18 Bosses, Lugs, and Cleanup

Most castings require some type of “cleanup” or finish machining to make a suitable part because the casting can not produce the precision, the detail, or the surface finish required. After all the casting is a very rough process with only a wooden pattern and a sand mold-form to hold dimensions. An example could be a gear machined from a casting. The smooth precision tooth faces and tight bore tolerance can not be used “as is” from the casting. Machining of the gear teeth and bore is a final step in manufacturing the gear.

Castings are almost always made with excess material for the cleanup and to assist the finish machining operations. This excess material can be in the form of bosses, lugs called “machining allowances” (frequently called cleanup allowance). Examples are machining allowance on the hub of a cast wheel (image 1), bosses on a base plate (image 2), and lugs on a cast metal elbow (image 3).

The cleanup allowance for sand cast parts (usually iron or steel) is 1/8 inch, and for die cast parts (aluminum, brass, etc) is 1/16 inch.

Lugs are provided on castings to enable the cast part to be held firmly. The lug on the cast elbow shown on the right, for example, allows the awkwardly shaped part to be held in a fixture for machining processes.

Image 1

Note the cast part on the left and its machined condition on the right.

Image 2 

Mounting bosses for base plate. Only requires machining boss surfaces.

Image 3

An astute designer will provide lugs in the blueprint that can be used to assist in holding or orientation of the casting for machining. Good designers are a machinist’s best friend.

Derived from The Virtual Machine Shop available and accessed at http://www.jjjtrain.com/vms/eng_castings/eng_castings_07.html
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