can I glue a flat (slightly arched) top and back to the flat kerf

[hugh153]

My octave Mandolin build (first time)is proceeding, thanks to advice from forum members.
My top and back will be slightly arched. Can these arched top and back be glued directly on to the flat kerf,or does the fact that there is now an arch, mean that I have to do something to the flat kerf to receive the top and back ( i.e angle it)?
Hope the question is clear. I'm building with only a limited knowledge of what I need to do, but enjoying it.

[sliebers]

From a guitar building perspective, arched tops and backs get the respective radius sanded into the rim with kerfing attached creating a dome to the entire surface. Radius dishes with sandpaper stuck to them are the usual method of achieving this, although other methods can work too.

I carve my octave mandolin tops and glue to flat rim, but my backs are guitar style with a 20' radius that I sand into the rim with a dish.

[tom.gibson]

You really need to shape the linings and the blocks to the same radius as the top or the top will deform or the glue joint will be bad. It's pretty simple to make a radius sanding dish to do the job.

[rcc56]

An easier alternative to a sanding dish is to cut your brace radius into a piece of 2" x 2" that's longer than the maximum dimension of your instrument body, and sand the face of it smooth and knock any sharp corners off the edges. Now you have a concave, radiused sanding block.

I have only a very modest shop with only a couple of power tools, so I roughed out a few of these on my cabinetmaker friend's bandsaw, and finished them up with a spindle sander chucked in my drill press.

You can glue a couple of inches of sandpaper to one end of the radiused face of this block, and leave the rest of the block bare. This will allow you to sand back and forth on one section at a time while the bare end of the face rests on the opposite side of the instrument.

You can use the same block to radius your braces. If you make a few of these blocks, you can also use them as radiused clamping cauls.

It's easier than making a dish, and will work just as well. Dishes are great if you have the time and equipment to make them, but for a "poor man's shop," the radiused blocks are a practical alternative.

[sunburst]

Martin guitar tops have been traditionally glued to a "flat" rim even though the top plate has a slight induced arch. (Backs, with their slightly higher induced arch, have generally been accommodated when surfacing the rim.) Guitar tops can be done either way (radius rim or not), and of course there are people who claim to hear a difference, and among those, some prefer the sound of the top glued to the flat rim.

[tom.gibson]

So I guess it depends what the OP means by a 'slight' arch. Maybe try fitting the top to a flat rim first and see how it looks. If it doesn't take much pressure to get a clean joint all around, it's probably okay.

[Tavy]

I think there are two ways to approach this - a continuous arch which extends out to the rim and ideally needs kerfing that matches the arch (lets call this the guitar method), or an arch with a slight recurve to it, so that the edges are flat despite a pronounced arch in the centre (I'd call this the Neapolitan method, as many bowl backs employed this approach). The latter works well for me, but there are many different ways to get to a good result.

There are lots of ways to produce the braces too: a sanding dish, a disk sander, spindle sander, or just some grit taped down to a flat surface. In the last 3 cases you mark out the profile first (mock it up on the computer and print out a template) and then carefully sand down to the marked line. All the methods have their pros and cons: sanding dishes are fast and accurate but only produce one radius and are a pain to make, disk and spindle sanders will keep the brace nice and square but there's a serious risk of a non-smooth curve, a sanding table is fast and produces a smooth curve, but there's a risk of things ending up non-square. So whatever you choose, the skill of the operator, and taking lots of care are in order!

[Tom Haywood]

What Tavy said. Another method to bevel the edge and kerfed lining (guitar method) is to use a sanding stick. Has it's own disadvantages, but it is quick and easy.

[Parker135]

I built three Gibson Army/Navy style mandolins from Crystal Forest plans. The designer suggests a thin piece of plywood or similar, about like a paint paddle, with a piece of quarter inch stock glued to one end to elevate it a little, and a strip of sandpiper glued to the other end. Rest the thickened end on the rim while sanding the opposite side. The angle pretty well matched the 12 ft radius of the top and back. Simple, but it seems better than doing nothing.

My current builds from Graham McDonald's plans use a radiused dish with a central hole that goes over a center post attached to a work board beneath the rim. I sprung for a commercial ukulele-sized dish with strips of sandpaper attached. This works really, really well. No going back now.