Official Luthiers Forum!

Couple of almost-semi-related questions:

1. I'm comfortable with the process of building a jig to route out a radius
dish. The only question I have is how do I lay out a segment of a 16' or
20' or 28' or n' radius circle onto the rails? I've seen Kinkead's method of
bending a straightedge held at each end with a nail, but I'm skeptical
enough to wonder if that's really a segment of a circle or a curve of a
different sort. I also wonder how far I would have to deflect the
straightedge to create a segment of a given radius. (I could do the trig,
but I can't find my calculator!!!)

2. When clamping braces in a go-bar deck with the radius dish
underneath, what type of material makes acceptable go-bar bars? I've
got some old "heart of pine", some poplar, some mahogany. Do you just
use whatever is laying around?

Thanks!

Sorry I can't help with the radius dish question but for the go bars you can use fiberglass rods used for kite building! 3/16" to 5/16' would do!
Or you can buy them for Blues Creek Guitars already made and save the hassle! Fiberglass can splinter and be a pain....it also smells horrible when you cut it!
BTW....if you go the "do it yourself" way don't cut the rods with a saw or scissors or cutter for that matter
Make sure you use a high speed cutter like a cutting wheel on a Dremel or it will cause the rods ends to split and it will be a mess!

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

I cut my 3/16" fiberglass rods 24 at a time on my bandsaw, no problem at all. 5/16 glass is way too heavy, it will poke right through your top. (and it'll kill you if it comes loose)


When I layed out my radius, I taped one end of my measuring tape to my drill press, pulled out 28' and used the tape as a radius to draw an arc on a piece of masonite.

Ron old man38761.9354976852

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

Ron, thats a great idea to get a radius!!

You are a genius! Aren't you?

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

You could just buy 2 of my radius gauges from HERE and draw the curve on your rails. Or better yet, you could just buy the dish and save yourself some time These radius gauges have so many uses around the shop, I find a new use for it every time I pull one out! After using my Brace Maker jig, I sand the brace in one of my dishes to get the spherical shape of the dish onto the bottom of the brace. Then I use my gauge to verify the radius. You can also check the top and back rims to see how accurate you got your sides profiled. Many more good uses, like making rails for making dishes. Good luck!
Tracy

Jay, I layed this out on cad several years ago, and here is a pdf to layout your own.

2006-02-13_231213_dish_layout_model.pdf

As for the go bars. use what ever you have lying around. I'm frugel and don't buy anything I could easily make for next to free. Rod True38761.9695717593

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Jay,

PM me and I'll send you an Excel-based arc-calculator or go here.

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

though you'll need your calculator for what's on the link.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

Greetings! - I just purchased a couple of radius dishes from Tracy at Luthiers Supply and they are really well done. This is just one of those areas that I found better in my situation to purchase, so unless you really just want to learn the process, I would recommend looking at his site and going that direction. With solid sheet sandpaper the right size and everything.

On the Go Bars, I use wooden dowels cut to the right length with rubber caps on each end to minimize slipping. All available at your local hardware store. I used 3/8" and have not had any problems, some folks like smaller stock. I calculated the radus or bend I wanted that coincided with the down pressure and cut them all to that length.

Thanks Rod and Pat for the input, now if i may, what do you use for the carriage and how thick is the carriage at the lowest part of the arc, in the center of the carriage ?

Serge

Simple Radius Calc HERE

Cheers

Kim

3/16" fiberglass rods from Into the Wind. 1/4" are too stiff, that's what I tried first. I replaced them all with 3/16" and it makes using the deck much easier.

They also sell a nice little Zona hacksaw and blades for cutting, which will come in handy if you need to cut carbon fiber later on.

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Jonathan Kendall, Siloam Springs AR

Jay,

For the dishes I make and sell I just had a buddy of mine who is good on AutoCAD draw out the arcs for me, 36" long. I use those arcs and then trace them to tracing paper. Then I transfer the lines onto my sled with carbon paper under the tracing paper. Saw out the arcs and sand them to the line. Check and adjust using the original AutoCAD drawings.

I use 3/16" fibreglass rods from "Into the Wind". I tried wood dowels and found that they were inconsistent in their strength and when using hide glue I don't have time to mess around with bar selection. I bought the 4 footers and cut them in half. I used an old japanse saw blade I had laying around, but after 25 cuts it is pretty dull. I bought 25 rods and 100 rubber ends. That gives me 50 go-bars. I just finished the x-braced back for my current guitar and it was a joy to use these go-bars. It was less than $60 delivered to middle 'a nowhere, Canada.

