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I am having a hard time understanding the different methods for making a radius dish. What method/jig do you employ for this? For the sled style as seen on Mr. Paulick's wonderful videos, I do not understand the straight slope on the runners. To translate a dome onto the dish wouldn't you have to have curved runners? Otherwise, the dish would just be a cone and not a sphere right? With the middle point in the center, whereas a radius dish has no middle point?

I would think you need curved runners .. mine are .

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

My runners are straight like Chris'. If the geometry is correct the router bit will scribe the desired arc. Just another way to skin the cat.

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Steve Smith
"Music is what feelings sound like"

Ah ... yes .. there is a way to get an arc from two straight runners ....like it says above the geometry needs to be right .. might as well just cut the arc IMO and be done with it.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Chris's jig is based on the "long compass" concept. As the two straightedges fixed at a shallow angle slide over the supports, a pencil at the apex scribes a perfect arc. Kind of hard to imagine how it works, but it does.

Did some reading about the long compass and still I do not understand how that could work with straight runners. Anyway, I want to make 28 foot for the top and 15 for the back. I figure I could use the long compass to trace an arc onto some mdf and freehand it out, then use this mdf as a template to shape the runners. How do you achieve the exact arc though? I remember seeing a chart a while ago for distances at the middle of the curve, is that what you use to get the right arc?

Edit: I also just thought, why not put two nails 24" apart on the mdf, then put the two boards together for the long compass and mount the router on the vertex of the boards, then I could simply route out the arc, cut it out to be used as a template then transfer onto the runners.

Seems like your adding in some unnecessary steps there.

I simply had two stops 24 inches apart, and then sprung a fibreglass go bar to the right deflection ... close enough ... works fine. mark this on a 2x4, cut, sand, then split it .. there are your two rails.

15' - .400
18' - .330
25' - .240
28' - .210

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

I think you are not only adding steps there with the nail method but it is very inaccurate as well - you templates and forms won't be kind to you later on. The way I did it was use your radius and fasten 6" or so wide pieces of plywood end to end and a router at one end. Use the router to swipe your arc directly onto the template and you are done... very accurate and free from imperfections from sanding etc...

Thanks Tony, I will write those numbers down.

Randolph I dont quite get your method, care to elaborate?

I did basically what Chris did with the sled and long compass method. I really didn't get how it was going to work until I got things together and actually saw it. Originally I was going to cut the arc, but was unsure if I was going to be able to accurately do it. I cut everything for the long compass on the table saw and as long as you assemble your sled properly it works.

Of course I made mine wide so I didn't have to remove the part of the rail Chris did. Doing it again I would follow Chris' plan to the letter. Storage would be easier and you have more open area to spin the dish.

There was a web site and a spreadsheet that was posted back when I did mine that helped a lot.

If you are having trouble visualizing how the long compass works try mocking it out on a piece of paper. Put two equal height boxes down and slide a triangle, point down, from one block to another with a pencil at the point. Perfect arc. It is the sort of thing they would have drowned you for showing them back in Salem. She's a Witch

John

Here's the site that will explain it and has the table to figure out the sagita. Which is the same as the rise when figuring the slope of an angle. The run would be 1/2 of your span.
http://www.liutaiomottola.com/formulae/sag.htm
I'm pretty sure this might be in the tut in the tut section along with some update possibly.
The long compass has been used in ship building for a very long time.
Also the center of the sled doesn't have to be removed for clearance as the router tip can just be extended to clear and it doesn't add to the arch with the LC but the radius remains the same and kind of just cuts in parallel. I didn't know this til after the tape was made. John Cross over at MIMF first posted this idea years ago. I was smart enough to print it out since it's lost in library limbo. Here's a drawing that may help you visualize how it works from John's post.

Also search for long compass here and find the discussion of John Killin's when he built his as there is some info there.
It's just a sled with straight rails across a span. I have another idea for the long compass too. If I build the jig I'll let you know if it works.

