# Routing an Arch



## Curmudgeon10 (Feb 24, 2013)

I am making an arched valance to be installed over the kitchen sink, in front of two good sized casement windows. It will span the distance between the cabinets that flank the walls on either side of the sink.

The joinery will be cope and stick, and the material poplar for rails and stiles; the two raised panels will be MDF. I have a Bench Dog router table, several routers, and all the bits needed to do this work; I also have the knowledge and experience (at least for the straight parts!). 

The valance is a rectangle, except the bottom rail is arched. The radius of this arch is about 6'5" at the bottom edge (I have calculated the exact numbers), and the chord across the bottom of the arched piece is ~ 4' 8". I've tried a quick and easy method (jigsaw cuts close to layout line, smoothing with belt sander) that doesn't work, because you can't really smooth the "inside" or concave curve well enough. 

It would be great to be able to generate somehow a template, and then use it to with a bearing/bit to true up my stock to the final dimensions. But how to produce a suitable template? What material? And if I had to construct a large, rather elaborate jig to swing the template stock past the bit at the correct radius to achieve the curve I need, why wouldn't I just use the 3/4" stock directly in the jig? Safety? Or should I make an accessory that swings the router by the stock at the correct radius... I'm perplexed..

I've attached my working drawing (the arch on the working drawing looks more ellipsoid, but that is just the best I could get from MS Word Draw tools).

Any ideas?


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## jw2170 (Jan 24, 2008)

Hi Rick, welcome to the forum.

I am not a maths genius so may not get the terminology correct.

File:Circle sagitta.PNG - Wikipedia, the free encyclopedia

If you know the distance (x) from the chord  to the circumference, can you put pins in those 3 spots and using a thin metal ruler or strip of timber, draw the arc and use that drawing to make a template? You do not need to work from the centre of the circle.


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## Phil P (Jul 25, 2010)

Hi Rick and welcome to the forum



Curmudgeon10 said:


> The radius of this arch is about 6'5" at the bottom edge (I have calculated the exact numbers), and the chord across the bottom of the arched piece is ~ 4' 8". I've tried a quick and easy method (jigsaw cuts close to layout line, smoothing with belt sander) that doesn't work, because you can't really smooth the "inside" or concave curve well enough.


I can suggest one possible method. Build a trammel arm of 6ft 5in radius (so about 7 feet long overall) from an 8 ft rip off the edge of a sheet of 1/2in plywood or 18mm (3/4in) MDF with the router screwed to one end and a screw at the other edn to act as a pivot. This is then used to make a full-size template in something like a piece of 12mm (1/2in) MDF. That template in turn is used to mark out the arch to be rough cut slightly oversize to with a jigsaw before being attached to the material (hort melt glue, screws which will be hidden by mouldings,clamps) and the final shape routed with a top bearing template trim cutter. I've used this method to make far larger radius curved servery counter componets in the past of up to 4.5 metre (14-1/2ft) radius. The main difference is that at that size it takes several rips with joins to make the trammel arm and using iit is a two man job. BTW the trammel "compass point" needs to pivot on a piece of material the same thickness as the workpiece and screwed down to the floor (for this sort of job that's where you need to work)

Incidentally, I can see one difficulty with your drawing - that particular shape is normally made-up from three separate radii - one at the start of the arch where it leaves the column, a second for the main part of the curve and a third for the centre of the arhc. Doing that requires three templates which are then used to make a single template before even starting on the actual material. A more complicated procedure as getting the transitions right can be quite tricky. have you tried laying this out in CAD?

So others can see it I've taken the liberty of PhotBucketing your drawing. Hope you don't mind:










I'll update this post in the morning to explain my technique in full, but it's past my bedtime now

Regards

Phil


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## CR1 (Aug 11, 2011)

If i I remember correctly that matthias wandel guy with woodgears.ca has a set up to do something like this


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## mgmine (Jan 16, 2012)

If you can draw the curve you want that will be 99% of the work. Make the template out of mdf you will find it much easier to sand than plywood. You can use 3/4" or you can use 1/4" or anything in between. I would use 1/4" which you can get at Home Depot because it will be easier to sand. Take your time when sanding and sand to the line.


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## MEBCWD (Jan 14, 2012)

Hi Rick,

This is what you need: Router Circle Cutting Jig for Woodworking


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## ham385 (Jan 29, 2013)

What about using a no-stretch cord, like mason line, to make a large ellipse? I guess that depends on whether you want the ellipse shape or more of a circle shape. I think you could use this 2 ways.
1 limit the travel of the router using the cord

Or 

2 draw the arc using the cord, draw a 2nd arc offset by the distance between the outside edge of your router plate and the outside edge of your bit.
2.a cut a thin 1" strip of hdf, 
2.b screw a bunch of small 1x1 blocks to the strip
2.c lay the strip on end, and bend to match your curve (the offset curve)
2.d screw the blocks to the scrap side of the workpiece.
2.e route away


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## Curmudgeon10 (Feb 24, 2013)

Thanks to everyone for many helpful ideas. To establish the correct proportions and get the Planning and Interior Decorating Department to sign off on this project before I committed significant time and $$, I constructed a mockup with PVC for the straight rail and stiles, and used cardboard for the arc and raised panels (just drew the panel lines on those pieces). Thus I was able to verfify the arc. I don't know about the multi arc requirement described; the one I made from builder's paper looks OK. 

Now, I have two choices --- I can carefully, smoothly and accurately cut the builders paper arc and laminate to a piece of MDF, and then go about perfecting a template, OR, I can go straight to the router mounted to the trammel, and swing the router to route away the small amounts of material left on the stock after rough cutting it.

It just so happens a guy is working in my kitchen now on a redo, and he is an experienced "radius man." He suggests the trammel, so that's what I'm goint to do. I think I'll take some pictures and let the forum know what happened (unless I loose a thumb or finger).


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## CR1 (Aug 11, 2011)

you can just make pattern from 3/8 ply and use a pattern bit with a bearing. the nice thing about making a pattern is you can go at it till you have it just so. Draw the lines and cut close, then sand to perfection. If you screw up, it's not a big deal just make another till you have it dialed in.


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## Curmudgeon10 (Feb 24, 2013)

My jig is made from a piece of red oak 1 x 6; one ends accepts 3/8" dowel pins for the two different radii; on the other end is a shop made router base for my old Porter Cable 3/4 router. I made the router base from 3/8" plywood and a piece of lexan I had in the shop (it came from a dog door I bought years ago). 

The pivot end of the trammel locates into a hole in a 6 x 9" x ~ 3" high box made from scraps (MDF and PVC on the top), which clamps into an old Black and Decker Workmate. The dimensions of the box were chosen so the swinging part (trammel) is exactly level with my work bench.

I have made one attempt so far, using poplar. The work is rough sawn with a recip saw close to the line, then screwed to the bench top at the corrent geometry for the trammel to operate.

Here's what I learned on the first attempt: I can rout the top arc without a problem, proceeding from right to left, and the finish is nice and smooth. On the bottom arc, I started from the right as well, but noticed that the router wanted to "run," so I switched off and began cutting from left to right. This worked OK until I got near the top of the arc (the middle). There, the router dug in hard to the grain and that pass was over.

I bought a new bit, and on the next try, I'll cut the top as before, but on the bottom, I'll start right to left, and exert a lot of control to keep the router from running away, until I'm well passed the mid point. Then I'll finish going left to right.

I'd post some pictures, but my camera battery died. New one on order.


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