Technical Information: Tool Life
Tool-life data have been developed experimentally for a wide variety of titanium alloys. A common way of representing such data is shown in is where tool life (as time) is plotted against cutting speed (fpm) for a given cutting tool material at a constant feed and depth in relation to Ti-6Al-4V. It can be seen that at a high cutting speed, tool life is extremely short. As the cutting speed decreases, tool life dramatically increases.
Titanium alloys are very sensitive to changes in feed. Industry generally operates at cutting speeds providing long tool life. Curve fitting of tool life to feed, speed, and other titanium machining parameters is commonly being done by means of computer techniques. However, in cases where no data base exists, certain rules of thumb should be recognized. For example, when cutting titanium, a high shear angle is produced between the workpiece and chip, resulting in a thin chip flowing at high velocity over the tool face. High temperatures develop, and, since titanium has low thermal conductivity, the chips have a tendency to gall and weld themselves to the tool cutting edges. This speeds up tool wear and failure. When dealing with high-fixed-cost machine tools production output may be much more important than a cutting tool’s life! It thus may be wise to work a tool at its maximum capacity, and then replace it as soon as its cutting efficiency starts to drop off noticeably, thereby maintaining uptime as much as possible.
When machining titanium in circumstances in which production costs are not of paramount concern, it is still unsound practice to allow tools to run to destruction. The other extreme, premature tool changing, may result in a low number of pieces per tool grind, but the lower the tool wear, the less expensive the regrinding.
Ideally, a tool should be permitted to continue cutting as long as possible without risking damage to the tool or the work but with the retention of surface integrity. The only way to find a safe stopping point is to check a few runs by counting the pieces produced and inspecting the surface finish, dimensions, and surface integrity. In this manner it can be established how many acceptable pieces can be produced before the tool fails.