Bronze Tolerances and Machining Allowance

Bronze foundries around the world use different tolerance / machining allowances to enable their customers to plan to use the minimum machining possible to create their part from cast bronze bar. Less machining means less time and also less chip or swarf left over at the end of the process. Here at AW Fraser we have our own machine shop, so the feedback between the foundry operation and the machine shop is a tight loop. This means that we are always trying to improve our cast bronze product and under our lean manufacturing projects we are looking to take cost out of the process and as a result our cast bronze customers benefit from good quality bar. At AW Fraser we combine the tolerance and machining allowance and add it on to the nominal size so that if you order a particular size, the bar should clean up to the size you have ordered.

The process of bronze continuous casting involves the transfer of molten bronze from an induction furnace via a crucible to a heated casting pot which has a die mounted to it. The die design determines the shape and size of the finished bar. The die is cooled to enable the bronze to form the desired shape. The cooling process means that the bronze bar will shrink as it cools. The bar is extracted from the casting pot by a set of rollers or jaws and the speed of extraction is a factor in the rate of cooling and this affects the final physical properties of the bar. The die itself is manufactured on-site from carbon and this material provides excellent non-friction properties meaning the bronze is unlikely to freeze or get stuck in the die. However the carbon does wear and so the size of the die will change over time. There are also small variations in size caused by the exact chemistry of the bronze and the exact temperature of the metal when it is poured from the crucible into the casting pot. A further factor to consider is that the bronze continuous casting method does produce slight variations in size as it pulls the bar from the machine. A further consideration is that some alloys, particularly aluminum bronze and manganese bronze have small surface cracking as a result of the casting process. This is normal and we have a different tolerance/machining allowance table to allow for this property. To allow for these small variations in size we have what we call the bar tolerance which is what we allow our production people to manufacture within. For example, let us look at 2" C83600 (LG2) bar. 2" 836 is one of our common sizes and is a stocked item for us. When the production order is given to the foundry, we say that the nominal bar size is 2", so they must manufacture with the combined tolerance and machining allowance which as can be seen from the below table is between 0.03" and 0.04" on bar below 4" outside diameter. For this example, the foundry will aim to make bar which measure 2.035". Given the variations described above, we will allow bar between 2.03" and 2.04" to be put into stock. The tolerance / machining allowance chart we use is as follows:

For any further questions, please contact us!!

1/             For alloys LG2, 660, PB1, PB2, CuSn12Ni, 844, LB2, 907, 922 927 Hollow and Solid Round Bars - Where tolerances are not specified by Customer or job sheet.

2/             For alloys 954, 864, 865, 955, 953, AB2, 857, 360, 352 Hollow and Solid Round Bars- Where tolerances are not specified by Customer or job sheet.

3/             Standard Bar Length: Where not specified by Customer or job sheet.

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