So now you have a clean rosette, ready to be glued. You either bought it or made it. Make sure you clean one side, so it can glue well in the top.
You should now have a rosette with one side still dirty (left), and one clean (right).
I assume you have a top joined, planed and scraped clean and with the outside face pretty clean, cut to shape (slightly bigger) but NOT thicknessed yet.
The goal is to cut a slot in the top very slightly wider than the rosette (0,2 to 0,3 mm.) and a little shallower (0,3 to 0,5 mm.). When you glue the rosette in the top, the glue makes it swell a little, leaving no gap whatsoever.
You can cut this slot in many ways, this is just one of them.
You will need a few tools:
A dial caliper, to precisely check the width and thickness of the rosette and the slot (i assume you know how to operate it).
A dremel router with a good bit, a base for it (like this one) and a circle cutter (like this one):
With a bigger router and a sturdier base, you can directly cut the slot; i do prefer to cut the slot first with the circle cutter because otherwise you can sometimes have a somewhat rough cut on endgrain.
Both the router base and the circle cutter i use pivot on a ¼” rod, so first i have to drill a ¼” hole where i want the center of the rosette. This is also when you decide if the 19th fret will be divided or not. I have two marks on my top template for divided and not. In this case i’ll drill it for the full 19th fret.
The blade of the circle cutter have to be sharp to be effective; it is very small but great care must be taken to shape it and sharpen it properly.
It is VERY important that the bevel on the circle cutter’s blade is towards the INSIDE of the slot, because otherwise the wall of the slot will not be 90 degrees.
When the blade is ready, clamp it in the right side, set the depth of cut as the rosette’s thickness and set the circle cutter to the appropriate diameter.
Having set the the circle cutter, you can proceed to cut.
Here i also lightly scribed the soundhole for reference, although i do not usually do it.
Flipped the blade over for the outside cut, set the right depth and check the diameter.
You can now proceed with the outside cut.
The top now is cut and you only need to clean the inside to an even depth.
To do this, i use a dremel with a base.
Set the depth of cut slightly shallower than the rosette.
And check it where the fingerboard will go, just in case...
Now you can rout in a few passes, depending how big your routing bit is.
Reaching the cuts, leave a small amount of wood to remove, and also stay a little shy of the cut.
If some wood is left against the cuts, clean it with a knife.
The top is now ready for receiving the rosette!
