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.
In general I think you’re deflecting. You’ve basically removed the previous limit of spindle power and are now running up against the machine strength limit (and motion limits where you’re losing steps). Those numbers from HSM don’t make sense to me for the forces. If I can, I’ll check some other sources when I’m back at work tomorrow.
If you want to test it, I ran through a simple deflection test here:
A few more things.
Although adaptive toolpaths can make use of it, be careful with WOC/stepovers of less than half the bit diameter. It will cause chip thinning where you are cutting less than you calculated chipload. As an example your last listed HSM screen is 18KRPM at 36IPM with a 33.3% WOC/stepover. It’s calculating that as a 0.002" chipload. That is correct without chip thinning (36/18000/1 - IPM/RPM/Flutes). However, with chip thinning at a 33% stepover you are actually only cutting 0.00188" chipload. That’s not too far off. Let’s say though that we are using the 6% from your first HSM screen. Then your actual chip is only 0.00095", less than half.
That can greatly confuse matters if you are trying to come up with limits based on chipload. It may also explain some of the issues you are having as the smaller the chipload the hotter your cut will get. You could also be rubbing/grinding instead of cutting if you are not hitting the minimum for the material/tool. That will both increase the heat and the cutting forces.
Be careful with this. The max torque for an ER11 collet is 18 ft-lb (24Nm) for a standard nut and 12 ft-lb (16Nm) for a mini. If you go over this not only are you potentially causing damage but you may be inducing runout which will make matters worse.
Also since I’ve run into it a few time here recently… Are you snapping your collet into the nut before using it?
In general I’ve never seen a cutter that can’t do at least 600 SFM in 6061. That works out to ~18K for the 1/8" and 9K for the 1/4". Most decent cutters can do at least 1000 SFM which is ~30K for the 1/8" and 15K for the 1/4". Specialty cutters can go much higher than that depending on what they are shooting for.
Hope that helps. Let us know how it goes.