SO, the columns are finished top and bottom. They would likely work this way but don't look very elegant. Let's cut the center 3/8" section to a nice visually attractive taper. They were kept at 3/8" till now so they could be held easily to finish the ends.
The plan was to grab the top in a four jaw on the lathe. Also put a small block at the base screwed on with a set screw as they mount to the plate while in use. The other end of the short block was center drilled for a tail stock center. This plan had a couple of problem. There is little space left at the top for the four jaw when space is left for the lathe tool. I had trouble grabbing the top adequately. At the tail stock there were other problems. The block tended to un-thread pushing the part towards the spindle and eventually coming loose. The coming loose part was pretty exciting, even on the Sherline.
I finally found a 1/4" collet that would grab the top better than the four jaw. I just used the threaded hole in the base for the tail stock center. It is just decorative so a few thousands isn't critical, and the center didn't harm the threads. The set up used is like this.
The G-code that CamBam produced reduces the diameter by 0.005" depth cuts. This gave a nice finish and didn't take too long. The taper was approximated with steps, then cut smooth. One finish pass further smoothed things out. In this pic you can see the stepped taper approx. nearing the end of the cycle.
Here is a picture of a column before thinning and one after.
I'm still building three engines, although I'm down to two cranks right now. Here is a picture of the three top plates with columns.
And finally a pic of one up to this point. I haven't cupped the screws to hold the crank, but this give an idea of the assembly.
Still a ways to go but they seem to be coming together.
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But I fear I've run out of time for this winter. The Traveling Sherline Show is going into storage till next fall. It takes up quite a bit of space on the road, but the tent to catch chips is too big to take. It's quite involved to set up the tent, so we need to be fixed for at least a few weeks. I don't see this happening till fall. So the Stirling is on hold till then.
Thanks for coming along.
Hugh