Our objective here is to cut three equally spaced and perfectly straight fin slots.  Of course they need to be the right length and width too!  The photographs below will show how this was done. You can click on any of the photos here to see a larger image.

Building this fixture took all evening. Setup was just a few minutes and cutting the perfect fin slot only took a few seconds!

The length of the slot can be precisely set by putting the router bit up against the end of the tube and then measuring the desired length from the edge of the sled.  The length is then marked on the top of one of the side pieces using a bit of blue tape.  Then it is just a matter of stopping the sled at the blue tape.

The sled is near the ending position as marked by a piece of blue tape.

When rotating the tube, it will likely move lengthwise a little bit, so it is necessary to double check the slot length each time.  The ending mark may have to be adjusted if the body tube is not positioned lengthwise to exactly the same point.

By the way, an astute observer will notice that the fin slot cut here is much longer than needed for the fins on Coldfire.  This is because Coldfire went through a redesign part way through the construction process in order to save weight.  Originally it was longer and had a different fin design with a much longer root edge. However, it became clear Coldfire would be too heavy and the design was changed.  This tube had already been slotted by then, but it could still be used by simply cutting the tube off at the end that was slotted. That left a shorter slot that was perfect for the new fin design.