# Acoustic....Radius Dishes???



## Antz_Marchin (Mar 31, 2006)

Just wondering if anybody who builds acoustics could explain Radius dishes to me. What exactly to they accomplish, are they completely necessary, where can I get them or can I build them?? I've heard a bunch of mention of them online but they are not mentioned at all in the book I have been reading up on . THanks


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## ajcoholic (Feb 5, 2006)

I have only built one acoustic, and I didnt buy the dishes but this is what they are for:

The top and back of an acoustic should be built with an arch - and the top and back are to be arched with different amounts of arch.

The braces are shaped with the proper arch (ie, not flat) and the top or back is placed onto the radiused (or dished) piece of MDF (the radious dish). which is then placed into the clamping fixture that uses the wood or fibreglass rods.

As the proper pressure is put onto the braces, the top or back is forced to conform to the shape of the dish. When the glue dries, the top and back will have the proper curved shape (thebraces hold the shape).

I used the method in the Natelson and Cumpiano book, I took a 1/8" cork cutaway except with a 1" perimeter so the top or back is basically unsupported except around the edges. WHen I glued my braces on the top and back will also curve down. However I can see with the radius dishes, the amount of curve is pretty much foolproof.

AJC


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## M_A_T_T (Jun 24, 2006)

Not only used on go-bar decks, but when I worked for an acoustic guitar luthier he had a radius dish with sandpaper glued to it setup on a machine that spun it quite slowly (around 120rpm or so). I actually did the bracing and body assembly while working for him, and I'd stick the assembled sides in the mold and carefully place the assembly in front of the sander and apply even pressure, checking often. This sanded the exact same curve on the edges that the go-bar deck put in the braced back. 

With the tops, a very slighlty curved go-bar deck dish was used, however a flat sanding dish was used with a small wedge jammed in between the mold and the sides at the tail end of the body on the front side. This caused slightly more material to be taked off the end edge area, so when the wedge was removed a slightly curved/radiused edge surface near the end was achieved, to give the top a more 'domed' shape as I recall.


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## WarrenG (Feb 3, 2006)

Antz_Marchin said:


> Just wondering if anybody who builds acoustics could explain Radius dishes to me. What exactly to they accomplish, are they completely necessary, where can I get them or can I build them?? I've heard a bunch of mention of them online but they are not mentioned at all in the book I have been reading up on . THanks


Antz,

Furthering the previous comments, the radius dish sands the curvature of the braces and the edges of the kerfed liner so that the back of the guitar can be bowed into, say a 15' radius. Why? A curved back is structurally stronger than a flat one. You can thin the wood more as a result. Is it absolutely necessary? No. But a flat back would have to be thicker to deal with the internal stresses. Thicker back = less movement, less bass, less volume. You get the picture.

Here's a photo of me sanding the kerfing in a 15' radius dish in preparation for gluing the back: 










Somewhat the same for the top although the radius is usually greater (26-28' foot). One caveat. For the top I only curve the upper bout portion of the X-bracing and the kerfed liner. Why? A strung-up guitar already pulls the top upwards at the bridge with the string tension alone. In fact, it tries to twist forward towards the soundhole. The precurved upper bout resists this allowing it to bow up evenly. Take a look at very old steel-string guitars and you will often see a lot of forward rotation and bowing behind the bridge. Almost like it's collapsing into the soundhole. These are true "flat-top" guitars and show the inherent problems over time. 

Hope this helped.


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## Antz_Marchin (Mar 31, 2006)

Awesome guys, thanks for all of the advice. I have 1 year of an MBA left and plan on getting bulding this summer so that it's something I can have the time to learn now and do as a hobby once I start working. 

