Search Results for: Radius

The backs of most modern musical instruments have a slight arch to them, they are not flat.  The slight arch makes the instrument stronger, and allows expansion and contraction with changes in humidity.   As the back swells with increased humidity the arch gets a little higher.  I use a pretty standard arch which is a circle with a radius of 15 feet.  The back braces are also cut with this 15 foot radius arch so that when they are glued to the back, the back has the desired arch.  The braces are glued to the back in a dish which has the same 15 foot radius cut into the surface.  One can buy these radius dishes, but they are (in my mind) pretty expensive, so I have cut my own.   As I build out the shop here in Florida, I need to cut some new dishes.

To cut a dish I made up a jig which is a channel just the width of a router base.  The bottom of the channel is made with some thin plastic trim molding which is bent along a 15 foot radius line.  Then as the router is run along, it cuts the 15 foot radius line.  The dish has a center pin, so you just rotate the dish and re-run the router along the channel.  A great project to do outside on a cool and breezy day because it makes a lot of sawdust cutting that dish out of a piece of plywood.  When you are done with the router a quick sanding to take off any unevenness and fuzz, a coat of shellac to seal the surface, and you are good to go.

I have received a number of requests for ukuleles with a radiused fingerboard (a slight convex curvature of the fingerboard).  This is supposed to make playing easier, particularly if you play bar chords, with a finger (naturally curved) across all four strings.  A 12 inch radius of curvature seems to be pretty well accepted. The issue is, how to easily and reliable cut the top surface of the fingerboard into the proper radius.  I have seen a number of jigs around, but a number of them were rather complex to construct.  I came up with the following design.

Using the laser cutter I cut 12″ radius curves into two matching acrylic side plates.  (The laser cutter allows one to make things very exactly, exactly a 12″ radius, and the two side plates are exactly the same.)  I realized that one only needs to have 2 points for these curved side plates to slide along to get a circle, so I used two sections of 3/4″ PVC pipe as it is very uniform, and rather slippery.  I routed two parallel groves into a pieces of really good plywood I came up with and bedded the PVC in auto bondo.  I epoxied the side plates to a top piece of good plywood, and added a threaded insert to the top with a corresponding hole in the acrylic baseplate of my router so I can easily affix the router to cut fingerboards, and away you go.  Works great, so in the future a radiused fingerboard will appear on some ukuleles, and will be an option for a custom order uke.

When you saw brace wood out of a board the wood fibers can (often) not run parallel to the faces of the brace. This is called ‘runout’. Under stress a brace with runout will tend to crack along the wood fiber lines, which are across the brace. If one looks at guitar repair videos where braces are being fixed, the crack is almost always across the brace, due to runout.

I have some nice sitka spruce boards which are nicely quartersawn which means that the growth rings are up and down in the board, ‘vertical grained’ which is what one wants for a brace. However, a little experiment demonstrated that the wood fibers did not run parallel to the face of the board.

As a way to make a stiffer, more stable brace I decided to saw this up into thinner strips and then laminate them together, alternating direction, to eliminate the runout problem. At the same time I decided to laminate them with a 15 foot radius curve which is what I use for back braces. That way I do not have to further process the brace to add the curved face.

I made a simple gluing block and laminated with a pre-catalyzed modified dry powder urea glue. This glue dries very hard, and once set is impassible to soften with water or heat. The perfect stuff for laminations that one never wants to take apart. I glued up some sections and then sawed the sections into brace stock. They held the 15 food radius well and seem very stiff which is good.

A little while back I built an octave mandolin. I built what is called a ‘flat top’ mandolin (as opposed to a carved top like a violin) though the top and back are not flat. Both the top and the back have a 15 foot radius. I have 15′ radius dishes from building ukuleles (the backs) so I was familiar with the radius of the plates.

I merged a couple of different plans, and settled on an X braced top. (I build ukuleles with X braced tops.) The plans also had an X braced back, so I did an X brace on the back as well.

On the next set of ukulele builds (tenors) I decided to try an X braced back with a single cross brace in the upper bout. It worked out really well. I think that the X holds the radius dome of the back better than the previously used 3 cross braces. I radiused the braces individually, glued the X together, and then re-sanded the entire X in the radius dish. With the X I could place the center of the dome exactly where I wanted it, and I was not relying on the radius profile of the sides to establish the top-to-bottom radius to match the side-to-side radius imparted by cross braces.

