Usual 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.
Even though this is solved I figured I might be able to answer some of the more technical questions that Redlander and Smorgasbord had.
Cutting forces
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For the same stepover the chip form will change along with the surface speed. So the forces can actually be slightly more or less for the larger tooling. It's more or less margin of error and trades though. Using Millalyzer, for the same cut it's Peak Transverse is 15.2lbf for 8mm, 15.5lbf for the 1/4". Peak Feed is 5.7 for the 8mm, and 5.5 for the 1/4". Power is 110W for the 8mm and 108W for the 1/4".
However, you do have to worry about the plunges. You can get away from a big part of it if you are ramping. If you aren’t though you are going to have a bigger spike of force on the plunge for the 8mm.
Deflection
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The deflection issue very much depends on the rest of the tool. A big part of this is that 8mm tooling is typically on a longer shank. Therefore unless you have an unlimited area to choke the tool, it's going to have a longer stickout. That means more deflection.
e.g. let’s take a 1/4" tool with a 1" cutting length and 2.5" overall length and a 8mm tool with the same spec but at 3" overall length. Now let’s assume that we can fit a max of 1" into the collet and spindle/router. That makes the 1/4" tool stickout 1.5" and have a deflection of 0.001" per 8.9lbf. The 8mm tool will be sticking out 2" and take 12.27lbf to deflect the 0.001". The crossover point is at ~2.28" where the 8mm tool will deflect just as much.
Alternatively if we change the length of cut on the 8mm tool to 1.5" and have the original 2" of stickout it will be weaker than the 1/4" tool and only take 8.11lbf to deflect the 0.001".
Mass
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This one is a lot of guess work and "your milage may vary". Obviously if there's more mass that more work for the bearings and the like. However, you also have other variables like runout which can greatly exacerbate the mass difference. The more "off center" the tool is spinning the more that mass difference makes to the bearings. Additionally assuming some part of that runout is angular runout, the longer the tool the greater the force on the bearings. All that is additive to the cubic material being removed.
Collets
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8mm collets in routers and ER11 and their clamping force and slip resistance are another issue. I can't really speak to the Makita or it's like as I don't really have good data for them other than that style of collet can only really produce force with ~4mm of the collet (tapered section). So all the more of the following.
While there are 8mm collets available for the ER11 they are NOT within original spec. RegoFix the inventors and the “R” in ER don’t make one. Added on to that you need progressively more force for the larger bore size of the collet within the same ER size. If I were to guess (and while an educated one, that’s what this is) there’s both not enough meat in that collet to keep things like the nut taper from trying to twist the collet leaves (or at least not enough margin that people like Rego are okay with it). Add/or that the amount of force required to get good slip resistance on that diameter is outside of the “safe” range of the ER11 collet nuts. Could also be that as they are so thin they just wear too fast for their preference.
“Better”
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In terms of 8mm being better it depends both on the above and that if you are following best practice. There are advantages to smaller diameter tooling depending on the material and machine.
e.g. Let’s say we are cutting wood and with a geometry that should be going 1200 SFM, a 0.004" chipload, at 0.25" deep, and a 0.125" stepover. The 1200 SFM works out to ~14,500 RPM on the 8mm and ~18,300 RPM on the 1/4". Meaning at the 0.004" chipload you are going ~146 IPM on the 1/4" and 119 IPM on the 8mm. The forces and deflection are dependent as per above (roughly same force, deflection dependent on tooling).
Assuming like for like tooling between 8mm and 1/4" is also unlikely correct. Realistically, you are probably going to have a number of geometry changes and different carbide grades from assumed use (barring application specific tooling). That will change things back and forth too.