1. Method for sizing current structures
Common hole sizing methods (blind holes, threaded holes, countersunk holes, countersunk hole sizing methods for chamfers);
❖ Blind hole
❖ Threaded hole
❖ Counterbore
❖ Countersunk hole
❖ Chamfer
2. Mechanical processing structure on parts
❖ Undercut groove and wheel overhang groove
When cutting workpieces, in order to facilitate tool removal and ensure that the mating surfaces of related workpieces are narrow during assembly, an undercut groove or grinding wheel overtravel groove should be pre-treated at the surface to be treated. .
The size of the undercut when turning the outer circle can generally be marked as “groove width ×; diameter” or “groove width × groove depth”. The protruding groove of the grinding wheel when grinding the outer circle or grinding the outer circle and end face.
❖ Drilling structure
The blind hole drilled with a drill bit has a taper angle of 120° at the bottom. Drilling depth refers to the depth of the cylindrical part, excluding the conical pit. At the step borehole transition, there is also a 120° angle cone, its drawing method and sizing method.
When drilling with a drill bit, the axis of the drill bit should be as perpendicular as possible to the end face to be drilled to ensure accurate drilling and avoid breakage of the drill bit. Correct construction of three drill end faces.
❖ Bosses and dimples
Contact surfaces between parts and other parts generally need to be treated. To reduce the processing area and ensure good contact between part surfaces, bosses and recesses are often designed on castings. Bolted bearing surface bosses or bearing surface pits in order to reduce the processing area, a groove structure is made.
3. Structure of common areas
❖ Shaft Sleeve Parts
These parts usually include shafts, bushings and other parts. When expressing views, as long as a basic view is drawn and the appropriate cross sections and dimensions are drawn, its main shape features and local structure can be expressed. To make it easier to view the drawing during processing, the axis is usually placed horizontally for projection. It is best to choose a position where the axis is a side vertical line.
When marking the dimensions of bushing parts, its axis is often used as a radial dimension reference. From there are taken φ14, φ11 (see section AA), etc. shown in the figure. This unifies the design requirements and process reference during processing (when shaft parts are processed on a lathe, use lugs on both ends to push against the center hole of the shaft). The large end face, mating surface (shoulder), or machined surface are often used as a lengthwise reference.
As shown in the figure, the straight shoulder with a surface roughness of Ra6.3 is selected as the main dimensional reference in the length direction, and sizes such as 13, 28, 1.5 and 26.5 are in it. pulled, then the right end of the axis; is used as an auxiliary base length direction, thus marking the total length of shaft 96.
❖ Disk cover parts
The basic shape of this type of parts is a flat disk, usually including end covers, valve covers, gears and other parts. Their main structure usually has a rotating body, usually with flanges of various shapes and uniformly distributed round holes and local structures. like the ribs. When selecting views, generally choose a section view through the plane of symmetry or axis of rotation as the primary view. At the same time, you need to add other suitable views (such as left view, right view or top view) to express the shape. and a uniform structure of the room. As shown in the figure, a left view is added to express the square flange with rounded corners and four evenly distributed through-holes.
When marking the dimensions of the disc cover parts, the axis passing through the shaft hole is generally selected as the radial dimension data, and the important end face is often selected as the main dimension data in the direction of length.
❖ Fork parts
These parts usually include shift forks, connecting rods, brackets and other parts. Due to their variable processing positions, the working position and shape characteristics are mainly taken into account when selecting the main view. The selection of other views often requires two or more basic views, and partial views, section views and other suitable expression methods are also used to express the local structure of the part. The view selection expression diagram shown in the foot seat parts diagram is concise and clear. To express the width of the bearing and rib, the right view is not necessary, but for the T-shaped rib, it is more appropriate to use a cross. -section.
When marking the dimensions of fork-type parts, the surface of the mounting base or the plane of symmetry of the part is generally used as the dimensional data. See figure for sizing methods.
❖ Box Parts
Generally speaking, the shape and structure of this type of parts are more complex than the previous three types of parts, and the processing positions change more. These parts generally include valve bodies, pump bodies, reducer housings and other parts. When choosing a main view, the main considerations are working location and form characteristics. When selecting other views, appropriate auxiliary views such as sections, sections, partial views 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 sizing, the axis required by the design, the large mounting surface, the contact surface (or processing surface), the symmetry surface (width, length) of certain main structures of the case, etc. are generally used as dimensions. reference. For those parts of the box that require cutting processing, the dimensions should be marked as much as possible to facilitate processing and inspection.
