Frequently Asked Question

High speed milling of Titanium alloys.
Last Updated 7 years ago

The limitation factor in milling titanium parts is tool temperature. Because of this limitation, spindle speeds are dramatically reduced. In this area of low spindle speeds, there’s no need for stability lobes because the distance between two peaks is so small and the difference between top and bottom of the peak as well. From time to time, the maximum depth of cut is 1mm when working on full diameter.

How can we increase radial and axial depths of cut dramatically for titanium parts in order to get high material removal rates?

While checking a range of low spindle speeds (500-800 rpm), the maximum depth of cut dropped from 1mm to 0.2mm (e.g. from 650rpm)at a certain speed in this range. How is this possible? I kept the depth of cut at 1mm (stable condition up to 650rpm) with a spindle speed of 800rpm (at this point 0.2mm was the maximum stable depth of cut) and it worked fine. Where’s the drop in depth of cut coming from and where we able to cut stable while the lobes indicated an instable condition?

Milling of Titanium and Inconnel at low speed is always a challenge, when the diameter of the cutter is small as well as the spindle speed. After the lobe number 7-8, the accuracy of chatter stability diminishes due to friction between the flanks and finish surface with vibration waves. This is called process damping and not possible to predict mathematically. It very much depends on the edge radius, clearance angle, speed, vibration frequency and amplitude, and work material. Researchers starting from Aachen and Leuven in 1960s have been unable to model this phenomenon mathematically.

The alternative is to use the knowledge in absolute stability (bottom border of lobes) and start testing the tool at that speed by increasing the depth of cut until chatter occurs. The chatter free depth of cut will increase as the speed is reduced after 7-8th lobe depending on the cutter geometry and vibration frequency. The productivity is improved only by controlling the temperature which depends on the radial depth of cut, feed and speed. First, you need to simulate temperature up to 90 degree immersion by dividing the radial depth of cut at 5 equal distances. Keep the cutting speed maximum same so as the feed, and record the tool life. Increase the cutting speed as you reduce the radial depth of cut by 20%. Decrease the radial depth by 20% and increase the speed and feed by the same amount). After simulations, try selected tests on the machine; make sure there is no chatter during cutting tests. Cut the material until the tool wear reaches to a limit, by measuring the wear at 4-5 minutes intervals. You will get a table of results which can be used in selecting proper cutting conditions in the factory.

Unfortunately, there is no magic way to estimate the tool life.

Note: The stability lobes at very low speed may be incorrectly calculated due to numerical problem caused by the resolution of FRF frequency. You need to ignore it.

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