Official Luthiers Forum!

Hello OLFians,

Can someone help me out with the radius curve depth for 28' and 20' for
a given length line (18", for example)?

I did the giant compass in my driveway, but the stretch factor of my twine
made it too wonky.

I'd like to switch to the method of flexing a metal rod between three
brads and was hoping someone has aleady calculated the depth of the
curves and could share

Thanks, Chris

I'd be interested too, for 15' and 30'   I thought about using a small linked chain and maybe a shawdow board to get the radius.

 if you use a 30' tape measure , it should work just fine with no stretch ,it  might sag in the middle , but if your driveway ( or work area  is relativly flat , you should be able to work it out ,, I know I am not too good with the math sooooo....  Jody

I had read about the tape measure method. The longest tape I currently
own is a 15' so was hoping to avoid buying a longer one that I'll never
use, though.

Thanks,
Chris

you can buy radius templates from our sponsors such as lmi or luthier suppliers, or print out paper templates off the web. much easier then futzing around with string and long tape measures.

Does this help?

http://www.scottdeegguitars.com/tools/arcCalc.php



It's best to use inches (or cm).  The tool will calculate the height
for points on the arc starting from the center (x=0) of the chord and
going to the end (x=l/2) in intervals of 1.  So, if you enter 300 for
the radius (25') and 24 for the length (2' chord) it'll give you values
for every inch starting at 0 (the center of the chord) and ending at 12
(half the full length).



Feedback accepted, frequently agreed with, but not always implemented.
:-)

Here is an alternative help; http://liutaiomottola.com/formulae/sag.htm

[QUOTE=ParamesB]Here is an alternative help; http://liutaiomottola.com/formulae/sag.htm[/QUOTE]

No disrespect to R.M. Mottola (I love his site), but this tool only calculates the height at the midpoint of the chord, which I found interesting but less then what I was looking for.  I'm sure there's something else out there that will also give you points in between, but I eventually got bored looking and just did the math/wrote the program.

enjoy

Thanks guys, what would you use and how would you excavate or dig out the dish?  Would you start with 3/4" MDF?

I'll give you the proverbial fish, but first I'll also teach you how to catch one.

At the risk of REALLY confusing someone, here is a calculation I've used in an excel file to calculate the exact arch height you are looking for.



= 240 - SQRT(240^2 - (18/2)^2)


what you might do is copy/paste this into an excel cell and see what it spits out.
240 is the number of inches in the 20' radius
18 is the number of inches in the total length of the "example"

change 240 (both times in the formula) to 336 (both times in the formula) for a 28' radius arch height



If you are not into Excel then here's the number you're looking for

0.168809368	4.287758

.168809368" or 4.287758mm for an 18" example at 20' radius

.112517584" or 2.857947mm for an 18" example at 28' radius

hope that helps

PM me with an e-mail and I can send you the file.

Dave

amazing...
we're quick with the help.
In the time it took me to dig out the file and type up a reply, three others did the same.

regardless of what you use....
good luck

[QUOTE=Chuck Hutchison]Thanks guys, what would you use and how would you excavate or dig out the dish?  Would you start with 3/4" MDF?[/QUOTE]
Hey Chuck, don't know how others do it, but I used my arch to build two rails about 3' long.  I then set them wide enough for my router to sit in.  I cut 1" MDF into a 2' circle, mounted it on a lazy susan frame, put it under the router on the curved rails, and slowly brought the router in while I controlled the rotation of the dish.  There's a discussion about this here which I found helpful.

http://www.woodenboatvb.com/vbulletin/upload/showthread.php? t=69702&highlight=radius+dish+jig

BTW: this makes an amazing amount of dust, and I build a little "dirty room" out of sheet plastic to contain things in my shop, and it was still everywhere!  Gotta get that central vac someday.

there are numerous threads on the forum dealing with making a jig to cut dished forms with a router, and plans for a version are available in the plans setion above.

