Official Luthiers Forum!

Well, I'm certainly no archtop expert--I've built a few and I have my ideas, but I'm still mostly a banjo guy. That said, banjos and archtops have a lot in common in terms of their overall physics of sound. An archtop plate and a banjo head are about the same when it comes to how the plate is excited by the energy of the strings. Yep, all of the archtops I've built have had a bit of spring in the bracing. It syncs up with what I do in tensioning a banjo head--although I like the idea of a totally "relaxed" build, a bit of tension on the soundboard (at least on banjos) seems to be a good thing. But I agree with what you're saying. Over time, as wood settles some of the voice can be lost. That's the big advantage of being a banjo builder. I can change the head tension on a banjo head whenever I want, and I can vary the tension with the different tension brackets to introduce extra tension in any direction that I want. Kind of like being able to change bracing patterns on the fly. Enough to give you guitar guys a bit of banjo envy, doesn't it!!??

I use a lot of carbon fiber in my banjos and have been planning to start using CF in archtop bracing too. I really like what it does in a banjo neck. But as I say, I'm a banjo builder and I'm a hack when it comes to archtops. I love building them and have my ideas, but those ideas are as likely to be complete BS as not, so take whatever I say here with a grain or two of salt!

Everything I've done with archtops has been based on either tradition or on my experience in banjo and resonator guitar building. All the same thing when it comes to the physics of the strings moving the bridge, but different in the nuances that separate the good from the mediocre. I've experimented a lot with banjo design over the years and hopefully can do the same sometime with archtops...

In the meantime, I'm glad there are folks out there doing the experimentation already!



Dave

Hi Bob,

I got it today and I am overwhelmed.......I need a math guy with cnc and electronic experience quickly! It is a 4 axis LNC but not nearly as self contained as I thought. I did not get an enclosure for it and the docs leave a lot to be desired for a diy.......

I now have about two weeks of intense dental pain to endure during a long procedure......no, not all at once either but long regardless so I won't be doing anything for that length of time. If I can concentrate while under the influence of the meds, I will try to consume the books.

Mike

After 20 years dealing with vibration and crack propagation, I take no chances when engineering. When thinking of doing an archtop I'm tempted, of course, to just program in some integrated bracing but I always go back to the idea that the grain orientation is not optimal and if a crack starts due to stress, it will propagate to failure from the top of the brace downward to the base. HOWEVER.....there is no reason the base of the brace shouldn't have runout as long as there is a correctly grain oriented CAP on the brace.

One option might be to leave the inside undercut by perhaps .05" and then glue separate, correct grain oriented braces into place....to be followed with a final cut that removes the extra material from the inside surface and does a preliminary shaping of the braces. This would give you 100% grain oriented braces and clean up the glue-line into a pretty tidy looking finished result. There would be a interesting look to the base of the brace when leaving a small fillet with a ball nosed cutter.

Furthermore...if you have integrated cam software I don't see any reason you can't develop a registration system into a vacuum fixture and do all your brace carving and tap testing on the machine. At some point, of course, substituting the cnc machine for a quick chiseling becomes ridiculous but if one were to keep the basic process of brace carving in mind when programming, and use a methodology that allows for quick alterations to the model...I'm certain the subtle alterations can be replicated on the machine. The point to this would be to have a visual record of each alteration and notations for each.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

[quote="Zlurgh"]After 20 years dealing with vibration and crack propagation, I take no chances when engineering. When thinking of doing an archtop I'm tempted, of course, to just program in some integrated bracing but I always go back to the idea that the grain orientation is not optimal and if a crack starts due to stress, it will propagate to failure from the top of the brace downward to the base. quote]

In other words, if the top starts to split from drying out, the brace will crack along with it. That makes sense.

Yikes! Industrial controls are a lot like electronics components: the manual is 'good' if there's enough information in there to get the thing hooked up...assuming you're already an expert. Put in a little messy translation and the fun gets ramped up!

You'll figure it out, just make sure to take the motors off the machine while you're messing with the control. Painful, crashy lessons learned when retrofitting the Fadal the first time! An oscilloscope helps a lot sometimes, but that's mainly if you don't have documentation on your servo drives.

Looks like some nice control panels and stuff...what did that puppy cost?

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I like how you always take it a few hundred steps 'too far', Stuart. I've been thinking of doing something similar since slightly before I got my first CNC, but I can never justify the cost of the scanning interferometer to do it the way I want to.

The fact that such a thing hasn't been done by anyone with the resources (a manufacturer) always leaves me comfortable that there'll be a million miles of ceiling for innovation in guitar construction for years to come.

_________________
Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

That might be a secondary benefit.

But...the main purpose of the brace is not to prevent cracking from humidity cycles...it's to add geometry that increases stability yet keeps things as light as possible. That means it will carry a sort of cantilever load...a bending load. For that kind of load fiber that is oriented directly to that load will be most efficient.

My point was that if the load along a brace is primarily a bending load then the stress concentration will be at the top of the brace where the material is thinnest and the leverage is highest. It will make little difference in the middle sections of the brace what the grain orientation is as long as the top is capped with a strip that correctly oriented. This is the fundamental principle in modern composite core construction. Fiber on the outside of a bend takes all the load (localized tensile load on the outside of the bend...localized compression load on the inside) and the core material need only maintain an even separation of the outside skins. The principle applies in any bending structure.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

That's a good thought.

I'm not saying mine is a great idea though...just something good enough to try and see. I know it would look very squared away when it was done. And the principle seems sound.

_________________
I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Bob I had time to read some online docs by another company and found them fairly straight forward. I also discovered that most of the electrical stuff will be wiring ends on cables and this is a lot more plug and play than I first thought. About the price......well the cheapest one I have found on line since I bought this, and it is only a model number different, is $3800. I am going to do some serious re evaluation before I start wiring and may end up selling most of my higher end stuff and just build a straight forward simple machine.

Mike