Dashboard issues with a Negative Rake Milling Cutter
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This was a very negative rake milling cutter that was running stable at 516 RPM, 0.236" Depth at full width of cut. The Dashboard showed red at these parameters.
The tool has a very negative rake angle (-11.2 degrees). This causes the insert to rub the workpiece and dramatically increase process damping by 5-10 times the conventional amount.
Here is the insert data. The manufacturer recommended a starting speed of 1154 RPM. Since the customer was running at half that speed, we can assume that though trial and error they kept slowing down until they were stable at 516 RPM.
Armed with this data and this assumption, we adjusted the process damping wavelength for the material up until the customer's speed (the blue dot) became stable.
More information is here:
Normally, we would congratulate the customer and say that is as good as it gets, but if you wanted to risk it, there is a stable lobe at 600 RPM that is right at the top of the depth of cut limit. There are additional lobes at 694 and 1034 RPM, but at lower depth limits. NOTE that the manufacturer's recommended starting speed 1154 is clearly unstable. You could try these speeds at lower depths of cut. For example, 1034 RPM at 0.170” DOC would yield a 60% increase in metal removal rate. The risk is that these speeds would not be robust