Rim Contouring

[allenb]

Hello,

I'm new to the forum and hopefully my question hasn't been answered a thousand times. If it has, please direct me to best thread that has already covered it.

I'm considering the Masterwork Guitar Mold System in conjunction with the Stand Alone Universal Contour Fixture but have some questions on creating the proper rim/kerfing angles. The back seems very straight forward since all of it is domed but the top has me stumped.

Following the instructions creates a sloped surface around 1.5 degrees (neck angle) from the waist up to the heel and the rim/kerfing ends up being flat. Doesn't the rim from the waist down to the tail block need to be angled to match the tops radius? The radius I'm referring to is from side to side, not spherical.

The question I have is how to merge the flat rim area from waist to heel with the angled rim edge from waist to tail?

Allen

[tippie53]

http://www.youtube.com/watch?v=QYHPCeVRUA4
If you want accuracy you can't beat the disks. I am closing in on 150 guitars and tried many different systems and none gave me the consistency of the disks. The backs radius usually run 15 to 20 foot radius and tops are more to match your neck angle. In most cases the 28 foot is used for a 1 1/2 degree. The key is to use this to set up the final action height on the top. You want to end up with a 1/2 string height at the front of the bridge.
I use a mold system same as Martin . This gives you a repeatable process.

[allenb]

I had read your article a while back (Neck Set Angle
Myth, Magic or Mathematics?©) and it helped me out on my first steel string build which was a 17" jumbo. One problem I ran into was putting a radius into the shoulder brace which I thought was necessary to blend with the top curvature imparted by the X braces. Because of this I had to sand a flat area for the fingerboard to have a continuous glue area. I used a 40' radius. Without it, it seemed it would be too much stress going from the domed shape of the top to the flat shoulder brace where the tops of the X meets up with the ends of the shoulder brace.

In your article it states "Brace tapered on the end to blend with X brace contour radius"

Is this being suggested or were you showing what some kit suppliers are telling their customers to do and is unnecessary?

On to my original question in my first post. I understand that the article states to sand the rim flat from tailblock to waist but I was assuming this was not the final step. If it stays flat with no taper to the rim edge wouldn't this cause me to have to force the domed top's edges to flatten out where they glue to the rim?

Allen

[kencierp]

Allen,
the taper on the shoulder brace is minuscule and frankly if you slope the rim from the sound hole back to the neck block properly the shoulder brace can be left flat. It would be hard for me to back track and tell you what you did wrong (seems you may have over did it tapering the brace ends) on your 17" jumbo but I really can't comment on that issue.

I think you are hung up on the premise that some how the X braces and the tone bars shape the sound-board into a perfect spherical section -- that is simple not true. The bracing does tend to raise the sound-board in the center but the perimeter is still relatively flat -- perhaps width-wise at a very,very slight angle -- the PM I sent you showed that .008" difference inner to outer edge of the rim.

This stuff is not new or even my idea -- its how its done in factories and premier custom builders all over the world --- Preston Thompson has exactly the same set up you are describing "stand alone and Masterworks" he is one of the best vintage replicators in the world.

[Darryl Young]

Hello,

Following the instructions creates a sloped surface around 1.5 degrees (neck angle) from the waist up to the heel and the rim/kerfing ends up being flat. Doesn't the rim from the waist down to the tail block need to be angled to match the tops radius? The radius I'm referring to is from side to side, not spherical.

The question I have is how to merge the flat rim area from waist to heel with the angled rim edge from waist to tail?

Allen

Allen, I'm a little confused by your use of the word "heel". The rim is sloped 1.5deg from the waist to the neck block (maybe that is what you meant by heel) and left level from waist to the tail block.

In addition, you have a choice of leaving the rim edges perfectly flat (from inside of lining to outside of sides)......or sloping/radiusing that rim edge to match the radius of the top. If you decide to radius the edge of the rim (say with a radius dish or similar), you will probably only do this from the waist to the tail block leaving the rim from the waist to the neck block flat. Alternatively, you could leave the rim flat around the entire perimeter and "spring" the radiused top onto the flat rim. That is not a problem and some folks do this on purpose believing it loosens the perimeter of the lower bout a bit giving a more traditional sound. I forgot the the model numbers but I've read where the main differnce between the Huss & Dalton traditional and modern models was leaving the rim flat on the traditional models and radiusing the rim on the modern models. Some folks state the radiused models are tighter and maybe a bit more complex overtones and the leaving the rim flat is warmer and more open. Some folks leave the rim flat on 000 and smaller models to get as much bass as possible (they usually already have very good treble and overtones) but radius the rim on dreadnaughts as they are so bass balanced with fewer overtones and this may add a bit of complexity/sparkle to their tone.

If you choose the radiused option, merging the flat part of the rim (upper bout) to the radius part of the rim (lower bout) is not a big deal......the difference is subtle. If I can find a link I'll share what one luthier does that works great.

[kencierp]

Thanks Darryl --- yeah, Huss and Dalton I knew there was at least one maker who had that rim sanding philosophy in their advertising I thought it was Hoffman or Santa Cruz, no wonder I could find nothing related at those sites.

[allenb]

Thanks Ken, John and Darryl for your replies to my question.

On my jumbo I had already shaped the shoulder brace to a radiused bottom throughout it's length from instructions in Cumpiano's book prior to seeing Ken's site. After seeing what Ken wrote the light went on and so it turned out to be a hybrid of sorts. My screw up. The jumbo turned out sounding wonderful BTW. Next build will be 1.5 degrees or so from waist to neck block (not heel) and sand flat from waist to tail. If it's good enough for the big guys it's good enough for me. It's funny how some of us look at something that's only a couple of thousandths and see it as a huge obstacle that can't be allowed but is really nothing.

Very interesting info about Huss and Dalton using radiused and non-radiused rim for affecting tone of the top. I had never seen anything on this before.

Thanks for clearing up something that's been bugging the heck out of me for a while!

Allen

[Darryl Young]

Allen, that's funny.......I was hung up on this exact thing on my first. I tend to obsess on little details like this as well. It's the main reason I take so long building. So my second should go much faster........however, I'm building from scratch (except for the neck) and now there are other things I've never done before!! <smile> First time to join panels (done the back on the 00 and I've done the top and back on the resonator) but still need to join the soundboard. I think I have a decent way of doing this now. I've already split the braces which was something else to learn to do as you need to make sure you follow the grain and have no runout.

Right now I'm drawing out the bracing details. Not using a standard plan so want everything laid out before I start (12 fret neck on a 14 fret 00 body). Then I'll have to do the rosette for the first time. Only issue here is that means I have to make a decision on the binding/purfling/rosette scheme.......and I'm terrible at asthetics!!!

[allenb]

It seems there are two primary camps of beginner level luthiers. One likes to just barrel on through to get to the finish to see the finished product and has the attitude that it may not be pretty but I learned a lot and after a few more I'll have it down. The other (including me) likes to go slow, obsess, ask tons of questions and when a mistake is in progress re-do it as many times as it takes till we think it's acceptable.

Either way furthers the learning process but I guess it's a little less costly on materials to go the second route. Both ways allows us to do what we love which is cutting and carving beautiful looking and smelling woods with real neat tools!

Allen

[tippie53]

I help a lot of people with the kit building and you are right . One piece of advice is , " you don't know ' what you don't know till you know it . "
Study what you are doing and if it makes sense to you ok if not ask questions , that is what this forum was made for .