HD28 Kit - First Build

[Rick S]

Is it clear that the radius dish is used to add a little angle to the sides and kerfing to make a good mating surface to glue to the top ?

That has nothing to do with the slope of the rims from the soundhole to the neck block.

I use a 40' radius top, which is so close to flat that I do not have to use a radius dish, for instance.

But the slope of the rims is the same each and every time, regardless of the radius of the top.

Get your slope right first, then if you need to radius the rims to provide a good mating surface with the top, then go ahead.

The dome need not trouble you. With proper rim preparation, it will fit nicely.

.02

Hello Dave. Thanks for the good advice and encouragement. It's not totally clear to me that the radiusing dish is solely to provide a good mating surface between the top and sides. I could be wrong but I believe I've heard John say that he's found that the 28 foot radius helps provide the 1 1/2 degree angle needed.

[Rick S]

I am on the east coast , If I am here I pick up the phone , or at least trip over the cats while running for it

John, thanks for the great information received over the phone. You're a gentleman and a scholar!

[Rick S]

Makes sense to me - at least on the upper bout where the fingerboard makes contact with the soundboard. OTOH I would think that a radius on the lower bout creates a dome that is structurally sound enough to counter the string tension.

Om my build I'm following some pretty detailed plans for the Official Luthier's Forum's medium jumbo and the luthier who drew them up made it abundantly clear that there was to be a 1.5º angle at the neck joint - which was further applied with a neck block extension that runs beneath the fingerboard extension up to the upper transverse brace. He also specified a 28' radius for the top though, so what I was going to do was to use a 28' dish to apply the radius, then go back with a flat sanding stick across the upper bout.

Using the 28' foot radius disc then flat sanding the upper bout is very similar to the process I used. John gave me similar instructions but, rather than using the upper transverse brace location as a reference, the reference is 1/16 rise at the saddle location. Good luck on your build!

[David L]

I do not radius the topside of the rim at all. Before I close the box I sand the whole topside of the rim dead flat with a flat bar privided with my KMG Radiusing fixture. I then make the proper adjustments to the fixture to sand the one and a half degree flat slope from the front of the sound hole to the neck block. The top of my box (prior to installing the soundboard) essentially has two flat spots, no radius. When I brace-up the soundboard, the only two braces that have a radius on them are the X-Braces, all other braces are flat bottomed. The X-Braces are radiused at 28', glueing that subtle of a radius (over such a short distance) to a flat (non-radiused Rim) is inconsequential. My top will have the radius without any forced stresses built into it. How do I know this? 'cause I'm a fart smeller, er, um, I mean a smart feller.

David L

[deadedith]

Well put David L!

[tippie53]

Point to consider , When you use any jig , Location is important . That is why you use the mold set on a parallel , this gives to a datum point to secure good numbers to assure critical dimensions . In plain words be sure you work is secure and not flopping around .
Have a good starting point to start to take measurements so you know the numbers you are getting area accurate. We like to see the questions you are asking , it means you is thinking . I know we often play around and have fun , that is what a hobby is about , but when setting something up that is critical to the final outcome , you need a process that is repeatable , and a way to measure so you can reproduce those numbers .
Sorry engineering floating on the surface . There is no one way that is the best for everyone . You need to find the techniques that works for you , Also , if you are building Martin style , that is different than gibson , which is different than taylor . All have gravitated to production techniques that work best for them .
Ken and I may not do things the same way , and that is good , as it gives different approaches to the same task . As you build keep a log book and note the things that happen and are right , and wrong. As you dial in your jigs, you will be surprised at how close you can work . I have had 4 dovetail necks just drop in without any fitting . Granted that isn't many out of over 100 , but you can see the more notes you take the better you can eliminate variability in process.
You agree Ken ?

[johnnparchem]

John and Ken,

Thanks to both of you. Through this discussion I am finally starting to get the reason behind the processes. I found both views and process very important toward understanding what I am doing.

[David L]

I also think that our two most experienced participants on this forum, having very different ways of doing things is an asset. I believe in math (I'm not very good at it), math doesn't lie, I lie, math doesn't, 1+1=2 always and that helps me to understand. I don't like doing things a certain way just because that's how it's done. By understanding why something is done a certain way helps me to be a better builder.

David L

[deadedith]

Here's another try, as I have visualized it to help me.
I think everyone understands it, but sometimes a diagram helps.

[Rick S]

Here's another try, as I have visualized it to help me.
I think everyone understands it, but sometimes a diagram helps.

Nice diagram Dave. Seems like an interesting jig idea.

I wasn't sure where the .392 arc angle factor came from so I had to tie into the .51 height using a different method. One minor adjustment that I would make is that the location of where to put the .51 inch block is actually a skosh less than 22.5 (22.494219). With a small angle of 1.3 degrees, it's hard to see the difference. I used the tangent function in Excel to determine that the slope at any point along the elevated radius line in your diagram is .024992. The slope .024992 times the run of 22.494219 equals the rise of 0.510465245.

Thanks Dave. It's all helping.