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one radius dish for both back and top?
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=19645		 Page 1 of 1
Author:  Tai Fu [ Thu Nov 13, 2008 1:10 pm ]
Post subject:  one radius dish for both back and top?
I think someone here suggested that one 25 foot dish for both top and back is good, can someone here elaborate on that, like is this a good idea?
Author:  Michael Dale Payne [ Thu Nov 13, 2008 1:28 pm ]
Post subject:  Re: one radius dish for both back and top?
I dout it would cause a significent sonic issue, but 25' is far flatter than 15' the increased dome of a 15' makes the guitar more comfortable to play and provides a bit more strenght in the back
Author:  Bob Garrish [ Thu Nov 13, 2008 1:41 pm ]
Post subject:  Re: one radius dish for both back and top?
The only 'high end' guys I know using one dish use a 15' dish for the back...and they leave the top flat.
Author:  Chris Paulick [ Thu Nov 13, 2008 9:49 pm ]
Post subject:  Re: one radius dish for both back and top?
Are 2 dishes glued back to back considered as 1 dish? If so I use one with 15' and 25' then. Eat Drink :D
Author:  Mike OMelia [ Thu Nov 13, 2008 10:48 pm ]
Post subject:  Re: one radius dish for both back and top?
That's the deal I understood. 25' on one side, 15' on the other. I am getting ready to build one this weekend. Is MDF really the right way to go? I dunno what it is, but other than use in sacrificial benchtops and blade guides, I don't like the stuff. But if some of yoose wise guys who know better, then I will use it. ;)

Mike
Author:  Rod True [ Thu Nov 13, 2008 11:18 pm ]
Post subject:  Re: one radius dish for both back and top?
Mike I like MDF for my dish, but I like to double it up. MDF can of course warp when it's thin and exposed to humidity fluctuations. So, I double a 3/4" sheet, then cut out the perimeter (usually 24" diameter) then route the dish. It's very messy so make sure you have dust collection or do it outside if it's nice (meaning not raining).

When you're finished making your dish(s) make sure and seal them with shellac or varithane so they remain stable.
Author:  Mike OMelia [ Fri Nov 14, 2008 1:18 am ]
Post subject:  Re: one radius dish for both back and top?
Rod,

Have you ever done a double sided (double thickness) dish for two radii, or would you prefer separate dishes for diiferent raddii?

Also, since my current project is the harp guitar, will a 24" diameter dish be satisfactory?

Mike
Author:  Kevin Gallagher [ Fri Nov 14, 2008 1:44 am ]
Post subject:  Re: one radius dish for both back and top?
I use a single dish that I cut a 15 ft. radius in one side and a 25 ft. radius in the other
to use on my top and back.

It's cut from a two inch thick piece of HDF and has 80 grit abrasive in each radius. I've
used it on almost 550 guitars including all of the prototypes that i've done over the years
to try new ideas on, but on about 475 guitars that actually ended up in the hands of players.

16 years of duty and it's still like new.

Regards,
Kevin Gallagher/Omega Guitars
Author:  Rod True [ Fri Nov 14, 2008 2:28 am ]
Post subject:  Re: one radius dish for both back and top?
Sorry Mike, I make true flat tops (I know I know) so I only have one radius dish requirement. Just the 15' for the back. BUT, if I chose to go to a radius top, I'd have no problem (as long as the dish was 1-1/2" thick min) to put a 25' on one side and 15' on the other.

I'm pretty sure that Shane Neifer at High Mountain sells a double sided dish.
Author:  Tai Fu [ Fri Nov 14, 2008 5:22 am ]
Post subject:  Re: one radius dish for both back and top?
I guess I will just go with one back dish and sand the top flat and use curved top braces... will try and make one with wedges and a thin board because I can't really route a dish, too much mess and the landlord isn't happy with me doing woodworking in my apartment.
Author:  Mike OMelia [ Fri Nov 14, 2008 9:01 am ]
Post subject:  Re: one radius dish for both back and top?
Rod True wrote:
Sorry Mike, I make true flat tops (I know I know) so I only have one radius dish requirement. Just the 15' for the back. BUT, if I chose to go to a radius top, I'd have no problem (as long as the dish was 1-1/2" thick min) to put a 25' on one side and 15' on the other.

I'm pretty sure that Shane Neifer at High Mountain sells a double sided dish.



Hey, I hear you. I have yet to see a Martin or a Taylor with any curvature in the top. I just thought that whilst I had the jig set up, I would go ahead and shape both sides.
Author:  Kevin Gallagher [ Fri Nov 14, 2008 4:40 pm ]
Post subject:  Re: one radius dish for both back and top?
Rod,
We're all building true flattops, but with the radius that has been used on their tops for a very long time.

