I'm a bit confused re having 1/8 stock but no 1/32 – not sure I see the relevance. As to how to hold 1/8 sheet to reduce to 3/32 must depend on how much you have and how much you need. Can you not just bolt/screw some to a backing piece (MDF?) by it’s edge and thin out enough from the centre for the requirement?

pgk

Superglue.

I have heard of success with superglue provided both surfaces are very well degreased first and are flat and not too polished. Heat is then used to break down the bond. Very sharp tools and fine cuts will lower the chance of premature separation.

I would use pgk pgk's method if there was enough spare sheet.

+1 Shellac has slow setting advantages when work has to be positioned accurately but superglue has stronger grip. I often use it for this sort of work. If the mandrel and brass are both clean the brass won't come off under moderate cutting forces.

I've used superglue to create a double headed penny by shaving two old coins down. They're made of Bronze, which is tougher than Brass, no problem.

To release the brass, boil in water. Slightly hotter than 100°C destroys the bond more effectively, so you might prefer to heat gently with a blowlamp. Any residual glue cleans off with Acetone or Nail Varnish remover.

Dave

I would have done it on the CNC mill. Usual procedure for me is to put blue masking tape on each surface then superglue them together – the tape makes it easier to remove and takes the glue with it. However it might be more secure to glue metal to metal. A good tip to separate them is to boil in water for a few minutes.