I made a flush arbor that would hold a 3" X .045" slitting saw. Turning the screw in the middle causes the arbor to expand into the saw's center hole. The idea was to not rip through the core material as it is only .005" thick. The slitting saw with it's fine teeth,( 180 teeth, I think), worked very well. It was run slow, about 170 rpm in a climb cut.
In the last pic you can see the construction of the old core. This is not a true honeycomb. The little channel for the water is created by runnung the strip brass through a crimper and assembling it row upon row so the ends could be pinched closed at the front and rear and pool soldered. (early form of wave soldering) There was actually 2 type of crimps, the one that was paired up to create the channel ,and the crimp size that made the connecting cooling fin. There were some little bumps in the crimp also that were there to create turbulence in the water tube. The minds that came up with this stuff in the teens and twenties were truly great.