Project How to make a mess - more jig building.

[boboreilly]

Although the top of an acoustic guitar may appear to be flat, it's not. It is actually domed with a very long radius (in my case, 28 feet). In order to be able to glue up the the top to the braces, and hold that curve, everything is sanded to the proper radius using, and then glued into place using, a radiused dish.

I first thought the 28ft radius would be easy to draw... just get a piece of string 28ft. long and tie a pencil to it, and draw the arc. No such luck. Too inconsistent and the string stretched. So, I drew it up in AutoCad and then printed a 3 ft. long section of it in lifesize. I glued it to a piece of wood for a template, bandsawed, and sanded to hit the line perfectly. Then I used that pattern and template routed 2 guides out of scrap poplar (one of those guides will later be used as a convex template for shaping the internal braces of the guitar).

Then I threw together a quick jig for my router to follow those guides, and made the biggest mess of my life.

Here's the jig. I drilled a center hole in the bottom of the 24" diameter mdf circles and a corresponding hole in the top of my cheapo mdf bench and used a metal dowel to make the dish pivot.



Here's the finished dish with a jointed board on top so you can see just how subtle the radius is.


It then got a quick sanding with the ROS, a light coat of shellac to seal the big fuzzy surface, and will subsequently get 80 grit sandpaper applied to the whole dish.

[NN]

Cool jig Bobor.









I could do that if I had one of those gas fired band saws.

[boboreilly]

That's water.

[NN]

Why heck that is even better.

[boboreilly]

I forgot to mention... I have 2 do a second one of these for the back of the guitar with a 20 foot radius.

I think I'm gonna do it outside.

[Boxman]

So...help me out here... When I send you 3/16ths thick slices of old growth cedar you're gonna sand them in this jig until they have a 28' radius to them?

[thepsdes]

You can readily draw an accurate arc if you know the arc length and the height of the arc (also known as the segment height). This process is called a Long Compass. To perform this place three brads in your workpiece the outer two at the sends of the arc and one in the approximate center slightly above or below the straight line connecting the outer points. The distance above or below the line should be equal to segment height. For discussion, say 1/8th inch. Take two straight sticks each slightly longer than the distance between the two outer points and resting the sticks on the outer brads and the inner brad they will form a V ( the sticks will overlap at the center brad). Fix the angle of the two sticks over the center brad. Place a pencil in the crotch of the V and keeping the sticks applied to the outer brads simply move the pencil left and right and you will draw a perfect arc. Now here is the problem, If you do not know the segment height it can be calculated knowing the radius of the circle and the length of the arc but it requires calculus. If any one wants to pursue this just let me know and will go through the calculations. John

[Jacko]

Very interesting Bobo........ continue with the progress.

[boboreilly]

[Boxman]

Beat me to it