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I want to make a radius dish. I think I know how to do it but please can someone help me through a bit of my confused thinking?
If I'm making a 25' radius and I have runners shaped to that curvature and the router bit protrudes below the runners by 1/2 '', doesn't that make the radius 25 1/2'? Do I have to make the runners to a radius of 24 1/2'?
Also, I want to make a jig to rout the curvature on the braces. If the dish has a 25' radius and the back plate is 0.1'' (for sake of argument) thick shouldn't the braces be shaped to 24.9' so that everything fits snuggly. Hope I'm accurate with the inches because I normally use metric.
It's strange that no where in UK sells radius dishes.

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We are all in the gutter but some of us are looking at guitars.

You're mixing up feet and inches when you add and subtract. You're worrying about 25' compared to 25.04' (or 25'-0 1/2"). Or 25' compared to 24.992' (25' minus .1"). It's insignificant, won't really make a difference.

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Jonathan Kendall, Siloam Springs AR

Johnathon, one of us must be confused, possibly both of us! I don't see where you got 25.04 from, I may be a metric man but I know that 1/2 '' equals 0.5 an inch.

I'm going to post again using metric and see if that's any clearer.
If I'm making a 762 cm radius and I have runners shaped to that curvature and the router bit protrudes below the runners by 1.2cm, doesn't that make the radius 763.2cm? Do I have to make the runners to a radius of 760.8cm?
Also, I want to make a jig to rout the curvature on the braces. If the dish has a 762cm radius and the back plate is 0.2cm (for sake of argument) thick shouldn't the braces be shaped to 761.8cm so that everything fits snuggly. It's strange that no where in UK sells radius dishes.

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We are all in the gutter but some of us are looking at guitars.

Yes, but 1/2" doesn't equal .5'.... You said 25' plus a 1/2" was 25 1/2' . You added a half foot instead of half an inch. Likewise, 25' minus .1" is not 24.9'.

1/2" is approximately .04'. (1/2 in X 1ft/12in) = .041666...

Anyway, I mainly wanted to show you that the differences were not that big and won't really make a difference. At that big of a radius, you'll still get a really tight fit with your parts.

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Jonathan Kendall, Siloam Springs AR

762 cm radius - 1.2cm error Your error is less than 2 thousands; you are over thinking this. My guess is that your runners and your use of them with the router will not be accurate within a couple thousands anyway.

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http://www.Harvestmoonguitars.com

aha, I got my ''''''''''' all mixed up, never mind, I'll stick to metric.
Anyway, It's the principle that counts. I want to strive for absolute accuracy. In principle should I make the runners slightly less of a radius to account for the extension of the router bit?

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We are all in the gutter but some of us are looking at guitars.

I'll do my best. I'm going to make a jig based on Chris Paulick's one.
In my eyes an extra 1.2 cm (or whatever the amount turns out to be) to a radius is quite a lot.
Anyway, I'm not asking the forum if my jig will work or be accurate. I quess I'm asking people who have made a radius dish before, did you take into account the router bit protruding below the router when you designed the jig?

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We are all in the gutter but some of us are looking at guitars.

The bit will travel in a parallel arc with the runner. Your router sled and support is creating the ark on its plane. The bit will be traveling on a different plane, but still follows the radius set buy your sled. Map it out on paper. When I did mine with the long compass technique I was worried about this same thing. I set up a test where I used a pencil at different lengths from the sled to draw test lines. It worked.

I'd be more worried about creating an accurate radius on your runner. Of course my logic could be off here because I used the long compass method and your setup may work differently.

I laid it out in Autocad to measure the difference between a dish routed at 25' exactly and one routed at 25'-0_1/2". In a standard size sanding dish (24" diameter), the 25' dish would be exactly 0.0004" deeper than the 25'-0_1/2" one.

It's good to be accurate, but you don't need to work to the same tolerance on everything. Save that kind of precision for the binding, fret work, and saddle placement.

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Jonathan Kendall, Siloam Springs AR

The arc of your dish will only be a bit over 61cm out of a 4785 cm circumference. Will your eyes see a .015 cm difference across your dish?

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http://www.Harvestmoonguitars.com

And so there is no confusion.......Jonathan's number of 4/10,000 of an inch equals roughly 1/100th of a millimeter.........not much and you won't build anything from wood that holds that type of accuracy.

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Formerly known as Adaboy.......

They don't call me Eagle Eye for nothing!
OK, you are all right. I've stopped stressing now and I'm going to start building tomorrow. Phew!
John, do you have any advice before I start? I don't have any plans or anything, I've just been watching Chris's video and trying to figure it out.
Thanks for all your replies, lads.

