Academic experience led to a return to the topic of developing a blank design as a casting. The selection of the casting mold division plane and the orientation of the casting, determination of the grade of allowance and accuracy class as well as the selection of foundry slopes are the main tasks of developing a cast design as a workpiece. In the following, I used the same example of a sleeve class as in an earlier related article: Workpiece as a casting in series production – Figures 1 and 2. According to the order, the number of pieces to be manufactured is 5000 pieces. We are therefore dealing with serial production. In the case of serial production (contractual, economic limit from 1000 pcs. up), sand casting is applied to machine and shell-molded sand molds. In piece (unit) and small-lot production from a dozen to several hundred pieces, casting for hand-molded sand molds is used.
In justified situations, the cast is also made for individual pieces when it results from constructional and technological requirements. Examples include the reconstruction of historic vehicles and machines. Another justified case is the need to obtain components with functional properties specific to cast parts.
The plane of division of the mold and casting
The choice of the mold division plane affects the distribution of casting slopes, which is important when determining and fixing the blank in the machining chuck. Depending on the orientation of the casting, a gating system is designed in the mold. There are 10 rules for selecting the division plane:
- ease of removing the casting from the mold;
- it should be flat, we avoid a broken surface;
- the height of the casting to be removed should be as small as possible;
- it is recommended to use natural inclinations of the casting, which significantly facilitates removing the model (pattern) from the mold;
- the casting should be oriented in the form so that its surfaces with higher accuracy of geometrical dimensions are all in one part of the molding box;
- ease of casting cleaning;
- cast surfaces to be subjected to further machining should be located on the top or side;
- the orientation of the casting in the mold should allow feeding the solidifying casting with metal from the risers;
- placing the casting in the mold should ensure effective filling of the mold cavity;
- the minimum number of cores should be used.
In the considered project I adopted the orientation of the blank casting with an axis perpendicular to the dividing plane (Fig. 3.b) – variant II. Figures 3.a and 7 show a different orientation of the casting, but variant I does not meet the above requirements as well.
Variant II (fig. 3.b) of the dividing plane (fig. 6) and the orientation of the casting in the mold have the following properties:
- Easy to take out of the mold.
- The dividing surface is flat.
- The height of the casting is smaller.
- The entire casting is in one lower part of the casting mold.
- Machining surfaces are located on the top and sides.
For the above reasons I chose option I (fig. 3.a and 6). The technological difference between figures 3 and 6 is that in the design under consideration the entire casting is in one, lower part of the casting mold.
Selection of machining allowance
Before determining the value of machining allowance for a given project, the grade of machining allowance must be determined in the first step. The following factors determine the grade of machining allowance:
- type of casting material;
- casting method.
Table 2 presents the ranges of allowance grades for selected materials and machining methods.
|Table 1. Based on technical standards PN-ISO 8062: February 1997|
|Method / material:||cast iron
|Sand molding hand-formed.||F÷H|
|Casting into molded sand molds
machine and shell.
In the case of the discussed workpiece (blank) casting project, when casting into machine-made sand molds for gray cast iron, the grade of allowance is in the range E ÷ G. I chose the F grade arbitrarily. The next table 2 shows the values of machining allowances for the selected grades and dimensional ranges. The value of the allowance is selected one for the entire casting on the basis of the largest external dimension of the part from the construction drawing of the part for which this project is being developed. When determining the dimension of the allowance on the diameter, its value selected from the table is based on side, not the diameter. So, for the outer diameter, the overall dimension increases by double the value of the allowance, and for the hole the diameter also decreases by double the value of the selected allowance. Remember that machining allowances should be treated as minimum values.
|Table 2. Based on a technical standard PN-ISO 8062: February 1997|
for machining [mm]
|Grade of machining allowance|
The largest construction dimension of the sleeve in our project is Ø174, so for the selected grade of allowance F the value of machining allowance is 2 mm.
Selection of casting radiuses
Table 3 shows the selected range of cast radius values based on technical standard PN-H-54215:1999. The values of casting radius depend on the sum of the thickness of the contacting walls of the casting and the pouring temperature. In our case, I determined that the pouring takes place at a temperature below 1300 ° C.
|Sum of thicknesses
of touching walls
|<1300 °C||>= 1300 °C|
|Angle between touching walls|
The sum of the walls (together with casting allowances and draft angles) is ≈50 mm, the angle between the walls is 90°, the pouring temperature is assumed to be <1300° C, so the casting radius is R5. In the case of the other end of the sleeve where there is no transition between the walls at a certain angle, the thickness of one wall of the casting is taken into account. In the case of our design, this wall thickness together with machining allowance and draft angles is ≈23 mm, i.e. the recommended minimum radius value is R2.5. I decided that for a sand form it is a small radius and I finally chose the radius R4.
