Finish/ machining allowance –
Extra material provided which is to be machined or cleaned for good surface finish and dimensional accuracy. It depends on
type of casting material
dimensions
finishing required
complexity of surface details
Range – 2 to 20 mm
To reduce the machining allowance, the entire casting should be kept in the drag flask such that dimensional variation and other defects due to parting plane can be reduced.
Draft –
Vertical faces of the pattern are to be made tapered to reduce the chances of damage to the mould cavity. It varies with the complexity of the job. Inner details require more allowance than outer. This allowance is more for hand moulding than machine moulding.
Shake allowance –
This is a negative allowance. Applied to those dimensions which are parallel to parting plane.
Distortion allowance –
Metals just solidified are very weak, which may be distorted. This allowance is given to the weaker sections like long flat portion, U & V sections, complicated casing, thin & long sections connected to thick sections. This is a trial and error method.
(B) Core prints:-
Core prints are required for casting where coring is required.
(C) Elimination of fine details:-
Type of details to be eliminated depends on
Required accuracy
Capability of the chosen casting process
Moulding method employed
Extra material provided which is to be machined or cleaned for good surface finish and dimensional accuracy. It depends on
type of casting material
dimensions
finishing required
complexity of surface details
Range – 2 to 20 mm
To reduce the machining allowance, the entire casting should be kept in the drag flask such that dimensional variation and other defects due to parting plane can be reduced.
Draft –
Vertical faces of the pattern are to be made tapered to reduce the chances of damage to the mould cavity. It varies with the complexity of the job. Inner details require more allowance than outer. This allowance is more for hand moulding than machine moulding.
Shake allowance –
This is a negative allowance. Applied to those dimensions which are parallel to parting plane.
Distortion allowance –
Metals just solidified are very weak, which may be distorted. This allowance is given to the weaker sections like long flat portion, U & V sections, complicated casing, thin & long sections connected to thick sections. This is a trial and error method.
(B) Core prints:-
Core prints are required for casting where coring is required.
(C) Elimination of fine details:-
Type of details to be eliminated depends on
Required accuracy
Capability of the chosen casting process
Moulding method employed
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