Arcs

[Bob Yarbrough]

Tips on laying out arcs & curves wanted. Looking at large radius curves where a compass will not work.

Once cut, tips on haw to sand to the "line" for a smooth finish.

Thanks,

Bob

[Todd Burch]

Bob,

I make my own trammels for laying out arcs and curves. The longest one I've laid out recently was was for an 11' 5" long radius.

I use a 1X2 on the flat. I drill a tight pencil hole in one end and jam a #2 pencil in it. I measure radius the I need from the center of the lead (or the center of the hole) and make a dot on the opposite end. I drill a thin hole pilot hole and then drive a screw thru from the other side. Viola - a trammel. (The only tricky part is getting the pencil lead and the screw tip on the same side!! DAMHIKT!)

I've used strings and two nails for ellipses too. While low-tech, it works quite nice and is quick.

As for smoothing, if you hook up a router on a trammel (think of a modified router baseplate with a long edge edge hanging off one side), there is no smoothing required, just a light sanding. I keep a 6' router-trammel handy hanging on the wall. It's made from a scrap of 1/4" masonite. I have a shorter one too.

What's your particular application?

Don't forget to search this forum - there is plenty more info on laying out arcs, etc.

[Ellen Benkin]

How big do you want it to be? I use a flexible ruler or a thin piece of wood, mark the ends and the midpoint and bend the ruler/wood to match. Then trace the line. I use a saw (band or jig) to cut away most of the waste and then a disk sander to get to the line. Usually this needs to be cleaned up by hand. If the curve is to be duplicated elsewhere, I make a template out of masonite and cut it out with a straight router bit with the bearing following the template. I spend a lot of time making the template as perfect as I can because this translates into less time fussing with the actual wood project. The router bit will duplicate the template exactly, including any dips or dings.

[Bob Yarbrough]

I am cutting shallow arcs on the base of a blanket chest. Went looking for a flexable arc this morning, (something I used in the far past for drafting before the advent of cad). Ended up with a $1.79 steel yeadstick from Menards and was able to draw a pretty smooth arc.

New blade on the Bosch jigsaw, 10 minutes with a 3M sanding sponge, and I am very pleased with the result.

Bob

[Dave Wright #2]

Bob,

I'm glad you got the job done. Most of my pieces have a curve or two in them somewhere. Over the years I picked up a few tips. Maybe some of them will help with your next curved surface.

LAYOUT

When the curve is larger in radius than your circle template or compass can draw, I often plot the curve using my CAD program and stick it to the workpiece with a light dusting of spray adhesive. Sometimes I make a large compass by putting the workpiece flat on my deck, driving a finish nail partially in as a center, and using pencil & string to draw the curve.

CUTTING

If more than one copy of the curve are in the project then I often form it in a scrap of plywood or MDF and template route the workpieces. That way I only have to fair and smooth the curve once.

Curves are usually roughed out at the bandsaw. It is sometimes faster to grab the coping saw for small ones. Often it is easier to bring the tool to the work; for those times I use the jigsaw as you did on this project.

FAIRING/SMOOTHING

Here's the surprise, or at least it was for me when I realized that it had become habit. I find fairing and smoothing to be easier, faster, and higher quality when done with rasps and files instead of sandpaper. This is particularly true of convex curves. I start with a Nicholson rasp; I would have to check if it's a #49 or #50. Had it for years. I then can usually finish with a cabinet file. The resulting surface is similar to 220 grit sanded (once you get the hang of keeping the file from gouging). Sometimes I go to a fine metal file, which leaves a wonderful polished surface. Give rasps & files a chance!

Hope This Helps,

Dave

[Bob Wingard]

I have a 3" square tube as an upright member of the blade guard/dust collector on my table saw. To this tube, I clamp a heavy-duty door hinge. To this I bolt an appropritely sized piced of scrap wood. To this, I attach (at the proper place) a small (8"X10") piece of plywood scrap with a 3/4" hole in the center. Calculate the radius of the arc, determine the start/end points on the project, set the jig, drop in a router with a 1/2" bit and a 3/4" bushing, put your stock on top of a piece of scrap to protect the saw, and rout away. If you don't trust the system at first, at least try using it to make a pattern/template that you can attach to your good stock & go from there. After a few tries, you'll gain enough confidence to use it directly on your stock.

[Keith Christopher]

I use a couple of different techniques depending on the type and size of the arc. The easiest by far for "odd" or long shallow arcs is:

(And no this is not stolen from DMarks but from my Grandfather who showed it to me long before DMarks was ever on TV.)

I take some thin stock and clamp it to something VERY heavy on each end and one in the middle. The laying the stock flat move the bricks or end pieces to match measured ref points or to simply find a curve you like.

The second involved using string and some nails but I would not work easily in this example and I'm not sure I would convey it correctly by typing it.


Keith

[Tom Hintz]

See the link below for the procedure one of my "plan testers", a retired cabinetmaker, showed me. I believe I have seen His Normness use a very similar method.
Anyway, it works for me in any situation and produces very clean, accurate arcs. Now that I know how to do it, it goes very fast as well.

http://www.newwoodworker.com/layoutarc.html

[Gary Sutherland]

Try this. I bought this tool, but it would be easy to make one. Quickly adjusts as needed.

[John Hart]

I do the same thing that Todd Burch does. Eventually, I made a dedicated piece with pivot holes spaced a 1/4 apart and use a finish nail as my pivot point and leave the pencil in a dedicated hole out on the end. If I need a large radius in a small piece of wood, I just clamp everything down on the bench and reference back to a pivot block out in space somewhere.

[Jay Knepper]

It seems that I have an arch in about everything that I make. Relatively tight radius curves, say 12" or less, I draw on the computer and print the result. Then I use the cut-out to make a pattern or parts depending on how many replicates there are. For longer radius curves I generally locate a spline, a long scrap of uniform thickness, 1/4" is about right. then I lay out the ends and high point of the arch and fix the spline so that I can use it to draw the smooth curve. I fix the spline by using quick grip clamps: two to block the ends and one to reach over the part or pattern and fix the spline at its apex.The spline method is much faster of the two if one can clamp the spline easily. For the tighter radius curves the 1/4" spline is too thick to smoothly follow the curve, and a thinner spline is too thin to easily clamp, so the printed pattern works better.