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I just had to make a radius dish for a repair I'm about to do. While I have the jig set up I thought I might make another one but wanted to see what the popular opinion was regarding usefulness of different raidii. So far I have 15 foot 25 foot and 28 foot. Stewmac sells a 40 foot radius dish they claim is for Martin guitars. I read on another forum somewhere that Martin used a 56 foot radius. In the same thread, most people agreed that Martin's radius was unknown as they never give it out. In Robert O'Briens steel string building course he uses a 30 foot dish which is not all that far off from the 28 foot I just made.

Anyway, I just wonder what a good complement would be to the assortment of dishes I already have. Thanks.

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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

Mine is 30'

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guitarsbygiltzow.com

30'

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Doug

Don't let fear or common sense stop you from trying to build something

25' top, 15' back...just because....

You could do a 60 ft radius for those times when you want to do a "flat" top.

I personally build with 28' for the tops and 15' for the backs. I make a pile of these every month. 15' is the most popular back radius I sell. for tops 28', 30' and 40' are also good sellers. I'm seeing increase in orders for 12'. Not sure what that means, but its interesting.

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Josh House

Canadian Luthier Supply
http://www.canadianluthiersupply.com
https://www.facebook.com/canadianluthiersupply?ref=hl
House Guitars - Custom Built Acoustic Instruments.
http://www.houseguitars.com

I have a 28' for tops and a 15 for backs plus a 60' bar for shaping the UTB.

That is what I got for my parlor because it is what was recommended for that guitar but will probably use it on others as well.

Bob

I was reading about Martin (in the old days) radiusing their braces to 25' and then forcing them onto a flat rim. Is this correct? Seems like you're asking for side cracks doing it that way. This is probably why all the information regarding Martin's top radius is so divided and generally unreliable. No one is really sure and Martin isn't giving up any info.

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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

No that is not correct , I learned a good bit when I was out at martin for a repair summit a few years ago.
Martin uses about a 50 foot radius. The top will have a 1 1/2 degree worked into the sides. In the old days this was more or less eyeballed into it. The sides were machined on a shaper and trimmed to a precise shape. This had the radius and top angle worked into it at this point. The dish was pretty much flat. I was fortunate to get to see the old jigs. Today it is more precise and they are laser cut . In this series of Photos I got to take a picture of the process during a Martin Repair summit.
The mold has a series of stops the pic of the sanding dish has a radius but that was "proprietary" . Then the operator sets the mold into the machine , as the wood is sanded away the stops will hit the resting points and it is removed.

Here is a link to an old movie at about 38sec you can see the operator planing the sides.
https://www.youtube.com/watch?v=Zs9miwi7CKE

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

We all have as a builder a choice but in reality it comes down to control of the neck angle . I use a modified top , I start with a 28 dish . This came about as the radius affords a good reliable 1 1/2 degree of neck angle. Here is a video on what I do.
https://www.youtube.com/watch?v=LrgRKKsxL-M
after the rim is prepped this is the 2nd stage on setting up the neck
https://www.youtube.com/watch?v=L3YJh9OT4mw

it doesn't matter if you use a dovetail or bold on , the geometry is still the same. Adjust the heel cheeks to get the ( Angle ) and ( Centerline)

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

"I was reading about Martin (in the old days) radiusing their braces to 25' and then forcing them onto a flat rim."

I don't know that much about Martin's methods, but some Classical builders build with a solera that is dished in the center, but has a flat rim. Also some steel string builders build guitars with a flat top for a more "open" sound. The tension of the strings pulls some dome into the top. If you were looking for another dish to build, a 60 ft. radius would give a small amount of doming and a little more stability than a truly flat top.

I'm no physics guy, but here's my understanding:

All other things being equal, more radius equals more stiffness in the top, which benefits high frequencies and hinders low frequencies. So something else probably has to give to get some of the lows back.

Gaining more stiffness in the top via a greater radius should allow for an overall thinner top to begin with and lighter bracing, leading to an instrument that has a quicker, more nuanced response to the touch.

Combining these factors makes for an instrument that takes a bit more time to build, and results in an instrument that needs more care from the owner / player. Because lighter bracing and thinner tops and trickier building and factories and warranties do not all get along very well, so we continue to make our own guitars:) And anyone who has played them knows that it's worth the trouble and care!

I've been using a 25' radius for the top. It's just where I started. At the time I thought it was standard, but after reading numerous discussion like this where people discuss the benefit of a 28' for geometry, I've wondered if it would be worth re-tooling a bit to get there. Is there a noticeable difference between the two? When you plot them out on paper, the it seems negligible.

I think a 30ft helps a bit with neck angle but only slightly over a 25. For me anyway, I don't think a 3ft diff would matter... I don't think I have that much accuracy in matching the dish arch to the top arch after assembly. Things move.
I've gone from 30ft to 45ft radius on my last few builds and am having some fun learning the relationship between bracing, top thickness/stiffness and arch.


