Pattern Allowances

A pattern is always made somewhat larger than the final job to be produced. This excess in dimensions is referred to as the pattern allowance. A number of allowances made on the pattern to ensure that the finished casting is dimensionally correct. The following allowances are usually provided in a pattern.

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Draft•Pattern draft is defined as the taper on vertical elements in a pattern which allows easy withdrawal of the pattern from the mold. •This draft is necessary in order that the pattern can be removed easily from the mold causing no damage to the sand. The amount of draft required will vary with the depth of the pattern. •pattern having no draft allowance when lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it.•Taper varies from about 1-3 degrees. The greater the taper, the easier it is to draw the pattern out of the sand

Amount of draft

The amount of draft depends upon the following factors : -Size and shape of patternDepth of mold cavityMethod used to withdraw patternPattern materialMold materialMolding procedure

ShrinkageThe pattern is intentionally made larger than the final] desired casting

dimensions to allow for solidification and cooling contraction of casting.

The total contraction of a casting takes place in three stages, and consists of:-

1. The contraction of the liquid from the pouring temperature to the freezing temperature,

2. The contraction associated with the change of phase from liquid to solid,

3. The contraction of the solid casting from the freezing temperature to the room temperature.

The total contraction is volumetric, but usually expressed linearly. This varies with each type of metal and the shape of the casting.

• The added dimensions are incorporated into the pattern by the pattern maker by using what is called shrink rulers. These rulers are made of steel and the shrinkage is compensated for by having been worked proportionately over its length. (shrink rule 12 inches long will be actually 12 + 3/16 inches long),

Pattern maker's contraction allowance• Cast iron 0.9-1.3%• Brass 1.3%• Gun-metal 1.0-1.6%• Phosphor bronze 1.0-1.6%• Aluminum alloys 1.3%• Zinc and zinc alloys 2.6%

Temperature as a function of time for the solidification of pure metals. Note that freezing takes place at a constant temperature. (b) Density

as a function of time.

Machining AllowanceThe extra material left on casting for any surface that will be machined called machining allowance.The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required.

Distortion or Camber AllowanceSome castings ,because of their shape and size may warp or distort during cooling. This is due uneven metal thickness. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look slightly inclined. To avoid this the shape of the pattern is slightly bent in the opposite direction.The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section of the casting and hindered contraction.

Measure taken to prevent the distortion in casting include: -

1. Modification of casting design

2. Providing sufficient machining allowance to cover the distortion affect

3. Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection)

Rapping and Shaking Allowances When pattern is removed from the mould , it is shaked from side to

side called rapping. The mould cavity may become large due to rapping. To avoid this

the pattern is made slightly smaller. This allowance given on the pattern is called rapping allowance.

In Small and medium size casting, this allowance can be neglected. But in large casting this allowance is considered by making the pattern slightly smaller than the casting.

Pattern Design considerationsA good pattern may produce a sound casting but a bad pattern will

always result in poor castings. Following factors should be considered while designing a pattern:

Pattern should be accurate as regard to its dimensions andpossess very good surface finish.

Proper allowance should be provided in pattern. Sharp corners of pattern should be avoided. Core prints provided for the pattern should be of

optimum in size. Changes made in the shape should be gradual, uniform

and smooth. Proper material for making the pattern should be selected. The wall thickness and section should be kept uniform. Pattern made for expected repeat orders should be

preserved and stored properly.

Color code for patterns

Pattern makers use a color code so that it is clear to the metal caster which surfaces are which. This is as follows:

cast' surfaces which are to be left unmachined - red or orange Surfaces which are to be machined - yellow Core prints for unmachined openings – black Core prints for machined openings - yellow stripes on black Seats for loose pieces and loose core prints - green.