Size Tolerance, Geometric Tolerance, Surface Roughness, Metal Casting and Defects

Size Tolerance, Geometric Tolerance, Surface Roughness, Metal Casting and Defects

1. Size tolerance - Size tolerance regulates the size of each dimension. – Limits specifying the allowed variation in each dimension (length, width, height, diameter, etc.) are given on the drawing.

• Limit Tolerances – (12.75/12.25 )
• Plus/Minus Tolerances

– Unilateral Tolerances - (12.00 + or - xxx)
– Bilateral Tolerances - (12.00 +xxx/- xxx)

2. Geometric tolerance - Geometric tolerance regulates the shape, parallelism, tilting, position, run-out, and other factors. 
• Allows for specification of tolerance for the geometry of a part separate from its size 
• GDT (Geometric Dimensioning and Tolerancing) uses special symbols to control different geometric features of a part.

3. Surface Roughness - it is defined as the irregularities which are inherent in the production process (e.g. cutting tool or abrasive grit). Roughness It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth.

Characteristics of Metal Casting and the defects easily to form in the casting process

Casting is an operation of shaping metal by pouring it in the liquid state into a mold followed by solidification.

Metal castings quality is determined by the casting properties of the metal:

(i). Fluidity – it is an ability of metals and alloys to flow through the gating system filling the cavity of the casting mold and conforming its shape.

(ii). Viscosity of the alloy (the lower viscosity of an alloy, the better its fluidity). As viscosity decreases with temperature, metals and alloys cast at increased temperature fill better the mold cavity.

(iii). Surface tension of the alloy. Low surface tension causes better filling of the mold cavity.

(iv). Shrinkage - Shrinkage is a contraction of alloy volume caused by:
> Contraction of the melt when it cools down to the liquidus temperature.
> Contraction of the alloy owing its solidification (cooling from liquidus temperature to solidus temperature).

(v). Surface quality- Surface quality is determined by:
> the surface quality of the mold walls,
> the mold coating material properties,
> gassing of the mold or coating materials,
> chemical reaction between the mold material and the alloy,
> casting pressure.
> Resistance to cracks formation- Casting cracks are result of internal stresses caused by a non-uniform cooling of the casting. Alloys having low ductility are the most susceptible to cracks formation.
> Hot cracks form during solidification of the alloy within the solidification interval.
> Cold cracks form in completely solidified casting during its consequent cooling.

Casting Defects - The unwanted irregularities that appear in the casting during metal casting process.

Types of Casting defects

1. Shift or Mismatch - The defect caused due to misalignment of upper and lower part of the casting and misplacement of the core at parting line.

2. Swell - It is the enlargement of the mold cavity because of the molten metal pressure, which results in localised or overall enlargement of the casting.

3. Blowholes - When gases entrapped on the surface of the casting due to solidifying metal, a rounded or oval cavity is formed called as blowholes. These defects are always present in the cope part of the mold.

4. Drop - Drop defect occurs when there is cracking on the upper surface of the sand and sand pieces fall into the molten metal.

5. Metal Penetration - These casting defects appear as an uneven and rough surface of the casting. When the size of sand grains is larges, the molten fuses into the sand and solidifies giving us metal penetration defect.

6. Pinholes - They are very small holes of about 2 mm in size which appears on the surface of the casting. This defect happens because of the dissolution of the hydrogen gases in the molten metal. When the molten metal is poured in the mold cavity and as it starts to solidify, the solubility of the hydrogen gas decreases and it starts escaping out the molten metal leaves behind small number of holes called as pinholes.

7. Shrinkage Cavity - The formation of cavity in the casting due to volumetric contraction is called as shrinkage cavity.

8. Cold Shut - It is a type of surface defects and a line on the surface can be seen. When the molten metal enters into the mold from two gates and when these two streams of molten metal meet at a junction with low temperatures than they do not fuse with each other and solidifies creating a cold shut (appear as line on the casting). It looks like a crack with round edge.

9. Misrun - When the molten metal solidifies before completely filling the mold cavity and leaves a space in the mold called as misrun.

10. Slag Inclusion - This defect is caused when the molten metal containing slag particles is poured in the mold cavity and it gets solidifies.

11. Hot Tears or Hot Cracks - when the metal is hot it is weak and the residual stress (tensile) in the material cause the casting fails as the molten metal cools down. The failure of casting in this case is looks like cracks and called as hot tears or hot cracking.

12. Hot Spot or Hard Spot - Hot spot defects occur when an area on the casting cools more rapidly than the surrounding materials. Hot spot are areas on the casting which is harder than the surrounding area. It is also called as hard spot.

13. Sand Holes - It is the holes created on the external surface or inside the casting. It occurs when loose sand washes into the mold cavity and fuses into the interior of the casting or rapid pouring of the molten metal.

14. Dirt - The embedding of particles of dust and sand in the casting surface, results in dirt defect.

15. Honeycombing or Sponginess - It is an external defect in which there is a number of small cavities in close proximity present in the metal casting.

16. Warpage - It is an accidental and unwanted deformation in the casting that happens during or after solidification. Due to this defect, the dimension of the final product changes.

17. Fins - A thin projection of metal, not considered as a part of casting is called as fins or fin. It is usually occurs at the parting of the mold or core section.

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