Casting – CT tolerance class selection
The determination of the tolerance class for the casting depends on:
- type of production (unit and low-volume, high-volume);
- material from which the casting is made;
- casting method.
In the case of large-scale production, casting into machine-formed and shell-formed sand molds is used. Table 4 presents the recommended ranges of CT accuracy classes for various casting materials and methods. In the case of piece production and casting into manually formed sand molds, the type of binder is important.
|Tabela 4. Based on a technical standard PN-ISO 8062: February 1997|
|Method / material:||Type
|Piece and low-volume production|
|Sand molding hand-formed.||loamy||13÷15||13÷15||13÷15||11÷13||13÷15|
|Mass (serial) production|
|Casting into molded sand molds
machine and shell.
|na. – not applicable;|
In the case of the discussed blank casting project for mass production, casting into machine-formed sand molds for gray cast iron, the range of accuracy classes is CT8 ÷ 12. Finally, I chose CT9. Table 5 shows selected tolerances for geometric dimensions. The tolerance field is selected separately for each casting dimension.
|Tabela 5. Based on a technical standard PN-ISO 8062: February 1997|
The basic dimension
|Values of the tolerance field for the casting [mm]|
|CT casting tolerance class|
After determining the machining allowances and tolerances, it is worth verifying the specified allowance and the accepted deviations within the tolerance of the geometric dimensions of the casting, whether the minimum external dimension does not reach the state in which the deviation “enters” into the material of the workpiece, which is no longer a machining allowance.
Casting inclinations (draft angles) are necessary to enable the free removal of the model (pattern) from the mold at the stage of its production. I used the provisions of the industry standard BN-76 / 4042-19, in which we distinguish three types of foundry slopes (Figure 4). Inclination type I seems to be the safest. Type I increases the dimensions of the casting.
However, if we are dealing with mass production, the use of other types of foundry slopes can be considered. Type II and III slopes allow savings on the material needed to manufacture the blank. Type II simultaneously enlarges and reduces the dimensions of the casting, while Type III only reduces them.
From the machining point of view, type I is the best.
The basis for determining the dimension n (figure 4) is the height or depth of the casting above or below the mold dividing plane (dimension H – figure 5). Table 6 contains selected n values for determining the draft angle.
Figure 5 shows a selected cut-out of the considered casting design as a blank. Dimensions H1 and H2 indicate n1 = 0.7 and n2 = 0.9 respectively. In the case of size n2 I placed it on the degree of change in the sleeve diameter. The goal was to get one tilt. I decided that the actual amount of allowance would not significantly increase the casting cost. For larger diameter differences, also grading in casting diameters should be used. Please note that the dimension associated with the foundry inclination for its clear understanding is preceded by the letter “p”.
Direct and indirect measurement
The use of the old industry standard is not mandatory and various types of documented principles can be applied, including internal company standards. In many cast drawings, the draft angles are defined by the angular dimension in [°]. With such dimensioning, the tilt angle is verified by measuring the angle using e.g. a universal protractor. This measurement is a direct measurement. In a situation where the rules from the industry standard BN-76 / 4042-19 have been applied, verification of the correctness of the casting slope is indirect and requires two measurements of two diameters for one draft angle.
|Variant of draft angles|
|Type of casting surface|
|Value of the dimension n|
Casting project as a blank
The final casting design, due to full symmetry, requires no additional projection except the main view. The construction drawing of the casting gives dimensions for draft angles and machining allowances. The casting project presented below in figure 6 is an educational project in the context of developing a technological process using defective manufacturing techniques. The real full casting design must also include the gating system. Information about allowances is not needed.
Figure 7 shows the design of the blank casting for the variant I of the division plane selection (figure 3.a). The dimension Ø100±1.1 in figure 7 includes machining allowance per side >2 mm due to alignment with the diameter of the rough (raw) surface. The cast as a blank is subject to the same economic production principles as the technological process at the end of which we obtain the final product. The increase in allowance is very small and has no significant effect on increasing the unit cost of the casting.
- Feld M., Podstawy projektowania procesów technologicznych typowych części maszyn, WNT
- Kapiński S., Skawiński P., Sobieszczański, Sobolewski J.Z., Projektowanie technologii maszyn, OWPW 2002
- Technical standard PN-ISO 8062:luty 1997
- Industry technical standard BN-76/4042-19 (Poland)
- Rudaś T., Horczyczak M., Morek R., Soroczyński A., Kochański A. – knowledge and teaching materials – ITW WIP PW