During the design classes I noticed that the analysis of the construction drawing in terms of technology is a challenge. I consider this skill to be the key from the point of view of the technological preparation of production. The constructional drawing enables identification of functional and free surfaces. Contemporary design principles, including roughness are extensive and allow direct indication of the machining method as well as the width of the machining allowance.
I would like to thank vigilant Mr. Philip, who pointed out a very important aspect regarding construction and executive drawings. The principles of developing technical drawings seemingly only remain unchanged. This causes that structural and technological documentation that were created in different periods can use, among others, different surface roughness descriptions.
Functional surfaces are primarily surfaces treated as part of the technological process and acting as technological bases. In the case of the first technological operation, the workpiece is determined using the raw surface of the blank (casting, forging, bars of various cross-sections, the pipe). If there are rough surfaces (not machined), one of these surfaces should be used as the main technological contact base in the first technological operation.
Figure 1 shows an example of a construction drawing of a part of a bush (sleeve) class, which was developed on the basis of the invalid PN-58/M-04252 technical standard.
Indication of the type of material indicates in a way, directly and indirectly, the production technique that should be used to produce a blank. In the case of the sleeve in the illustration we have a specific steel. In the series production, the forging can be used, and in the unit production a rod with a properly selected diameter. We use castings in mass production for materials predisposed with their properties for this production technique, e.g. cast iron. However, in unit production, the use of cast iron is not recommended due to economic considerations. In such a situation, another material (steel) should be chosen in agreement with the client or constructor.
Analysis of the construction drawing is important at the stage of selection and development of the blank design. It is necessary to recognize the functional (essential) surfaces that will be machined. Depending on the type of blank (casting, forging, profile), machining allowances are selected.
In the first place, we pay attention to general information about surface roughness, which should be in the upper right corner of the construction drawing (figure 2) or near the drawing table.
Roughness marks enclosed in brackets are values that should be plotted on the actual construction drawing (fig. 3).
There are also surfaces on the construction drawing that do not have any assigned roughness marking. In this case, the roughness of such surfaces is determined by the roughness sign preceding the brackets in the upper right corner (figures 4.a and 4.b).
The symbol in figure 4.a means that the given surface retains its properties, including roughness from the previous production stage of a given production process. In the case of serial production of the sleeve in question, this will be the process of manufacturing the blank. For series production, it will be a cast or forging. For piece production, for example, a rod or a 3D print from metals.
The symbol in figure 4.b specifies that the surfaces in the construction drawing devoid of individually defined roughness are to be machined. The obtained roughness is to be in accordance with the symbol in figure 4.b.
In the case of the currently recommended PN-EN ISO 1302: 2004 standard, the roughness indication is slightly different, and the location of the general information (fig. 2) has been moved closer to the drawing table. In the PN-EN ISO 1302: 2004 standard, among others you can meet the following roughness marks (figure 5).
Figure 5 shows selected, exemplary roughness marks according to PE-EN 1302: 2004:
a.) the surface can not be machined or subjected to other treatments (laser texturing, 3D printing);
b.) the surface should not be machined, but previous operations should ensure a specific surface roughness;
c.) the surface should be processed and the roughness after machining should be Ra 5;
d.) the surface should be processed, grinded, and the roughness after machining should be Ra 5 – a strictly defined machining method is indicated;
Fig. 5.e.) presents an example of general information about the surface roughness of an object within a construction drawing as well as an executive drawing. If any surface in the drawing does not have an individual roughness marking assigned, the reference preceding the brackets applies. Roughness marks in brackets are the ones that can be found in the drawing itself. A general description of the roughness is placed near the drawing table.
Accuracy of geometric dimensions
Roughness and dimensional accuracy are correlated technologically. From the user’s point of view, there may be requirements for very low roughness (high smoothness) of a given surface, with no requirement for accuracy of machining. If the construction drawing is a dimension devoid of deviations (fig. 6) then in this case the accuracy class is IT14÷15. In the metal and machine industry, these classes are referred to as “workshop classes”. Large deviations mean that it is not considered to be accurate.
Let us note the dimensioning of the dimension itself without deviations. In the technological reality, each production technique allows to achieve certain roughness and geometric accuracy. Consequently, it is not possible to obtain a repeatable one and only one value of a given dimension. The value of the dimension devoid of deviations somehow emphasizes the low accuracy requirements. In the case of a part of the sleeve class, such dimensions often apply to surfaces that are not machined.
In the case of illustration 6, the Ø44 dimension concerns the technological groove, which provides us with the possibility of machining the Ø40H8 borehole. The groove provides us with conditions for the exit of the tool without contact with another surface. Without this groove, it would not be possible to machine the hole to the desired dimension with a defined roughness (finishing). In turn, the Ø46 dimension refers to a technological groove or technological undercut which has the same task, i.e. to enable machining of the external cylindrical surface to the dimension of Ø50h6. Here, too, the tool is provided with an exit and a departure from the workpiece.
The dimension, the value of which is supplemented with an alphanumeric description, eg H9 or g6, makes it possible to recognize functional surfaces that eventually cooperate with the surfaces of another part in the machine or device. In this way the type of fitting is determined. However, this does not mean high accuracy (narrow tolerance).
Transversal holes, splineways
Transversal holes (fig. 7) and various types of key grooves or splines affect, among others on whether it will be possible in the case of part of the class, the sleeve to make the main hole (in the axis of the object) ready (finally) in the first operation. All kinds of surface cuts (grooves, transverse holes) usually mean the introduction of a deburring operation or deburring. Deburring is an operation carried out both at the locksmith station using trimers, with the use of a power tool as well as on CNC machine tools.
The construction drawing is not only a graphic description of the structure (GPS – Geometrical Product Specification) but at the same time it is the most important source for the technologist responsible for the technological preparation of production.
- Own notes of the author.
- Consultations and knowledge Tadeusz Rudaś Ph.D. and Maciej Horczyczak Ph.D. – Warsaw University of Technology
- Reference books of Systemy i Technologie Mechaniczne Sp. z o.o.