1. The through-hole and threaded hole of two connected parts do not match;
During assembly, it is required to expand the threaded hole and replace the screw.
During assembly, secondary processing is required for reaming.
It is recommended to use threaded clearance holes for design
2. Ignore the parameters of purchased parts;
Note: M14 is indicated as ordinary coarse thread, and its pitch is 2.
Therefore, the thread does not match the purchased part during assembly.
This case eventually leads to the scrapping of the designed connector.
3. The designed parts do not have process fillets and process grooves, and cannot be processed by common processing methods.
Note: As the design in Fig. 1 does not consider the process fillet, its shape can only be processed by EDM or other special processing methods, but it cannot be processed by conventional processing methods.
If the shape shown in Fig. 2 is designed with process fillet, the countersunk surface can be milled by line, and then 10 milling cutters can be used to clean the corner, which is convenient for processing and cost saving!
4. When the holes and shafts are matched with each other, the tolerance is not taken into account, which leads to the failure of installation.
Note: When the design of parts involves the need to put the shaft into the hole, it includes the installation of bearing and bearing seat, and the installation of shaft and bearing bore.
If the diameter of the hole is the same as that of the shaft, tolerance fit shall be considered, except for interference fit and partial transition fit (rarely used).
The principle is that the upper deviation of the shaft diameter shall not be greater than the lower deviation of the hole diameter, otherwise the shaft will not fit into the hole.
5. The locating lip was not considered when designing the fitting of parts and welding parts, resulting in inconvenient processing operation.
Note: As shown in the figure above, the design without locating port is easy to weld the flange and steel pipe out of alignment (not concentric) during assembly, which is easy to cause new problems.
With the design of locating seam, the welding operator can easily and accurately position and weld by placing the steel pipe into the seam when assembling.
In the part design, it is also necessary to consider whether it is convenient for the assembly personnel to assemble, so it is often necessary to consider the design of positioning lip.
6. When designing parts, it is not considered whether conventional means can be processed.
7. There is no machining allowance when designing weldments.
Note: As shown in the above figure, a weldment needs to be designed.
The final machining height is required to be 100mm, and the following is the part drawing.
The total height of the weldment will be 100mm before splicing and welding according to the design in the left figure.
However, due to welding deformation and welding shrinkage, the total height of the weldment will be less than 100mm.
When the upper and lower flange surfaces are machined to the required plane requirements, the final total length will be far different from the original design requirements.
The right figure considers the machining allowance when designing the part drawing, and the total height of the parts after splicing is 104mm, so that the upper and lower flange faces have 2mm machining allowance respectively, which is enough to process them to the target size.
Newcomers tend to neglect the allowance when designing such weldments, which will eventually lead to the scrapping of parts.
8. The welding deformation and welding assembly error were not considered when designing the weldment.
The installation hole of the weldment was drilled on the part before welding.
After welding, it was found that the position of the hole was wrong, which led to the failure of installation.
Note: If the installation holes on the weldments as shown in the above figure are machined when designing the welding parts, the requirements for the relative position positioning of several section steels are very high during welding.
If the welding positioning accuracy cannot be reached, the corresponding positions of the installation holes on the two steel bars are prone to large errors, which ultimately lead to the inability to install.
In principle, the installation holes of welding parts should be processed after welding, which not only simplifies welding but also ensures the accuracy of empty position of parts.
9. Surface treatment was not considered when designing parts, such as Q235, 45 #, castings and other rusty materials.
Note: There are many commonly used materials that are easy to be corroded and oxidized in mechanical engineering materials.
When selecting such materials to process parts, attention should be paid to adding surface treatment in the technical requirements.
Such as: blackening, chrome plating, painting, plastic spraying, sand blasting, etc.
10. Unreasonable selection of parts design materials.
Note: There are many kinds of mechanical engineering materials, each of which has its own unique performance, such as welding performance, anti-corrosion performance, thermal conductivity, self-lubricating performance, heat treatment performance, etc., but newcomers are often prone to make some typical mistakes in the process of selecting materials:
(1) The welding parts of stainless steel weldments shall be 304（ ×）
Stainless steel 304L has good welding performance.