Of course. What I was talking about was only to make a template for a 28' and 15' radius. Once you have this, the battle is half done. Think of scribing an arc with a 28' long radius. At one end you have a pin (nail or screw) and at the other a pencil. I used 6" wide plywood strips attached end to end to get my 28' long piece, drove a screw into one end and attached a router to the other end. The router scribes the arc. With a straight bit in you can trim a 3/4" piece of plywood to a 28' radius as you scribe the arc. This way your template is complete after you trim it. You can then make as many offspring as you want with a flush trim bit. This method would be a good one if you are building a curved cradle type machine to make your dishes (that's what I did) They came out flawlessly by the way. I'm sure the other method would produce just as good a result. Hope this makes better sense.

Chris, I understand the logic behind the long compass, but do not understand how it could be employed directly into the jig. Wouldn't you have to have an angle at both ends of your runners?
On the other hand I am wondering what kind of results I would acheive with tracing the long compass directly onto the runners for the jig.

P.s. Sorry about all the confusion, I get it now .

You must of done this outdoors, seems like a big operation and alot of plywood.

Or for the 28' r. you could have take two of the 6" x 48" pieces of plywood and stacked them together and measured over 18" from one end and put a mark on the edge. Then from the same corner you measured the 18" from measure up .964" and put a mark. Draw a line between the marks and cut that line and you have the two rails for your router long compass sled and just span them 36". If you want use a rail to draw the arch by taping a pencil at the apex and drive 2 nails 36" apart and use it as the long compass. Although with those measurements you won't be able to draw the full arch. You would have to center it.

Chris you mean cut from 18" in from the corner back to .964 up from the corner and that will be used make a 28 foot radius? What about 15 foot? You make your dishes 36" not 24"? Sorry for all the questions . If I were not so broke I would just buy some.

I make my dishes 24" but 18" to center gives you 6" to from the span rail to the edge of the dish. You can make the span smaller if you want but I wouldn't go less then 30". Go to that link and use the table to find out the saggita for whatever radius you want.
I use 25' top and 15' for back. The 25' top will give you the 1 1/2 degree angle so the fretboard extension will sit flat on the top.
One trick is to take the saggita and double it like I told you in the above post so that you only have to make one cut.
Read through this discussion
viewtopic.php?f=10101&t=19589&hilit=+long+compass
Then if you have some questions ask. And check out the table link. There is also some other good stuff on that site.
I could give you the numbers for the 15' radius but you aren't going to learn anything that way. Read those links and then if you have a question I'll see if I can answer it for you. John Killin should be up on this stuff too.

Thanks alot Chris, I think I finally fully understand it. You want the middle of your rail to be the apex of the triangle, and in order to do so you cut an angle from the centre to the saggita distance up from the corner. So you do not have to do two cuts for each rail you simply double it up at one end . But 25 foot radius gives you the proper arch for the 1 1/2 degree neck angle? It says this on the Blues Creek website : "What you decide on as a radius is up to you. I like to use a 1 ½ degree neck angle so I can do a set up of ½ in at the bridge and a 28 foot arc will equate numerically to that angle."

I don't think you will see very much difference in angle between 25 and 28 foot radiused tops. If I'm looking at this correctly, the height difference in the center of a 24" diameter dish is only 0.030".

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Steve Smith
"Music is what feelings sound like"

I don't think you are going to notice much difference between the 25 and the 28 in the real world and then you side could be off some too.
You don't want your apex in the center of your sled rails but one third from the end. And the rails should be 1 1/2 times the span long. I think that's discussed in the past discussion link.

edit:
28' =.214
25' = .240

That's the saggita/rise across a 24" span if I punched the correct numbers in the table. So the difference is .026" center or 12" back. So you would have to consider where the bridge/saddle is relative to the span from neck to tail block. So it might even be less. Your splitting hairs here. The top probably pulls up more then that.
Do what I do and use a Doolin adjustable neck joint and just dial in your neck angle.

You need to crawl before you can walk... before you can run

If you can do a bolt on neck you can do an adjustable neck joint. Hit the ground running!

ok I think I got it all, can you please check my (lousy..) drawing, to see if it's all correct??

it's a 24inch dish with the edge of the 'guide' being at 18 (half of 24=12 plus 6 clearance from the dish edge).