I am wondering if anyone knows about the physics of modern carbide cutting enough to say if there is a minimum feed, rpm speed.
I know it’s different for materials I’m most concerned with aluminum 6061, cutting with lapped Aluminum geometry end mills. I’m wondering if I can run at 10% feed and rpm. So instead of 10000rpm at 1000mm/min, run at 1000rpm at 100mm/min. Or is there a point where the end mill just stops cutting because the edge just isn’t moving fast enough.
Absolute speed and feed numbers are less important than feed per tooth as far as the cutter is concerned. Too little feed per tooth and the tool will rub instead of cut which will heat up the work and the cutter. In harder materials that results in work hardening and tool dulling. Aluminum isn’t likely to do either but if it gets too hot it will weld to the tool and eventually the tool will simply break off.
The minimum feed per tooth will depend on the tool itself. Some sizes/geometries want a couple thousandths (inch) as a minimum while others are ground such that a few tenths is enough. Where the practical limit is for a given tool is probably only found through trial and (expensive) error but for most, I don’t go below .0005-.0008" in aluminum, and then, only with a new and very sharp (uncoated) tool.
As for a practical lower limit on rpm and feed, this is going to be more a function of the machine’s capabilities than the tool’s. Your machine isn’t going to spin at 1rpm, and the linear motion is unlikely to be very smooth at .1mm/min due to the nature of steppers. Outside of that extreme example, you also have to consider available spindle torque based on the motor’s torque curve. While I realize that electric motors theoretically have 100% torque available at 0RPM and that only goes down as speed increases, real world is a often a bit different when inertia is taken into account. I’ve had my spindle stall because I was spinning it too slowly, usually when tapping a deep hole. Higher speed allowed it to push through the chip and finish the cut.
Something to consider when thinking about cutters and materials. Hand cranked mills, drills and lathes have been used for many decades before high speed cnc machines. In short you will almost never hurt much by slower feed speeds and almost always increase drama with faster feed speeds.
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On the 770MX, 1.5hp. I’ve found that 4-6k rpm is in the range that minimizes wear and tear on everything. Motor efficiency, rigidity bla bla bla. Takes a little longer but safe and sane. Below 1/4” diam, rpms go up of course. Still hit some harmonics depending on part.
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