Losing my marbles

[Malcolm Wheeler]

I have concluded that the following is impossible to achieve, but before I admit that, I figured I should ask here for some advice.

I am building a marble machine, and I need advice on how to route a narrow small radius inside (concave) surface, using a plunge ball bit. The objective is to make open topped, partially enclosed tunnels in the narrow curved edge (one inch wide) of a block of wood. (see router bit picture below to see what I mean).

I have attached two pictures of the kind of radius I am talking about to try and illustrate the concept. *HOWEVER* the pictures are misleading in that they have an open topped channel (easy to route with an internal bull nose bit) but my goal is the partially enclosed tunnel.
concave1.jpgconcave2.jpg

The radius I need is about 4 inches, but please note I do not have a small trim router – my router motor is 4 inches in diameter. I thought initially that I could build two templates either side of the block of wood for the router motor to ride on ( with the base plate removed ) but I don’t think the router bit will extend out far enough. I have seen bit extensions for sale, but don’t they vibrate like crazy?
As I said – I think it is impossible to achieve (unless bit extensions really really work) – I think I will have to route two surfaces with an internal bull nose, glue them, then partially cut one side off on the band saw.

Here is a picture of the bit I want to use from the Lee Valley website.


Here is a picture of the bit I think I will be forced to use at the end of the day and glue...


Thanks for any advice!
regards
Malcolm

[Jim Matthews]

Rip the board in half, route the channel and reassemble with glue?

There should be plenty of ways to hide the glue line.

[Anthony Whitesell]

Router bit extensions do work. Though some accuracy is lost due to slop and the geometeric math of being further away from your reference point. Even so, the marble bit in the first picture has its depth set by the base plate of the route. So in general terms, only works on flat surfaces. I'm not sure how you would control its depth on an side corner.

As for the second bit, nothing says the bit must be fully buried in the wood as shown in the example cut. If you allow the top portion of cutter (shank side as shown) to protrude from the stock, you can easily get one half of the cut as shown with the first bit. As Jim said, then glue the two halves together.

[Ken Shoemaker]

+1 for Rip, Route, and Glue