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Hi guys,

I'm looking for information about making your own radius dishes. Along the way, I found this cool video on youtube showing Chris Paulick's set up, or jig for making a radius dish. Here's the link:

http://www.youtube.com/watch?v=tvGemvizrz4

Great video, by the way. Anyway, he mentions using a long compass to come up with his arc to make his jig. So I looked up a long compass and found this link:

http://gicl.cs.drexel.edu/people/sevy/l ... mpass.html

Another great source of information, but I'm still curious as to how to get a measurement to begin with. How far apart do you make your two outer points, and how much higher do you make the inner point?

Any advice on this would be greatly appreciated. Thanks.


Sean

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change isn't synonymous with improvement
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Here's another link Sean. http://homepage.ntlworld.com/coleman005/Dish%20jig.htm

Instead of using a long compass to draw an arc, I mounted the router on a long compass and cut my dishes that way. There was a discussion on the MIMF a couple years back - might be in the library by now...

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Gabriel Regalbuto
Carlsbad, California
Freedom 35!

This diagram may help. 36" is your outside, 24" the dish, 18" to center and 2*s is your saggita.

Just to be sure you know, the sled is not riding on an arc but an angle cut from the middle to the end. So you basically take a straight board, mark a point to be center (allowing enough room to move side to side) then make a mark 18" from the center and measuring up from there you mark the distance of the sagitta. Draw a line from the center to that point, going right off the end, and cut to the line.

Is that clear?

Thanks so much. Your idea is very simple, quick, and I really appreciate the spreadsheet for figuring the arc of the jig.

Very helpful, thanks again.

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But how did you make your long compass? How did you figure your measurements of the long compass?

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change isn't synonymous with improvement
instagram.com/bucketguitars/

Still a little confusing. I don't understand "saggita", or 2*s.???

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change isn't synonymous with improvement
instagram.com/bucketguitars/

The geometry I'm sure you'll be able to google. This is how I put mine together:

I just swapped the router mount from one edge to the other. You can see the different angles in the wood for 15' and 25' radius. Mine is four foot wide, in theory you only need 3', but that was one more cut, and the extra slack means that you don't have to get to the end of your sled.

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Gabriel Regalbuto
Carlsbad, California
Freedom 35!

Liutaio Motolla's Lutherie Information website has some useful formulas.

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

I used this one to calculate the radius when laying out my 30 ft, 360 inch radiused workboard.

You can find the heights of offset from a straight line drawn from any to points on the arc.

You can layout your points at 1 inch intervals and draw the arc.

Pat

I made mine with my table saw and finished with a small plane. I cut the circles on a band saw using a fixed point 90 degrees from the blade to get nice 24 inch circles. I took the cutoffs and fixed them to a square sheet of mdf. Using the formulas from the website posted above I dropped this contraption on the center of the table saw and clamped it down for the first deepest cut. I started in the center and worked my way out at 1 inch intervals lowering the blade a little each time based on the formula. If I was smart I would have taped some cardboard around my tablesaws raising and lowering handle so I could index all the heights so I could make all my measurements at one time and use half inch intervals. I found my dado blade worked the best and a combination blade didn't work well at all. I had to vac all the dust up at each 1 inch interval. If you try this at home make sure you clamp the unit down each time. If it were to get away from you that would be bad. I chose to do it this way as I have better dust control on the table saw. Using a router may have been better. I had though of making a way to rotate the dish in the form so you could run a router back and forth on an arched support. If I had to make many dishes that is what I would do.

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the table saw is just not a safe tool for this. Please use some common sense as it is best not to have to pick up fingers off the floor or dig a stick out of your stomach.

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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

Also look at
http://dunwellguitar.com/
and click on the Luthier Pages button. In the section under Tools and Jigs go to the bit about making hollow forms. You may also want to look at the powered sander rig as well.

Alan D.

Do a search here and there are past discussions that give you all the info you need and the links . You all need to use the search more often instead of re hashing the same topic.

Sometimes it's enlightening to rehash a topic. I've read all the previous threads when they occurred but I don't recall anything about RM's site, for example. Besides, I think that almost everything on basic building techniques has been gone over a bunch of times and traffic on the forum would be pretty low if everybody found what they needed from searches. Where's the fun in that?