I also think that an X braced back puts less strain on the instrument with humidity changes. With a cross brace the back wood is expanding/contracting across the length of the brace. But the brace does not get much longer/shorter with humidity changes since wood does not change much in length. This always has bothered me. The back wood is expanding/contracting and trying to break free of the brace. With an X brace the stress should be less because the X is at 45 degrees to the direction of wood movement so there should be less movement per-unit-length of the brace. The X brace is longer so the wood movement is spread out over a longer distance.

I see no reason to go back to cross braced backs, at least on tenor or baritone sized instruments.

A little while back I wanted to do something different. Being a fan of Celtic music, I decided to build an octave mandolin. An octave mandolin is like a regular mandolin, 4 pairs of strings, only it is tuned a full octave lower. The Irish Bouzouki is an octave mandolin. I built what is called a ‘flat top’ mandolin (as opposed to a carved top like a violin) though the top and back are not flat. Both the top and the back have a 15 foot radius.

This was also a chance for a bit of experimentation. I laminated the walnut sides. Three pieces of 1/16” walnut veneer, pre-bent on a hot pipe, and then laminated together using the octave mandolin form I made. They came out great, very stiff and accurate in shape with no spring-back. (You can never have too many clamps!)

The top is Alaskan yellow cedar with an X braced system that is the merging of a couple of different plans. I also did a ‘Picasso’ rosette, gold mini-guitar tuners and frets.

The instrument came out really well. Lovely to look at (I really like that teardrop shape) and it is really loud and has sustain that goes on forever. I had a chance to show it to a Celtic band and the bagpipe player said he likes mine better than the commercial octave mandolin his band-mate was playing.  You can see the instrument here, look under “Ukuleles – Gallery”  (where else to put pictures?)

I thought I would go into a bit of detail as to how I cut a spiral rosette. To do this I use three different jigs that I developed and cut out of ¼ inch acrylic on a laser cutter (which I use at out local community college, a great resource.)

The first jig is the spiral template itself. This is used to cut the outside of the spiral. It is indexed to a center hole which fits over a 1/8” pin on a routing board which comes through a corresponding hole in the top. This pin will also be the center of the inner circle of the rosette, and the smaller eventual sound hole. The spiral jig is positioned so the the point of the spiral is out from under the area that will be covered by the tail of the fretboard. The spiral jig is held in place by clamping a straightedge to the routing board along one side of the jig. Since the jig also is fitted over the centering pin this holds it steady.

The second jig is a base for a StewMac dremmel router base. This is a simple base, with a bit of 3/16” brass tubing glued to a hole in the center. This tubing extends a bit below the base allowing the base to ride on top of the spiral jig and follow the outside profile. A small dremmel bit is fitted to the dremmel and extends through the tube just far enough to cut the desired rosette channel, the depth of which is set by the thickness of the pearl which will make the rosette.

Using this setup the dremmel router is run along the outside edge of the spiral jig to cut the outside of the spiral.

When this is done one shifts to the third jig, which is the dremmel router base with which I cut my other more circular rosettes. This base has a series of holes which fit over the indexing pin and when the dremmel router rotates around the pin a circle is cut. The holes are spaced and numbered such that moving from one hole to the next larger increases the diameter of the circle cut by 1/16”, or only 1/32” increment in the radius.

Using this base, and a small router bit, one cuts circles aiming to just meet the tip of the spiral path already cut. One can start with a slightly larger diameter and run the cutter up to the spiral path. One then decreases the size of the circle by using successively lower numbered holes till the circle just meets the spiral.

The very pointy tip of the spiral is cut out using a scalpel. When this is done you use successively larger numbered holes in the router base to clean out the inside of the spiral channel.

Now that the spiral channel is cut, all that remains is to line it with purfling and cut pearl to fit. Cutting the pearl is actually easier than doing a completely round, even with rosette. Since the spiral is tapered, one cuts a piece of pearl and can then slide it around from a larger part to the smaller part, compressing the purfling tight against the side of the channel. The very end of the spiral will be hidden under the tail of the fingerboard.

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