Are you saying that you use no radius in your top bracing at all? If so, your guitars are trouble waiting to happen
since that arc provided by the bracing being cut to a radius creates a lot of the rigidity that enables a top to work
well and maintain its shape for a long time.

If you are using arched bracing that is cut to a 25 foot radius or something in that area, but aren't cutting the
rim made up of the sides, blocks and kerfed lining to a matching radius to receive it and provide a tension free,
full contact joint around the perimeter of the guitar, you're still building a flattop guitar with the typical arch that
we all use.

Mike, you need to realize that every Martin, every Taylor, every Gibson and every guitar made by a major manufacturer
or the huge majority of small shops that you see has a radius cut into the bracing that pulls the top into an arc. They
may look flat, but they all have it. If you lay a straight edge across the lower bout of any of those guitars, the radius
would be immediately noticeable to you.

Guitars built without any arc will have one of two things common to them....either they will be braced more
heavily than would be necessary to ensure longevity than those with an arch......or they will fail by giving in to
string tension and showing the typical signs of a noticeable hump behind the bridge and a noticeable drop in
front of the bridge as the torque on the bridge is allowed to have its way with the flat top more quickly or readily
than those with the arch in the top.

Regards,
Kevin Gallagher/Omega Guitars
Author:  Rod True [ Fri Nov 14, 2008 5:23 pm ]
Post subject:  Re: one radius dish for both back and top?
Kevin first you have to know that I respect your experience and knowledge regarding guitar construction and building.

Now...I know of at least 3 well respected small shop luthier's who all make "True" flat top guitars with no radius in the top. That would be Jim Olson, Kevin Ryan and Charlie Hoffman, also I'm pretty sure that Bill Wise makes a true flat top as well, but he's much newer to the game.

Also, reading on the Godin website, they make their Seagull guitars (Similar to a Takamine') with only the upper portion of the top arched. Everthing above the soundhole is arched to resist the torque of the neck and everything below the soundhole is flat.

When I was making my first guitar, I talked at length over the phone with Jim Olson about his design of the true flat top and I asked him how many of his guitars he's had come back due to failure (because of the flat top design). He said only one, it was one of Phil Keaggy's, who is well known for his alturnate tunings and hard use of the guitar. Apart from that, he's had no issues with building a fully flat topped guitar (this was 5 years ago). I'm pretty sure he's over 1200 guitars built with a "True" flat top with some of those being 30 years old, so I think his method is pretty sound.

The front of the top (from the soundhole forward) is angled to allow for the set of the neck, but apart from that downward angle, the entire top is braced dead flat.

That's what I've based my construction on, the backs of these guys who seem to have success with it. I don't have the answers or wish to debate at all the merrit's of the arched top vs the flat top, but I trust that it works because of the history it has with 3 very respected builders. I know it's not the norm but that's ok :) I trust it to work just fine.
Author:  Tai Fu [ Fri Nov 14, 2008 5:50 pm ]
Post subject:  Re: one radius dish for both back and top?
Todd Stock wrote:
Martin builds their M, J, and D bodies with 28' top and 25' back radius...why not just build both back and top to 25' and eliminate a dish. First minor change in humidity will cause more of a change than that.


It sounds like there isn't a whole lot of difference between 28' and 25' radius too, in fact I kinda question if there was much of a difference between 20' and 25'... the drop is probably a difference of about 3mm or so... (I have to check) which is easily taken up by several layers of cardboard.
Author:  Bob Garrish [ Fri Nov 14, 2008 7:53 pm ]
Post subject:  Re: one radius dish for both back and top?
The distance between the peaks, over a 16" span (width of guitar) is:
Flat to 28' : 0.191" (~4.75mm)
28' to 25' : 0.023" (~0.6mm)
25' to 20' : 0.053" (~1.35mm)
Author:  Alexandru Marian [ Fri Nov 14, 2008 9:14 pm ]
Post subject:  Re: one radius dish for both back and top?
How do you calculate this stuff, a program? Math on paper takes a while. I was curious 0.12 height over a 14 span equals what radius :)
Author:  Mike OMelia [ Fri Nov 14, 2008 11:37 pm ]
Post subject:  Re: one radius dish for both back and top?
Alexandru Marian wrote:
How do you calculate this stuff, a program? Math on paper takes a while. I was curious 0.12 height over a 14 span equals what radius :)


I posted the equation some 6 weeks ago... its in here somewheres. I love this debate about flat tops vs curved tops. It always gets folks riled up (in a friendly, loving kind of way).

One of my other favorite hobbies is the 427 AC Cobra (got the engine sitting in my shop). Here, its the big block vs small block debate.

