1. Bushing parts
This kind of parts generally have shafts, bushings and other parts, in the view expression, as long as a basic view is drawn plus appropriate cross-sectional drawings and dimensions, its main shape features and local structure can be expressed. In order to facilitate the viewing of the drawing during processing, the axis is generally placed horizontally for projection, and it is best to choose the position where the axis is the perpendicular line.
When dimensioning the bushing parts, its axis is often used as the radial size datum. From this, the Ф14 and Ф11 shown in the figure (see A-A section) are noted. In this way, the design requirements are unified with the process standard at the time of machining (when the shaft parts are machined on the lathe, the two ends are pressed against the center hole of the shaft with thimbles). The reference in the length direction is often an important end face, contact surface (shoulder) or machined surface.
As shown in the figure, the right axis shoulder with a surface roughness of Ra6.3 was selected as the main size reference in the length direction, from which the dimensions of 13, 28, 1.5 and 26.5 were injected, and the right shaft end was used as the auxiliary base in the length direction, so that the total length of the shaft 96 was marked.
2. Disc cover parts
The basic shape of this kind of parts is a flat disc, generally with end covers, valve covers, gears and other parts, their main structure is generally a rotary body, usually with various shapes of flanges, evenly distributed round holes and ribs and other local structures. When selecting a view, a cross-sectional view with a symmetrical plane or a axis of rotation is generally selected as the main view, and appropriate other views (such as left, right, or top view) need to be added to express the shape and uniform structure of the part. As shown in the image, a left view has been added to represent the square flange with rounded corners and the four evenly distributed vias.
When marking the size of the disc cover parts, the axis through the shaft hole is usually selected as the radial size datum, and the main size datum in the length direction is often the important end face.
3. Fork frame parts
Such parts generally include forks, connecting rods, bearings and other parts. Due to their variable machining positions, the main consideration is the work position and shape features when selecting the main view. The selection of other views often requires two or more basic views, and the local structure of the part should be expressed with appropriate local views, cross-sectional views and other expressions. The view selection scheme shown in the pedal parts diagram is concise and clearFor the width of the bearing and rib, the right view is not necessary, and for the T-shaped rib, the section is more appropriate.
When dimensioning fork frame parts, the mounting base surface or the symmetrical surface of the part is usually selected as the size datum. The dimensioning method is shown in Fig.
4. Box parts
Generally speaking, the shape and structure of such parts are more complex than the previous three types of parts, and the processing position changes more. Such parts generally include valve body, pump body, reducer box and other parts. When selecting a home view, the work location and shape features are primarily considered. When selecting other views, a variety of auxiliary views such as cross-sectional, cross-sectional, partial and oblique views should be used according to the actual situation to clearly express the internal and external structure of the part.
In terms of dimensioning, the axis required by the design, the important mounting surface, the contact surface (or the processing surface), and the symmetrical surface (width and length) of some main structures of the box are usually selected as the size benchmark. For the part of the box that needs to be cut, the size should be marked as far as possible according to the requirements of convenient processing and inspection.
5. Dimension notation of common structures of parts
Dimensioning of common holes (blind holes, threaded holes, counterbores, countersinking holes), and dimensioning of chamfers.
1. Blind holes
2 threaded holes
3. Counterbore
4. Countersink flat hole
5. Chamfer
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1. Introduce the concept of surface roughness and the main evaluation parameters
1. The concept of surface roughness
The microgeometry of peaks and valleys with small spacing on the surface of a part is called surface roughness. This is mainly due to the knife marks left by the tool on the surface of the part and the plastic deformation of the surface metal when cutting and splitting when machining parts. The surface roughness of the part is also a technical index to evaluate the surface quality of the part, which has an impact on the fitting properties, working accuracy, wear resistance, corrosion resistance, sealing, appearance and so on. On the premise of ensuring the performance of the machine, in order to obtain the corresponding surface roughness of the parts, the appropriate processing method should be selected according to the role of the parts to reduce the production cost as much as possible. Generally speaking, where there are parts with matching requirements or surfaces with relative motion, the surface roughness parameter value should be small.
2. The code, symbol and marking of surface roughness
GB/T 131-2006 specifies the code for surface roughness and its annotation. The symbols on the drawings indicating the surface roughness of the parts are shown in the following table.
3. The main evaluation parameters of surface roughness
The evaluation parameters of the surface roughness of the parts are:
1) Arithmetic mean deviation (Ra) of the profile – the arithmetic mean of the absolute value of the profile offset over the length of the sample. The values of Ra and the sampling length l are shown in the table.
2)) Contour Maximum Height (Rz) - The distance between the contour peak line and the contour peak base line within the sampling length.
The Ra parameter is preferred.
4. Example of code marking of surface roughness
When the surface roughness height parameters Ra, Rz, and Ry are marked with numerical values in the code, except for the parameter code Ra, which can be omitted, the corresponding parameter code Rz or Ry should be marked before the parameter value, and the annotation example is shown in the table.