Sent from my iPhone using Tapatalk

Pulled 30' put of the air 10 years ago and stuck with it. I sand the upper bout of the completed rim set with a jig For the proper neck geometry.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Leave it to me to throw a wrench in the works. I use a 10' dish on the back and 10' cylindrical radius on the top. Plenty of bass, treble AND mids. Figuring the neck angle is a snap with a cylindrical radius...

That's interesting. So, by cylindrical, you mean it has a radius from bass to treble side? Or tail block to neck block?


Sent from my iPad using Tapatalk

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"I'm not going to say that perfection has never been achieved. However, if it has, it probably went unnoticed due to it's lack of character."

Radius is across the box, bass to treble. It is flat from dovetail to endpin block.

Is that X braced or ladder?
Must be fun fitting the braces to the radius if it's X braced!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Both.
Not bad at all, I have marks on the "dish", and get them close on the 6 x 89 sander.

AFAIK, Martin has never used that much radius on top braces. They were much flatter. Willi Henkes has stated that he thinks the braces were flat, and any radius was achieved by drying out the tops before bracing them. AFAIK, the rims were flat on the top side.
I use an asymmetrical radius on my tops (25 foot radius above the bridge, flat below) which works out to about 40 foot overall. I have always clamped to a flat rim using a flat plate, and I have not experienced any problems from doing this.

The only 'eyeballing' in the old days was to radius the back surface using a block plane. The planing on the top surface was done to flatten the kerfing and make it flush with the sides. The same procedure was used when I first visited the Martin factory in the early-1980's. Before the early-1990's there was no 1 1/2 degree taper to the upper bout. Before then, pre-profiled sides were dead straight on the top edge.

IMHO, no.

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John

It's easy to look at a Martin (of any age) and see that the rim is flat and does not have and angle in the top rim down towards the neckblock. Try it, just sight down the body. They do not have any angle planed into the top rim.

I glue an 28' arched top to a flat rim (credit to John Arnold for turning me onto this). By flat, I mean it'll sit flat on a table. The kerfed lining has a bit of a angle to it to accept the "domed" top.

Sorry to say but the above statement about the Martin process is simply not factual. I have been there many times, carry on communications with their engineering department and for that matter Dick Boak too.

The Martin process is described in this link along with pictures of the machine that does the operation and Chris makes verbal video confirmation of the process. Perfectly flat with a 1.5 degree slope from the sound hole to the neck block.

http://kennethmichaelguitars.com/neckangle.html

It is true that prior to the dawn of Martin cutaway models 1980 ish there was little concern regarding the hump/slope or whatever you want to call the poor fit at the body. And this is not secret stuff if you call the Martin factory they will proudly state that they make "flat top" guitars and that the braces are contoured to 52' to prevent a concave appearance and also to make surface area of the sound board slightly larger than the perimeter of the instrument profile -- this helps to prevent cracks not by adding strength but rather allowing the top to "flatten" before it cracks

If you were to purchase a set of braces from the Guitar makers Connection one can simply check out the contour radius. And don't forget reverse engineering is how our offshore manufacturing concerns manage to kick the butts of USA companies. Checking the contour on a guitar is pretty simple, de-tune it lay a straight edge across the sound board or back get the measurements along the edge and do the simple math. Problem with that for the top is there is some residual contour from the tension of the strings deforming the braces, that many times will end up at about 40'.

There is certainly no harm in sanding a bevel on the rim, the reality is that the bevel (of 28') is about .008" inside to outside of the rim, pretty close to flat in wood working terms (.005"). -- Santa Cruz advertising indicates they build both ways and that the bevel (.008") tends to accentuate the trebles. Gotta wonder about that? In the link above you can also see the sloping procedure used by some other makers using a routing fixture.


https://www.youtube.com/watch?v=rAu3Nw4KYdo

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Ken Cierp

http://www.kennethmichaelguitars.com/

Welp, Ken probably knows better than I do...but when I get home, I'm going to take my Martin, take a piece of thin flexible something or other (that will give a straight edge when run along the side of a guitar) and start at the butt, going up to the neck and see if the rim drops at the neck block.

They sure as hell look flat when I sight down the body, I think I'd see approx. 7/64" of drop.

Once I do this, I'll be convinced, one way or the other.

Martin changed many of their processes in the early-1990's. This was deemed the 'New Technology', and made much more use of CNC processes. Body shapes changed, the nuts on most models became flat on the bottom rather than angled, bridges on the lesser models had a radical change in the design, bolted mortise and tenon neck joints were introduced, A-frame bracing was introduced, etc.
That change includes using a radius sanding dish to radius the back edge of the rims. That is when the 1.5 degree bevel on the top edge of the rim in the upper bout started.
Before that, the rims were flat on the top edge.
The difference in the back arching is pretty evident, if you are used to looking at older Martins. The bodies are noticeably deeper in the waist area.

Sanding a bevel on the kerfing is not the same as radiusing the entire rim surface. In the latter case, the edge of the rim is not coplanar, and results in a body that is deeper in the waist area when compared to the ends.
I prefer to radius the UTB to achieve a straight fingerboard.

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John