Pattern

A pattern is a mould-forming tool used in foundries. It is a model or replica of the object to be cast. Except for various allowances, a pattern closely resembles the final casting.

A pattern can be defined as a model or form around which sand is packed to create a mould cavity. When molten metal is poured into this cavity and allowed to solidify, it results in the final cast object.

Functions of a Pattern:

1. It creates a mould cavity for casting.
2. If the casting requires a core to be hollow, the pattern includes core prints to support the core inside the mould.
3. Runners, gates, and risers, which help introduce and feed molten metal to the mould cavity, may be part of the pattern.
4. It helps establish locating points on the mould to ensure proper casting dimensions.
5. It determines the parting line and parting surfaces in the mould.
6. A well-made pattern with a smooth finish reduces casting defects.
7. Properly constructed patterns help in minimizing the overall cost of the casting process.

Pattern Making Machines:

Besides the hand tools, a modern pattern makers shop needs some power-driven machines also. These machines help the pattern maker in • Increasing production. • Improving accuracy and maintaining consistency in the patterns. • Performing various operations more conveniently.

Tools Used in Pattern Making:

Different tools are used in pattern making based on their function.

(a) Measuring, Making and Layout Tools
(i) Steel rule (ii) Shrinkage rule
(ii) Caliper (iv) Divider
(v) Marking gauge (vi) Trammels
(vii) Try square (viii) T-bevel
(ix) Combination square.

(b) Tools for clamping purposes:
(i) Hand vice (ii) Pattern maker’s vice
(iii) Bar clamp (iv) C-clamp
(v) Hand screw (vi) Pinch dog (to hold wooden pieces together for joining etc.).

(c) Sawing Tools:
(i) Coping saw (ii) Bow saw
(iii) Compass saw (iv) Rip saw
(v) Crosscut saw (vi) panel saw
(vii) Back saw (viii) Dovetail saw
(ix) Miter saw with miter box.

Allowances:

A pattern is always made slightly larger than the final casting to account for various mechanical and metallurgical factors. These allowances ensure that the final casting has the required shape and dimensions. The different types of pattern allowances are:

1. Shrinkage or Contraction Allowance: Compensates for the shrinkage of metal as it cools.
2. Machining or Finishing Allowance: Extra material is added to allow for machining and finishing.
3. Draft or Taper Allowance: Ensures easy removal of the pattern from the mould without damaging it.
4. Distortion or Camber Allowance: Given to counteract possible warping or distortion of the casting during cooling.
5. Shake or Rapping Allowance: Accounts for the slight movement of the pattern while removing it from the mould.