1. The dimensioning method of common structure
Common hole dimensioning method：
Blind Hole
Threaded Hole
Counterbore
Countersink Hole
The dimensioning method of the chamfer
2. Machining Structure On Parts
Undercut and grinding wheel overtravel groove
When parts are cut, in order to facilitate the withdrawal of the tool and ensure that the contact surfaces of the relevant parts are close during assembly, the undercut groove or the grinding wheel overtravel groove should be pre-machined at the step of the processed surface.
The size of the undercut when turning the outer circle can generally be marked in the way of “groove width × diameter” or “groove width × groove depth”. Grinding wheel overtravel groove when grinding outer circle or grinding outer circle and end face.
Drilling Structure
A blind hole drilled with a drill bit has a cone angle of 120° at the bottom. The drilling depth refers to the depth of the cylindrical part, excluding the cone pit. At the transition of the stepped drilling, there is also a cone angle of 120° round table, its drawing and size notation.
When drilling with a drill, it is required that the axis of the drill is as perpendicular to the end face of the drilled hole as possible to ensure the accuracy of the drilling and avoid the drill from breaking. The correct structure of the three drilling end faces.
Bosses And Pits
The contact surface of a part with other parts is generally 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 boss or support surface recessed form. in order to reduce the processing area, a groove structure is made.
3 Common Part Structure
Shaft Parts
Such parts generally include shafts, bushings and other parts. When expressing the views, as long as you draw a basic view and add appropriate cross-sectional views and dimensions, you can express its main shape features and local structure. In order to facilitate the viewing of pictures during processing, the axis is generally placed horizontally for projection, and it is best to choose the position where the axis is the lateral vertical line.
When marking the size of bushing parts, its axis is often used as the radial size benchmark. From this, the Ф14 and Ф11 shown in the figure (see A-A section), etc. are noted. In this way, the design requirements and the process reference during processing (when the shaft parts are processed on the lathe, the center hole of the shaft is held by the thimble at both ends) is unified. The length direction datum often chooses important end faces, contact faces (shoulders) or machined faces.
The right shoulder with a surface roughness of Ra6.3 as shown in the figure is selected as the main dimension reference in the length direction, and dimensions such as 13, 28, 1.5 and 26.5 are injected.
Then take the right shaft end as the auxiliary base in the length direction to mark the total shaft length 96.
Disc Cover Parts
The basic shape of this kind of parts is a flat disc, generally including end caps, valve caps, gears and other parts. Their main structure is basically a rotating body, usually with various shapes of flanges and uniformly distributed circular holes. And ribs and other local structures. When selecting a view, generally select the cross-sectional view of the symmetry plane or the axis of rotation as the front view, and also 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 a square flange with rounded corners and four evenly distributed through holes.
When marking the size of disc cover parts, the axis through the shaft hole is usually selected as the radial dimension reference, and the main dimension reference in the length direction is often the important end face.
Fork frame parts
Such parts generally include forks, connecting rods, bearings and other parts. Due to their variable processing positions, when choosing the main view, the main consideration is the working position and shape characteristics. For the selection of other views, two or more basic views are often required, and appropriate local views, cross-sectional views, etc. must be used to express the local structure of the part. The view shown in the footrest parts drawing is refined and clear. The right view is not necessary for expressing the width of the bearing and ribs. For T-shaped ribs, the cross-section is more appropriate.
When marking the dimensions of fork bracket parts, the installation base surface or the symmetry plane of the part is usually selected as the dimension reference. Refer to the figure for the dimensioning method.
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 position changes more. Such parts generally include valve body, pump body, reducer box and other parts. When choosing the main view, the working position and shape characteristics are mainly considered. When selecting other views, appropriate sectional views, cross-sections, partial views, and oblique views should be adopted according to the actual situation to clearly express the internal and external structure of the part.
In terms of dimensioning, the axis, important installation surface, contact surface (or processing surface), and the symmetry surface (width, length) of some main structures of the box are usually selected as the dimensional reference. For the parts on the box that need to be cut, the dimensions should be marked as far as possible to facilitate processing and inspection.
4 Surface Roughness
The micro-geometric characteristics of the peaks and valleys with small spacing on the surface of the part are called surface roughness. This is mainly due to the tool marks left by the tool on the surface of the part and the plastic deformation of the surface metal during cutting.
The surface roughness of parts is also a technical index for evaluating the surface quality of parts. It has an impact on the mating properties, working accuracy, wear resistance, corrosion resistance, airtightness and appearance of the parts.
The code, symbol and mark of surface roughness
GB/T 131-1993 specifies the surface roughness code and its notation. The symbols on the drawing that indicate the surface roughness of the parts are shown in the table below.
Main evaluation parameters of part surface roughness
1) The arithmetic mean deviation of contour (Ra)
Within the sampling length, the arithmetic mean of the absolute value of the contour offset.
2) Maximum profile height (Rz)
The distance between the top line of the contour peak and the bottom line of the contour peak within the sampling length.
Note: The Ra parameter is preferred when using it.
Marking method of surface roughness symbols on drawings
1) The surface roughness code (symbol) should generally be noted on the visible contour line, the boundary line or their extension, 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 as required.
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 narrow or it is inconvenient to label, it can lead to the label. When all the surfaces of the part have the same surface roughness requirements, they can be uniformly marked on the upper right corner of the drawing. When most of the parts have the same surface roughness requirements, the code (symbol) that is used most can be used. Note at the same time in the upper right corner of the drawing, and add the words “the rest”. The height of the uniformly marked surface roughness code (symbol) and explanatory text should be 1.4 times that of the drawing mark.
The surface roughness code (symbol) number of the continuous surface on the part, the surface of the repeated elements (such as holes, teeth, grooves, etc.) and the discontinuous surface connected by a thin solid line are only noted once.
When there are different surface roughness requirements on the same surface, use thin solid lines to draw the dividing line, and note the corresponding surface roughness code and size.
When the tooth (tooth) shape is not drawn on the working surface of gears, threads, etc., the surface roughness code (symbol) notation method is shown in the figure.
The working surface of the center hole, the working surface of the keyway, the surface roughness code of the chamfer, and the rounded corner can be simplified and marked.
When the parts need to be partially heat-treated or partially plated (coated), thick dotted lines should be used to draw the range and mark the corresponding dimensions. The requirements can also be written on the horizontal line of the long side of the surface roughness symbol.
5 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 is composed of two elements: standard tolerance and basic deviation. The standard tolerance determines the size of the tolerance zone, and the basic deviation determines the location of the tolerance zone.
1) Standard Tolerance (IT)
The value of the standard tolerance is determined by the basic size and tolerance class. The tolerance level is a mark to determine the accuracy of the size. The standard tolerance is divided into 20 levels, namely IT01, IT0, IT1,…, IT18. The accuracy of its size decreases from IT01 to IT18. The specific values of standard tolerances can be found in relevant standards.
2) Basic deviation
The basic deviation refers to the upper or lower deviation of the tolerance zone relative to the zero line position in the standard limit and fit, generally refers to the deviation close to the zero line. When the tolerance zone is above the zero line, the basic deviation is the lower deviation; otherwise, it is the upper deviation. There are 28 basic deviations in total, and the code is expressed in Latin letters, with uppercase as hole and lowercase as shaft.
The basic deviation and standard tolerance have the following calculation formulas according to the definition of dimensional tolerance:
ES=EI+IT or EI=ES-IT