I want to make a wood slice magnet which an extra thickness magnet is inserted on a 5mm thickness wood slice.
Here is my material.
The trouble is that I couldn't find a suitable forstner for it, and a pilot hole may break the thin slice. After deep research, I found someone use router:
(The left one)
It looks nice, but soon I found holding a small wood slice for routing is different case.
Is there a better tool for my need?
If you have a plunge router, I'd clamp the wood slice from above with a larger chunk of wood.
To help a little with maintaining position, I'd tack two tracks onto your worktop. (Make them shorter than your wood slice, of course.
If you're doing more than a couple, I'd screw the larger chunk of wood to the base of the router for perfect alignment. (If that's too bothersome, plunge the bit into the hole you've made -- power cord unplugged, of course -- to maintain alignment when you're clamping everything in place. This will take a few attempts, as you can no longer see your slice for alignment.)
Then clamp everything up and go for it. If you haven't screwed the guide wood to your router, then you'll need to clamp the router down as well.
With the hole established in the larger chunk of wood, you should be able to put crosshairs on the slice and eyeball the alignment.
Either a router or a Forstner (or similar) drill bit are both good ways to make the recesses you want, but in a small piece of wood I think a drill bit is clearly the better option. If you can find a suitable flat bit/spade bit cheaply you might also like to try this proposed modification from a previous Answer.
Regardless of the method used to mill the recess your main difficulty is in holding such small pieces so that the wood is secure and the operation is safe, so that's the thing to concentrate on.
With a router you'd need to build a custom clamping rig with jaws that were the same thickness as your slices or slightly thinner, and additionally the wood needs to be held tighter because of the very high speeds that routers run at.
It's much simpler with a drill bit since the clamping device or aid can be thicker than the workpiece (much thicker in fact because of the long shaft on the bit) and it won't get in the way.
One of the standard ways woodworkers hold small parts is with a handscrew:
These both show routing operations but you can see how you could just as easily do the same for drilling.
You probably don't own a handscrew and although a simple version is easy enough to make at home a number of other clamp types are capable of holding the work in a similar way. F-clamps, quick clamps and even a shorter sash clamp could all be used for this without any problem.
Personally I think you could safely hold the workpiece by hand if you have a drill stand or pillar drill, but if you want to be more careful than that another trick would be to attach the slices to a long piece of wood that you can clamp firmly off to one side or hold by hand with your fingers safely away from the spinning bit.
There are various ways to temporarily bond one piece of wood to another, and here double-sided tape may work perfectly well. But if you find it's not secure enough I suggest you try using superglue between two strips of tape as I describe in How do I temporarily attach two pieces of wood together for machining?
Using a Forstner or brad point bit or router, you can cut the recess before cutting out your circle and slicing it to 5mm thick. Optionally make a guide and grind off the center spur, if using a Forstner bit.
Alternately, drill all the way through, then slice thinner slices and glue each O-shaped slice onto a circle of the same diameter.
