Preface: I’m with PreciseBits. So while I try to only post general information take everything I say with the understanding that I have a bias.
I won’t talk too much about routers as I have a very strong bias in this type towards the 611. But in general the 2 biggest “failure” points are usually brushes and bearings. Brushes are replaceable and cheap. Bearing failure is basically throwing out the router or investing in tools for bearing changes. With that said, I’d stick to a brand name that will likely use at least decent bearings that are spec’ed to the RPM and load expected.
I’m also obviously bias in tooling but can provide a lot more general information here. The biggest limit from my perspective is holding down the cutting forces. So you have a combination of trying to actually “cut” a chip and keep the cutting forces down to keep it “on path”.
The short version of this would be use a 1 or 2 flute cutter, preferably no larger than 1/8", with a high helix (flute twist), and preferably an up-cut. Why?
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With the max feed and minimum RPM of the system you can't really support a higher flute count and still "cut". You can still remove material in the shape of the tool. But you will be grinding or rubbing it out of the way. The minimum it takes to actually "cut" material is dependent on the tool geometry and the material. Softer more flexible material will need a higher chipload (feed).
The single flute will have an addition advantage here as it’s not nearly as susceptible to runout (tool spin off the center axis of the router). Runout in multi-flute tools will cause the chipload (feed) to fluctuate across the flutes. Or if the runout is greater than the chipload, push all of the feed onto a single flute (in the case of a 2 flute cutter).
For the same tool geometry cutting forces increase for any increase in tool diameter, chipload (feed), and pass depth. As you are limited by the ability to resist cutting forces the only real reason to go larger than a 1/8" is if you need to cut deeper.
The higher the helix (flute twist) the more force is driven into the “Z”. Keep in mind though that that force is also going to be exerted on the material and that could lead to tear out. Probably the worst thing would be straight flute cutters. They engage the entire length of the flute at the same time and cause large spikes of force.
The up-cut is because that direction will try to force the Maslow through the material it’s cutting. As opposed to a down-cut where the force will be trying to lift the Maslow off the material. So the up-cut especially with a higher helix tool should provide more rigidity.
If someone wants the long version let me know. I can go more into the the force changes, chipload, runouts effect on the cut, general rule of thumb for material minimum, tool geometry, etc.
You should be able to ballpark it with chipload. Like for like any cut that would work with say a single flute tool would work at double the feed with a 2 flute. You won’t have a true like for like though due tool geometry chanes like flute rake, helix, core, etc. But it should get you close for similar geometries.
If anyone doesn’t know the simple version of chipload is the widest part of the chip taken by a single flute in a single flute rotation. So, Chipload = Feed / RPM / Number of flutes.
Hope that’s useful. Let me know if there’s something I can help with or expand on.