Tips on Machining Vol. 28
How to prevent chatter in long-overhang face milling

(1) Increase rake angle

Fig. 1 shows the comparison of cutting resistances between rake angles of 0° and 30°. The main cutting force (a force in the spindle rotation direction) is decreased by 33%, feed force (one in the feeding direction) by 37%, and thrust force (one in the spindle pushing-up direction) by 80%, respectively when the rake angle is increased to 30° from 0°.
This means increasing a rake angle can reduce cutting resistance and as a result suppresses chatter. However, it should be noted that depending on the material, cutting resistance may not be decreased even when a rake angle is increased.

Fig. 1: Comparison of cutting resistance between different rake angles (material: S50C)

(2) Reduce cutting edge angle

Tool tips with different cutting edge angles (15°, 45°, 75° and 90°) were attached to our original cutting tool to check cutting resistance at different cutting edge angles. The test results are shown in Fig. 2. Although almost no change was observed on the main cutting force, the feed force increased and the thrust force decreased as the cutting edge angle increased.
Chatter is likely to occur when a holder is long and tends to cause bending when feed force is large. This means chatter can be suppressed by the use of a tool with a small cutting edge angle. Running a tool in such a way that the feed force becomes small and thrust force becomes large is also effective in chatter suppression.

Fig. 2: Difference in cutting resistance (material: S45C)

(3) Keep the number of simultaneously engaged cutting edges constant

When looking at individual tips of a tool with multiple cutting edges like a face mill, you can notice that the tips perform cutting and non-cutting operations repeatedly. “The number of simultaneously engaged cutting edges” is the number of tool tips that come in contact with a workpiece at the same time (Fig. 3).
Changing the edge numbers causes great fluctuations (intermittence) of cutting resistance and thus leads to chatter.
Given these facts, keeping the number of simultaneously engaged cutting edges constant is important to prevent chatter. This is made possible by adjustment of the number of cutting edges and the relative positions of the face mill and the workpiece.
It should also be noted that, cutting resistance increases as the number of simultaneously engaged cutting edges increases. So, the number of the edges must be decided based on the magnitude and direction of cutting resistance.

Fig. 3: The relationship between the number of simultaneously engaged cutting edges and cutting force

*Source: SUMITOMO ELECTRIC HARDMETAL “Technical Guidance/References in Cutting Tools General Catalog 2019-2020”

(4) Criteria of stable machining by holder lengths

We also conducted a test to see how the tool holder length can affect chatter. A face mill with a diameter of 63 mm and cutting edge angle of 45° was attached to three holders in different lengths (60, 90 and 150 mm) and measurements were taken at points where chatter occurred.

With the 60- and 90-mm holder, machining was performed properly even when the spindle output reached close to its maximum. With the 150-mm holder, chatter was observed when the material removal rate reached about one third of that of the other holders.
The result reveals that with a short holder, chatter-free machining is possible even when spindle output is increased close to its maximum, however, with a long holder, the value at which chatter occurs is the threshold of chatter.
We also checked the material removal rates of 150-mm holders with different rake angles (8° and 23°). The 23° rake angle achieved an approximately 1.7 times greater removal rate than that of the 8° angle. This demonstrates even when holder length is the same, a larger rake angle can reduce both feed force and main cutting force and raise the threshold of chatter.

Fig. 4

Machine tools and cutting tools are evolving rapidly, but the fundamental problems that arise at the shop floor such as chatter and tool breakage remain the same.
What is important in achieving successful machining operation is to well understand the impact the three basic elements – a rake angle, cutting edge angle, and the number of simultaneously engaged cutting edges – can have on cutting resistance and to determine optimal conditions for each machining.
DMG MORI conducts various experiments focusing on the fundamentals of cutting phenomenon. If you have problems with your machining operation, please feel free to contact us.

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