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In October 2008 we acquired a new mount to hold the telescope in our backyard observatory. A 10"-diameter adapter plate is used to attach the telescope mount to a concrete pier. Four holes needed to be drilled and counterbored in this plate to match the ½" bolts embedded in the pier.
This is child's play if the milling machine is large enough to accommodate the plate. Unfortunately, my milling machine is not large enough. However, there's plenty of space for the plate to protrude off to the side. I found a scrap length of aluminum, then drilled and tapped two holes to match existing holes in the plate. I bolted this "handle" to the bottom of the plate, and clamped it in the milling machine vise. After some trial and error, the plate was properly positioned under the milling machine spindle.
First I drilled the hole all the way through, then switched to an end mill and counterbored for the bolt head. For the next hole, I unbolted the plate from the "handle" still clamped in the vise, rotated the plate 90°, and bolted it to the handle again. I repeated this process until all four holes were done.
Here is the adapter plate installed on the concrete pier. The socket head cap screws thread into ¼"-20 holes drilled and tapped into the ends of the ½" pier bolts.
Updated September 7, 2016