I'm building a copy of a Stickley 89-1026 bedside cabinet (although to use as a printer cabinet).

I'm building it out of bamboo plywood.

I decided to use the MFS as an arc jig for the router (credit Brice for the idea) to make a 1/4" template for the route with a flush trim bit.

As previously mentioned in This thread, I had problems adding new connecting hardware to the templates for easy attachment of the pivot point piece.

Festool customer service has been great and is sending me new connecting hardware, but I decided I needed to use my weekend time to be productive.

So I decided on a workaround where I clamped the other side of MFS profile in place to hold it while making the arc.

Pictures follow, hopefully self explanatory.

I used a combination of MFS joiners as well as regular rail joiners to join everything together. It took all my profiles (from both an MFS 700 and MFS 400 kit), but I was able to make a nearly 6 foot long (1900 mm +) arc compass.

Camelot? Let's not...tis a silly place!

Thanks Greg.

But the point of this exercise was to give me a hard, perfectly cut curve in the plywood to use as router template. Drawing (which I've done many times) would have left me with something that needed to be bandsawed and then faired, and they never come out very well.

Cool idea, how do you handle offset between the pivot and the cutter ?

All that matters is the straight line measurement between the cutter and pivot.  The offset dosen't come in to play. 

Seeing as this is about arches, this is where you need to start. If you don't understand the layout use and fabrication of arches after reading these, go back to straight lines.;

http://www.thisiscarpentry.com/author/wm-todd-murdoc/

fritter,

As Brice pointed out, the length of the radius in a simple arc is all that matters. As long as everything is set up on the centerline, life is good. The biggest issue is the pivot point and router diameter. This affects the arc, the size of an arc is different on the inside and outside of a cut by a 1/4'" diameter router bit.

Alan,

If you want a bracket to set the pivot point and the router bit inline, use the gauging bracket from the LR 32 system. It will clamp to the rail and the spring loaded edge stop pin works as the pivot. That is fine for small arches, very hard to keep it from pulling out of the hole on larger arches.

Brice,

They pivot through the rail is a 5/16" x 24 bolt 1 1/4" long, so the rail rotates on the shoulder, not the threads. I use a nylock nut, set to "snug" the rail.

The stops for the router are the rail stops that come with the saw. I set up to demonstrate how to make this process work, then realized I only own 1 stop. DOOOOHHHHhhhhh (Now if that Tom guy from Tool Home could remember I forgot [and never told him I needed] to order another stop, that would be customer service.) Once set with 2 stops the router will not move, but it will lift straight up.

I have routed 1" cherry with this set up. 2 passes. No issues what so ever. Seeing as it is easily adjusted, you could make a clearing cut, then adjust to the proper dimension and make the final cut. Biggest issue I've had with real long radiuses is the surface your working on shifting a little. That is why this is on the floor.

I neglected to mention that the vertical flex in the guide rail will affect the cut. You ned to support the intermediate sections of the rail with material the same thickness as the pivot support and the piece being machined (these should be the same thickness). I space mine about every 2'.

Sorry about the lousy pictures, all taken with my iPhone. Sometimes I can't hold it steady enough when I click the shutter.
Tom

You can view the set up as a right triangle, the base being the distance from the center of the pivot point of the rail to the center of the router bit, the rise being the distance from the base line to the center of the bit, the hypotenuse being the center of the pivot point to the center of the router bit.

If you need an arc with a 4" rise, and a cord of 20" you have to use a 14 1/2" radius. Using the 14 1/2" hypotenuse you will have a right triangle with a 4" rise and a 13 15/16 base. A straight line is a straight line does not matter if it is the base or hypotenuse, as long as the number is correct.

I know I used the center in the above example and there will be an adjustment for bit diameter. The 14 1/2" hypotenuse to cut on the inner edge will need to be 14 5/8".

Tom