Surface Roughness Annotation of the direction of numbers and symbols in Surface Roughness
5 The annotation method of surface roughness code (symbol) on the drawing
1) The surface roughness code (symbol) should generally be noted on the visible contour line, the dimension boundary line or their extension line, and the tip of the symbol must point from the outside of the material to the surface.
2) The direction of the numbers and symbols in the surface roughness code must be marked according to the regulations.
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2. Annotation example of surface roughness
On the same drawing, each surface is generally marked with a code (symbol) only once, and as close as possible to the relevant dimension line. When the space is small or it is inconvenient to label, you can elicit the label. When all surfaces of the part have the same surface roughness requirements, they can be uniformly marked in the upper right corner of the drawing, and when most of the surfaces of the parts have the same surface roughness requirements, the most used code (symbol) can be noted in the upper right corner of the drawing at the same time, and the word "rest" is added. The height of the uniformly marked surface roughness code (character) and explanatory text should be 1.4 times that of the drawing.
The surface roughness code (symbol) of a continuous surface, the surface of a repeating element (such as holes, teeth, grooves, etc.) on a part, and the same surface that is connected discontinuously with a thin solid line is noted only once.
When there are different surface roughness requirements on the same surface, the dividing line should be drawn with a thin solid line, and the corresponding surface roughness code and size should be noted.
When the tooth (tooth) shape is not drawn on the working surface such as gear and thread, the surface roughness code (symbol) note is shown in the figure.
The surface roughness code of the working surface of the center hole, the working surface of the keyway, the chamfer, and the fillet can be simplified.
When the parts need to be partially heat treated or partially plated (coated), the range should be drawn with thick dots and marked with the corresponding size, and the requirements can also be written on the horizontal line on the long side of the surface roughness symbol.
6. Standard tolerances and basic deviations
In order to facilitate production, realize the interchangeability of parts and meet different use requirements, the national standard "Limit and Fit" stipulates that the tolerance zone is composed of two elements: standard tolerance and basic deviation. The standard tolerance determines the size of the tolerance zone, while the fundamental deviation determines the position of the tolerance zone.
1 Standard Tolerance (IT)
The value of the standard tolerance is determined by the basic dimensions and the tolerance class. The tolerance class is a mark that determines the degree of dimensional accuracy. The standard tolerances are divided into 20 grades, i.e. IT01, IT0, IT1,... ，IT18。 Its dimensional accuracy decreases sequentially from IT01 to IT18. The specific values of the standard tolerances are shown in the relevant standards.
2 Basic deviation
The basic deviation refers to the upper or lower deviation of the position of the tolerance zone relative to the zero line in the standard limit and fit, and generally refers to the deviation close to the zero line. When the tolerance band is above the zero line, the fundamental deviation is the lower deviation, and vice versa, the upper deviation. There are a total of 28 fundamental deviations, and the code is indicated in Latin letters, with holes in uppercase and shafts in lowercase. It can be seen from the basic deviation series diagram that the basic deviation of the hole A~H and the basic deviation of the shaft k~zc are the lower deviations, the basic deviation of the hole K~ZC and the basic deviation of the shaft a~h are the upper deviations, and the tolerance zones of JS and js are symmetrically distributed on both sides of the zero line, and the upper and lower deviations of the holes and the shafts are +IT/2 and -IT/2 respectively. The basic deviation series diagram only represents the position of the tolerance zone, not the magnitude of the tolerance, so one end of the tolerance zone is an opening, and the other end of the opening is defined by the standard tolerance.
The basic deviation and standard tolerance, according to the definition of dimensional tolerance, are calculated as follows:
ES=EI+IT 或 EI=ES-IT ei=es-IT或 es=ei+IT
The tolerance zone codes for holes and shafts are composed of the basic deviation code and the tolerance zone class code.
7. Cooperation
The relationship between a bore and a shaft tolerance zone that is bonded to each other of the same basic dimensions is called a fit. According to the different requirements of use, the fit between the hole and the shaft is loose and tight, so the national standard stipulates the type of fit:
1. Clearance fit
When the hole is assembled with the shaft, there is a fit with clearance, including a minimum clearance equal to zero. The tolerance zone of the hole is above the tolerance zone of the shaft.
2. Transition fit
When the bore is assembled with the shaft, there may be a gap or interference fit. The tolerance zone of the hole overlaps the tolerance zone of the shaft.
3. Interference fit
The fit of the bore and shaft assembly with interference (including a minimum interference equal to zero). The tolerance zone of the hole is below the tolerance zone of the shaft.
8. Benchmark system:
When manufacturing mated parts, the system in which one of the parts is used as a reference part, and its basic deviation is certain, and the system of obtaining a variety of different properties of the fit by changing the basic deviation of another non-reference part is called the benchmark system. According to the actual needs of production, the national standard stipulates two benchmark systems.
1) Base hole system (as shown in the figure below)
Base hole system - refers to a system in which the tolerance zone of a certain hole with a certain basic deviation forms various combinations with the tolerance zone of the shaft with different basic deviations. See diagram below. The hole of the base hole system is called the reference hole, and its basic deviation is coded H, and its lower deviation is zero.