4. Surface roughness
❖ Concept of surface roughness
Microscopic geometric shape features composed of closely spaced peaks and valleys on the surface of the part are called surface roughness. This is mainly due to the tool marks left on the workpiece surface during workpiece processing and the plastic deformation of the surface metal during cutting and splitting.
The surface roughness of parts is also a technical indicator to evaluate the surface quality of parts. It has an impact on fitting properties, working precision, wear resistance, corrosion resistance, sealing, appearance, etc.
❖ Surface roughness codes, symbols and markings
GB/T 131-1993 specifies the surface roughness code and its rating method. Symbols indicating the surface roughness of parts in the drawing are shown in the table below.
❖ Main parameters for evaluating surface roughness
The parameters for evaluating the surface roughness of the part are:
1) Arithmetic mean deviation of the contour (Ra)
Arithmetic mean of the absolute value of the contour offset in the sampling length. The value of Ra and the sampling length l are shown in the table.
2) Maximum profile height (Rz)
In the sample length, the distance between the upper line of the contour peak and the lower line of the contour peak.
Note: The Ra setting is preferred when using.
❖ Labeling requirements for surface roughness
1) Example of coded labeling for surface roughness
When the surface roughness height parameters Ra, Rz and Ry are marked with numerical values in the code, except that the parameter code Ra may be omitted, the corresponding parameter code Rz or Ry shall be marked before the value of the setting. table for labeling examples.
2) Marking surface roughness. Surface roughness numbers and symbols.
❖ How to mark surface roughness symbols on drawings
1) The surface roughness code (symbol) should generally be marked on visible contour lines, dimension lines or their extension lines. The tip of the symbol should point from the outside of the material toward the surface.
2) The direction of numbers and symbols in the surface texture code shall be marked according to regulations.
Surface Roughness Labeling Example
On the same drawing, each surface is generally marked with a single generation (symbol) and as close as possible to the dimension line concerned. When the space is small or it is inconvenient to mark, you can draw the mark. When all surfaces in a part have the same surface roughness requirements, they can be marked uniformly in the upper right corner of the drawing. When most surfaces in the part have the same surface roughness requirements, the most commonly used code (symbol) may be. At the same time, write it down in the upper right corner of the drawing and add the word “rest”. The height of all uniformly marked surface roughness symbols (symbols) and explanatory text should be 1.4 times higher than that of the drawing markings.
The surface roughness code (symbol) of the continuous surface of the part, the surface of repeated elements (such as holes, teeth, grooves, etc.) and the discontinuous surface connected by thin solid lines is not noted only once.
When there are different surface roughness requirements on the same surface, a thin continuous line should be used to draw the dividing line, and the corresponding surface roughness code and size should be noted.
When the tooth shape (tooth) is not drawn on the working surface of gears, threads, etc., the surface roughness code (symbol) is displayed in the figure.
Surface roughness codes for center hole working surface, keyway working surface, chamfers and fillets can simplify labeling.
When parts are to be partially heat treated or partially plated (coated), the range should be drawn with thick dotted lines and the corresponding dimensions should be marked on the horizontal line on the long side of the surface roughness. symbol.
5. Standard tolerances and fundamental deviations
In order to facilitate production, realize the interchangeability of parts and meet different usage requirements, the national standard “Limits and adjustments” stipulates that the tolerance zone consists of two elements: standard tolerance and deviation basic. The standard tolerance determines the size of the tolerance box, while the base deviation determines the location of the tolerance box.
1) Standard Tolerance (IT)
The standard tolerance value is determined by the base size and tolerance class. The tolerance level is a mark that determines the accuracy of dimensions. The standard tolerance is divided into 20 levels, namely IT01, IT0, IT1,…, IT18. Dimensional accuracy decreases from IT01 to IT18. For specific values of standard tolerances, see the relevant standards.
2) Baseline Gap
Baseline deviation refers to the upper or lower deviation of the tolerance zone from the zero line within the standard limits and coordination, generally referring to the deviation close to the zero line. When the tolerance zone is above the zero line, the base deviation is a lower deviation, otherwise it is an upper deviation. There are a total of 28 fundamental deviations and the codes are expressed in Latin letters, with uppercase letters for holes and lowercase letters for trees.
