Cutting an oval (elliptical) rosette and soundhole with a circle-cutting jig

[Mark McLean]

I tried an oval soundhole on my last build. I was asked how to cut the rosette and soundhole. Here is the method that I came up with using the familiar Stewart-MacDonald circle-cutting jig and a Dremmel tool, which is how I usually do circular cuts.

Oval soundhole shapes are nothing new. Mandolin/Bouzouki/Cittern family instruments have been using them for a century and a half. It was also popular in some Gibson guitar models of the 1920’s, and with other makers of that era, like Maccaferri. The traditional way to cut the oval hole is using an oval shaped template to guide a cutting tool. Then a simple rosette channel could be cut using a gramil tool registering off the inside of the soundhole. Here is a pictorial of that traditional method from Dan Koentopp.
http://koentoppguitars.com/blog/constru ... chicagoan/

However, guitar makers are used to cutting circular rosettes and inlaying them using a circle-cutting jig incorporating some sort of routing tool. So – if that is your favourite method - can you use the same sort of kit to cut an ellipse (oval) instead of a circle. The answer is YES. Setting up a router to cut an ellipse is not too difficult. Instead of a single pivot point you will need two pivots, and a cross-shaped channel in which they can run. This 45 second video demonstrates the basic principle:
https://www.youtube.com/watch?v=FsxDVhoS-o8

If that has got you interested, this slightly longer video gives some more useful information about how a jig would be set up.
https://www.youtube.com/watch?v=ZJ09XPqBX28

These examples demonstrate how to route a big ellipse shape, like a table top. We need to scale it down to make a guitar rosette and soundhole. Here is my adaptation of the Stew-Mac circle router using a Dremmel tool. The usual axle pin is supplemented by a second (a drill bit) held snug in a small block of wood and secured to the jib by tape (double-sided tape, or the superglue and masking tape trick).

Some basic mathematics – the area of an ellipse is (π x R1 x R2), where R1 and R2 are the radii of the short and long axes of the ellipse. You can choose dimensions which will give an equivalent area to a normal circular soundhole (e.g. 120mm x 80mm gives an area almost exactly that of a 98mm diameter circle). Once you have decided on the dimensions of the ellipse you set the two pins at a distance from the Dremmel bit that equals half the diameter that you want to achieve. For example, if you wanted a hole of 120mm x 80mm you would set the pins at 40mm and 60mm from the bit. You need to consider whether to take your measurement from the near side of the cutting edge (if you are cutting out a rosette disc) or from the far side (cutting a rosette channel or the soundhole) – but if you have experience cutting circles with this jig you will already have worked that out.

You then need to make a cross-shaped track for the pins to run in. Thin MDF or Masonite works well for this. The cut-out needs to be a tight fit for the diameter of the axle pins so that they don’t wobble about as they run. I screwed a solid block of wood to the underside of this so that I can mount it securely in a vise at bench height. Then the guitar soundboard sits on top with an initial small hole cut just so that you can see the jig underneath and get the alignment right. Some corresponding marks on the soundboard help get it straight. An oval that is slanted a bit wouldn't look too good.

In this job I cut the outside edge of the rosette channel first. Then you could adjust the diameter to do the inside edge of the rosette, and then use some method (Dremmel freehand, or chisel) to hog out the material between the channels. My rosette was going to extend all the way to the soundhole edge so I didn’t need to do a second channel for the inside edge.

Use a similar process to cut out your rosette material. I glue it in with a Perspex caul in the go-bar deck.

oval jig 5.jpg (93.81 KiB) Viewed 32063 times

oval jig 6.jpg (136.09 KiB) Viewed 32063 times

Once it is dry you can cut out the soundhole to final dimensions. In this case it was a bit oversized so that a soundhole binding could be added to bring it to final dimensions.

I found the cutting process a bit stressful (high pucker factor as Robbie O'Brien would say). The tracking of the cutter can get a slight wobble as it flips from pivoting on one axis to the other. You need to go slow, especially at the ends of the ellipse (where you are also most likely to get tear-out of the grain). I had to tidy up some slight irregularities. As always - do lots of practice on scrap first!

[lamanoditrento]

Thanks for a great post! That is going straight to the pool room (or bookmarks).

[Fisherman]

Great tutorial, Mark. I'm looking forward to tring this out (on scrap first!).

After having used a gramil to cut binding channels and for one (old fashioned round...) rosette I'm keen to avoid it again if at all possible...

[simso]

Came out really nice

Steve