When entering and exiting the processing site, can you understand all the complicated process drawings? When designing a processing plan for a customer, do you have any questions about the dimensions? This time the editor brings you a different classic - knowledge about dimensioning in mechanical design! No more worrying about not understanding the drawings!
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Dimensioning methods for common structures
Dimensioning methods for common holes (blind holes, threaded holes, countersunk holes, countersunk holes); dimensioning methods for chamfers.
❖ Blind hole
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❖ Threaded hole
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❖ Counterbore
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❖ Countersinking hole
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❖ Chamfer
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Machined structures on the part
❖ Undercut groove and grinding wheel overtravel groove
When cutting parts, in order to facilitate the withdrawal of the tool and ensure that the contact surfaces of related parts are close during assembly, an undercut groove or a grinding wheel overtravel groove should be pre-processed at the step of the surface to be processed.
The size of the undercut when turning the outer circle can generally be marked in the form of "groove width × diameter" or "groove width × groove depth". The grinding wheel overtravel groove when grinding the outer circle or grinding the outer circle and end face.
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❖ Drilling structure
The blind hole drilled with a drill bit has a 120° taper angle at the bottom. The drilling depth refers to the depth of the cylindrical part, excluding the taper pit. At the transition of the stepped drill hole, there is also a 120° cone angle cone, its drawing method and dimensioning method.
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When drilling with a drill bit, the drill bit axis is required to be as perpendicular to the end face being drilled as possible to ensure accurate drilling and avoid drill bit breakage. Correct construction of three drill end faces.
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❖ Bosses and dimples
The contact surfaces between parts and other parts generally need to be processed. In order to reduce the processing area and ensure good contact between the surfaces of the parts, bosses and pits are often designed on the castings. Bolted support surface bosses or support surface pits; in order to reduce the processing area, a groove structure is made.
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Common part structures
❖ Shaft sleeve parts
Such parts generally include shafts, bushings and other parts. When expressing views, as long as a basic view is drawn and appropriate cross-sections and dimensions are drawn, its main shape features and local structure can be expressed. In order to facilitate viewing of the drawing during processing, the axis is generally 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 the radial dimension benchmark. From this, Ф14, Ф11 (see section A-A), etc. shown in the figure are drawn. This unifies the design requirements and the process benchmark during processing (when shaft parts are processed on a lathe, use thimbles at both ends to push against the center hole of the shaft). The important end face, contact surface (shoulder) or machined surface is often used as the benchmark in the length direction.
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As shown in the figure, the right shoulder with a surface roughness of Ra6.3 is selected as the main dimension reference in the length direction, and sizes such as 13, 28, 1.5 and 26.5 are drawn from this; then the right axis end is used as the length direction. auxiliary base, thus marking the total length of the shaft 96.
❖ Disk cover parts
The basic shape of this type of parts is a flat disk, generally including end covers, valve covers, gears and other parts. Their main structure generally has a rotary body, usually with flanges of various shapes and evenly distributed round holes. and local structures such as ribs. When selecting views, generally choose a section view through the symmetry plane or axis of rotation as the main view. At the same time, you need to add appropriate other views (such as left view, right view or top view) to express the shape and uniform structure of the part. As shown in the figure, a left view is added to express the square flange with rounded corners and four evenly distributed through holes.
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When marking the dimensions of disc cover parts, the axis passing through the shaft hole is usually selected as the radial dimension datum, and the important end face is often used as the main dimension datum in the length direction.
❖ Fork parts
Such parts generally include shift forks, connecting rods, supports and other parts. Due to their variable processing positions, the working position and shape characteristics are mainly considered when selecting the main view. The selection of other views often requires two or more basic views, and appropriate partial views, section views and other expression methods are also used to express the local structure of the part. The view selection shown in the pedal seat parts diagram is concise and clear. For expressing the width of the bearing and rib, the right view is not necessary, but for the T-shaped rib, the cross-section is more appropriate.
