Official Luthiers Forum!

I have searched the library on the use of radius dishes and am still a little confused. I have not built a steel string with a radius dish before.

From what I understand, the radius of the top in the upper bout should approximate the angle at which the neck will join the body. If done correctly, there will not be a gap under the fingerboard extension where it joins the top. Since there are so many variables involved here, how do make sure that you wind up with the proper neck angle, select an appropriate top radius, and get a perfect fit of the neck extension to the top? If, when you get around to setting the neck angle and you don't get a perfect fit of the neck extension to the top, how do you fix it?

I am building the Stew-Mac OOO 12 fret kit but I want to use a radius dish rather than the sanding stick method they suggest.

I have built 2 classicals and this issue did not come up. Making a classical seems a little easier for this part of the build since the neck angle is in the same plane as the top and the upper bout is flat (in general, although some classical builders do it differently).

Am I missing something or overthinking it?

I am sure I will be corrected if I am wrong, but I think , it is usualy neccessary to fit the neck , by shaving areas of the heel, and perhaps some sanding, of the top under the finger board, or the underside of the fingerboard it self, and even some shims if you are using the old martin style neck joint... Jody

if you look into the Yairi shop tour thread on this forum ,, its shows a little about setting a neck with the dove tail joint ,,, Jody

Thanks for the info but let me expand my question a little bit to clarify.

If you were to use a 60 foot radius on the top as opposed to say a 30 ft radius, this would effect how the fingerboard fits on the top. Is the difference so small that sanding the top (when you fit the neck) would allow for a tight fight no matter what the radius?

Or to put it another way, do different radius's approximate different angles and is that something that should be planned in advance?

Yes. This is why many builders arrive at what ever top radius that they prefer when combined with the building style that they use.

For example, I flatten the upper bout by using a flat upper transverse brace and flattening the kerfed linings from the neck block to the waist. The neck block is flattened too.

Some builders use a radiused transverse brace that is flattened under the fret board extension. And some may flatten the upper bout of the top so that the fret board extension lays down properly.

There is also the issue of if a slight drop off, .010 - .020 of the fret board extension is desirable and some of us calculate this factor in too.

And... of course there is the neck angle, bridge and saddle height and a partridge and a pear tree.......

I can tell you this that a 25' radius works well for me but I think that a 28' or 30' would work fine too if I also adjusted some of the other things that I do. Any of these top radiuses would work well for you too I am sure.

I'll take a quick stab at this- the understanding I've gained is that you want the bridge to be in the
neighborhood of 5/16-3/8 inch thick, and the saddle to extend another 3/32 to 1/8 above that. Ideally
then, you want a straightedge laid across the top of the frets to land right on the top of the bridge. If
checking without the fretboard or frets in place, you need to subtract the thickness of these parts to
give you a height to check. If the fretboard is 1/4" thick, and the frets 1/16" tall, then you subtract
the total of these from the bridge height 3/8'= 6/16 -(1/4 + 1/16)= 1/16". So if you lay a straight edge
along the neck, you want it to touch at the top of the body, and be 1/16" above the top at the bridge
location. The angle works out to be around 1 1/2 degrees for 20'-25' radius tops and "standard" sized
guitars. So you leave the heel of the neck a little long to start, and start trimming to 1) set the angle
as above, and 2) end up with the 12th or14th fret (depending on the guitar design) right above the edge
of the body.

As Hesh mentions, a lot of builders take the radius off the upper kerfings, from the soundhole to the top
of the body, making that section of the top a true flat plane- it sounds difficult, but it's easy, and doesn't
seem to affect the sound, although you'll get a lot of opinions on that one

_________________
Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

My first guitar build was a 000-12 Stew Mac kit. I used a sanding dish ( that turned out to be very inacurate) and really made the neck set a mess. For your first guitar I would suggest you use the technique Frank Finnoccio used in his class.

This is a two step process. First, with a 24" square flat board covered with 80 grip sand paper sand the top side of the kerfed rim flat. Second, double stick tape a 1/4" block to the tail block so that the lower bout rides above the paper and sand the area of the top above the sound hole to put about a 1 1/2 degree flat ramp in that area.

The domed top will flex to fit this shape and the neck set will be much easier.

Steve

I use a 28 foot bowl for the top, and radius everything. The neck angle comes out somewhere around 1.5 degrees (i dont measure it in degrees, it is whatever it is with a sliding bevel) and the fingerboard will sit flat on the top after you give that area a little flat sand. The neck angle is really irrelevant , it could be whatever, depending on how the rim was built. It is really nothing but the angle the neck blank needs o be cut at to sit on the sides.

This leaves a bridge in 5/16 to 3/8 range, and nice healthy 1/8 of saddle. The lie of the frets sits just above the bridge top.

Dont set your saw or whatever to cut the neck at 1.5 degrees. Measure the angle with a sliding bevel, then take that and use it to set the mitre gauge on your table saw - I leave the angle a little shy of what it measures, in order to initially set the neck with a forward pitch. This gives you the advantage of being able to pull sandpaper between the heel and body (floss the neck heel some call it) and have the neck slowy pitch back into alignment all at the same time with fitting the heel to the body. Handy when your neck area is a rounded shape, and the neck heel cut is flat to start out.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Thanks everyone for your replies. This answers alot of questions for me (but raises others as it always seems to do).



Tony - if I am reading correctly it sounds as though you build the body before you cut the angle on the heel of the neck. When you measure with the bevel, I am assuming this means you hold the bevel on the guitar side and the top and that is the angle you use (if that angle is say 91.5 degrees, the heel block would be cut at 88.5 degrees??) I guess this can only be accomplished when everything is planned out in advance (bridge height, saddle height, dome height etc.). This was kind of what I was getting at in my original question - the need to plan this in advance (or use someone elses good plans).


Guest (Steve)- Thanks for the tip that Frank Finocchio uses. That makes sense to me. I can't say enough good things about Frank. I took his archtop class as well as a few one day workshops. His attention to detail is amazing.

Steve, did the neck happen to have too much backset? I noticed on those plans, that if you put a straight edge across the top of the frets, it overshoots the bridge by a HUGE amount. But I noticed that if I drew in the arch that the braces have onto the plans, it seems to bring up the dome of the top to where it needs to be.

I like the idea that Frank uses. You don't have to thin the top in the upper bout. I don't like having to flatten the upper bout to get the geometry right, unless it's already close...I'd rather have the bracing correct instead. I finally figured out that a 1/32" offset on the UTB gets me really close on my dreads.



Tony, do you use the same radius on the UTB as the other top bracing? I drew out the angles according to the plans, and they appear to be correct (StewMac plans) even though it looks visually to be a hefty amount, but it matches the radius of the other top bracing. During my build I am thinking about gluing on the top first, and then doing some kind of preliminary testing of the neck angle before gluing on the back; just in case I need to change the UTB radius.

_________________
Brad Tucker