Here are some excerpts from an American Machinist online article.
=============================================================Rounding Up the Usual Suspects 12/28/2009 By Matt Tegelman
When boring performance falls off, the cause may be any one or a combination of factors. These include workpiece stability, adequate stock allowance, tooling rigidity, insert grade and geometry and the matching of speeds and feeds to cutter capability.
Part stability Although the machining center and fixtures usually are not the first factors shops may consider, these can seriously impact tool performance if the part is not stable when being machined.
Often, operators are unsure of how much stock to leave for boring applications. This is especially true in rough boring applications where a twin cutter is used. It is not uncommon to see drills that are so close to the final size of a part that only 0.020- to 0.030-in. of stock is left on diameter, which isn?t enough material to engage both insert tips on a twin cutter. This leads to chatter and poor tool performance.
Tool assembly rigidity
Modular tooling systems offer an infinite number of combinations for maximum rigidity and usage. In cases where tool length is needed, it is important to start with a larger base size and then reduce the tool diameter as necessary, rather than using the same bar diameter for the entire length of the tool.
Insert grade and geometry
The most rigid boring assembly, complete with a balanced boring head, can perform miserably if the insert is not suited to the application.
Speeds & feeds
Ideally, the boring bar is run at high speeds and more moderate feedrates; but again, this can be limited depending on the conditions previously cited. A common mistake during rough boring is simply to multiply singlepoint feedrates by two when using a twin cutter. This usually is not an effective calculation; twin cutters can operate at feeds four times faster than those of finishing tools for the same diameter because a much larger nose radius can be used. Feeding a twin cutter too slowly will lead to chatter without the right stock allowance. Rough cutters are designed for heavier cuts, requiring more material and harder feedrates.
Matt Tegelman is an application manager at BIG Kaiser Precision Tooling Inc. =============================================================