Common hole size injection method (blind hole, threaded hole, counterbore, countersink hole); chamfer size injection method.

 

 

 

 

 

When parts are cut, in order to facilitate withdrawal of tool and ensure that contact surfaces of relevant parts are tight during assembly, undercut groove or grinding wheel overtravel groove should be pre-machined at the step of machined surface.

 

Size of undercut when turning outer circle can generally be marked in the way of "groove width*diameter" or "groove width*groove depth". Grinding wheel overtravel grooves when grinding external circles or grinding external circles and end faces.

Blind hole drilled with a drill bit has a cone angle of 120° at the bottom. Drilling depth refers to depth of cylindrical part, excluding cone pit. At transition of 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 axis of drill is as perpendicular to end face of drilled hole as possible to ensure accuracy of drilling and avoid drill from breaking. Following figure shows correct structure of three drilling end faces.

 

Contact surface of a part with other parts is generally processed. In order to reduce processing area and ensure good contact between surface of parts, bosses and pits are often designed on castings. Bolted support surface boss or support surface recessed form; in order to reduce processing area, it is made into a groove structure.

Such parts generally include shafts, bushings and other parts. When expressing 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 viewing of picture during processing, axis is generally placed horizontally for projection, and it is best to choose position where axis is lateral vertical line.
When marking size of bushing parts, its axis is often used as benchmark for radial dimensions. From this, Ф14 and Ф11 shown in figure (see A-A section), etc. are noted. In this way, design requirements and process reference during processing (when shaft parts are processed on lathe, center hole of shaft is held by thimble at both ends) is unified. Length direction datum often chooses important end faces, contact faces (shaft shoulders), or machined faces.

 

As shown in figure, right shoulder with a surface roughness of Ra6.3 is selected as main dimension reference in length direction, from which dimensions such as 13, 28, 1.5 and 26.5 are noted; then take right end of shaft as auxiliary base in length direction to mark total length of shaft 96.

Basic shape of this kind of parts is a flat disc shape, generally including end caps, valve caps, gears and other parts. Their main structure is basically a rotating body, usually with various shapes of flanges, uniformly distributed circular holes, ribs and other partial structures. When selecting a view, generally select cross-sectional view of symmetry plane or axis of rotation as front view, also need to add appropriate other views (such as left view, right view or top view) to express shape and uniform structure of part. As shown in figure, a left view has been added to express a square flange with rounded corners and four evenly distributed through holes.

 

When marking size of disc cover parts, axis through shaft hole is usually selected as radial dimension reference, main dimension reference in length direction is often important end face.

Such parts generally include forks, connecting rods, supports and other parts. Because their processing positions are changeable, when choosing main view, main consideration is working position and shape characteristics. For selection of other views, two or more basic views are often required, appropriate local views, cross-sectional views, and other expression methods must be used to express local structure of part. View shown in footrest parts drawing is refined and clear. Right view is not necessary for expressing width of bearing and ribs. For T-shaped ribs, cross-section is more appropriate.

 

When marking size of fork frame parts, installation base surface or symmetry plane of part is usually selected as size reference. Refer to figure for dimensioning method.

Generally speaking, shape and structure of this type of parts are more complex than previous three types of parts, and processing position changes more. Such parts generally include valve body, pump body, reducer box and other parts. When choosing main view, 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 actual situation to clearly express internal and external structure of part.

 

In terms of dimensioning, usually axis, important installation surface, contact surface (or processing surface) required in design, symmetry surface (width, length) of some main structures of box are used as size reference. For parts on box that need to be cut, dimensions should be marked as far as possible in accordance with requirements for easy processing and inspection.

Surface roughness is composed of microscopic geometric shape characteristics of peaks and valleys with small spacing on the surface. This is mainly due to tool marks left by tool on the surface of part and plastic deformation of surface metal when cutting is split.
Surface roughness of parts is also a technical index for evaluating surface quality of parts. It has an impact on mating properties, working accuracy, wear resistance, corrosion resistance, airtightness and appearance of parts.