This is the chart you want .
http://www.liutaiomottola.com/formulae/sag.htm
And it has been in every post I gave about my method.

it took me a while before i checked this topic out for that very reason. it was, however, my choice to look at it. just as easy to stay away. the whole point of this forum is to have someone answer your question personally, rather than just read an encyclopedia of luthiery information. thats how i see it anyway.

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sweat the small stuff.

I see it as doing some work on reasearching it first to see if the info is out there and then ask the question. A lot of things have been discussed pretty well in the past and that's the value of the forums having an archive or library and tutorial section. So there isn't always a need to keep re hashing it unless there is something not understood or if you have something to contribute. The forum is a place to discuss and get/ find info on things and not to get personal attention. At least that's my take on it.
Don't take this wrong, I'm all for helping people if I can but I just want them to do some of the work first first and use the resources at hand.

Edward Taylor wrote:
This diagram may help. 36" is your outside, 24" the dish, 18" to center and 2*s is your saggita.

Just to be sure you know, the sled is not riding on an arc but an angle cut from the middle to the end. So you basically take a straight board, mark a point to be center (allowing enough room to move side to side) then make a mark 18" from the center and measuring up from there you mark the distance of the sagitta. Draw a line from the center to that point, going right off the end, and cut to the line.




Although the math is relatively straightforward here, the only problem that I seem to be having is with the sagitta designation used by Edward Taylor in his post from about a year ago. It's one of the few examples that walks you through this process. Here's the problem: For example, the sagitta of a 15' radius board should be .336" (using Matolla's website calculator). Is Ed suggesting that 2*s is .336", or is 2*s actually "2 times s" or 0.672" ? Other than that, this looks like a great system and should be relatively easy to build.

Thanx

Mitch

No it's 2 X the sagitta or .672". that's so you only have to make one cut using the cut edge as one cut. Other wise you would go from the apex over 18" in each direction and up the sagita of .336"in this case. and draw your lines and make two cuts . Doubling the sagitta allows for one cut. Also take 1/2 your span and in this case 18" and add 2" to it make that the center point of your apex from one edge and if cutting a 24" dish add 12" or 1/2 the dish dia to that and that's how much you need for the other side of the apex to edge. In the above example the rails of the sled would be 52" as a minimum might be 48" but the sled wouldn't have any overhang an would fall off the span rails. Also if you use a box and have the heigth of the span rails setup for a 25' radius and then remove the 25' rail for your 15' rails it's not a bad idea to add a spacer to the one span rail the differance of the 2XSag. 25' and 2XSag. 15'. That will allow you to have your router bit at about the same place.

Chris;

Thanx for the clarification of (2*s), and the suggestion on how to tweak the rail length to keep the sled from falling off of the rails. I should be able to take it from here.

Thanx again,
Mitch

I think if you find the sagitta for a the 20" or 1/2 the 40" span and did the 2X Sagitta you could just measure in 20" and up the factory edge and draw your line too instead of the 18" and squaring it up. I believe once you have the correct apex angle it doesn't matter if you change the span. I believe it will cut the same radius. Changing the angle changes the radius. I thought about inverting the angle and have the router cut the outside radius or concave. Be a neat way the make a jig for a fretboard radius. Not that I haven't thought about that for a while.

Agreed. This thing is simply an inverted right triangle with the sagittal distance (2*s) becoming the short leg of the triangle, ½ the beam distance becoming the long leg, and the length that rides along the box edge (not sure what to call it) becomes the hypotenuse. It doesn’t matter what the beam length is. As you stated, the critical value is the angle that’s created between the short leg (sagittal distance) and the hypotenuse.

As for using something like this for fretboard radii, I believe that if you simply invert the sled so that the hypotenuse is facing upward, and you use springs to push it up on to a rail (rather than weights to push it down on to the box side), this thing might work in principle). The problem that seems obvious is the relatively short width of the fretboard, so this might need to be completely re-engineered using a smaller router and sled. Just a thought.

M

I just cut a new dish using a setup that is pretty much just like the one Chris showed. I actually reworked mine so it looks even more like his today.

One thing to think about when you are calculating the measurement of your rails is that the sag is based on the span between the fixed rails or the box that the sled rides on.

The last dish I made I calculated the SAG based on the fixed rails. I then went to measure the dish I cut and I was off. I couldn't believe it. Then I realized that I needed to calculate the sag for a 24" span (size of my dish) to check my work. After that I was spot on.

Pretty much like the router jig with curved sides but with straight sides sorta like this but you could use pipes for the jig to slide on. Pic not to scale.