Here is a photo of some octagonal spindles that have been dried in my kiln. Along with the spindles are a tenon cutter and a block of wood with holes in it that correspond to the size holes that I drill for the spindles.
Normally I am not interested in working with jigs where basic skill can be employed. The skill that I practice often opens doors to new and exciting abilities that I'd hate to sacrifice for the certainty of a jig. However, this is one instance where I have found a simple way to speed my process and improve my results.
The way that I have always formed the tenons on my spindles is by shaving them until they fit snugly in a hole in the block. It works fine and is wonderfully cheap. The problem came when I called on my students to do the same and found it to be a stumbling block. Often I saw beautifully made spindles lost in the last act. So I started using the tenon cutters, not to form the whole tenon (this wouldn't work anyway because they don't follow the fibers), but to put a circular reference directly on the end of the tenon. With this reference, all of the excess wood is made entirely visible and can be removed quickly with a drawknife. By leaving the spindle in the octagonal shape, the efforts is further reduced.
As you can see, the tenon that I form with the cutter (in a Jacobs chuck in my lathe) is only about 1/8 of an inch long. It is important that the end of the spindle is cut square, otherwise the tenon may form off to one side a bit.
After shaving the octagon a hair larger than the tenon, I round down the tenon and go back to my trusty block of wood to finish it off. The image below shows the tenon in the hole. I always hold the piece up to the light so that I can see where the light shines through. This reveals the high spots for me to shave.
After a good amount of use, the hole in the block becomes oversized and I use it for my initial sizing and drill a fresh hole for the final shaving. It is important to note that drill bits can drill different sized holes in different species of wood. I always test my first tenon in the actual seat as well as the block to understand the relationship between the holes. I must admit that at first I told myself that this was a good solution for students but not for me, perhaps a bit of misplaced ego! Now I use this on almost every chair and enjoy the simplified action of getting beautifully tight tenons,