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When marking the dimensions of fork-type parts, the mounting base surface or the symmetry plane of the part is usually used as the dimensional datum. See the figure for dimensioning 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. Such parts generally include valve bodies, pump bodies, reducer boxes and other parts. When choosing a main view, the main considerations are working location and shape 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.
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In terms of dimensioning, the axis required by the design, the important mounting surface, the contact surface (or processing surface), the symmetry plane (width, length) of some main structures of the box, etc. are usually used as the dimensional benchmark. For the parts of the box that require cutting processing, the dimensions should be marked as far as possible to facilitate processing and inspection.
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Surface roughness
❖ Concept of surface roughness
The microscopic geometric shape characteristics composed of peaks and valleys with small spacing on the surface of the part are called surface roughness. This is mainly caused by the knife marks left by the tool on the surface of the part when processing parts and the plastic deformation of the surface metal during cutting and splitting.
The surface roughness of parts is also a technical indicator for evaluating the surface quality of parts. It has an impact on the matching properties, working accuracy, wear resistance, corrosion resistance, sealing, appearance, etc. of the parts.
❖ Surface roughness codes, symbols and markings
GB/T 131-1993 specifies the surface roughness code and its notation method. The symbols indicating the surface roughness of the parts on the drawing are shown in the table below.
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❖ Main evaluation parameters of surface roughness
The evaluation parameters of part surface roughness are:
1) Arithmetic mean deviation of contour (Ra)
The arithmetic mean of the absolute value of the contour offset within the sampling length. The value of Ra and the sampling length l are shown in the table.
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2) Maximum height of outline (Rz)
Within the sampling length, the distance between the top line of the contour peak and the bottom line of the contour peak.
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Note: Ra parameter is preferred when using.
❖ Labeling requirements for surface roughness
1) Example of surface roughness code labeling
When the surface roughness height parameters Ra, Rz, and Ry are marked with numerical values in the code, except that the parameter code Ra can be omitted, the corresponding parameter code Rz or Ry must be marked before the parameter value. See the table for examples of labeling.
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2) Marking of surface roughness. The method of numbers and symbols in surface roughness.
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❖ How to mark surface roughness symbols on drawings
1) The surface roughness symbol (symbol) should generally be marked on the visible contour lines, dimension lines or their extension lines. 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 regulations.
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Example of labeling surface roughness
On the same drawing, each surface is generally marked with only one generation (symbol) and as close as possible to the relevant dimension line. When the space is small or it is inconvenient to mark, you can draw out the mark. When all surfaces of a part have the same surface roughness requirements, they can be marked uniformly in the upper right corner of the drawing. When most surfaces of the part have the same surface roughness requirements, the most commonly used code (symbol) can be At the same time, note it 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 that of the drawing markings.
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The surface roughness code (symbol) of the continuous surface on the part, the surface of repeated elements (such as holes, teeth, grooves, etc.) and the discontinuous surface connected by thin solid lines is only noted once.
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When there are different surface roughness requirements on the same surface, a thin solid line should be used to draw the dividing line, and the corresponding surface roughness code and size should be noted.
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When the tooth (tooth) shape is not drawn on the working surface of gears, threads, etc., the surface roughness code (symbol) is shown in the figure.
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The surface roughness codes of the working surface of the center hole, the working surface of the keyway, chamfers, and fillets can simplify the labeling.
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When parts need 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. The requirements can also be written on the horizontal line on the long side of the surface roughness symbol.
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Standard tolerances and basic deviations
In order to facilitate production, realize the interchangeability of parts and meet different usage requirements, the national standard "Limits and Fits" stipulates that the tolerance zone consists of two elements: standard tolerance and basic deviation. The standard tolerance determines the size of the tolerance zone, while the basic deviation determines the location of the tolerance zone.
1) Standard tolerance (IT)
The value of standard tolerance is determined by the basic 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. The dimensional accuracy decreases from IT01 to IT18. For specific values of standard tolerances, see relevant standards.
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2) Basic deviation
The basic deviation refers to the upper deviation or lower deviation of the tolerance zone relative to the zero line in 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 basic deviation is a lower deviation; otherwise, it is an upper deviation. There are a total of 28 basic deviations, and the codes are expressed in Latin letters, with uppercase for holes and lowercase for shafts.