Under the unified normal use environment of the company, 304L is selected for all stainless steel materials that need welding, and 304 is selected for stainless steel parts that do not need welding.
(2) The following figure shows a part that can only be completed by special machining such as wire cutting or electric discharge, but the material is non-metallic (nylon 1010)（ ×）.
The vast majority of non-metallic materials are insulated, as is nylon 1010.
Its insulating characteristics decide that it cannot be used for wire cutting or EDM.
Generally, the characteristics of shock absorption and lubrication are considered when selecting non-metal materials.
Therefore, considering its special shape and processing difficulty, for example, brass H62 can be selected as the material with relatively similar characteristics.
(3) The following figure shows a part that can only be completed by special machining such as wire cutting or electric discharge, but the material is non-metallic (nylon 1010)（ ×）.
The vast majority of non-metallic materials are insulated, as is nylon 1010. Its insulating characteristics decide that it cannot be used for wire cutting or EDM.
The selection of non-metal is generally based on the characteristics of shock absorption and lubrication.
Therefore, considering its special shape, the material selection of brass H62 is relatively close.
11. Geometric tolerance is not considered.
Note: novices are easy to ignore their installation requirements and installation accuracy when designing parts, so form and position tolerances are not considered.
When newcomers do not know how to mark geometric tolerances, they should ask more senior brothers and experienced colleagues, first master the ability to judge whether the designed parts need to add geometric tolerances, and then gradually master the query manual method to master the accuracy of geometric tolerances.
12. There are few dimensions and diameters in drawings“ φ”.
Note: It is easy to leave out the size and diameter marks when drawing new people“ φ”
13. Unable to write technical requirements.
Note: When drawing, new people usually have no or only one sentence of “acute angle deburring” and rarely can actively add technical requirements such as modulation treatment, surface treatment, and processing precautions.
The technical requirements are for the drawing reader (the person who needs to see the drawing during the whole production process of the parts in the drawing).
From the perspective of the drawing user, it is necessary to try to explain the part of the drawing that cannot be easily understood, so as to help the drawing reader better understand the drawing and understand the original intention of the designer.
14. The paint is not painted according to the standard color number.
Note: During the process of surface anti-corrosion treatment (painting) required by the design, the technical requirements of the new person are as follows:
“Spraying white paint on the surface”, “spraying black paint on the surface”, “spraying blue paint on the surface”, etc.
It does not meet the requirements. In the industrial standard, each color has many derivative colors, so the color code on the standard color card needs to be added.
For example, “Spraying white paint on the surface: RAL9001”, so that the paint sprayers can match colors according to the originally designed color card.
15. The dimensions are not standardized, for example, the datum plane is confused with the datum axis.
16. The selection of dimensioning datum is not correct, and it is not based on the mounting surface, which causes problems in the final assembly.
For example, suppose that the two boards in the following figure need to be installed together by the mounting holes on their views and the four sides are required to be flush.
There are two groups of cases in the following figure:
Note: As shown in the above figure, if the dimension datum of two parts is selected as shown in the left figure, the overall hole position deviation may occur due to the free tolerance deviation of the overall dimension.
This will eventually lead to the inability to assemble the two parts.
When dimensioning, full consideration should be given to the selection of the benchmark of the marked size to reduce the final assembly problems caused by cumulative errors.
17. Process trench is not considered when designing parts
Note: Newcomers usually do not draw undercuts when designing parts. It is easy to ignore undercuts because they do not know enough about them.
In fact, it is very important.
For example, if there is no undercut designed at the root of internal and external threads, then the threaded connector matched with it cannot be screwed into contact with the end face of the root of the thread, which usually leads to the unavailability of the designed parts.
In addition, the undercut needs to be designed at the root of the shaft shoulder when machining high-precision mounting surfaces and stepped shafts.
The figure above explains the role of the undercut and enables the processing of props to the entire cylindrical surface.
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