I have a circular puck of stainless steel that is 9.5 mm in diameter. In the center, there is already a 5 um pinhole. My objective is to basically cut the steel along the dotted line, approximately 0.5 mm from the pinhole. Can you advise on what techniques to can make this cut precisely?
The reason I want to do this is to be able to align two of these so that I can have 2 pinholes that are ~1 mm apart from one another, as in the following image.
Calling a small disc that is 9.5 mm DIA and 1.0 mm thick "a thin sheet of steel" is a bit of a stretch.
Any machinist would simply clamp the disc in a suitable vise and mill away the unwanted part. Precision would be a couple orders of magnitude better than your requirement.
If you have patience and only need a small number of them done, a pair of pliers and a grinding wheel could accomplish the same thing.
The tool I would use is metal cutting scissors (or Snips). It's remarkably easy to get a straight cut with these after a little practice. I would first make a cut far from the line (somewhere in the area you're going to discard), to get a feeling for a particular tool cutting steel of a particular thickness.
With some experience it is possible to cut as close as 1mm to the line, which reduces the amount grinding work needed.
Probably the simplest way is to cut it a few mm oversized and grind or file off the excess.
I would say that 0.25mm is not very difficult to do.
You can use Water jet cutter to be really accurate or if you don't have this possibility, you can simply use a mill or CNC with specific drill.
