The most common method to make the channel(s) for the rosette on the top plate of a guitar is to route them using a down cut/down spiral router bit on a plunge router. A foolproof way to do this is with a router baseplate specially designed for routing circles. This page contains instructions for doing this using the Jasper M200 or M400 circle guides. It also contains a calculator that can be used to do the tedious calculations needed to select the pivot hole in the baseplate for each cut.
Initial appearance: February 7, 2019
Last updated: February 07, 2019
Routing rosette channels is a lot easier and quicker than cutting the walls of the channels with a circle cutter with a knife blade (called a compass gramil) and then excavating the waste from the channel with chisels. This is particularly the case for steel string guitar rosettes which often feature relatively thin rings in the rosette patten, which must be tediously excavated using a narrow chisel. But straight or up spiral router bits produce very fuzzy channel edges when cutting spruce. Down cut/down spiral bits work very well in this application, leaving a nice clean edge. The problem when using these bits is that they eject chips down toward the floor of the channel when cutting, and this action tends to lift the router up off the work. Although it is certainly possible to use such bits in a small circle cutting jig designed for a Dremel or other tiny grinder, you generally will have to take a number of very shallow passes and hold the router firmly to the surface of the plate to prevent it from being lifted during routing.
I like to use a heavy router for cutting rosette channels instead. The weight of the router alone prevents lifting and keeps the router in contact with the top. Fortunately there are commercially available circle guide router base plates that can be used to route circles of all the diameters needed for guitar rosette channels and soundholes. The one I use is the Jasper M400 Pro from Jasper Tools. It comes pre-drilled for mounting on most routers and will cut a range of diameters in 1/16" increments. It comes with a 1/8" diameter pivot pin, which plugs into holes on the baseplate for each circle diameter to be cut. I use a 2 HP plunge router. I'll generally use a 2mm diameter bit to cut the thin BWB rosette rings in one pass, and then use this or a thicker bit to cut any wider channels. I don't recommend using any bit wider than 1/8" because wider down spiral bits tend to lift even a heavy router.
The baseplate comes with mounting instructions but I suggest one of two additional modifications: 1. Add a thin un-drilled plate between the router and the drilled baseplate, or 2. Make a longer pivot pin than the one that comes with the baseplate. Here is the issue. Some of the holes in the baseplate for the pivot pin end up under the router base. If you plug the pivot pin into some of these holes it will contact solid metal of the router base. This situation is fine. But behind some of these holes there are channels or pockets in the router base, and it is possible to push the pin right through the base so none of its length extends. In use you will stick the pivot pin into the appropriate hole in the baseplate and then position the router so that the pin engages and plugs into a hole you have drilled at the center of all the circles that need to be cut. When you lower the router you hope the pin is actually going into that center hole, but it may actually just be pushing through the baseplate and into a pocket in the router base that you can't see. This situation is a disaster waiting to happen when you start cutting the circle.
One solution is to just make yourself a longer pin, so no matter which baseplate hole you use the pin will always be long enough to fully engage the center hole in the work. The other thing you can do is to build a solid plastic disc of about 1/16" thickness that will mount between the router base and the circle guide base plate. This way, the pin can't go through the baseplate no matter which hole it is inserted into. Such a plate is easy to make. Transfer the diameter of the baseplate and the location of mounting holes and bit clearance hole to a sheet of plastic and cut and drill.
Big routers generally do not have 1/8" diameter collets, so the bit will have to be mounted using a 1/8" collet adapter. Set the plunge stop on the router for less than the thickness of the top plate. I like to keep the floor of the pocket about 0.02" thick. Clamp the top plate (usually rectangular at this point) to a flat backer board using four clamps at the corners. The clamps must be placed so they don't interfere with the rotation of the router about the pivot pin. Mark the location of the center of the soundhole, prick the mark with an awl, and drill a 1/8" diameter hole at this point using a bradpoint bit. The hole goes right through the plate and well into the backer board. Put the pin in a hole in the baseplate and check to be sure the center hole just drilled is deep enough. You want the router baseplate sitting firmly on the top plate.
The baseplate is marked for a variety of cutout/channel outside diameters. Unfortunately these marks are not for disc/channel inside diameters and they are only accurate for cutouts if a 1/4" router bit is used. Fortunately the manufacturer of the baseplate provides the math necessary to indicate which pivot pin hole to use for the diameter of cutout or disc you want. Unfortunately not all router bit diameters will give you exactly the desired circle diameters, something the manufacturer's instructions don't tell you. When routing the simplest rosette plus the soundhole you need to do the math three times - once for the outside edge of the rosette channel, once for the inside edge, and once for the soundhole. A typical steel string rosette with three rings, plus the soundhole, requires doing the math for five cuts. This is too much for my elderly brain and I always end up making at least one arithmetic mistake. So I made the following calculator to do the math.
The calculator takes the following arguments:
And the calculator returns:
Here's the calculator:
Note that the calculator can be used for any circle cutting operation. It does not check for values outside of the range of pivot pin holes provided by the baseplate. See the circle size range in the manufacturer's instructions for your particular baseplate. Note also that the actual diameter cut will be the same as your desired diameter only if both the desired diameter and the diameter of the router bit are evenly divisible by 1/16". In all other cases the actual diameter will be off by as much as 1/32". Note that this means that bits and circles in rational millimeter sizes will almost always end up a bit different than desired. Even given these limitations I find I can cut appropriate channels for all rosettes.