The blank (workpiece) in the form of a cast is economically justified in the case of series production (from 5,000 pieces). This article presents the basics of casting design in the context of further technological operations using mainly machining. The design of gray cast iron castings in sand molded by machine or by hand is presented below.

The design of the workpiece in the context of further technological operations requires identifying which surfaces will be machined in subsequent operations and which will remain in the raw casting state. It is necessary to choose the tolerance class of the casting and machining allowances. In the case of a structural drawing of the casting, it is not necessary to determine oversizes, but for teaching reasons it is worth doing. Below is an example of a blank drawing with marked allowances (additionally highlighted in red) – figure 1 [4].

In the intersection, the machining allowances are a dashed area with double density relative to the areas of objects that do not constitute machining allowances. According to the old recommendations, the surface in the case of the top surface of the object (at the dividing surface) should have a wider allowance (1.5x). Such a recommendation resulted from the fact that the upper parts of the lighter impurities accumulated during the casting process. The increased overspray prevented the occurrence of undesired impurities in the volume of the object as the final product. Modern recommendations do not include such tips.

The choice of the pitch plane (surface) depends on the complexity of the shape. Fig. 1 and 2 show examples of pitch planes on the upper surfaces of the casting. The cast is completely placed in the bottom of the mold. However, it should be remembered that this will not always be the case. It is enough that the blank has two flanges, and then the dividing plane contains the main axis of the object. Each project is considered individually.

Casting tolerance class

The currently applicable technical standard is PN-ISO 8062: February 1997, which allows you to choose machining allowances and tolerances for the dimensions of the cast. Currently, no standard is mandatory, with the exception of health and safety standards and other safety aspects. Figure 1 is the tolerance class CT11 and the level of machining allowances F. PN-ISO 8062: February 1997 in the case of casting in machine-shaped and shell-shaped sand molds, for gray iron, ductile iron and malleable cast iron, tolerance class CT 8÷12 (for large-scale production).

In the case of production of short series and in the case of piece production, casting in sand molded by hand is used. Then, the tolerance classes for gray, ductile and ductile cast iron as well as other casting materials are CT13 ÷ 15 for a clay adhesive. For the chemical binder in the case of gray, spheroidal and malleable cast iron, the type CT11 ÷ 13 is assumed (table 1).

Table.1 Based on a technical standard PN-ISO 8062: February 1997
Basic dimension of the rough casting		Tolerances range of the cast [mm]
		Grade of tolerance of the cast CT
above	to and inclusive	8	9	10	11	12	13
10	16	1,1	1,6	2,2	3	4,4	–
16	25	1,2	1,7	2,4	3,2	4,6	6,0
25	40	1,3	1,8	2,6	3,6	5,0	7,0
40	63	1,4	2	2,8	4,0	5,6	8,0
63	100	1,6	2,2	3,2	4,4	6,0	9,0
100	160	1,8	2,5	3,6	5,0	7,0	10,0
160	250	2	2,8	4,0	5,6	8,0	11,0
250	400	2,2	3,2	4,4	6,2	9,0	12,0

 

Specific tolerance values are selected from tabular data based on a ranked primary dimension – for each dimension separately. The tolerance field is the sum of absolute values – modules – upper and lower deviations. The accepted deviations, usually symmetrical, are to be within the accepted tolerance.

Figure 2 shows the blank (wokrpiece) design in the form of a cast part of the sleeve class in which the tolerance class CT12 was adopted and the machining allowance grade (level) G.

Machining allowances

The degree of machining allowances is determined for a given cast material and a casting method. In the case of gray, spheroidal and ductile cast iron for casting in sand and shell molded molds (large-scale production), the degree of machining allowance E-G is assumed. In the case of small lot production, unitary (casting in sand molded by hand), the degree of finishing allowance F-H is recommended (table 2).

The value of the allowance is selected based on the largest dimension of the external casting associated with the object for a given class of machining allowances. When specifying the dimension of the allowance on the diameter, the value of the allowance selected from the table is on the page, not on the diameter.

Table 2. Based on a technical standard PN-ISO 8062: February 1997
largest dimension [mm]		Machining allowance for machining (turning, milling, drilling etc.) [mm]
		Machining allowance ratio
above	to and inclusive	F	G	H
40	63	0,5	0,7	1,0
63	100	1,0	1,4	2,0
100	160	1,5	2,2	3,0
160	250	2,0	2,8	4,0
250	400	2,5	3,5	5,0
400	630	3,0	4,0	6,0

 

After specifying machining allowances and tolerances, it is worth to verify the matched machining allowances and accepted deviations of the cast geometric dimensions (tolerances), or if the minimum external dimension does not reach a state where the deviation “enters” into the material of the object, which is no longer a machining allowance.

As a rule, casting for making blanks (workpieces) is used for several thousand pieces when the costs related to starting such production are at the level of economic profitability. Sometimes, however, in the case of short series and unit orders, it is not possible to change the material and make the item from other materials. A cast is required. Therefore, the PN-ISO 8062: 1997 technical standard provides for such situations. However, the economic justification for such choices on an industrial scale is not recommended.

The design from figure 2 was developed for the sleeve design from figure 3 [4]:

Data on the design of workpieces (blanks) in the form of castings can also be found in [1] in the new editions. The technological processes can also be developed on the basis of the general technological processes for parts of the bushing and disk class contained in [1].

Sources

1. Feld M., Podstawy projektowania procesów technologicznych typowych części maszyn, WNT 2000
2. Kapiński S., Skawiński P., Sobieszczański, Sobolewski J.Z., Projektowanie technologii maszyn, OWPW 2002
3. Technical standard PN-ISO 8062:luty 1997
4. Rudaś T., Horczyczak M., Morek R., technical academic knowledge – Warsaw University of Technology