Pattern allowances
Casting Technology • Engineering Study Notes
PATTERN ALLOWANCES – NOTES
Important allowances provided on a pattern to obtain the required final casting dimensions, shape and surface finish.
Key Idea:Pattern is always made with some increments and decrements
in dimension compared to the final casting.
Pattern allowances are as follows.
1Shrinkage Allowances:
The shrinkage allowance is provided to take care of the contraction of casting. When the casting gets solidified, it shrinks or contracts in size. To compensate this dimensional shrinkage problem we make a pattern larger in size than the original casting to be produced.
2Draft or taper allowance:
A draft or taper is provided on all vertical surfaces of the pattern to allow the removal of the pattern without damaging the sides of the mould.
3Machining allowances:
After the solidification is over the casting has a poor surface. So it needs a machining process to get a good surface finish. So to ensure this machining allowance is given somewhat on the pattern size. This is known as machining allowances.
4Distortion allowance:
Sometimes casting gets distorted during solidification due to internal stresses and uneven cooling. To overcome this dimensional distortion some changes provided on pattern size. This is known as distortion or camber allowances.
SUMMARY TABLE
| Allowance Type | Purpose | Effect on Pattern Size | Applied On |
|---|---|---|---|
| ● Shrinkage Allowances | Compensate shrinkage of casting during solidification | Increases pattern size | All dimensions |
| ● Draft or taper allowance | Allows removal of pattern without damaging mould | Provides draft angle on vertical faces | Vertical surfaces |
| ● Machining allowances | Extra material for machining to get good surface finish | Increases pattern size | All machined surfaces |
| ● Distortion allowance | Compensate distortion due to internal stresses and uneven cooling | Cambered or distorted pattern | As required by shape |
☑ Remember
- All allowances are important for producing accurate and defect-free castings.
- Proper consideration of allowances reduces machining cost and improves quality.
- Always specify allowance values based on material, casting size, and complexity.