Designing Castings to Include Machining Allowances

What are machining allowances and why do we need them in the casting process?

Some features and faces of your casting may need machining after casting, to achieve tighter flatness tolerance than can be achieved in a raw casting, or to remove taper, gate witnesses, or rough cast faces.

To enable your finished machined dimensions to be achieved consistently, you need to make sure that there is sufficient cast material on those faces to then be removed by the machining operation.

By making sure that your machining allowances are precise, the final application of your casting will benefit from finely tuned operational capability.

How are the machining allowances of castings calculated?

When deciding on the dimensional relationship between different areas on a casting, you will first need to consider the die and mould construction. To get this right, it’s essential for the machinist and die designer to work together.

There is also an opportunity here to reduce the cost of finished castings, by arranging for casting feeders to be located on areas which are to be machined; this minimises the need for fettling and eliminates feeder witnesses on the finished component.

The amount of material to be added is related to the overall size of the casting, rather than varying with individual dimensions. We’ve created this table to help:

Minimum Machine Finish

Designers should select the line on the table showing the range within which the casting’s greatest dimension falls.

The values given in the above table are based on maximum ‘build-up’ or a ‘safety factor’ for average casting variations in flatness, squareness, concentricity, etc, as well as linear tolerance.

The values are the minimum recommended. Taper, when required, must be added to the above values.

Casting Processes CTA


Ready to find out more?

Our ‘Notes for Designers’ guide goes into even more detail – Covering linear tolerances, flatness tolerances, minimum achievable section thickness, machine finish allowances and more.

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