At the beginning of the design, the structural shape of the parts needs to meet two major requirements: design requirements and process requirements. At the same time, the structural design of the parts should consider not only industrial aesthetics and styling, but also the possibility of craftsmanship. The common structures on parts are mostly obtained through casting (or forging) and mechanical processing, so they are called process structures. Understanding the common process structures on parts is the foundation for learning part drawings.
Casting process structure on parts
1. Casting rounded corners
In order to facilitate casting modeling, avoid sand dropping at the corner of the sand mold when starting from the sand mold, and to prevent casting defects such as cracks, loose structure, and shrinkage at the corner of the casting during pouring, the intersection of adjacent surfaces on the casting should be made into rounded corners. For compression molded parts, their rounded corners can ensure that the raw materials are fully filled into the mold and facilitate the removal of the parts from the mold.
The casting fillet radius is generally taken as 0.2-0.4 times the wall thickness, which can be found in relevant standards. The size of the fillet radius of the same casting should be as close or equal as possible.
2. Starting angle of the mold
When shaping, in order to facilitate the removal of the wooden mold from the sand mold, a certain slope is often designed along the mold lifting direction on the inner and outer walls of the casting, which is called the mold lifting slope (or casting slope). The size of the starting angle of the mold is usually 1:100-1:20. When expressed in angles, the handmade wooden mold is 1 ° -3 °, the metal mold is 1 ° -2 °, and the mechanical mold is 0.5 ° -1 °.
Due to the presence of casting rounded corners at the intersection of casting surfaces, the intersection lines on the surface become less obvious. In order to distinguish different surfaces when looking at the picture, the intersection lines in the picture still need to be drawn, which are usually referred to as transition lines. The drawing method of transition lines is basically the same as that of intersecting lines without rounded corners.
3. Casting wall thickness
To ensure the casting quality of castings and prevent the formation of tissue porosity and shrinkage due to uneven wall thickness and different cooling crystallization rates, as well as the formation of cracks in thin and thick areas, the wall thickness of castings should be uniform or gradually changed to avoid sudden changes in wall thickness and local hypertrophy. The difference in wall thickness should not be too large, so a transition slope can be set at the intersection of the two walls. Sometimes the wall thickness may not be indicated in the diagram, but rather in the technical requirements.
In order to facilitate mold making, shaping, sand cleaning, removal of sprues and risers, and mechanical processing, the shape of castings should be simplified as much as possible, and the outer shape should be as straight as possible. The inner wall should reduce concave convex structures. Excessive thickness of castings can lead to casting defects such as cracks and shrinkage, but excessive thickness also results in insufficient strength of the castings. To avoid the impact of thickness reduction on strength, reinforcement ribs can be used to compensate.
