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  Porter Cable Router Fence  
  Model # 4269 & Model # 42700  
The Porter Cable Router Guide is a well-built, sturdy, and highly accurate accessory. For cutting radiuses, I've often used a Porter Cable router with a PC Router Guide attached to a 1x4 trammel arm.

To use the fence with a trammel arm, the guide must be reversed then fastened to the to the trammel arm.  I made a wooden transition block, which I screwed to the guide.
I also notched the transitional block to accept a 1x4, so it's easy to change out the trammel arm (which happens for almost every job). The transition block is about 1 1/4 in. thick, that allows the trammel arm to sit flat on the work table, while the router is sitting on 3/4 in. stock.
Remember, when you lay out the holes in your trammel arm, add for the thickness of the router bit. 
The outside of the router bit cuts the inside of the radius; the inside of the router bit cuts the outside radius.  I use an aggressive up-cutting 1/2 in. bit, so the trammel-point hole spacing for 3 1/2 in. casing is 4 in. The other holes in that trammel arm are for flutes, which are cut with the same set-up, but a different bit.
The PC router guide works so well that we've adapted them for use with our Bosch 1613 Plunge Routers (a much more user-friendly tool).  We tapped the bar stock at the centers for the Bosch router, and drilled holes to fasten the adaptor to the router guide.
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