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What is Machining Tolerances?

Machining tolerances are meaning how many dimensional allowances the casting producers need to leave for the machining works. Please see the following diagram. On the surfaces after machining, the rough casting producers need to leave 2 to 10 mm machining allowance for the machining.

Why we need to consider the machining allowances?

Some buyers may think the allowances should be considered by rough casting producers, and it has no relations with buyers. This is not total correct for the following some reasons.

1. If the casting producers just supply the rough castings, then buyers have designated other machining workshops to machine for them. Then, the buyers or machining workshops have to tell the rough casting producers about the machining allowances, in case of the insufficient allowance for the machining.

2. Even the casting foundry in charge of machining in-house. If the buyers could provide more information about the machining allowances, then it will be more helpful for casting foundries to reduce the sampling time, and reduce more wastage. If the cast foundries use metal patterns, then the allowance should be considered carefully since the modification to the metal patterns will be more difficult.

3. The metal foundries consider their rough casting costs based on the real rough weights of castings, which means if the machining allowances are too large, then their production costs will be increased, and so their prices will be higher. If the buyers could give correct information to the allowances, then it will be very helpful to reduce the casting prices.

How to consider the machining allowances?

Most machining allowances should be considered by the experiences. Normally, as for the automatic molding line, molding machines and shell molding processes, then 2 to 3mm will be suitable. As for the manual green sand and resin sand casting process, then 3mm to 5mm will be proper. However, as for the large castings, or for large surfaces, then the machining allowance should be 5mm to 10mm. This is because large castings have larger dimensional tolerances, and more surface casting defects, so larger machining allowance will be necessary to remove defects and avoid allowance insufficiency.

Of course, the metal foundries should have more experiences than buyers, but at least, the buyers should understand why the suppliers always care about the rough casting weights, and machined weights.

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