This can be seen from the base gap series diagram: the hole base gap A～H and the shaft base gap k～zc are the hole base gap K～ZC and the base gap of the hole; the tree a～h are the upper gap, JS. The tolerance zones de and js are distributed symmetrically on both sides of the zero line. The upper and lower gaps of the hole and the shaft are +IT/2 and -IT/2 respectively. The basic deviation series chart only shows the position of the tolerance zone, not the size of the tolerance. Therefore, 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 the standard tolerance, according to the definition of the dimensional tolerance, have the following calculation formula: ES=EI+IT or EI=ES-IT ei=es-IT or es=ei+IT The code of tolerance zone of the hole. and the tree uses the base gap code. It consists of the tolerance zone quality code.
6. Cooperation
The relationship between the tolerance zones of holes and shafts having the same basic dimensions and combined with each other is called fit. Depending on the usage requirements, the fit between the hole and the shaft can be loose or tight, so the national standard stipulates the fit type.
1) Clearance fit: When the hole and shaft are assembled, there is a fit (including the minimum clearance equal to zero). The tolerance zone of the hole is above the tolerance zone of the shaft.
2) Transition fit: When the hole and shaft are assembled, there may be a gap or interference fit. The tolerance zone of the hole overlaps the tolerance zone of the shaft.
3) Fit with interference: There is interference (including the minimum interference equal to zero) when the hole and the shaft are assembled. The tolerance zone of the hole is less than the tolerance zone of the shaft.
❖ In the reference system, when manufacturing matching parts, one of the parts is used as the reference part and its base deviation is certain. The system that achieves different types of adjustments with different properties by changing the base deviation of another non-standard part. This part is called the reference system. Depending on actual production needs, national standards provide for two reference systems.
1) Basic hole system: This is a system in which the tolerance area of a hole with a certain basic deviation and the tolerance area of a shaft with different basic deviations form various fits. See photo below left. The hole consisting of the base hole is called the reference hole, its base deviation code is H, and its lower deviation is zero.
2) Basis shaft system: This is a system in which the tolerance area of a shaft with a certain base deviation and the tolerance area of a hole with different base deviations form various fits . See the image below on the right. The axis of the base axis system is called the reference axis, its base deviation code is h, and the upper deviation is zero.
❖ Cooperation code
The fit code consists of the hole and shaft tolerance zone codes and is written as a fraction. The numerator is the tolerance zone code of the hole and the denominator is the tolerance zone code of the shaft. Any combination containing H in the numerator is a basic hole system, and any combination containing h in the denominator is a basic axis system.
For example 1: φ25H7/g6 means the base size of the fit is φ25, the clearance fit of the base hole system, the tolerance area of the reference hole is H7 (the base gap is H , the tolerance level is level 7 ), and the tolerance zone of the shaft is g6 (the basic deviation is g, the tolerance level is level 6).
For example 2: φ25N7/h6 means the basic adjustment size is φ25, the transition adjustment of the basic axis, the tolerance zone of the reference axis is h6 (the basic deviation is h, the tolerance level is level 6), and the tolerance area of the hole is N7 (the basic deviation is N, the tolerance level is level 7).
❖ Marking of tolerances and adjustments on plans
1) Mark tolerances and fits on the assembly drawing, using the combined marking method.
2) There are three forms of marking methods on part drawings.
7. Geometric tolerance
After the parts are processed, there are not only dimensional errors, but also errors in geometric shape and mutual position. Even if the cylinder is of suitable size, it may be large at one end and small at the other end, or thin in the middle and thick at both ends, etc., and its cross section may not be round, which is a problem. form error. For stepped shafts, each shaft segment may have different axes after processing, which constitutes a position error. Therefore, 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 called geometric tolerances.
Geometric tolerance chips
❖ Shape and position tolerance codes
The national standard GB/T 1182-1996 stipulates the use of codes to mark shape and position tolerances. In actual production, when the geometric tolerance cannot be marked by a code, it is permitted to use a textual description in the technical requirements.
Geometric tolerance codes include: symbols for each geometric tolerance element, geometric tolerance frames and guidelines, geometric tolerance values and other associated symbols, and reference codes, etc. The height h of the font in the frame is the same as the size number in the design.
❖ Example of geometric tolerance marking
For a valve stem, text added near the geometric tolerance marked in the figure is repeated for the purpose of explanation to the reader only and need not be repeated in the drawing itself.
Daguang focuses on providing solutions such as precision CNC machining services (3-axis, 4-axis, 5-axis machining), CNC milling, 3D printing and rapid prototyping services.
