L/D is a ration between the length by which a boring bar hangs out from the tool post to the actual cutting edge (tool overhang: L) and the tool diameter
For boring, the deeper the hole to be machined is, the longer the boring bar needs to be. However, if the boring bar is too long then vibrations, chatter, will occur. Therefore it is important to take into consideration the L/D when setting cutting conditions and selecting the shank material.
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The materials that can be used for shanks of boring bars include steel, heavy metal and cemented carbide. For a boring bar with a steel shank, if the L/D is greater than 4-5, then it is prone to vibrations and as such limits the depth of cut and feed rate.
When vibrations occur, this not only results in poor surface finish it also causes micro-chipping of the cutting edge and as such the tool life can decrease.
Heavy metal boring bars and cemented carbide boring bars are used to increase the rigidity of the boring bar and as such prevent vibrations thus enabling them to machine to larger L/D ratios when compared to steel shank boring bars.