GB/T 131-1993 specifies surface roughness code and its notation. Symbols on drawing that indicate surface roughness of parts are shown in table below.
Symbol table for surface roughness

Evaluation parameters of surface roughness of parts are:

Within sampling length, arithmetic mean of absolute value of contour offset. See table for value of Ra and sampling length l.
Selected values of Ra and 1 and 1n (GB/T1032-1995)

Note: 1, 1n are lengths in X-axis direction of profile being assessed, including one or several sampling lengths.
2. Bold font in Ra value is the first series, which should be preferred.

Distance between top line of contour peak and bottom line of contour peak within sampling length.

 

Remarks: Ra parameter is preferred when using it.

When surface roughness height parameters Ra, Rz, Ry are marked with numerical values in code, except that parameter code Ra can be omitted, the other parameters must be marked with corresponding parameter code Rz or Ry before parameter value. Refer to table for labeling examples.
An example of dimensioning of surface roughness height parameter

 

1) Surface roughness code (symbol) should generally be noted on visible contour line, size boundary line or their extension line, tip of symbol must point from outside of material to surface.
2) Direction of numbers and symbols in the surface roughness code must be marked as required.

 

Labeling example of surface roughness
On same drawing, each surface is generally marked with a code (symbol) only once, and as close to relevant dimension line as possible. When space is narrow or it is inconvenient to label, it can lead to label. When all surfaces of part have same surface roughness requirements, they can be uniformly marked on upper right corner of drawing. When most of surfaces of parts have same surface roughness requirements, most used code (symbol) can be marked on upper right corner of drawing at the same time, and words "the rest" can be added. Height of uniformly marked surface roughness code (symbol) and explanatory text should be 1.4 times that of drawing mark.

 

Surface roughness code (symbol) number of continuous surface on part, surface of repeated elements (such as holes, teeth, grooves, etc.) and discontinuous surface connected by a thin solid line are only noted once.

 

When there are different surface roughness requirements on same surface, thin solid lines should be used to draw dividing line, corresponding surface roughness code and size should be noted.

 

When tooth (tooth) shape is not drawn on working surface of gears, threads, etc., surface roughness code (symbol) notation method is shown in figure.

 

Working surface of center hole, working surface of keyway, surface roughness code of chamfer, rounded corner can be simplified and marked.

 

When parts need to be partially heat-treated or partially plated (coated), use thick dotted lines to draw range and mark corresponding size, or requirements can be written on horizontal line of long side of surface roughness symbol.
In order to facilitate production, realize interchangeability of parts and meet different usage requirements, national standard "Limits and Fits" stipulates that tolerance zone is composed of two elements: standard tolerance and basic deviation. Standard tolerance determines size of tolerance zone, basic deviation determines location of tolerance zone.

Value of standard tolerance is determined by basic size and tolerance class. Tolerance level is a mark to determine accuracy of size. Standard tolerance is divided into 20 levels, namely IT01, IT0, IT1,..., IT18. Accuracy of its size decreases from IT01 to IT18. Specific values of standard tolerances can be found in relevant standards.

 

Basic deviation refers to upper or lower deviation of tolerance zone relative to position of zero line in standard limit and fit, generally refers to deviation near zero line. When tolerance zone is above zero line, basic deviation is lower deviation; otherwise, it is upper deviation. There are 28 basic deviations in total, code names are expressed in Latin letters, with uppercase as hole and lowercase as shaft.
It can be seen from series of basic deviations that: basic deviation A～H of hole and basic deviation k～zc of shaft are lower deviation; basic deviation K～ZC of hole and basic deviation a～h of axis are upper deviation. Tolerance zones of JS and js are symmetrically distributed on both sides of zero line, upper and lower deviations of hole and shaft are +IT/2 and -IT/2 respectively. Basic deviation series diagram only shows position of tolerance zone, not size of tolerance. Therefore, one end of tolerance zone is an opening, and the other end of opening is limited by standard tolerance.

