Starfire Fin Slotting

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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! This shows the body tube after slotting it for the fins. The photographs below will show how this was done.

You can click on any of the photos here to see a larger image.

This is a simple wood fixture for slotting 7.5" body tubing using a router. The bottom is a piece of 1x12 hardwood that is four feet long. The sides are 1x 8 and also four feet long. The "sled" on top is made from some 1x3 and 1x2 pieces as well as a square piece of 1/4" thick acrylic. Holes are drilled into the acrylic to allow it to be bolted to my router base. There is a 1" diameter center hole in the acrylic for the router bit and collar to extend through. The transparent acrylic makes it very convenient to be able to see the alignment of the router bit to a mark on the body tube.

The extra width on the bottom piece that extends past the sides makes it very convenient to clamp the whole fixture to a table top.

The fixture was built so that a standard 7.5" body tube with three layers of glass on it will exactly fit.

The blue tape around the end of the tube is my way of marking three equally spaced positions for starting the fin slots. The blue tape is first wrapped around the tube and marked at the overlap point. I then remove the tape and lay it out in a straight line. The marks at the overlap point are now at opposite ends of the tape. It is then a simple matter to divide the length between these marks into thirds. Each 1/3 distance is then marked with a line across the tape. The tape is then rewrapped around the body tube being careful to realign the original marks at the overlap point. The result is three equally spaced marks for starting the fin slots. One can be seen in this photo.

My Craftsman router is mounted to the "sled" with three small bolts. (Not shown.)

This shows how the router bit is positioned for cutting the fin slot. I used a 3/16" straight cut bit. The fins are 1/8" thick so this bit produces a slot that is slightly wider than the fins to allow for tolerances and alignment errors between the slots and fins.

It is a good idea to friction fit a tight fitting bulkhead into the tube before cutting the slots. This helps to support the sections of the tube that are cut apart. Otherwise, they can bend inward and will get nicked by the bit and the resulting slot width will not be as precise. Of course the router will slot right through the bulkhead, but the bulkhead can still be reused many times. I drilled two finger holes in the bulkhead to make it easier to take in and out and to help with positioning it.

You can also (sort of) see the two clamps I used at the opposite end of the tube to clamp the tube to the sides of the fixture to keep it from rotating. These clamps can also be seen from inside the body tube in the previous photo just above. Click on these photos to see a larger image.

Everything is ready to go.

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. Then it is just a matter of stopping the sled at the mark.

Building the fixture took all evening, setup was just a few minutes, but cutting the perfect fin slot only takes a few seconds!

Another view. The sled is stopped at the ending position as marked by a piece of blue tape.

Rotate and repeat as needed!

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.

The finished article.

The slotted body tube slides over the top of the fin can section.

A perfect fit!

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