What is tool life?

Tool life is the time a cutting edge cuts before it reaches a defined end-of-life criterion. That criterion can be a maximum flank wear, a dimension that starts to drift, a finish that can no longer be held, a changed chip form or rising cutting force; chipping and breakage end tool life without warning. Shops express it in minutes of cutting, in parts per edge or in metres of cutting path. For a tool that is reground, tool life is the time between two regrinds; total life is the sum of them.

What does tool life depend on?

Tool life depends above all on cutting speed, and also on feed and depth of cut, the tool material and coating, the workpiece material and its hardness, coolant, vibration and interrupted cuts. Taylor described the relationship with cutting speed as vc × T^n = C, with T the tool life in minutes and n an exponent, not the spindle speed. The constants n and C hold for one combination of tool and workpiece material, at fixed feed and depth of cut, and only within the tested range. Because the exponent n is small, a modest rise in vc costs a relatively large share of tool life. In the classic cost model, the cutting speed with the lowest cost per part therefore lies below the speed with the highest output.

What does the standard say about tool life?

ISO 3685 sets out tool-life testing with single-point turning tools and defines wear forms and criteria such as average flank wear VB; ISO 8688-1 and ISO 8688-2 do the same for face milling and end milling. These criteria are meant for comparative tests under controlled conditions. On the shop floor you set your own criterion for when an edge is changed, depending on tolerance, finish and the risk of breakage.

How does tool life work in practice?

Tool life affects cost through tool cost per part and through time: every edge change stops the machine. In unattended hours a worn or broken tool can scrap the rest of the batch; tool counters, sister tools and breakage detection reduce that risk. In most aluminium alloys tool life is long and the cost item small; in stainless steel, hardened steel or titanium it can become noticeable per part. The tool life used in an estimate belongs with the cutting data in the same estimate: different cutting data shift the balance between machining time and tool consumption.

What is tool life often confused with?

Tool life is not wear: wear is the process, tool life the time to the limit you set. Nor is it the life of the holder or of a reground tool as a whole: the holder lasts for years, the edge for minutes to hours, and a solid end mill has several tool lives across its regrinds.