[QUOTE=smdeeg] [QUOTE=Chuck Hutchison]Thanks guys, what would you use and how would you excavate or dig out the dish?  Would you start with 3/4" MDF?[/QUOTE]
Hey Chuck, don't know how others do it, but I used my arch to build two rails about 3' long.  I then set them wide enough for my router to sit in.  I cut 1" MDF into a 2' circle, mounted it on a lazy susan frame, put it under the router on the curved rails, and slowly brought the router in while I controlled the rotation of the dish.  There's a discussion about this here which I found helpful.

http://www.woodenboatvb.com/vbulletin/upload/showthread.php? t=69702&highlight=radius+dish+jig

BTW: this makes an amazing amount of dust, and I build a little "dirty room" out of sheet plastic to contain things in my shop, and it was still everywhere!  Gotta get that central vac someday.
[/QUOTE]

Wow, this was exactly what I was looking for.  Something simple for a simple man.  You just saved me about 200 bucks.  Thanks man!

By the way Scot Deeg, welcome to the forum.  Nice looking guitar you have on your site.  Once again, thanks for the help.

[QUOTE=Chuck Hutchison]By the way Scot Deeg, welcome to the forum.  Nice looking guitar you have on your site.  Once again, thanks for the help.[/QUOTE]
Thanks for the welcome Chuck, I'm happy to be here, and thanks for the compliment on my guitar.  It's nowhere near perfect, and I have a ton to learn, but for a first instrument I'm pretty proud of it.

[QUOTE=smdeeg]Does this help?

http://www.scottdeegguitars.com/tools/arcCalc.php

It's best to use inches (or cm).  The tool will calculate the height for points on the arc starting from the center (x=0) of the chord and going to the end (x=l/2) in intervals of 1.  So, if you enter 300 for the radius (25') and 24 for the length (2' chord) it'll give you values for every inch starting at 0 (the center of the chord) and ending at 12 (half the full length).

Feedback accepted, frequently agreed with, but not always implemented. :-) [/QUOTE]

Perfect!  Thanks a zillion Scott, and all who posted helpful advice.  I'm new to OLF and am working on my first guitar.  This a great group of people!

 

Thanks,

Chris

also try this. i will also say, and this is just my opinion, but both the long string in the middle of your driveway and the rod bent between nails are worthless. well, maybe not worthless, but not nearly as accurate as the long-compass.  youll notice in the picture they show a pencil used to draw the arch. instead of a pencil, i just made a carrier for my router and routed out a perfect arch. you can then use that as a template to make the rails for your dish-making jig. if its un clear maybe i can take some photos.

_________________
sweat the small stuff.

I used my wooden deck, and limited my arch to 25', the length of my tape.   

By tacking a slice of thin maple to my deck, and using a pencil to draw an arc, perfect every time, no math.

I did the same for 15'. Took the maple thin to the bandsaw and sawed out the line, then smoothed it a bit to make it useable.

I like how you guys think.

"How many luthiers does it take to make an arched template?" only one..... who has a long enough tape.

I remember these days...

_________________
http://www.dickeyguitars.com

Here is a link to a thread and post that I made almost two years ago that has a spreadsheet that you can download to your computer and save it for your own use. The explanation of the spreadsheet is in the thread along with a screen shot.

   http://www.luthiersforum.com/forum/forum_p osts.asp?TID=5275&KW=Peter+J#forumTop

After you have your calculations for depth, use a fiberglass rod (Lowe's or Home Depot for $1.98) pound three nails in a board; two at the extremes of your desired baseline length and the third at the center of your chord and just below the line representing the total depth you have calculated. Place the fiberglass rod along the arch described by the three points you have identified ( the 3 nails) and the rod will conform to a uniform arc that you can scribe with a pencil and use as a template.

_________________
Peter

Ooops... the link didn't work. Here, this one should work OK.

http://www.luthiersforum.com/forum/forum_posts.asp?TID=5275& KW=Peter+J#forumTop

_________________
Peter

Chuck I have plans in the "Library of Plans" at the top of the page for a bowl radius jig. If you don't know all fund go towards supporting the forum

I use a 24" spread instead of 18" because thats how big my radius dishes are.

Here's what I come up with using 24"

for a 28' radius you get .2144" height
for a 20' radius you get .3002"

for 30' you get .2001"
for 15' you get .4004"

I use Autocad, draw a circle with the radius you want and 2 lines 24" apart and measure the height.

When I made my templates I used a 4 foot aluminum straight edge (on edge) instead of rods to trace my radius.

_________________
Horton, MI

I just double-stick taped  a 48" length of 80 grit to the hood of my(then) '82 Olds Cutlass, and sanded the rails for my original, and still in use, dish. Whatever that radius is, is what I have always used....


B.Bubba: Improvise

Or was it an '81 Cutlass?