Crank up the bore: General Industry Coverage

VIDEO

Click here to watch a demonstration of Kennametal’s asymmetric line boring bar as it is applied on a machining center.

There’s more than one way to accurately finish a multiple-journal crank bore in an engine block. The best solution, in terms of robustness and results, is a conventional line boring bar, according to Marcus Paul, global product manager for holemaking at Kennametal Shared Services GmbH, Fürth, Germany. The finished bore has a high geometric quality because the tool is supported on both ends, but the boring process requires a special machine to lift the block so the tool can move in and out of the bore, as well as in and out of a counter-bearing on the fixture. This limits flexibility.

Another option is a multiple-blade reamer with guide pads, which can be applied on a standard machining center, but the reamer’s guide pads continuously contact the workpiece, so cycle time, or machining process, is longer and chips can create quality issues if they squeeze between the pads and workpiece. “It is not as robust as the line boring bar solution,” said Michael Hacker, technical program manager for Kennametal.

Automaker BMW challenged the toolmaker to develop a tool that accentuates the advantages of reaming and line boring while virtually eliminating the disadvantages. Kennametal accepted the task and developed its asymmetric line boring bar, Hacker said.

Paul noted the asymmetric tool’s two guide pads are positioned so they only contact the last journal, enabling the boring bar to be quickly fed in and out of the multijournal bore without touching the workpiece. This geometry allows the bar to be fed through the raw, or unfinished, bores on an eccentric path. “That is our trick,” he said, adding that the last journal functions like a counter-bearing.