Regarding the radius dishes though, I hear making them is a mess and accuracy is very critical. I don't have a large enough workshop to make these or the woodworking expertise at this point to make something that has to be so precise and that I iwll be using for quite a while on a number of guitars. Does anybody know of any dealers located in Canada through which these can be purchased? Or would any of you more experienced guys be willing to build/sell a couple. Just a thought. Thanks again for all the help, I'm getting really excited as I gather more tools/knowledge. -BJ


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## mandocaster (Jan 11, 2007)

".....Here come old flat-top, he come grooooovin up slowly....."

I wonder if that was a reference to bridge sag?


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## ajcoholic (Feb 5, 2006)

Antz_Marchin said:


> Awesome guys, thanks for all of the advice. I have 1 year of an MBA left and plan on getting bulding this summer so that it's something I can have the time to learn now and do as a hobby once I start working.
> 
> Regarding the radius dishes though, I hear making them is a mess and accuracy is very critical. I don't have a large enough workshop to make these or the woodworking expertise at this point to make something that has to be so precise and that I iwll be using for quite a while on a number of guitars. Does anybody know of any dealers located in Canada through which these can be purchased? Or would any of you more experienced guys be willing to build/sell a couple. Just a thought. Thanks again for all the help, I'm getting really excited as I gather more tools/knowledge. -BJ



I find dealing with Luthier's Mercantile or Stewart Macdonald in the USA both very hassle free, if you cant get them in Canada.

I'd think the easiest most precise way to make these is on a CNC machining center, but a large wood lathe wood do. Mine only takes up to 16" so I cant make them.


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## Antz_Marchin (Mar 31, 2006)

ajcoholic said:


> I find dealing with Luthier's Mercantile or Stewart Macdonald in the USA both very hassle free, if you cant get them in Canada.
> 
> I'd think the easiest most precise way to make these is on a CNC machining center, but a large wood lathe wood do. Mine only takes up to 16" so I cant make them.


Yeah they both seem to have great stuff but shipping to Canada for something this size is going to be as much as a dish would cost on it's own. I'll look for a CNC center in the area and see what they can do. THanks


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## WarrenG (Feb 3, 2006)

Antz,

Talk to Mark Kett. He used to make a tonne of jigs and fixtures and ran a Web site called guitarjigs.com.

http://www.kettguitars.com/

Warren





Antz_Marchin said:


> Awesome guys, thanks for all of the advice. I have 1 year of an MBA left and plan on getting bulding this summer so that it's something I can have the time to learn now and do as a hobby once I start working.
> 
> Regarding the radius dishes though, I hear making them is a mess and accuracy is very critical. I don't have a large enough workshop to make these or the woodworking expertise at this point to make something that has to be so precise and that I iwll be using for quite a while on a number of guitars. Does anybody know of any dealers located in Canada through which these can be purchased? Or would any of you more experienced guys be willing to build/sell a couple. Just a thought. Thanks again for all the help, I'm getting really excited as I gather more tools/knowledge. -BJ


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## Antz_Marchin (Mar 31, 2006)

^^Thanks Warren, I shot him an email this afternoon. I'll wait to hear back. I found a local CNC shop as well, I'm gonna give them a call tomorrow. See if I can find some waterproof 3/4" MDF at Home Depot or something and see if the guys at this shop know haw to make a 25' radius in it


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## cougar2 (Mar 10, 2007)

Antz,
This is the first time on this site and saw what you want to do. One thing to remember is the $$ amount for the program to do the job. If you are looking at a shop rate and the program it may be a little much. To just order one from Stewmac will run about $105.00 in Canadian plus shipping and such. Im not up on what mdf is now but lets say you get 4 radius dishes for around $75.00 and machining is $150.00 to $200.00. You are now at around $305.00 and shill have to get a program, you may be up to around the same price as buying from Stewmac. Find out how much they want for the programing to do the job. I had a CNC mill in my shop and made my own dishes, 20' for the backs and 28' for the tops. MDF will warp if you get it near a humidifire and it won't go back no mater what. I had one that did this but all the others were fine, I just kept them away from any high humidity. Let me know what price they give you, I may be able to help you with a copy of mine. 
Dennis


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