Boring operations
Boring operations involving rotating tools are applied to machine holes that have been made through methods such as pre-machining, casting, forging, extrusion, flame-cutting, etc. Roughing operations are performed to open up the exisiting hole to within large tolerances and usually to prepare for finishing, which makes the hole to within tolerance and surface finish limits.
Typically, boring operations are performed in machining centres and vertical boring machines. The rotating tool is fed axially through the hole. Most holes are through-holes, often in prismatic or round components. External boring operations can be accomplished using specially adapted boring tools.
The most common size holes being bored are 30 to 100 mm diameter. The range of Sandvik Coromant boring tools covers 23 to 550 mm for roughing and 3 to 975 mm for finishing.
As regards hole depths, four times the hole diameter is generally the maximum recommended depth. Various tools have individual maximum hole depths. For rough and fine boring of deeper holes – of six times the diameter – damped boring tools with tuned tool bodies should be employed.
As regards hole quality, finish boring can typically achieve holes within tolerances of IT 9 with some achieving IT6. Surface finishes better than Ra 1 micron can be achieved. Finish tools have adjustment possibilities to accurately pre-set cutting edges. In principal, however, a boring tool designed for roughing can also be used to achieve holes with a high surface finish and to within close tolerances as only one of the inserts generates the surface.

Source:SANDVIK – Metalcutting Technical Guide

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