Uses for a long compass?

[John Coloccia]

I use it to make radius dishes for sanding/gluing flattop guitars.

I'm curious what other uses does the long compass have in the shop?

[Steve Rozmiarek]

John, I use a large beam compass, just a stick with a couple Starrett or Stanley trammel points attached for layout sometimes. Big arches can be "tuned" in full scale this way. Used it on a gazebo to set the brace arches, and several projects to find my choice of curves for corbels.

[Frank Drew]

Laying out big curves or circles? Like table tops, architectural features, etc.

[harry strasil]

Using the 3-4-5 method to scribe short arcs to check corner squareness of plywood sheets.

Laying out a door to fit an unsquare opening.

Cork stopper on the sharp end and then using as a back scratcher, a framing square works well too.

[John Coloccia]

It just occured to me that many probably aren't familiar with the long compass. I wasn't until I needed to be. Here's a good explanation of this handy device:

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

So I personally use this technique to make an accurate 24' radius dish (that's a 24' radius on a 22" diameter dish...not a 24' diameter dish!), something that would be pretty difficult to accurately cut otherwise.

[Lewis Cobb]

It just occured to me that many probably aren't familiar with the long compass. I wasn't until I needed to be. Here's a good explanation of this handy device:

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

So I personally use this technique to make an accurate 24' radius dish (that's a 24' radius on a 22" diameter dish...not a 24' diameter dish!), something that would be pretty difficult to accurately cut otherwise.


I for one was wondering "what's this lad going on about" - thanks for the link !! :D

[harry strasil]

Been using that method of striking an Arc since the 60's while in the navy, I used it lots in civilian life to cut sheet steel to match the OD of grain bins and oil field tanks among other things. Just never heard it called a long compass before.

To determine how far above the outer points to place the center one, I always took the chord height measurement.

[Mike Henderson]

To me, the long compass is best for really a big radius when all you want is an arc and not a complete circle.

For my work I generally use a couple of trammel points on a beam. I haven't had the need for really big circles but a friend of mine used a long compass for making a curved top driveway gate. He only wanted an arc and not a half circle so the radius was huge. I think he used a couple of 2x4's and had some helpers when he laid out the cut.

I never heard the term "long compass" before, either, but I don't remember what people called it.

Mike

[Chaz Alexopoulos]

That is a great tip, John. I could of used that for the sides of the shade canopy I built for a porch swing. I ended up wasting half-an-hour sketching it freehand. Great link.

[Steve Rozmiarek]

Hmmmm.... I can't think of any benefits for that approach over trammels, or a string or whatever. Seems that the structural integrity of the rig would limit it's size, and it would become pretty clumsy with any size at all. For smaller stuff, seems more trouble then it's worth. Clever though. What am I missing?

[John Coloccia]

Hmmmm.... I can't think of any benefits for that approach over trammels, or a string or whatever. Seems that the structural integrity of the rig would limit it's size, and it would become pretty clumsy with any size at all. For smaller stuff, seems more trouble then it's worth. Clever though. What am I missing?

What makes it cool is that you can make rigid fixtures out of them. For example, I put a router in mine and slide the whole assembly back and forth to make a large radius.

[Steve Rozmiarek]

What makes it cool is that you can make rigid fixtures out of them. For example, I put a router in mine and slide the whole assembly back and forth to make a large radius.


See, I knew I was missing something!

[Farhad Mahini]

Dear John,
I been searching the entire net for info on radius dish making. So far there are ambiguities in my min. I’m glad you have experience on this. I had seen the site u mentioned. What is missing there is the vertical distance between the inner point and the outer point which is not mentioned. Guess it is the “ BD “ that the leftbrainluthier.com formulates and can be calculated thru ArcCals.xls .
The leftbainluthier goes on to explain that “ If the dish will be say, 18" wide then we need an arc probably 24" wide “ ! A statement which totally adds to my confusion !! Is this math or statistics ??
Even more so he talks about the “ width “, which I’m not sure if it is the length of the guitar body or the lower bout he is talking about. In the example he mentions an OM with 47 cm of width which I’ve never come across such a guitar unless he means the body length.
Another issue is that the highest point – the apex of the curve in all the guitars I’ve seen is not in the middle of the back, but rather more towards the heel side. One more thing here is that on my Ibanez AEF 18, there is a about 10-12mm ( ¼” ) of rim along the back where the curvature comes to almost zero and then backs up again to sit on the angled kerfs.
Would appreciate if you comment on these issues, and help me out to finally build(rout) my arched dishes for 28 and 15’ curvatures, and what is important to be able to use it for a 00 , OM, Jumbo and Jumbo 12 equally.
Farhad

[Myk Rian]

I used a 16' radius to form the curves on this dictionary stand.

233941

[John Coloccia]

The compass is made from 2 "pivots" and two pieces of wood joined at an angle. Whatever your arc is, the two pivots fall on the arc, and the fulcrum of the angle formed by the two boards falls on the arc. It's that simple.

To calculate the "inner" point, you need to calculate the sagitta of the arc. It's actually the versine function (you've heard of sine and cosine? Now you've heard of versine). Maybe you can find something online about this. Essentially, given a radius of a circle, and a distance (the distance between ANY two points on the arc...in your case the distance between the two pivots), if you drop a perpendicular line from line formed by the two pivots to the furthest point of the arc, the sagitta is that distance.

So if you take a circle, and draw a line....ANY line, through the circle (not necessarily through the center), you divide the circle into two arcs. The sagitta is how far "up" the shorter arc rises away from that line at it's furthest point.


http://www.youtube.com/watch?v=tvGemvizrz4

This may make it more clear. It's very difficult to describe. If I had you in my shop for 10 minutes I could just show you with two sticks exactly how to do it. It's very easy, and generally it's a very nice and simple way to make an arbitrarily large section of a circle. This is one of those things that you just need to fool around with for a bit, and think about for a bit.

My suggestion:
Buy the stupid radius dish if you wish to use one. I don't use one anymore, but I've bought subsequent ones and would never make one again. Ever. You couldn't pay me enough. It's an incredible waste of time, the jig to make it is an incredible waste of space, the the whole thing is far better made by a CNC machine and delivered to your house in a nice little box.