At the beginning of the design, the structural shape of the part needs to meet two major requirements, one is the design requirements, and the other is the process requirements. At the same time, the structural design of parts should not only consider industrial aesthetics and modeling, but also consider the possibility of technology. Most of the common structures on parts are obtained by casting (or forging) and machining, so they are called process structures. Understanding the common process structure of parts is the basis for learning part drawings.
Casting process structure on parts
1. Cast fillet
In order to facilitate the casting modeling, avoid sand falling from the sand mold corner when pulling out the mold from the sand mold, and wash the corner when pouring, and prevent the casting defects such as cracks, organizational porosity and shrinkage cavity from occurring at the corner of the casting, so the intersection of adjacent surfaces on the casting shall be made into rounded corners. For compression molded parts, the fillets can ensure that the raw materials are filled with the die, and it is convenient to take the parts out of the die.
The casting fillet radius is generally 0.2-0.4 times of the wall thickness, which can be found in relevant standards. The fillet radius of the same casting shall be the same or close to each other.
2. Lifting angle
During molding, in order to take the wood mold out of the sand mold, a certain slope is often designed on the inner and outer walls of the casting along the mold lifting direction, which is called the mold lifting slope (or casting slope). The mold lifting angle is usually 1:100-1:20. When expressed by angle, the wood pattern for manual modeling is 1 ° - 3 °, the metal pattern is 1 ° - 2 °, and the metal pattern for mechanism modeling is 0.5 ° - 1 °.
Because there are casting fillets at the intersection of the casting surface, the intersection lines on the surface become less obvious. In order to distinguish different surfaces when looking at the drawing, the intersection lines in the drawing should still be drawn, which are usually called transition lines. The drawing method of transition line is basically the same as that of intersection line without fillet.
3. Casting wall thickness
In order to ensure the casting quality of the castings, prevent the shrinkage cavity caused by the loose structure outside the wall thickness due to the different cooling and crystallization rates due to the uneven wall thickness, and the cracks at the thin and thick phases, the wall thickness of the castings shall be uniform or gradually changed to avoid sudden changes in wall thickness and local hypertrophy. The difference of wall thickness should not be too large, so the transition slope can be set at the intersection of two walls. The wall thickness may not be indicated in the drawing, but shall be indicated in the technical requirements.
In order to facilitate mold making, molding, sand cleaning, removal of gating and riser and machining, the shape of castings shall be simplified as much as possible, the shape shall be straight as much as possible, and the concave convex structure shall be reduced on the inner wall. The casting with too thick thickness is easy to produce casting defects such as cracks and shrinkage cavities, but the casting with too thin thickness is not strong enough. In order to avoid the influence of thickness thinning on strength, reinforcing ribs can be used to compensate.