Making a radius dish- an alternative way

Construction and repair of Classical Guitar and related instruments
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singularity
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Making a radius dish- an alternative way

Post by singularity » Mon Oct 27, 2014 8:31 pm

Not sure if it was discussed before but here it goes. There are several step-by-step procedures that you can find online on how to make radius dish but to be honest I wasn't happy with any of them, either because they involved CNC, carriage with router or simply drilling in the center 3-5mm (depending if it's for back or top) and then sanding by hand and go "Yeaaah, that's 'bout right"
Here is my alternative way of making a radius dish, and I'm sure someone, somewhere already came up with this same idea but I came up with it on my own (re-invented the wheel comes to mind). This will work with wide belt sander or router:
1. Cut 2 MDF boards to the size that you need for your plantila, add couple of inches extra to cut it to shape
2. Place a spacer in the middle of the bottom board and place the first board carefully on top of the spacer and other board. Alternatively you can just tape the spacer to the bottom board. I used couple of washers for this.
4. Use fasteners to attach both boards to each other (in circular fashion, around the perimeter). This will create a bump in the middle of the top board. Counter sink those.
5. Now you can run it through the wide belt sander or you can use router with dado attachment to get rid of the bump. I run it until I get scratch marks around the perimeter of the board
6. Remove the screws, and whoila... If you like you can seal or laminate it

It's fairly easy to control the depth of the dish with shims.

I hope this helps some of you, these dishes are not exactly cheap.
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riemsesy
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Re: Making a radius dish- an alternative way

Post by riemsesy » Mon Oct 27, 2014 9:09 pm

well done :)
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Rahoul Waghmare
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Re: Making a radius dish- an alternative way

Post by Rahoul Waghmare » Thu Oct 30, 2014 1:36 pm

really simple

chiral3
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Re: Making a radius dish- an alternative way

Post by chiral3 » Thu Oct 30, 2014 1:59 pm

Stepping down a dimension: there was a thread not too long ago, can't recall when exactly, by people that radius braces by deforming the brace similarly and then planing the top. After releasing the ends you have a roughly radiused brace. I suppose going up one dimension one could, if strong enough, deform a cube and plane it into a sphere. (That was a math joke. I promise math people are laughing. Somewhere.)

Thanks for posting the pics.
"Life is under no obligation to give us what we expect" - Margaret Mitchell

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singularity
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Re: Making a radius dish- an alternative way

Post by singularity » Thu Oct 30, 2014 2:22 pm

chiral3 wrote:Stepping down a dimension: there was a thread not too long ago, can't recall when exactly, by people that radius braces by deforming the brace similarly and then planing the top. After releasing the ends you have a roughly radiused brace. I suppose going up one dimension one could, if strong enough, deform a cube and plane it into a sphere. (That was a math joke. I promise math people are laughing. Somewhere.)

Thanks for posting the pics.
Yeah, at first I was concerned about that too (hence I put screws in circular fashion) but as you can see from the picture partially sanded board looks very "round". You could also use thinner material attached to thicker bottom board and more screws if you're concerned about having "perfect radius". This method certainly beats the sanding-by-hand method but it doesn't say nowhere that you can't "touch it up" by hand, around the perimeter for example.
I do bend the braces and sand them on edge sander, I keep them taller so that when I finish sand them on the radius board they're stiff enough to keep the shape and don't bend as the brace is pressed down. Later they're cut down to right size.
Thanks for comments guys.

mqbernardo
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Re: Making a radius dish- an alternative way

Post by mqbernardo » Thu Oct 30, 2014 4:40 pm

ingenious. and i´m no math guy, but i laughed.

JohnPierce

Re: Making a radius dish- an alternative way

Post by JohnPierce » Thu Oct 30, 2014 4:58 pm

chiral3 wrote:... deform a cube and plane it into a sphere....
There was a white cat sitting in my lap when I read that. It is now a very annoyed white cat with tea stains on its back sitting on the other side of the room. :lol:

Sequoia
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Re: Making a radius dish- an alternative way

Post by Sequoia » Thu Nov 20, 2014 8:03 pm

I'm voting for this man to be the next president. Singularity, the party.

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tom0311
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Re: Making a radius dish- an alternative way

Post by tom0311 » Fri Nov 21, 2014 8:21 am

Really simple. Reminds me of James Listers' method of planing braces. I like it, and I'm going to use it on my next build I reckon.
“There are two means of refuge from the misery of life - music and cats.”

Jim Frieson

Re: Making a radius dish- an alternative way

Post by Jim Frieson » Fri Nov 21, 2014 5:31 pm

Smart and it works . That is what I used to do to make dishes . Figured it out from planing braces .
Now I have a different way . I am in Japan and I have about 25 wooden planes , good steel .
It took time to shape them and repair them - got 'em used - but they now do a multitude of jobs with versatility including solera etc .

StevSmar
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Re: Making a radius dish- an alternative way

Post by StevSmar » Sat Nov 22, 2014 1:54 pm

What a great idea- so simple!
Regards,

Steven from Winnipeg

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rastaferraille
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Re: Making a radius dish- an alternative way

Post by rastaferraille » Tue May 15, 2018 7:55 pm

Thanks for the tip, really good idea !

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rastaferraille
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Re: Making a radius dish- an alternative way

Post by rastaferraille » Wed May 16, 2018 7:30 pm

Hello,

What about the method that uses two pieces of plywood, one thick as a base and the other thiner (10mm) aimed to become the radius dish itself ? You put some pieces of wood all around the thiker one, then you use a bolt at the center to force the dish to rest on the base. Not shure if I'm clear... Did you understand what I'm trying to say ?

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singularity
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Re: Making a radius dish- an alternative way

Post by singularity » Fri May 18, 2018 7:41 pm

Oh yeah, I understand and think that'll work too. The only thing I would be concerned about is how much clamping pleasure to use around the areas where there's a void between the center and perimeter of the circle, too much force and you get somewhat deformed arc (spline?). Otherwise excellent idea, I might try this myself.
Thanks
s

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geoff-bristol
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Re: Making a radius dish- an alternative way

Post by geoff-bristol » Fri May 18, 2018 9:16 pm

Problem is - to my mind the dish needs to be oval !
I do it with small planes and curved scrapers - and yes - I'll trust my feel with my fingers. There is more brain power in fingertips than you might think.
But I like your method - as it makes a more natural curve than a true mathematical radius - the word I think is catenery ? We often made arch profiles for cellos by 'draping' a length of fine chain between two nails and tracing the shape. The curve you get is not a constant radius.

Always better to kill cat before skinning - whichever way you do skin it :D

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