2) Base shaft system (as shown in the figure below)
Base shaft system - refers to a system in which the tolerance zone of a certain shaft with a certain basic deviation forms various combinations with the tolerance zone of the hole with different basic deviations. See diagram below. The shaft of the base shaft system is called the reference shaft, and its basic deviation is coded H, and its upper deviation is zero.
9. Compounding substituency
The fitting code consists of the tolerance band code of the hole and the shaft, written in the form of fractions, with the numerator being the tolerance band code of the hole and the denominator being the tolerance band code of the shaft. All those containing H in the molecule are the base hole system fit, and all the ones containing h in the denominator are the base shaft system fit.
For example, the meaning of φ25H7/g6 means that the basic size of the fit is φ25, the gap fit of the base hole system, the tolerance zone of the reference hole is H7, (the basic deviation is H tolerance grade 7), and the tolerance zone of the shaft is g6 (the basic deviation is g, and the tolerance grade is 6).
For example, the meaning of φ25N7/h6 means that the basic size of the fit is φ25, the base shaft system transition fit, the tolerance zone of the reference shaft is h6, (the basic deviation is h, the tolerance grade is 6), and the tolerance zone of the hole is N7 (the basic deviation is N, and the tolerance grade is 7).
10. Tolerance and fit are marked on the drawing
1) Mark the tolerance and fit on the assembly drawing, and adopt the combined injection method.
2) There are three forms of annotation methods on the part drawing.
11. Geometric tolerance
After the parts are processed, there are not only dimensional errors, but also errors in geometry and mutual position. Even if the size of the cylinder is qualified, there may be a situation such as one end is large, the other end is small, or the middle is thin and the two ends are thick, and its cross-section may not be round, which is an error in shape. For stepped shafts, there may be different axes of each axis section after machining, which is an error in position. Therefore, the shape tolerance refers to the allowable variation of the actual shape from the ideal shape. Position tolerance refers to the allowable variation of the actual position from the ideal position. Both are referred to as geometric tolerances.
Geometric tolerance bullets
1. Code for shape and position tolerances
The national standard GB/T 1182-2008 stipulates that the shape and position tolerances are marked by codes. In actual production, when it is not possible to mark the geometric tolerances with the code, it is allowed to use words in the technical requirements. The geometric tolerance code includes: the symbol of each item of the geometric tolerance, the geometric tolerance frame and guide line, the geometric tolerance value and other related symbols, as well as the reference code. The height h of the font in the box is equal to the size number in the drawing.
2. Example of geometric tolerance annotation
A valve stem, the text added near the geometric tolerance marked in the diagram is only repeated for the purpose of explanation to the reader, and there is no need to repeat the annotation in the actual drawing.
1. Casting structure on parts
1 Cast fillet corners
When the blank of the part is a casting, due to the requirements of the casting process, the corners where each surface of the casting intersects should be made into a fillet. Casting fillet prevents sand shakeout at the corners of castings and avoids shrinkage porosity and cracks when the metal cools. The size of the casting fillet is generally taken as R=3~5mm, which can be uniformly indicated in the technical requirements.
2 Mould inclination
When manufacturing parts blanks by casting, in order to facilitate the removal of the pattern in the sand mold, an slope of about 1:20 is generally made along the draft direction of the pattern, which is called the draft slope. Therefore, there is also a corresponding draft slope on the casting, which can not be marked on the drawing, nor may it be drawn, as shown in the figure below;
3. Casting thickness
When the wall thickness of the casting is not uniform, cracks and shrinkage holes will occur due to the different cooling rates of the metal in various places after casting. Therefore, the wall thickness of the casting should be as uniform as possible, as shown in the figure above, and when it is necessary to use different wall thicknesses to connect, a gradual transition should be adopted, as shown in the figure above. The wall thickness of the casting is generally injected directly.
2. Machined structure on parts
1. Retract groove and grinding wheel overstep groove
In the cutting of parts, in order to facilitate the withdrawal of the tool and ensure that the contact surface of the relevant parts is close during assembly, the retract groove or the grinding wheel overstep groove should be pre-machined at the surface step to be machined. When turning the outer circle, the size of the retract groove can generally be marked according to the method of "groove width× diameter" or "groove width ×groove depth". Wheel overpass grooves when grinding the outer cylinder or grinding the outer cylinder and end face.
2. Drilling structure
Blind holes drilled with drill bits have a cone angle of 120° at the bottom, and the drilling depth refers to the depth of the cylindrical part, excluding the cone pit. At the transition of the stepped borehole, there is also a round table with a cone angle of 120°, and its drawing method and size injection method.
When drilling with a drill bit, the axis of the drill bit is required to be perpendicular to the end face of the drilled as much as possible to ensure the accuracy of the drilling and avoid the breakage of the drill bit. Correct structure of the three borehole end faces.
3. Bosses and pits
The contact surface of the part with other parts is generally machined. In order to reduce the processing area and ensure good contact between the surface of the parts, bosses and pits are often designed on the castings. In the form of bolted support surface bosses or support surface pits, in order to reduce the processing area, the groove structure is made.