 

Basic deviation and standard tolerance, according to definition of dimensional tolerance, have following calculation formulas:
ES=EI+IT or EI=ES-IT
ei=es-IT or es=ei+IT
Tolerance zone code of hole and shaft is composed of basic deviation code and tolerance zone grade code.
Relationship between holes and shaft tolerance zone that have same basic size and are combined with each other is called a fit. According to different requirements of use, fit between hole and shaft is tight or loose, so national standard stipulates type of fit:

When hole is assembled with shaft, there is a clearance (including minimum clearance equal to zero). Tolerance zone of hole is above tolerance zone of shaft.

When hole is assembled with shaft, there may be a clearance or interference fit. Tolerance zone of hole and tolerance zone of shaft overlap each other.

When hole is assembled with shaft, there is an interference fit (including minimum interference equal to zero). Tolerance zone of hole is below tolerance zone of shaft.

 

When manufacturing matching parts, one of parts is used as reference part, and its basic deviation is fixed. Sstem of obtaining various coordinations of different properties by changing basic deviation of another non-reference part is called reference system. According to actual needs of production, national standard stipulates two benchmark systems.

Base hole system--refers to a system in which tolerance zone of a hole with a certain basic deviation and tolerance zone of a shaft with different basic deviations form a variety of coordination. See below. Hole made by base hole is called reference hole, its basic deviation code is H, and its lower deviation is zero.

 

① Basic hole system

Base shaft system-refers to a system in which tolerance zone of shaft with a certain basic deviation and the tolerance zone of the hole with different basic deviations form a variety of coordination. See below. Axis of basic axis system is called reference axis, its basic deviation code is h, and its upper deviation is zero.

 

② Base shaft system

Matching code is composed of tolerance zone code of hole and shaft, written in the form of a component number, numerator is tolerance zone code of hole, denominator is tolerance zone code of shaft. Anything that contains H in molecule is a basic pore system coordination, and any that contains h in denominator is a basic axis system coordination.
For example 1: φ25H7/g6 means that basic size of fit is φ25, clearance fit of base hole, tolerance zone of reference hole is H7, (basic deviation is H tolerance level is 7), tolerance zone of shaft is g6 (basic deviation is g, and tolerance level is 6).
For example 2: φ25N7/h6 means that basic size of fit is φ25, base shaft transition fit, tolerance zone of reference shaft is h6, (basic deviation is h, tolerance level is 6), and tolerance zone of hole is N7 (Basic deviation is N, and tolerance level is 7).

1) Mark tolerances and fits on assembly drawing, using combined notation method.
2) There are three types of marking methods on part drawing.

 

After parts are processed, there are not only dimensional errors, but also geometrical shape and mutual position errors. Even if size of cylinder is qualified, it may have one end large, the other end small, or middle thin and two ends thick. Cross section may not be round, which is a shape error. For stepped shafts, there may be different axes of each shaft segment after machining, which is a position error. Therefore, shape tolerance refers to allowable variation of actual shape from ideal shape. Position tolerance refers to allowable variation of actual position from ideal position. Both are referred to as form and position tolerance.

 

Geometric tolerance bullet

National standard GB/T 1182-1996 stipulates that shape and position tolerances shall be marked with codes. In actual production, when geometric tolerance cannot be marked with a code, it is allowed to use text in technical requirements.
Shape and position tolerance codes include: each item of shape and position tolerance symbols, shape and position tolerance frame, guide line, shape and position tolerance values, other related symbols, and reference code, etc. Height h of font in frame is same as size number in drawing.

 

For a valve stem, text added near geometric tolerance marked in figure is repeated for reader's explanation, and there is no need to repeat annotation in actual drawing.