Mike
Author:  Alexandru Marian [ Fri Nov 14, 2008 11:56 pm ]
Post subject:  Re: one radius dish for both back and top?
Found it...well it's fine as long as you know the radius and want to extract the height, but the other way around..pff. It's a cubic function hidden in, so there will be a good deal of math to do on paper, no thanks I had enough in school :) i can still remember..the pain.. [headinwall] :lol:
Author:  John Killin [ Sat Nov 15, 2008 12:21 am ]
Post subject:  Re: one radius dish for both back and top?
Chris pointed me to the link for this calculator

http://www.liutaiomottola.com/formulae/sag.htm

This is what I have been using to do my calculations for my dish. I am going to be cutting one dish for a 15' and another dish for a 25' radius. I plan to cut a radius on both sides and having one with sand paper and one side without.

I’m working out the final details for my jig and should be cutting the radiuses into the blanks by Sunday.

John
Author:  Tai Fu [ Sat Nov 15, 2008 9:02 am ]
Post subject:  Re: one radius dish for both back and top?
I just thought of something... On the Taylor website they said that they radius the back a little less than needed and radius the top a little more since the string tension (and possibly moisture and stuff) will reduce the top radius and increase the back radius which will give them the radius they want...
Author:  Kevin Gallagher [ Sun Nov 16, 2008 1:08 am ]
Post subject:  Re: one radius dish for both back and top?
Rod,
You've picked three of the best possible backs to lean on when it comes to selecting design ideas and
construction mehtods. I'd never discussed top bracing or the arc in a top with Jim, Charlie or Kevin, but
if they are building with a completely flat top and no arch in the bracing....that's good enough for me.

I built a few completely flat tops back in the day when I was yet to break the 100 mark, but felt that
the arched top and bracing gave me more rigidity in the assembly and adopted that method as my
standard offering. They had the same preparation as the method you're talking about with the area
toward the neck being sloped to provide the proper fingerboard approach to the bridge, but were flat
otherwise.

Come to think of it....none of the guitars that I built with flat tops ever had a problem or needed
any repair or attention in that area and are still being played regularly by their owners.

If it's working.....

Thanks,
Kevin Gallagher/Omega Guitars
Author:  Flori F. [ Sun Dec 14, 2008 12:18 pm ]
Post subject:  Re: one radius dish for both back and top?
Kevin Gallagher wrote:
Rod,
You've picked three of the best possible backs to lean on when it comes to selecting design ideas and
construction mehtods. I'd never discussed top bracing or the arc in a top with Jim, Charlie or Kevin, but
if they are building with a completely flat top and no arch in the bracing....that's good enough for me.

I built a few completely flat tops back in the day when I was yet to break the 100 mark, but felt that
the arched top and bracing gave me more rigidity in the assembly and adopted that method as my
standard offering. They had the same preparation as the method you're talking about with the area
toward the neck being sloped to provide the proper fingerboard approach to the bridge, but were flat
otherwise.

Come to think of it....none of the guitars that I built with flat tops ever had a problem or needed
any repair or attention in that area and are still being played regularly by their owners.

If it's working.....

Thanks,
Kevin Gallagher/Omega Guitars


Hi guys, sorry for coming so late to this discussion, but I recently read a few things that made me want to ask a few questions. (Please bear with me. This reply is a little long winded!)

One of the things I read suggested a flat top will be more responsive in the bass end. I also read something else on here a few weeks back, a study of some sort, that suggested that (all other things being equal) the flatter a top is, the more responsive it is.

My general question is: If either or both of these claims are true, does the use of a top radius just lead builders into a situation where they have to compensate elsewhere to achieve the same responsiveness (or bass response) as on an unradiused top?

For instance, if (all things being equal) a radius imparts rigidity and decreases responsiveness, might builders use thinner tops and/or bracing in order to recapture responsiveness? And on the other side of things, if one built with a flat top, would the top and/or bracing have to be left stiffer than they would on a radiused top in order to make the guitar structurally sound?

And now for the second part of my question: If it's true that radiusing decreases responsiveness (or bass response), it seems there should be a way of mathematically determining how and whether a flat top with stiffer bracing (in order to impart structural integrity) or a radiused top (with lighter bracing/thinner top) will be more responsive. Has this been sorted out?

Finally, all that leads me to wonder: Assuming a flat top is more responsive, is there a minimum radius at which structural integrity has been enhanced "sufficiently" and past which one is just adding unnecessary rigidity? How much stronger is a 50, 40, 30, 25, 20, etc radius than a flat top?

My apologies for this mess of questions. Any help in understanding all this would be greatly appreciated! And if I'm totally misguided, please be gentle.

- Flori
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