It's been about a year and a half since I saw it running, but if I recall it was a 20HP Matsuura vertical machining center. And the Walter F4041 has only 4 inserts on a 2" cutter, the outgoing Seco had five. At the same spindle speed and feed rate in inches per minute, that means that the Walter cutter was at a 20% higher feed per tooth. They couldn't go bigger in diameter because of the restrictions on the part geometry. Neither cutter was pulling even 30% load with fresh inserts, but that's still about 5.5HP.

I found a great load calculator on the Walter website "downloads" section, and have it on my phone, tablet and home PC.

You ask a good question Willie. The home shop machinist may have everything from a Harbor Freight 1/2HP drill/mill to a war period Cincinnati Universal to a full blown mutli axis CNC, and many points in between. I think the key indicator as to what diameter and type of cutter one can use, along with cutting parameters, would be the spindle adaptation.

A Morse taper spindle probably shouldn't use carbide insert tooling at all. An R8 spindle with 2HP (what I have) can run up to a 2-1/2" lead angle face mill or a 2" shoulder mill, but I push mine harder than I probably should and have 3" face mill and up to 2-1/2" shoulder mill. An NMTB30 would be in that same class, an NMTB40 might be able to step up a little bit. The key isn't HP, it's torque that matters. Chip load per tooth matters. You don't want such a light feed rate that you are rubbing inserts to death, so better to be in the torque range of your spindle and get some load rather than go high speed and have no power.

On my machine in a typical 1018 steel, I can face with my 2-1/2" 4-insert 45º lead cutter up to about a .075" (2.0mm) depth of cut at 1.5" width of cut and feed at 20 inches per minute. The machine is protesting, but doing it. If I go to the F4041 shoulder mill, I'm more limited on material removal rate so with that one I'll use it's deeper shoulder milling capacity and run up to .400" (10mm) deep but engage it by only .050" (1.2mm) width. Or, go the other way around, My cutters were selected with the mindset of getting at least a 10HP CNC mill in a couple of years.

Try to think of getting more cutter by virtue of lowering the cutting force in high positive cutting geometry. A cutter using those TPG inserts has very little axial shear angle and ZERO radial That's why they hammer on the spindle, especially when using a cermet insert which will have a heavier edge hone than a ground carbide TPG. Forces are greater, cuts use more HP. Get modern cutters with positive geometry, high shear and rake angles, and you can get higher material removal rates per HP. A 1HP Bridgeport (like I had until last year) can easily run a 1-1/2" or 2" cutter with 3 or 4 inserts and the right style of cutter & inserts.