dimensional tolerance

To assign dimensions and tolerances to parts produced by castings, consideration should be given to functional requirements of finished parts and allowances for production requirements such as machining tolerances and drafts that may be included when producing finished parts.

The acceptance values for main steel products and the main tolerance considerations for the production of parts as main steel products are as follows, and a series of allowable error ratings have been introduced to facilitate exchange of physicians for tolerances with this information.

subtraction gradient (Draft, Taper Allowance)

Definition: the angle at which the mould wall should be allowed to be removed from all vertical planes of the pattern without damaging it.

The angle of subtraction depends on the size of the casting, the method of production, and whether the molding is performed by hand or machine.

Minimal tolerances are required for machine molding

The inner surface of a manual sand form usually requires more tolerances than the outer surface.

The recommended angle under normal conditions is approximately 3/16 in (approximately 1.5°) per ft.

The subtraction angle is one of the important issues to be discussed with the casting technician who will produce the casting.

processing margin

Definition: the angle at which the mould wall should be allowed to be removed from all vertical planes of the pattern without damaging it.

Generally, the machining margin depends on the size and shape of the casting, the surface to be machined, the hardness of the steel, the roughness of the casting surface, the tendency of twisting and the type of tool used for processing.

Standard finishing tolerances for steel casting are different for each casting, making it difficult to formulate.

Unable to generalize allowable values for one casting into another product.

Generally, the machining margin varies from 3/16 in to 3/4 in, depending on the variables listed above, and the machining tolerances must be determined after discussion with the casting technician to supply the casting.

Variables Affecting Dimension Tolerance

Dimension tolerance is the allowable deviation of parts manufactured from dimensions specified in specific areas, and the size of tolerable tolerances varies depending on the factors below

• 1)
  how many castings are produced in a single neck/gold type or in large quantities
• 2)
  Design and Dimension Types of Casting
• 3)
  Top and bottom neck/ molds, patterns used in heavy-duty production, and core box equipment life
• 4)
  Processes used in the production of upper and lower neck/ molds and heavy porcelain
• 5)
  Cast Weight and Dimension Length
• 6)
  whether the casting is dimensioned and dimensioned as part of the right-cutting and inspection work (whether it is more strictly or more leisurely).

Casting Design and Core

The design of castings, the complexity of the cast geometry by many Chinese, is another consideration for determining tolerances that can be met.

The accuracy and precision of the medium dimensions may depend on the size, dimensions, shape, and ability to make strong heavy materials, such as bending, deflection, and breaking due to pressure from surrounding metals.

Problems with Parting Lines

For cast neck/ molds, how to set up a parting line, such as top·lower middle, can affect the finished casting in many areas, such as machining margins, subtraction gradients, casting tolerances, etc.

A better solution to these issues can be achieved when consulting with a casting expert.

Neck/Mold and Middle Neck/Mold (pattern)

The conformity of casting dimensions and design dimensions is of paramount importance because, in the manufacture of castings, the type of pattern used is significantly different and must be made into a dimension that takes into account the shrinkage of the casting within the mold made of patterns (neck/ molds).

In addition, the tolerances of castings produced by different foundries in the same pattern are usually not the same.

Foundries selected to make certain castings are encouraged to consult patterns to ensure that foundry experience does not cause errors and rework problems with regard to the shrinkage of castings.

The life of a pattern is another consideration when choosing a pattern material.

In the production of large quantities of castings, the dimensional variation of the pattern due to wear is greater in the pattern of wood than in the pattern of metal. (Mold < Aluminum < Suzy 로 Wood foam Styrofoam

< 패턴 재료에 따른 주조 공차 변수와 가격의 상관관계 >

Loose wood pattern Pine pattern, mounted on cope and drag boards Hard wood pattern, mounted on cope and drag boards Plastic pattern, mounted on cope and drag boards Metal pattern, mounted on cope and drag boards Metal Matchplate

Greatest variation Least variation

Lowest Cost Highest Cost

Source: SFSA 2000 steel casting tolerance

Patterns and tolerances of the castings according to the material of the finished casting: Since each material has its own density/weight, shrinkage due to the solidification cooling of the metal should be considered. Large castings with heavy-handed materials may harden as the medium hardens, preventing the metal from shrinking, resulting in dimensional error and defect.

moulding molding process (construction)

Another important factor determining the tolerance range in many castings is the moulding process and production technology.

Mold molding equipment also affects dimensional tolerances. For example, high-pressure sand molding produces a more rigid mold than normal construction. Therefore, tighter tolerances are possible.

• General injection casting
• high-pressure die casting
• Shell Mould Process
• Lost Wax

< Dimension accuracy is higher as you go right >

Cast Weight and Dimension Length Tolerance

Casting weight has a very important effect on dimensional change.

Both the casting weight and the dimension length affect the process capability relative to the dimensional tolerance in a nonlinear manner, and the shrinkage may be incorrect depending on the density of the molten metal.

Flatness (straightness) error (Flatness Tolerance)

Hand molding 6 to 8

Sand cast machine molding and shell molding 5 to 7

Investment casting 4 to 6

Measurement and Dimension Upgrade

Permissible error in the dimensions of the casting surface is a matter of prior consultation between the manufacturer and the consumer.

However, in order to minimize dimensional errors, certain specified tolerances should be considered in conjunction with the production method.

The tolerances that are more stringent than the capacity of the production method need to be partially machined to pass the dimensions of the casting.

Cast tolerance after heat treatment

In the case of heat-treated castings, deformation may be caused by warping, deflection, crushing or complexity of shape due to hot heat treatment.

Heat treatment jigs suitable to product characteristics are required to solve the above problems, and precision dimensional deformation of machined parts is difficult to avoid when processing heat.