I make a lot of ornamental birdhouses and a few skep looking beehives. When turning the bases of the houses and the top of the hives I have always used a wooden shop made mandrel. I have actually broken the wooden one in the past. The pic on the left is of a repaired break.
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The wooden one has served me well making onsey twosey or even half a dozen ornaments but I recently got an order for 92 of them. I use a one inch carbide Forstner bit to bore the inside of the houses, works great except that the ID of of the bores even with the same bit varies slightly with wood specie. My solution in the past has been to add or remove a layer of plastic electrical tape to maintain a tight enough fit for turning the wood. Long story short, I got tired of taping and un-taping. I decided I needed an adjustable mandrel so I chucked up a chunk of aluminum in my metal lathe, with some turning, drilling, and sawing I came up with this.

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It has slots in the side walls that vane like pieces of 1/8" UHMW can be pushed out against the inner wall of the BH blank. These two pics only show one set of slots but another set was added 90degrees to this set after the photo. You can see two of the four vanes in the pic above. The end cap screws onto the end with an internal thread and captures the four sections of aluminum plus the ends of the UHMW vanes. The second pic shows the cap in place. The extra aluminum in the chuck was turned away later.
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This tapered pen is eventually threaded just behind the taper and is screwed into and out of the mandrel to raise and lower the vanes. The vanes were initially cut over size in diameter of the mandrel, they were sticking out of the mandrel by about an 1/8". I set the initial lowered height of the vanes by having the tapered pin retracted to its max retract position then turning off all of exposed UHMW flush with the aluminum of the mandrel.
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Then I cut slots around the circumference of the mandrel, through the aluminum and the vanes to fit in o-rings. The o-rings are 5-10 thousandths proud of the vanes and mandrel with the whole mandrel being just under one inch in diameter. When the tapered pen is screwed in it lifts the vanes and the o-rings against the ornament blank wall holding the blank tightly on the mandrel. The o-rings serve two purposes, they add friction plus they hold one end of the vanes inside the mandrel body, as mentioned a cap holds the other ends.
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The hardest part of making this thing was cutting the 1-1/4"-8tpi threads in the lathe end of the mandrel. My shoulders were sore for two days after completing that task.
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Since making and using the mandrel I have turned about 20 ornaments with it and it works great, absolutely no tape required. lol ;-)

Thanks for looking and C&C appreciated.