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This may be a stupid question, but I have looked all over and can't seem to find an answer.

I basically want to start making my own radius dishes and various other tools shaped to a radii. However, I can't seem to find a practical way to make a template for a radii. I see a lot of tutorials saying "take your 15ft radius jig and do this" but I can't seem to find any way to make a perfect radii in the first place. Any advice on how to make these?

I quite often see the same idea used to check a radius of braces and the arch of a back etc.

Chris made an excellent tutorial on this.

http://www.luthiersforum.com/forum/viewtopic.php?f=10117&t=18097

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The method I will be using when I make radius dishes soon is Chris Paulick's long compass router jig.
Youtube search making a radius dish and you should find the video tutorial he posted (should also be in the tut section).
It is basically a sled with runners cut at an angle that corresponds to the radius desired.
Here is a good diagram someone else posted a while back:


The 36 inches is for the width of your jig (which is basically a box or tray), the 24 is the diameter of your dish, the 18 is to the middle of the jig where your dish will be mounted on a pin or dowel, and the 2*s is for the height at the end of the runner from the center of the runner that gives you the angle that corresponds with the radius (times two, so you only have to make one cut on the runners). There is a handy chart somewhere here with all those heights to radius', cant find it saved on my computer so do a search for radius dish.

You can use this same long compass concept to make your radius gauges. It seems complicated at first, but the whole thing is pretty simple really. If there was any part of this post that is not clear please say so.

Can't you just tape or nail down a string that is 15' long in your garage or driveway, and use the loose end to help you draw an arc ?

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Chris has a nice toot on making a radius dish .It would be nice to know what angle he cut on the sled to get the 15 ft. radius though.Perhaps someone can chime in on how to get the proper angle for a given radius.

Sure or even better use a carpenters tape measure which won't stretch. Most folks use the other methods to build jigs for routing the radius dishes. I made my jig using the long-compass which I think is what Chris' video shows.

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For some theory and the geometry of this topic, see:
http://liutaiomottola.com/formulae/sag.htm

You can download printable (.pdf) radius files (from Steve Senseny) at http://www.mimf.com/library/radii.pdf. You can take the file on a memory stick to any copy centre to save the trouble of taping together smaller sheets from your home printer.

Cheers
John

Yep....just screw your board to the living room floor. Drill a 3/4 in.Dia hole to depth in the center and set your router depth (1/4 fluted bit)...suspend your router 15ft from the peak of the vaulted ceiling with a piece if wire (tape the cord to the wire)....arrange to take the wife to someplace nice..... turn on the router and come back in 2 days..and broom up the mess.

Make sure you leave the dog/cat with the neighbor.

Ok... Kids....don't try this at home!
Kent

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Hi Peter,

If I'm reading your question correctly you are wondering how to go about creating the arc template to use to create your dish. If you do a search for Long Compass Method you'll find one approach. Here is one link for you http://gicl.cs.drexel.edu/people/sevy/luthierie/compass/Long_compass.html.

Good luck.
Rick

Kent,
That is too, too good!
Johnny Depp has to demonstrate it though.
Coe Franklin

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I did it by fastening two 3/4" x 5" x 8' pieces of plywood together end to end. I attached a router with a straight bit to one end and a nail to the other (at the appropriate distance). It is easy to swing the arc and the router will cut your actual template very acurately. Mine came out perfectly.

This is easier than it seems, folks.
Get a pice of "something" to use as a base fore the raduis. Plywood, sheetrock, what have you.
Lay it on a level surface.
Drive a small finish nail somewhere near the middle.
Wieght this piece down via bricks, cinder blocks, your truck, or merely have someone else kneel on it (to hold the tape measure).
Hook your tape to it and stretch it out to beyond the radius you desire.
Lay your pattern material out so that it comes into range of the tape.
Hold the pattern material down so that it doesn`t move.
Hold your pencil at the appropriate measurement.
Scribe your line/arc while holding the pencil in the exact place on the tape as you go.
Once you try it, you`ll find out that it works very well.
Coe Franklin

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Check my oil too, if you don`t mind,,,

Thanks for all the info guys. I knew that I could use a tape measure and measure out 15ft and use that but I wondered if thee was a more practical method I could use. Some good ideas and the long compass seems pretty straight forward.


Thanks.

And, you can easily build a 'quick and dirty' jig with a router/lam trimmer where the pencil would go in the long compass.
Once you have a smooth pattern in something like 1/4 ply, you can make heavier patterns for routing dishes, etc with a pattern bit in the router.

Cheers
John