Dimensional tolerance of castings refers to the difference between two allowable limit dimensions of nominal dimensions of castings. Within two allowable limit dimensions, castings can meet the requirements of machining, assembly and use.
The fixture is a large-scale production of sand casting machine modeling. According to table 6-25 in the foundry engineer’s Manual:
The dimension tolerance of bearing is ct8-12, and ct9 is taken.
The boundary dimension of the support is 285mm * 140mm * 120mm. According to table 1-9 in the foundry process design, it can be concluded that:
The tolerance value of bearing dimension is 3.2mm.
1. Casting shrinkage
Casting shrinkage is also called casting linear shrinkage, which is expressed by the difference between the length of pattern and casting divided by the percentage of pattern length
ε=[（L1-L2）/L1]*100％
ε – casting shrinkage
L1 – pattern length
L2 – casting length
According to table 6-24 in the foundry engineer’s manual, the blocked shrinkage rate of bearing can be obtained
Blocked shrinkage 1.5%
2. Formwork lifting angle
In order to facilitate the mold lifting, a certain slope shall be reserved in the direction of the pattern and core box to avoid damaging the sand mold or core. This slope is called the starting slope. The lift angle shall be applied on the surface perpendicular to the parting surface where there is no structural angle on the casting.
The draft angle of the preliminary design is as follows:
According to table 6-39 in the foundry engineer’s manual, we can see that the height of surface a of the external mold (as shown in the figure) is 52mm
The inclination of the external surface of clay sand molding is a = 0 ° 30 ‘, a = 1.0mm
According to 6-39 of the foundry engineer’s manual, the lift angle of the B-side of the external mold (as shown in the figure) with a height of 100mm can be found as follows:
The inclination of the external surface of clay sand molding is a = 0 ° 25 ‘, a = 1.2mm
However, for the same casting, the same die inclination shall be selected as far as possible to avoid frequent tool replacement during metal die processing. So the inclination of the same die is a = 0 ° 30 ‘, a = 1.0mm
3. Minimum cast holes and grooves
Whether the holes, grooves, steps, etc. on the parts are cast or machined should be considered from the quality and economic aspects. Generally speaking, large holes, grooves, etc. should be cast out to save metal and processing time, and at the same time, the hot spot caused by local over thickness of castings can be avoided to improve the quality of castings. Smaller holes, grooves or thick casting walls are not easy to cast holes, but it is convenient to directly rely on processing.
According to the contour size of the fixture 285mm * 140mm * 120mm, it can be found from table 6-45 in the foundry engineer’s manual that the minimum cast hole of the steel casting is about 60mm in diameter.
The diameter of the large hole is 65mm and the wall thickness is 24mm considering the machining allowance. If the hole has a large diameter and a small height diameter ratio, it should be cast.
The diameter of the small hole Φ 30 is 24mm considering the machining allowance, which is smaller than the requirement of the minimum cast hole 60mm, and the wall thickness is 10 mm. The diameter of the hole is small, the height diameter ratio is large, it should not be cast out, and the machining is more economical and convenient.
Casting weight tolerance
Casting weight tolerance is the allowable range of casting weight change expressed as a percentage of the nominal weight of the casting.
The nominal weight of the bearing is about 11kg and the dimensional tolerance is grade MT9.
According to table 8-4 of the foundry engineer’s manual, the weight tolerance of the bearing is MT9, and the weight tolerance value is 10% according to table 8-9 of the manual.
4. Negative number of typing
For dry sand mold, surface drying mold and large size wet sand mold, the parting surface is generally not smooth due to baking, finishing and other reasons, and the upper and lower contact surfaces are not strict. In order to prevent fire during pouring, asbestos rope and mud strip should be padded between the parting surfaces before closing the box, so that the size of the casting is increased obviously at the parting surface. In order to ensure the accuracy of casting size, it is called negative parting number to subtract the corresponding size from the pattern in order to offset the increased size of the casting in the proposed process. The negative number of parting is not considered because the support is of wet type and small casting.
5. Reverse deformation
Because of the nonuniformity of cooling speed, the castings often deform after cooling. In order to solve the problem of flexure deformation, the reverse deformation pattern is made according to the opposite direction of the possible deformation of the casting during the manufacturing of the pattern, so that the amount of pre deformation made on the pattern is called the reverse deformation. However, there is no large flat plate in the support, so there is no need to set the reverse deformation.
6. Negative allowance of non machining wall thickness
In the process of manual clay sand molding and core making, in order to take out the wood mold, it is necessary to knock the mold, and the wood mold will expand when it is damped. All these conditions will enlarge the cavity size, thus increasing the non processed wall thickness, and making the size and weight of the casting exceed the tolerance requirements. In order to ensure the accuracy of the casting size, the thickness of the rib plate in the wood mold or core box that forms the non machined wall thickness should be reduced, that is, smaller than the drawing size. The reduced thickness dimension is called the negative allowance of the non machined wall thickness. The bearing sand core belongs to machine core making, and the molding belongs to machine modeling. Therefore, it is not necessary to set negative allowance for non machining wall thickness.