Failure of a machine tool three jaw chuck set

Gagg, Colin (2010). Failure of a machine tool three jaw chuck set. In: Fourth International Conference on Engineering Failure Analysis (ICEFA-IV), 04-07 Jul 2010, Churchill College, Cambridge, UK.

URL: http://www.icefa.elsevier.com/

Abstract

A three jaw chuck set from a lathe machine tool failed whilst in use. At some prior point in time the case-hardened chuck jaw set had sustained damage to its gripping serrations, thus requiring refurbishment of the jaws. Shortly after re-entering service, the refurbished jaw number one (of three) failed traumatically. Sudden loss of grip allowed a large rapidly rotating work piece to be thrown free, hitting and inflicting seriously injury to the machine tool operator. Analysis of the broken jaw revealed sub-surface case-cracking, initiated as a result of metallurgical changes induced by thermal processing undertaken as part of the refurbishment cycle. Fatigue propagation of a case-crack had then ensued, driving the crack to final traumatic failure. Further analysis revealed stress raising features on a second jaw. This particular jaw had initiated a longitudinal crack that, at the time of observation, had not propagated to failure. Therefore, both thermal processing and design shortcomings were found to be at issue with this incident. When considering the safety critical nature of lathe chuck jaws, a question to consider is whether a more prudent route of action is required for the future. Rather than attempting refurbishment, it would be reasonable simply to scrap worn jaws. This is considered a sensible course of future action, rather than running the risk of being overtaken by a similar failure. Finally, the issue of ineffective or non-existent machine tool guarding was considered, with the outcomes effectively reducing injury compensation awarded to the machine tool operator.

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