allowance can be summarized as: (1) the dimension tolerance of the previous process (Ta). (2) Surface roughness and surface defect layer depth (Ra+Da) formed in the previous process. (3) Shape error and position error (△x, △w) formed in the previous process. (4) The clamping error of this process (εb). 3. Method of determining machining allowance (1) Look-up table correction method (currently the most widely used. (2) Empirical estimation method (only applicable to single piece and small batch production. (3) Analytical calculation method (less used at present. )
Question 9: What is the machining allowance of mechanical properties samples of metal materials? The mechanical properties samples of metal materials specified in national standards have certain size and surface quality requirements. Therefore, when making a sample, the sample is roughly made into a sample slightly larger than the standard sample, and then finely processed into the standard sample. This larger size is the machining allowance, which is removed by subsequent finishing. See GB228-2002 tensile test for details.