Hello,
I'm wondering if anyone can recommend a cutting bit (preferably with a 1/4" shank) that mills nomex honeycomb nicely.
Thanks!

https://www.neuhaeuser-controx.de/services/composite-tools/honeycomb
Very expensive, really necessary 5-axis HS spindle included. I tested 4 flutes carbid router on 3-axis machine, it really doesn´t work. No cutting rather pushing...
The carbide burrs Chris talking about are for cured panel machining, not for honeycomb cores.
If the core shape is quite complicated I don´t know another method how to machine it.  If the core is flat, resp. constant thickness all around the area and you would like only an edge bevel all around, for example chamfer 3x45°, I use angle grinder with sanding wheel P120. Bond it using spray glue on MDF pattern and sand it manually. If I have a complicated core that´s the reason for Rohacell using instead the honeycomb. There is no problem with 3D machining or thermoforming.