My dishes are double laminated 3/4 MDF that is spun into a perfect circle before I dish them. Going this way makes the dishes very stable and well balanced, you should consider the same if you can.

Good luck

Shane

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Canada

I just cut a bunch of go bars from scraps of clear (ish) Pine I had laying around. 3/4" X 1/4" or so works great for my 4 foot deck. I cut some from Maple too, but they are softer & take on a "set" much quicker than the pine.
With the large surface on the ends, they don't slip like the dowels did on my old deck.

Thanks for the link Kim!

Serge

If calculate the rise of an arc for a given chord length and take three pins, Two at the ends of that chord and one dead center but the rise height ofset and bend a piece of straight metal or wood to the shape you do indeed have a true arc segment the force on the material being bent will be distributed across the three nodes points (pins) thus forming an true arc.

Wood dowels and slats have been used for hundreds of years for go-bars but do not reall supply a equal force because of defferences in the grain structure between each slat. Fiberglass on the other hand is very consistant. 3/16 rods work very well. a 22 1/2" long 3/16 fg rond with 1" of compression bend will provide 8# of down force which is near perfect for clamping braces. increase the bend or loading and you increase the down force delivered at the clamp point. The resin and choped fiberglass used in these rods is molded under pressure is not subject to splintering. I have cut at lease a hundred and have never had one snap or splinter when cut a a 90 deg or 45 deg angle.

if you buy the radius templates or make then it is then a very simple matter to use said template as a routing template to form the runners for your dish jig.

The three-nail setup ( as in Kinkeads's book) doesn't give you a circular arc. The batten has more curvature at the center nail and is locally straight near the end nails.
You can do it exactly with a batten and four nails. Use two closely spaced nails at each end, each pair equally spaced, to curve the batten. Here is a sketch, and the formula to figure how much deflection you need for the radius you want.
The batten will form a perfect circular arc in the center section between the two inside nails.



Phil

Hey Welcome to the forum Phil.

Thanks for helping out so soon!!

Shane

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Canada

I used 3 nodes to define the arc this is a perfect example. The only thing I would suggest is that you put a nail to the opposite side of the arc at the middle of the chord so that you keep the rod from pushing off while tracing. If tracing the inside arc middlle nail to the outside of the arc. If tracing the outside, middle nail to the inside.MichaelP38762.523912037

Attached is a PDF file for the dish curve template layout. Hope this is what you are looking for.2006-02-14_131325_Dish_Curves.pdf

Many thanks to everyone who has provided advice on both questions.

I would certainly buy a couple of dishes from one of the sponsors had I the extra $$$ lying around. However, as it stands I have quite a bit of extra 3/4" plywood and a nice router, so I'm going to have a go at making the dishes. I know plywood isn't the ideal material, but I imagine it'll do for a while.

I'll be using PVA glue for the braces, so I figure I'll have more open time than if I was using hide glue. I use PVA strictly for convenience - it's what I have from furniture building and I'm comfortable with it.

I think I'll try cutting out some wooden bars to see how they do, and if that's not satisfactory, I'll go with fiberglass rods. Again, a matter of trying to use what I've got available in lieu of spending cash I don't really have!

Again, thanks for all the advice - REALLY helpful!

Take a look at my jig for cuttting bowles in the Library of Plans. It makes bowl cutting just about as easy as it gets.

Also I recamend Luthiers Suppier's brace profile cutting jig. ver quick and ver accurate braces in a flash I did two guitar sets in about 3 min.

Here's a way to draw the arcs, using a "long compass" without having to use a "long compass".


http://gicl.cs.drexel.edu/people/sevy/luthierie/compass/Long _compass.html

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now known around here as Pat Foster
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http://www.patfosterguitars.com

[QUOTE=MichaelP] I used 3 nodes to define the arc this is a perfect example. The only thing I would suggest is that you put a nail to the opposite side of the arc at the middle of the chord so that you keep the rod from pushing off while tracing. If tracing the inside arc middlle nail to the outside of the arc. If tracing the outside, middle nail to the inside.[/QUOTE]

Exactly like my PDF up top.

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"There's really no wrong way, as long as the results are what's desired." Charles Fox

"We have to constantly remind ourselves what we're doing....No Luthier is putting a man on the moon!" Harry Fleishman

"Generosity is always different in the eye of the person who didn't receive anything, but who wanted some." Waddy Thomson