It can be seen from the basic deviation series diagram: the basic deviation of the hole A～H and the basic deviation of the shaft k～zc are the lower deviation; the basic deviation of the hole K～ZC and the basic deviation of the shaft a～h are the upper deviation, JS The tolerance zones of and js are symmetrically distributed on both sides of the zero line. The upper and lower deviations of the hole and the shaft are +IT/2 and -IT/2 respectively. The basic deviation series diagram 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.
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Basic deviation and standard tolerance, according to the definition of dimensional tolerance, have the following calculation formula:
ES=EI+IT or EI=ES-IT
ei=es-IT or es=ei+IT
The tolerance zone code of the hole and shaft is composed of the basic deviation code and the tolerance zone grade code.
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Cooperate
The relationship between the tolerance zones of holes and shafts that have the same basic dimensions and are combined with each other is called a fit. Depending on the usage requirements, the fit between the hole and the shaft may be loose or tight, so the national standard stipulates the fit types:
1) Clearance fit
When assembling the hole and the shaft, there should be a fit with clearance (including the minimum clearance equal to zero). The tolerance zone of the hole is above the tolerance zone of the shaft.
2) Transitional cooperation
When the hole and shaft are assembled, there may be gaps or interference fits. The tolerance zone of the hole overlaps the tolerance zone of the shaft.
3) Interference fit
There is interference (including minimum interference equal to zero) when assembling the hole and the shaft. The tolerance zone of the hole is below the tolerance zone of the shaft.
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❖ Benchmark system
When manufacturing matching parts, one of the parts is used as a datum part, and its basic deviation is certain. The system of obtaining various types of fits with different properties by changing the basic deviation of another non-datum part is called the datum system. According to actual production needs, national standards stipulate two benchmark systems.
1) Basic hole system (as shown in the picture below left)
Basic hole system - refers to a system in which the tolerance zone of a hole with a certain basic deviation and the tolerance zone of a shaft with different basic deviations form various fits. See picture below left. The hole made of the basic hole is called the reference hole, its basic deviation code is H, and its lower deviation is zero.
2) Basic shaft system (as shown in the picture below right)
Basic shaft system - refers to a system in which the tolerance zone of a shaft with a certain basic deviation and the tolerance zone of a hole with different basic deviations form various fits. See picture below right. The axis of the basic axis system is called the datum axis, its basic deviation code is h, and the upper deviation is zero.
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①Picture of base hole system
②Basic shaft system
❖ Cooperation code
The fit code consists of the tolerance zone code of the hole and the shaft, and is written in fraction form. 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 that the basic size of the fit is φ25, the clearance fit of the base hole system, the tolerance zone of the reference hole is H7, (the basic deviation 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 that the basic size of the fit is φ25, the basic axis transition fit, the tolerance zone of the datum axis is h6, (the basic deviation is h, the tolerance level is level 6), and the tolerance zone of the hole is N7 (the basic deviation is N, the tolerance level is level 7).
❖ Marking of tolerances and fits on drawings
1) Mark tolerances and fits on the assembly drawing, using the combined injection method.
2) There are three forms of marking methods on parts drawings.
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Geometric tolerance
After the parts are processed, there are not only dimensional errors, but also geometric shape and mutual position errors. Even if the cylinder is of qualified 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 an error in shape. For stepped shafts, each shaft segment may have different axes after processing, which is 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 referred to as geometric tolerances.
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Geometric Tolerance Bullets
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❖ Codes for shape and position tolerances
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 with a code, it is allowed to use text description in the technical requirements.
Geometric tolerance codes include: symbols for each item of geometric tolerance, geometric tolerance frames and guide lines, geometric tolerance values and other related symbols, as well as datum codes, etc. The height h of the font in the frame is the same as the size number in the drawing.
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❖ Example of geometric tolerance marking
For a valve stem, the text added near the geometric tolerance marked in the figure is only repeated for the purpose of explaining to the reader, and does not need to be repeated in the actual drawing.
