Bit deflection help

For the past week I have been running sample blocks trying to dial in the best setting to cut HDPE. because of some narrow cuts I have to make, I can't use a bit larger than 1/8" (Might be able to do 4mm, but haven't looked into that yet). Anyways, to make a long story shorter, I have found a bit that cuts HDPE beautifully, however it deflects very bad at the bottom 5% of the material. Here is an exaggerated drawing of what I am dealing with. Test piece is a 2" diameter, 1" thickness with a 5/16 hole in the center



The bit is a single flute twist up cut.
Speed 13750 rpm
Feed 1397 mm/min
DoC 1.5875
Inside hole is cut conventional, out side is cut climb (2" diam)

The finish is very smooth and chatter free, but no mater what I try I can not get rid of the deflection. I can slow it down, speed it up, change the DOC and no difference. The best progress I have made is lowering the plunge rate to about 200 mm/min. Helps a bit but doesn't eliminate it. Also, oddly doing a finish pass, or repeat pass has absolutely no affect on it.
I suspect the way this bit is designed is part of the problem. I think the twist in the bit causes it to "pull". I haven't been able to find a single flute straight bit in 1/8" yet so haven't tried that.

I have also tried a 2 flute straight bit but the edge isn't as clean and the deflection is still there, but not as bad. I have tried a 2 flute upcut twist bit and that gives me a straight edge, but its rough as well and the double twist leaves a very slight hill-vally surface.

I'm out of ideas to try. Maybe cut 1 or 2mm past the bottom???
Could the deflection be coming from the gantry and not the bit? I have tightened and re-tightened everything on the gantry, In fact I am afraid that everything is too tight.
Is router placement important, as far as how high it is on the gantry?

Thanks for reading.

 

Found a very informative page, might help others
Who is Afraid of Tool Deflection? [2020: 4 Evils + The Cures]

 

Thanks for the info - I plan to do just that. I have new bits coming.
As for the bit length, yes it's only about 1/16 extra.
After reading the info in that link I posted, I'm thinking the reason I get such bad deflection with the best cutting bit is 2 fold, that bit is HSS, and single flute, making it flex even more.

 

For anyone who is interested, I figured it out. - I was ignoring the Chip Load.
My chip load was way to light. I was messing around with a 3 flute bit today and the deflection was a lot less, but still couldn't get rid of it. Since slowing it down didn't seem to help (and I actually noticed vibration was worse going slower) I said screw it and set the speed to 4000 mm/min. The vibration disappeared and so did the deflection. (but 3 flutes leaves a fairly rough edge)

I changed back to the single flute bit, recalculated the F&S for max chip load (I was running the lightest thinking it wouldn't deflect as much), set the depth to 1.75mm and like magic - no vibration and a fairly clean edge (could be cleaner) and most importantly - straight and square with the surface. The only vibration I get is on it's final pass because it's only cutting a fraction of the step down so I will recalculate my step down distance to equal out over the full 25.4 thickness.

Thanks to everyone listening to me think out loud!