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We are all in the gutter but some of us are looking at guitars.

Joe,

Check out this thread.

viewtopic.php?f=10101&t=19589&st=0&sk=t&sd=a

I asked Chris a bunch of questions as I was working on my setup. At the end of the thread we talk about what you are asking in this thread.

I actually changed my setup to look more like the one Chris showed in the video. Originally I had made mine with a sled that was wider than the dish. That was not the best move. It was awkward to advance with the clamp. Chris used the more compact version and that has worked well for me.

For the box that supports the sled I have mine so that the dish rests on a 12” wide board. If I had to do it again, I’d support the dish with a platform that is large enough for the entire dish sit flush on.
Good luck,

John

John that's really helpful, thanks a lot. I did try a search on this forum for such a thread put couldn't see it. Haven't had a chance to read that thread yet but I can see at a glance it'll be useful. I love Chris's videos. I copied my binding channel router from his video.
Thanks again, your a good man, this forum needs people like you!

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We are all in the gutter but some of us are looking at guitars.

Here's my setup; the rails that the router base rides on are shaped to the desired radius. The whole thing rotates around the dish blank registered by the end pieces. It takes quite a few passes- ya know, you route a channel, move the device, route a channel, move... etc. I smoothed the dish with a ROS, and shellacked everything.
pvg

If you make them yourself, I would highly recommend figuring out a way to attach a shop vac to it. That MDF dust gets everywhere. I made mine outside on my porch without a shop vac and it made a mess of everything within a 10ft (3m) radius.

I like your setup PVG, that looks pretty slick.

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Jonathan Kendall, Siloam Springs AR

Here's a shot of the jig I use, curves laid out using the shipwright's compass (AKA long compass):

...details (in metric, mostly!) in the usual place.

Joe, if you're building in Geordie land, make sure you pay attention to your humidity control. Fog on the Tyne and all that..., otherwise the difference between 25' and 25.04' will be the least of your worries.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

This is what mine ended up looking like in the end:



If you look on the right hand side of the sled, where its hanging over the edge of the frame, you can see the upside down radius that causes the sled to dip down, creating the same radius in the MDF discs.

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Jason McGowan
M&S Guitars
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United States of America, looks like another silent night
As we're sung to sleep by philosophies that save the trees and kill the children....
Casting Crowns

John, I've just read your dialogue with chris and I have a headache!
I had the idea of using the long compass method before watching Chris's video. It's a beutiful, simple method. I didn't realise you could double the height of the sag. at one end of the sled and leave the other without a rise. That makes it even simpler (famous last words!). I'll try to take some pics to show you how I get on.
Trevor, humidity is a problem here. I have an average humidity of about 70%, although I'm not that confident in the acuracy of my hygrometer. I've just moved into a new house and set up my first guitar making workshop in the garage. I'm saving up to buy a good dehumidifier. When I have the heating on the humidity drops to 30%, when we boil potatoes in the kitchen next door it rises to 100%. That poor wood. Although maybe the starch will stiffen the wood!

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We are all in the gutter but some of us are looking at guitars.

Joe,

I had a headache for the duration of that thread. I started that right after Chris posted his videos on making the dish so in the thread there is quite a bit of me trying to understand the logic of the system and also some discussion of tweaks to the finer points of the setup. Hopefully by the end it made sense.

After making my original dishes I went back to make a dish with a 60' radius. The beauty of the system used by Chris is that you really only need to cut an accurate straight line. I have a table saw so that felt like a good approach. The main thing is setting it up so that you make the same cut on both boards. Well for the 60' dish I cut the line on the rail without the table saw and it was quick and easy. I used my router to do it.

On the rail I marked points for zero point and the rise (doubled). I then took a straight edge and attached it to the rail connecting these two lines. I cut the rail close to the line and then used a pattern routing bit to do the final cut. For the next board I just screwed the first rail to the second and repeated the process with the pattern bit. It only took a few minutes to pull it off.

Oh and for the dust. It does make a lot of dust. I have started doing this outside and wear a respirator while I’m doing it. One thing with your carriage setup is that you need to make sure the sled sits flush to the box. If you setup a vacuum, make sure it doesn’t interfere with this. Put the same thought to where your router’s power cord is going.


Good luck,

John

Thanks John. I nearly finished making a 24'' radius. I did things a little different than you and Chris and I'm not convinced it's going to work. I'll try tomorrow.
Actually, I've had this idea for a sled hinged in the middle so you can change the radius real easy. The middle would have to be removed to avoid going into the dish. I think I know how. Planing on trying it out this week. If it works I'll post a thread on the forum.

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We are all in the gutter but some of us are looking at guitars.